I started as a contributor for the blog network addictionblog.org.
So the Huffington Post runs a sub-blog on Addiction and Recovery and sometimes they present excellent reporting (for example, the piece on opioid addiction by Jason Cherkis who actually interviewed my boss, Dr. Mary Jeanne Kreek, for the article). But more often than not, they present quite variable reporting on addiction. A recent interview with psychologist Marc Lewis, PhD is one such example.
Based on my own neuroscience of addiction background, I unfortunately find a number of Dr. Lewis’s claims not supported by scientific evidence and I believe the spread of such false statements can have the exact opposite of his intended effect—hurting more addicts rather than helping them. I do not claim to be the consensus voice of the addiction field but present my own arguments based on my own research and work done in the field. I also admit have not read any of Dr. Lewis’s books and am merely responding to the statements made in his interview. I include references at the end of the post.
The original interview between Carolyn Gregoire, Senior Health and Science Writer for Huffington Post and psychologist Marc Lewis, PhD
The questions (Q) by Carolyn Gregoire in the original interview are in bold, Dr. Lewis’s response (L) is italicized, and my response (S) is the un-italicized larger-size text.
Q: What’s wrong with the disease model of addiction?
L: I know what scientists are looking at when they say addiction is a disease. I don’t dispute the findings, but I dispute the interpretation of them. They see addiction as a chronic brain disease — that’s how they define it in very explicit terms.
My training is in emotional and personality development. I see addiction as a developmental process. So the brain changes that people talk about and have shown reliably in research can be seen as changes that are due to learning, to recurrent and deep learning experiences. But it’s not an abnormal experience and there’s nothing static or chronic about it, because people continue to change when they recover and come out of addiction. So the chronic label doesn’t make much sense.
S: The brain is a physical organ that operates under defined molecular biological principles. Drugs are physical chemical substances that perturb the molecular function of the brain. It is true that addiction is a process that can take months or even years to develop but the end result is a physical neurobiological change in how the brain functions [1, 2]. And when neuroscientists say chronic brain disease—or what my lab says A disease of the brain with behavioral manifestations—what we mean is that repeated drug use has caused a change is brain function which in turn results in a change in behavior. That doesn’t mean that this change is irreversible but, like other diseases, the first step to treatment is recognizing the underlying biological cause. Defining addiction as a chronic brain disease is not a judgment or interpretation of the development of addiction (which definitely does involve a learning and memory component [3, 4]) but is a statement asserting that drug addiction and drug cravings, compulsive drug use, and relapse are ultimately based on physical changes in the brain. It is important that we recognize this because otherwise we would not be able to treat it with effective and safe medications, in combination with other behavioral and psychological therapies.
Q: What’s problematic about the way we treat addiction, based on the disease model?
L: Well, lots. The rehab industry is a terrible mess — you either wait on a long list for state-sponsored rehabs that are poorly run or almost entirely 12-Step, or else you pay vast amounts of money for residential rehabs that usually last for 30-90 days and people often go about five to six times. It’s very difficult to maintain your sobriety when you go home and you’re back in your lonely little apartment.
What I emphasize is that the disease label makes it worse. You have experts saying, “You have a chronic brain disease and you need to get it treated. Why don’t you come here and spend $100,000 and we’ll help you treat it?” There’s a very strong motivation from the family, if not the individual, to go through this process, and then the treatments offered in these places are very seldom evidence-based, and the success rates are low.
S: I strongly agree with this assessment. The rehab industry and many 12-step programs are ineffective, expensive, and rarely based on scientific evidence. The primary reason is that for decades addiction was thought of a problem of “spiritual weakness” or “lack of will power”. In reality addiction is a medical disorder based on physical neurobiological processes that make it seem like an addict has no “will power”, when in reality that addict’s brain has been hijacked to crave the drug compulsively and practically uncontrollably. However, again, I disagree that calling addiction a disease is what funnels people into rehab clinics. I believe it is the stigmatization of addiction that precludes treatment by doctors (unlike for every other disease), which in turn fuels admission into the rehab industry. Sadly, effective medications exist (such as methadone and buprenorphine for opioid addicts) that can flick a switch off in an addicts brain, satisfying their craving and allow them to live a normal live [5, 6]. Or medications such as naltrexone may be effective at reducing drinking in alcohol addicts but is not widely used [7, 8]. It is only recently that public acknowledgement of the biological basis of addiction and appropriate shifts in public policy are beginning to take place. Importantly, addiction medicine is beginning to become incorporated into medical school education and the first accredited residency programs in addiction medicine have been announced.
Q: There are lots of ways to trigger a humanistic response besides calling something a disease. So you would say that telling people who are in recovery for addiction that they have a “chronic disease” is actually doing them a disservice?
L: Well, the chronic part is really a yoke that people carry around their necks. [Proponents of the disease model] say that this is important because this is how to prevent the stigmatization of addicts, which has been a standard part of our culture since Victorian times.
But I think that’s just bullshit. I don’t think it feels good when someone tells you that you have a chronic disease that makes you do bad things. There are ways to reduce stigmatization by recognizing the humanity involved in addiction, the fact that it happens to many people and the fact that people really do try to get better — and most of them do. There are lots of ways to trigger a humanistic response besides calling something a disease.
S: I agree that stigma is a huge problem with the treatment of drug addiction and mental health. Admitting you are an addict or depressed or know someone who suffers from these disorders is accompanied with unnecessary shame and fear of admission of the problem. I disagree that acknowledgement of medical/neurobiological basis of these disorders (ie calling them diseases) increases stigma but in fact do humanize patients. It helps alleviates shaming–both public and self–and can help an addict to seek evidence-based, medical treatment. Acknowledging the chronic nature of the disorder is not intended to make people feel bad but is merely truthfully stating the nature of the problem in hopes that it can be properly treated; denial can be lead to false and ineffective treatments.
Q: It can be difficult to comprehend the idea that something as severe as a heroin addiction is a developmental process. Can you explain that?
L: First of all, let’s include the whole bouquet of addictions. So there’s substances — drugs and alcohol — and there’s gambling, sex, porn and some eating disorders. The main brain changes that we see in addiction are common to all of them, so they’re not specific to taking a drug like heroin, which creates a physical dependence. We see similar brain changes in a region called the striatum, which is an area that’s very central to addiction, which is involved in attraction and motivational drive. You see that with gambling as much as you do with cocaine or heroin. So that’s the first step of the argument — it’s not drugs, per se.
From there, it’s important to recognize that certain drugs, like opiates, create physical dependency. There’s a double whammy there. They’re hard to get off because they’re addictive, like sex or porn is, but they also make you uncomfortable when you stop taking them. People try to go off of them and get extremely uncomfortable and then they’re drawn back to it — now for physical as well as psychological reasons.
S: It is true that all addictions involve the striatum and there are similarities between the different addictions but to say that ALL addictions affect the brain in the exact same way is an absurd simplification. Different drugs absolutely DO affect the brain differently and have differences in addiction potential and relapse potential. To say addiction to heroin is identical to addiction to alcohol is identical to gambling addiction and therefore has nothing to do with the specific drug or behavior is just plain wrong. A wealth of evidence is gathering that addictions to different drugs progress differently and effect different brain systems, despite certain changes common to all . For example, even opioids such as morphine and oxycodone, whose pharmacology are probably the best understood of any drug of abuse (they interact with mu opioid receptors ), have different behavioral and neurobiological effects that may affect addictions to the individual drugs (see my blog post). In a paper published by the lab I work for, the Kreek lab, cocaine administration in drug naïve mice (mice that have never had cocaine in their system) results in a rapid release of dopamine . In contrast, some studies show that self-administration of an opioid drug only increases dopamine in rats that have already been exposed to the drug and not naïve animals . The differences in the dopamine profiles between cocaine and opioids obviously means that how these two drugs affect the brain is different and is drug-specific! These are just a few small examples demonstrating the scientific inaccuracy of lumping all addictions into one general category or making the false claim that addiction has “nothing to do with the drug” (just as reducing cancer to a single disease is entirely inaccurate and harmful for its treatment).
Q: In the case of any type of addiction, what’s going on in the brain?
L: The main region of interest is the striatum, and the nucleus accumbens, which is a part of the striatum. That region is responsible for goal pursuit, and it’s been around since before mammals. When we are attracted to goals, that region becomes activated by cues that tell you that the goal is available, in response to a stimulus. So you feel attraction, excitement and anticipation in response to this stimulus, and then you keep going after it. The more you go after that stimulus, the more you activate the system and the more you build and then refine synaptic pathways within the system.
The other part of the brain here that’s very important is the prefrontal cortex, which is involved in conscious, deliberate control — reflection, judgment and decision-making. Usually there’s a balance between the prefrontal cortex and the striatum, so that you don’t get carried away by your impulses. With all kinds of addictions — drugs, behavior, people — the prefrontal system becomes less involved in the behavior because the behavior is repeated so many times. It becomes automatic, like riding a bike.
S: Dr. Lewis’s assessment is basically correct. The core of the reward circuit involves dopamine-releasing neurons of the ventral tegmental area (VTA) projecting to the nucleus accumbens (NAc; a part of the ventral striatum), which primarily drives motivated behavior and is involved in reinforcement of drug taking behavior. Conversely, the prefrontal cortex acts as a “stop” against this system and one model of addiction is the motivated-drive to seek the drug overpowers the “stop” signal from the prefrontal cortex. However, addiction is far more complex beyond just this basic system. Numerous other circuits and systems (hippocampus, amygdala, hypothalamus, just to name a few) are also involved and each individual drug or rewarding stimuli can affect these circuits in disparate ways .
Q: What would a scientifically informed approach to addiction look like?
L: That’s a really hard question because the fact that we know what’s happening in the brain doesn’t mean that we know what to do about it.
A lot of recent voices have emphasized that addiction tends to be a social problem. Often addicts are isolated; they very often have difficult backgrounds in terms of childhood trauma, stress, abuse or neglect — so they’re struggling with some degree of depression or anxiety — and then they are socially isolated, they don’t know how to make friends and they don’t know how to feel good without their addiction.
S: As I’ve stated above, a scientifically informed approach to addiction treatment already exists but is not widely used. However, one day an addict will hopefully be able to consult with a medical doctor to receive appropriate medications specific to their addiction, which will be combined with individual counseling by a psychiatrist or psychologist and a specific cognitive behavioral therapy or other psychological/behavioral therapy. The combination of medications and psychological therapy administered by trained medical professionals will be the future of evidence-based addiction medicine. Development of additional medications and/or psychological therapies for future treatment absolutely requires solid scientific evidence supporting their efficacy, which includes use of randomized control trials, prior to widespread implementation.
But to call addiction primarily a social problem once again ignores all the basic neuroscience research that shows the powerful effects drugs have on the brain. It also ignores the prominent effect of genetics and how, due to a random role of the dice, an individual’s risk of becoming an addict can drastically increase [2, 13]. Plus the opioid epidemic that is currently sweeping the nation effects nearly every strata of society regardless of socioeconomic status, age, gender or race, and therefore cannot be explained simply by the hypothesis that addicts are people that are socially isolated. Why someone starts using drugs in the first place and how exactly they progress from a casual drug user to an addict are incredibly complex questions that scientists all over the world are attempting to answer through rigorous research. Being socially isolated or experiencing childhood trauma may certainly be factors that eschew some people towards the development of addiction but are definitely not the only ones.
Q: So what can we do about that?
L: Other than certain drugs that can reduce withdrawal symptoms, there’s nothing much medicine can offer, so we have to turn to psychology, and psychology actually offers a fair bit. There’s cognitive behavioral therapy, motivational interviewing, dialectic behavioral therapy, and now there are mindfulness-based approaches, which I think are really exciting.
There’s been good research from Sarah Bowen in Seattle [on Mindfulness-Based Relapse Prevention] showing that mindfulness practices can have a significant impact on people, even on people who are deeply addicted to opiates.
S: This is a completely false statement: medications for treatment of addictions exist ! Once again, comprehensive systematic reviews of methadone and buprenorphine, two medications used for treatment of opioid cravings, have indisputably shown that these medications are effective at keeping addicts off of heroin compared to no medication [5, 6]. Furthermore, a number of other drugs are currently being explored for treatments to alcohol and cocaine addiction [15, 16]. Some people may consider methadone or buprenorphine replacing “one drug with another” but this is naïve view of how powerfully addictive opioid drugs can be and how use of these FDA-approved medications in combination with individual psychological counseling, can lead to gradual dose reduction and amelioration of cravings. Medication-assisted addiction treatment is designed to help addicts fight their craving so that they can live a normal life. With time, dose can be reduced and cravings can become less intense.
The study Dr. Lewis cites regarding mindfulness is well designed and intriguing. However, the study did not compare mindfulness-based approaches to medication-based approaches and is therefore incomplete . Nevertheless, it is an interesting approach that may be able to be combined with medication-based treatment but definitely requires more research before its efficacy can be confirmed.
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- Kreek MJ, et al. Opiate addiction and cocaine addiction: underlying molecular neurobiology and genetics. The Journal of clinical investigation. 2012;122(10):3387-93.
- Kelley AE. Memory and addiction: shared neural circuitry and molecular mechanisms. Neuron. 2004;44(1):161-79.
- Tronson NC, Taylor JR. Addiction: a drug-induced disorder of memory reconsolidation. Current opinion in neurobiology. 2013;23(4):573-80.
- Mattick RP, et al. Methadone maintenance therapy versus no opioid replacement therapy for opioid dependence. The Cochrane database of systematic reviews. 2009(3):CD002209.
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- Hartung DM, et al. Extended-release naltrexone for alcohol and opioid dependence: a meta-analysis of healthcare utilization studies. Journal of substance abuse treatment. 2014;47(2):113-21.
- Badiani A, et al. Opiate versus psychostimulant addiction: the differences do matter. Nature reviews Neuroscience. 2011;12(11):685-700.
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- Zhang Y, et al. Effect of acute binge cocaine on levels of extracellular dopamine in the caudate putamen and nucleus accumbens in male C57BL/6J and 129/J mice. Brain research. 2001;923(1-2):172-7.
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- Bidlack JM. Mixed kappa/mu partial opioid agonists as potential treatments for cocaine dependence. Advances in pharmacology. 2014;69:387-418.
- Bowen S, et al. Relative efficacy of mindfulness-based relapse prevention, standard relapse prevention, and treatment as usual for substance use disorders: a randomized clinical trial. JAMA psychiatry. 2014;71(5):547-56.
Why is it that one person becomes an addict and another does not?
This is a central question in addiction field and one that I’ve touched on in some of my posts (and will continue to explore in the future). Two recent papers may help to shed more light on this difficult and complicated question. Both studies have revealed changes that occur in the brain as a result of childhood trauma that may cause an individual to be more susceptible to risky behavior such as drug abuse.
Both papers are neuroimaging studies meaning they use living human subjects and look at brain activity in response to different scenarios. There are many ways to image a living brain but these studies both use functional magnetic resonance imaging (fMRI). Basically, fMRI measures blood flow into the brain. As neurons turn “on” (that is, when they conduct an electrical signal), they require energy. Neurons use glucose as their primary energy source, which is delivered to them through blood flow. Therefore, the more blood flowing to a region of the brain = the more energy required by neurons = more neurons “firing”.
The analysis of fMRI data is very complicated and beyond the scope of my knowledge or this discussion. But in essence, when you think or read about something, certain areas of your brain process that information. Using fMRI, you can actually visualize regions of the brain that are turning “on” or “off” when a patient thinks about a particular situation! Watch these YouTube videos for additional explanations on fMRI.
In both of the studies featured in today’s post, subjects would read different scripts while in the fMRI scanner and the scientists would image the entire brain and identify the regions that were active during the test. Then data from multiple subjects can be compiled and a composite image that represents the averages all the subjects can be produced. The picture to the right is an example of this type of composite image. Finally, you can see which regions of the brain are active for most of the patients during the different experiments. Keep this information in mind as I go over the papers.
The first paper performed fMRI scans on adolescents that had or had not experienced maltreatment or trauma during childhood (less than 18 years old). 67 subjects were recruited from a larger study looking at disadvantaged youth and 64 were eventually used in the study. The adolescents filled out a standard survey that allowed the scientists to learn which of the subjects had experienced maltreatment/trauma during childhood.
The experiment involved having the different subjects read a script about either a stressful moment, their favorite food, or something neutral or relaxing while their brains were being imaged in the fMRI scanner.
Amazingly, for the stressful scenario, a difference in brain activity was detected in multiple regions of the prefrontal cortex only in subjects that had experienced childhood maltreatment! What this means is those youths that were abused as kids responded to stress differently than youths that were not abused. Their brain function has literally been changed later in life as a result of the abuse they suffered as children.
The prefrontal cortex is a part of the mesocorticolimbic system, a group of brain areas especially involved in addiction. The prefrontal cortex is also involved in decision making, impulsivity, and other functions. It’s not clear what this change in prefrontal cortex activity actually means but it is possible that the altered activity could make the youth more vulnerable to stress or more likely to engage in risky activities, such as drug abuse.
The second study was also interested in subjects that had experienced maltreatment or trauma during childhood but it instead of adolescents, this study used subjects that are adult men dependent on cocaine. Similarly, the subjects were grouped into those that had been mistreated as kids and those that had not.
In a parallel design to the other study, the subjects read a script describing a situation while being scanned in the fMRI machine. The scripts in this study included stress, cocaine-associated, and neutral. Interestingly, an increase in activity in a specific region of the prefrontal cortex and an area of the brain involved in motor activity were detected in the subjects that had been abused during childhood. And even more important, these changes were correlated to enhanced drug craving. These results suggest that childhood trauma can affect drug craving for addicts, which may be relevant factor in triggering relapse. That is to say, addicts that have been abused as children may be more vulnerable to not only acquiring addiction but also relapse.
It is important to keep in mind that, like the previous study, the real functional importance of these different changes in unknown. However, clearly there are real changes that occur in the brain as a result of abuse/maltreatment during childhood. Imaging data must be taken with a grain of salt because it is difficult to show real causality. Yet, both studies (and many others) suggest long-lasting changes in brain activity, especially in response to stress, as a result of childhood trauma/maltreatment.
The conclusions we can draw from these studies is that childhood mistreatment, or trauma can have lasting changes on the brain. How these changes affect behavior is a much more difficult question to answer. Nevertheless, the changes that occur may be one of the factors that can contribute to susceptibility to addiction. These studies are supported by a previous post in which animal studies have shown that stress during early age leads to greater drug use as an adult.
And a broader point, these two neuroimaging studies help to put a different perspective on disadvantaged youth and importance of a stable home life, the lack of which can significantly affect you as an adult and may even contribute to susceptibility of become a drug addict.
This is part three on my series of posts looking at Stress and Addiction. To recap: we’ve seen that, in laboratory studies, stress increases susceptibility to drug addiction. Stress not only increases the self-administration of drugs in adult rats but stress during an early age can have a long-lasting effect on drug-taking behavior. Today, we’ll wrap up by looking at some molecular changes that might help to explain why this effect exists. I’ll conclude by addressing some questions that might have occurred during the course of this discussion.
The first paper is examining the effects of stress and cocaine on dopamine. Dopamine is a very important neurotransmitter. All drugs of abuse cause increases in dopamine in an important region of the brain called the mesolimbic pathway. I will discuss this system in detail in the next post but for now don’t worry about the details. All you need to know is that drugs can increase dopamine.
Dopamine levels can be measured directly in the brain using the technique microdialysis (I discuss this technique in more detail in my post The Scientist’s Toolbox: Techniques in Addiction). In this first paper, the scientists use a type of stress called foot-shock stress. It is very similar to tail-pinch stress (see Part 1). The animals are placed on a grid that is connected to an electrical supply. The scientists administer a small amount of electric current to the grid, which gives the animals feet a little shock and stresses them out.
The microdialysis technique was used on rats that underwent foot shock stress in order to measure dopamine levels (in a region of the brain called the striatum) after the stress test. As you can see in the top graph of Figure 1, foot shock stress causes an immediate increase in the amount of dopamine released and this eventually returns to normal. The different symbols mean different stress intensity with the most intense stress represented as black squares. Interestingly, as you can see in the lower graph of Figure 1, a more intense foot shock (ie a more intense stress) causes more dopamine to be released.
Remember that I said that cocaine also causes dopamine release? So maybe stress makes cocaine feel better because it works together with cocaine to create a larger release in dopamine than cocaine would by itself. Next, the investigators decided to test this idea.
In this experiment, mice were exposed to a weak foot-shock stress then given an injection of cocaine and the amount of dopamine released was measured. Figure 2 shows much more dopamine was released in the striatum in rats that received cocaine + stress (black squares) compared to cocaine + no stress (white squares) or just stress by itself (black circles). Perhaps the hypothesis that stress makes cocaine more pleasurable because its boosts dopamine released might be true?
Recall from Part 1, that stress activates the HPA axis, which results in release of the stress hormone cortisol (corticosterone in rats and mice). But do drugs of abuse also activate the HPA axis? This next paper—done in lab that I work in—takes a look at this question.
Cocaine was given to rats under a number of different conditions. In the first experiment, cocaine effects on the HPA axis were examined in the short term (acute cocaine use). Rats were injected with either saline for two days, cocaine for 1 day, cocaine for 1 day and saline for 1 day, or cocaine for 2 days. After the injections, blood was drawn from the animals and the corticosterone in the serum was measured.
*Technical notes: 1) Serum is the liquid part of blood and it does not contain the red blood cells and clotting proteins. Serum is often used when measuring hormones in the blood. 2) Corticosterone can be measured multiple ways but this experiment used something called a radioimmunoassay (RIA). I’ll save the explanation of it for a future Scientist’s Toolbox post.
As you can see in Figure 1, immediately after the rats receive cocaine (either 1 day or 2 days) corticosterone increases. This means that cocaine has resulted in activation of the HPA axis. Interestingly, the animals received 1 day of cocaine and 1 day of saline did not show high corticosterone levels which means that the levels have returned to normal after the cocaine.
But what happens with repeated cocaine use (chronic cocaine use)? Addiction develops because of chronic use of the drug so are any changes occurring after many days of cocaine use?
Interestingly, in Figure 2, corticosterone is high after 3 days of cocaine but after 14 days of cocaine corticosterone levels are much lower! What’s going on here? What these data suggest is that 14 days of cocaine use has caused a change in the HPA axis activity. The cocaine has activated the HPA axis so frequently the axis has compensated for this over activation. The activity of the HPA axis response has been blunted because of the repeated cocaine use.
This one small example of how drugs can cause long lasting molecular adaptations and changes in the brain. Perhaps this is why stress helps to make someone more vulnerable to addiction, because changes occur both at the level of dopamine release (paper #1) and in HPA axis activity (paper #2). Both drugs and stress have similar molecular effects that may work together! I’d like to very briefly discuss one more paper that combines both of these concepts.
This paper is complicated but I’m just going to present a small amount of the data. The key points you need to know is that the scientists are using social stress (see Part 1) in this paper for two key experiments 1) self-administration to measure cocaine taking behavior and 2) microdialysis to measure dopamine release. However, they also inject a chemical compound directly into the rat’s brain that blocks HPA axis activity. This chemical acts at the starting point in the HPA axis: the activity of CRF is blocked (the chemical name is abbreviated as CP). Let’s see what happens in this experiment!
*Technical notes: 1) the drug actually prevents the action of CRF interacting with its receptor. Chemicals that do this are called antagonists. Therefore, the scientists are injecting a CRF Receptor antagonist into the rat brains. 2) as a control, an inactive solution is also injected into some animals. This is called artificial cerebral spinal fluid (aCSF). For the drug studies, the correct comparision is CP vs aCSF.
Like we saw in other papers, stress increases self-administration (Figure 1, black circles) compared to no stress (white triangles). However, when you give the CP at a high dose (light grey circles) compared to a low dose (dark grey circles) it reduces the self-administration! This means that blocking HPA axis activity reduces the effects of the stress on the cocaine self-administration. Cool!
Next, they did a very similar experiment but only this time measure the interaction between stress, cocaine, and the CRF antagonist on dopamine release. The results are presented in Figure 2. Animals that were stressed and than given a dose of cocaine but not the CP (stress + cocaine + aCSF, black circles) released a large amount of dopamine compared to animals that were only given the cocaine injection (white triangles), which is consistent with findings from Paper #1. Amazingly animals that were stressed and then given cocaine + the anti-HPA axis drug CP showed reduced amounts of dopamine released at bot a low dose of CP (dark grey circles) and high dose (light grey circles). These experiments show that the effect of stress on cocaine taking behavior might be because the stress activates the HPA axis which causes more dopamine to be released.
*Technical note: I described this experiments very briefly but they are extremely technically challenging and probably required months of hard work just to make the two little graphs!
Finally, if we summarize the papers from Part 1, 2 and 3 we can come up with a little mechanism to help explain the different results from the different papers. Based on the data, stress can contribute to the vulnerability of becoming an addict because it activates the HPA axis and increase the dopamine released, which may cause the drug to feel better to a person and make them want to take more of it. There may be a synergy between stress and drugs that changes brain function so that addictive drugs feel more addictive.
You probably noticed I used the word “may” many times and this is because our proposed mechanism requires a lot more testing. In fact, we barely even scratched the surface with this discussion! There are literally hundreds more papers looking at many other details just on stress and addiction. Hopefully this post and the previous two can give you a little appreciation for the difficultly in learning anything about how addiction really works and what specific changes occur in the brain from drug use! Science is a challenging and time-consuming pursuit but also totally worth it!
To wrap up our discussion on stress and addiction, I’ll address some questions/criticisms that you might have with the research papers in this and previous two posts.
Q & A
Some questions about the research you might have and my answers:
Q: Only the psychostimulants cocaine and amphetamine were looked at in these papers. Does stress have the same effects on other drugs of abuse?
A: Yes. The effect of stress is the same with nearly all drugs of abuse tested including the opioid morphine and heroin, alcohol, and nicotine. The neural machinery that is responsible for enhancing the addictive powers of drugs is common to all drugs of abuse.
Q: Only the initial stages of drug taking were looked at in these papers. That is to say, the role of stress was only discussed in the initiation of addiction. How does this translate into progression to full blown addiction?
A: The effect of stress is consistent regardless of where you are on the addiction continuum: stress enhances the reinforcing properties of drugs of abuse. That is to say, stress makes the pleasurable feeling from drugs more pleasurable. However, in humans, you will never get as clear of an effect (that means, easily testable) as you will in laboratory animals. Humans experience many different types of stress throughout a single day and the specific effect of stress on drug taking depends on the type/length/frequency of the stress and other environmental factors. Nevertheless, in controlled clinical studies, changes in HPA axis function as a result of drug use have been widely reported. A feed-forward mechanism exists in which stress promotes drug taking and then drug effects the stress response so that the next stressor has a greater effect on drug taking, etc.
Q: Can stress trigger relapse?
A: Yes, this is one of the most well studied effects of stress on drug taking: stress can trigger drug cravings in abstinent individuals. In the laboratory, an animal can be taught to lose its self-administration behavior by switching the drug to a neutral substance like saline. Therefore, when the animal nose pokes it does not get drug and eventually it doesn’t nose poke at all. This is called extinction. Amazingly, if you stress an animal with foot-shocks or some other phase and then test it’s self-administration behavior the animal will go back to lever pressing again!
Thanks again for reading! If you stuck through all three of Stress and Addiction posts please comment or email me. I would love to know!
Next time: Doping on Dopamine.
This is part 2 of a series that deals with Stress and Addiction. It one part of a multi-part series of posts in which I attempt to provide a detailed analysis of all facets of one the most important questions in the addiction field: Why does one person become and an addict and another does not?
In Part 1 I showed how some forms of stress used in the laboratory, tail-pinch and social-stress, were able to increase the amount of psychostimulants self-administered by rats. Let’s expand this discussion to consider stress that may have parallels in human society.
Note: It is important to keep in mind that l am not making the claiming that stress exposure is the cause of addiction. I am merely providing evidence that your environment, specifically stress exposure, can increase your susceptibility to becoming an addict.
Can stress during childhood effect your susceptibility to becoming a drug addict?
It’s an intriguing question with many societal implications, given the disparities in the environments that children from different socio-economic backgrounds experience in the United States (I recommend reading Jonathan Kozol if you’re interested in learning about these disparities). But I’m not a sociologist, economist, public policy expert, etc. so I’m not capable of giving you my professional opinion on all the facets of this complicated issue but I can discuss the science behind the effects of early-life stress on drug-taking behavior.
This first paper looks at rats and the effects of stress during very early life on self-administration of cocaine as an adult but first, a little primer on the rodent life cycle. Rats and mice are mammals and like all mammals, give birth to live young. Litter size can be anywhere from 4-8 depending on the strain of animal. Animals born in the same litter are called littermates (note: this term doesn’t really have a human equivalent since humans typically only give birth to a single child at a time. Though we do have the terms twins, triplets, etc.).
Newborn animals (including humans) are called neonates. The mother rodent will then nurture her pups but feeding them breast milk through her nipples. All mammals are raised in this manner (even whales). This generally occurs for about three weeks in mice. In the lab, pups are then separated from the mother by the scientist, so they can become acclimated to an adult diet (i.e. not breast milk) but naturally, young mice will stop drinking breast milk once they are old enough The process of switching from breast milk to solid foods is a called weaning. Rodents are still considered in the adolescent stage for another three weeks after weaning. They go through puberty—the biological process in which mammals reach sexual maturity—at about 6-7 weeks of age and are then considered adults. *Note: these approximate times are for mice. Rat stages fall a little later.
The first study stresses rats during the neonatal phase by handling the newborn pups and separating them from their mother for 1 hour a day for days 2-9 after the date of birth. The authors call this neonatal isolation stress. The rats are then allowed to reach adulthood. At day 100 (3 months after the end of neonatal isolation stress) the stressed rats and rats from the same litter that were not handled (control group) underwent catheter implantation surgeries and then were tested for acquisition of cocaine self-administration for several didn’t doses.
Interestingly, as shown in Figure 1, the rats that underwent the neonatal isolation stress self-administered more cocaine at lower doses (0.125mg/nose poke) and 0.25mg/nose poke). However, this effect was not seen at a higher dose (0.5mg/nose poke). These experiments suggest that early life stress can have an effect on adult rats and increases the pleasure they get from the drug and makes them more likely to self-administer cocaine at a dose they otherwise might not be interested in. The self-administration data are summarized in Figure 2.
What’s really fascinating is how a relatively brief period of stress (1hr a day for 7 days) can have such a drastic effect on the rats’ behavior 100 days later! This study suggests that stress during early life can have permanent (or at least, very long lasting) effects on brain function.
The second study takes a similar approach but looks at different developmental time period. This study isolates rats (meaning rats are placed in individual cages rather than with their littermates) during adolescence, days 24-42 after birth (the experimental group, ISO). The control animals were socially housed during this time (SOC). This time frame falls after weaning but before puberty. On day 43, ISO animals were than housed together and a few weeks later, at 12weeks of age (well into adulthood) ISO and SOC rats were tested for self-administration of cocaine.
Similar to the other study, the authors found that isolated rats acquired cocaine self-administration at a low dose of cocaine (0.0625mg/nose poke), which means that animals that underwent the social isolation stress self-administered more cocaine at a low dose when compared to the socially housed control animals. This is seen in the left half of the graphs in Figure 1 (the left panel shows nose pokes while the right panels shows the total amount of drug self-administered). However, at a higher dose of cocaine (0.25mg/nose poke), the right half of the graphs in Figure 1, there was no difference in self-administration between the two groups.
Importantly, the isolation stress also had an impact on motivation to take the drug. In the next experiment, rats were tested on a progressive ratio self-administration where they have to nose poke multiple times in order to get a single dose of drug. The number of pokes required increases everyday until the rat is no longer willing to try to get the drug. This limit where the animal gives up is called the break point and it is a measure of how hard the animal is willing to work to get the drug (ie how motivated is the animal for the drug).
As you can see in Figure 2, rats that underwent the social isolation stress during adolescence had higher break points, which indicates they were willing to nose poke more times in order to get the drug (more motivated).
In summary: these experiments also showed that stress that occurs early in the rats life (during adolescence) can have a long lasting impact on the rat brain. The stress made the rats more likely—more susceptible—to acquire self-administration behavior. That is to say, early life stress caused the drug to appear to be more pleasurable to those animals (they wanted to self-administer more of the drug) than for the control, socially housed animals.
In conclusion, these papers have shown that, in rats, stress during early life can have significant effects on an animal’s susceptibility to becoming an addict. Many other papers have identified similar findings. As I alluded to at the beginning of the post, this knowledge has disturbing implications for humans raised in drastically different environments.
However, let’s briefly discuss some caveats to these studies. You are probably wondering, “well, that’s good and well for rats, but has this effect been proven in humans?” First, one of the reasons we run these types of experiments in mice and rats because it is much easier to control for all the other variables that would make interpreting the experiment extremely difficult. Fortunately, we can’t take a bunch of kids, stress them out during their childhood, and then see how much drugs they take at as an adult! But there are other types of analyses and experiments that could be run using data gathered from the “real” world.
Are there studies in humans that confirm the animal studies, that early life stress can increase the susceptibility to addiction?
The answer is yes! Lots of them! I don’t have time to review them all but thankfully many other scientists have. Check out these two review papers for a summary of some these studies. If you are interested in stress and addiction studies in humans, please let me know! I would be happy to devote a post or two to this topic.
Finally, I would just like to end on a broader point, these studies once again confirm how brain development during early life can have far-reaching effects on adult hood (many other fields look at the general top of early life brain development). Indeed, the conditions under which we are raised are an important contributor to how we turn out as adults. But let’s not forget the role of genetics (this will be saved for a future discussion)!
Next post I’ll wrap up the Stress and Addiction discussion by looking at some of the molecular details of how and why stress increases susceptibility to addiction.
Thanks for reading!
Why does one person become an addict and another person does not?
The vulnerability/susceptibility to addiction is one of the most important questions in the addiction field and also one of most difficult to answer. Is it genetics, the environment, or the addictive power of the drug itself? Spoiler alert: the answer is all three! But rather than trying to explain the answer in mere blog post (which is impossible), I think it’s better to tackle different aspects of the question in multiple posts (well, I probably could do it in one but I’m scientist: I would be a doing a disservice to you and to myself if I didn’t do a thorough job). This is the first post in this series.
Over the years, a lot of research has been done that has been able to show that stress can contribute to why one person becomes an addict and why another person does not. But how do we know that stress is important? And what is “stress” anyways? Let’s get our information straight from the horse’s mouth so to speak: a review of a few research papers that look at this question.
What is Stress?
Stress is one of those terms that is used often but may not be well understood. At one point or another we’ve all described our day as “stressful” and we all understand what this means but just take a moment and try to describe what “stressful” means in words that apply to ALL “stressful” situations. It’s tough, right? That’s because stress can mean any number of things in a number of different contexts.
In biology, we have a specific definition of stress: a response (usually immediate and automatic) to an environmental condition or factor, a stimulus, or other type of challenge. The body has several systems in place that mediate the stress response. For example, you probably have heard of “fight-or-flight”, which is one of the body’s stress responses.
Another of the key components of the body’s response to stress is the activation of the hypothalamic-pituitary-adrenal (HPA) axis. See the diagram. The HPA axis is hormonal system that involves chemical communications between several organs.
- First, something happens that requires the body to respond to it, this could be sudden change in temperature, or an attack by an aggressor, or some other challenge. This factor is called a stressor.
- Second, the stressor causes the hypothalamus, a region of the brain that controls many of the body’s functions, to release a small protein molecule called corticotropin releasing factor or hormone (CRF or CRH)
- Third, CRF acts on the anterior pituitary gland, a small organ that secretes many different hormones. CRF stimulates the pituitary to release another small protein called adrenocorticotropic hormone (ACTH). ACTH then enters the blood stream.
- Fourth, ACTH travel through the bloodstream until it finds its way to the adrenal glands, small organs that are located on top of the kidneys.
- Finally, ACTH causes the adrenal gland to release cortisol (corticosterone in rodents), the “stress hormone.” Cortisol has many effects on many different organs throughout the body. Cortisol can also act on the hypothalamus and the pituitary gland themselves in order to inhibit their release and turn the HPA axis “off” until the next stressor. This is called a negative feedback loop.
Note: This is of course, a simplified model and there is a whole field of research devoted to working out the precise molecular mechanisms that regulate the HPA axis and how it responds to many different kinds of stressors.
Stress plays an important role in addiction. Stressors can make a drug seem more appetizing or even make it even feel better (more pleasurable). Anecdotally, after a stressful day, did you ever feel like you really needed a drink? Or, for the current and/or former smokers, how a cigarette was especially satisfying after a particularly jarring event? There’s a neurobiological reason for that feeling!
How do we know stress is important in addiction?
We are going to examine a few research papers that span over two decades (this discussion will be split over two posts). Each paper will reveal a little piece of the puzzle about why stress makes drugs more addictive. However, a Google Scholar search for “stress and addiction” gives you 527,000 hits! Basically, I chose these ones because they are easy to explain and, more or less, fit together in a sequence. Also, they all use one or more of the techniques that I described in my last post: The Scientist’s Toolbox: Techniques in Addiction Research, Part 1. I encourage you to read it before proceeding.
As we go through, try to keep the question we are trying to answer at the back or your mind: Does exposure to stress make it easier to become an addict and, if so, how does it do this? But this is a big question so it’s broken down into little pieces that each paper will try to answer. By the end of the second post, all the little pieces should add up to the bigger story.
Both of the papers we’ll go over today look at what effect stress has on the behaviors of rats exposed to psychostimulants, either amphetamine or cocaine.
As I described in The Scientist’s Toolbox, psychostimulants cause an animal to move around more, and subsequent doses, over a period of a few days, increase that movement. Recall that this phenomenon is called locomotor sensitization.
In the first paper, rats are given 4 injections of amphetamine, one injection of amphetamine every three days and, sure enough, after the fourth injection exhibit greater locomotor activity; these rats are exhibiting locomotor sensitization to amphetamines. These results are shown in the left panel of Figure 1: black circles (4th dose of amphetamine) vs white circles (1st dose). Similarly, rats were given the same regimen of injections and 24hrs after the fourth injection self-administration of amphetamine was tested. As show in the right panel of Figure 1, only animals that were previously exposed to amphetamine (black circles) compared to saline-exposed rats (white triangles) self-administered amphetamine (nose-poked in order to receive drug infusions).
For the next experiment, there are two groups of rats: one group is exposed to stress and other is not. The type of stressor used in these experiments is called tail-pinch and it is exactly what it sounds like: a device is set to deliver a quick squeeze to the rat’s tail. This causes just a mild amount of pain and is very unexpected to the animals, thus it “stresses them out”. This means, as shown in other studies, that tail-pinch activates the HPA axis (increased cortisol secretion). In this experiment, no apparatus is used so instead the rats are placed in a bowl one at a time and then the scientist pinches the tail using forceps (tweezers).
Each animal in the stress group is exposed to 1min of tail pinch, 4times/day for 15days. This represents a chronic stress. The non-stress group rats are also placed in the bowl but no tail-pinch is applied. This is important to make sure that simply being handled or being put in the bowl is not having an effect. This non-stress group is an essential part of the experiment because it allows us to compare the effects of the stress test to animals that did not receive the test. It is called a control group. Controls are necessary for every experiment so that the scientist can make a useful comparison and allows him/her to interpret the experimental results.
Back to the experiment: 24hrs after the last tail pinch, both groups of animals are give an injection of amphetamine and their locomotor activity is measured. As you can see in the left panel of Figure 2, amphetamine caused greater movement in the animals that were stressed (black triangles) compared to the non-stressed control group (white triangles). This means, the ability of amphetamine to affect the animal’s movement was enhanced by stress.
In the second part of this experiment, the same stress exposure procedure is done but then the animals undergo a self-administration experiment (if you’re interested in the details, the catheter surgeries are completed before the stress exposure is started). As shown in the right panel of Figure 2, the stress group (black triangles) successfully acquired self-administration, meaning they gradually self-administered more and more amphetamine every day of the experiment. This behavior is similar to how human addiction begins, escalation in the amount of drug taken each time. Interestingly, the non-stress control group (white triangles) self-administered amphetamine for the first two days but gradually stopped and didn’t really seem interested in receiving the drug by day 5.
In Figure 3 the authors of this study compared the effect of prior exposure (sensitization) to stress for both the locomotor and self-administration experiments. They did this by dividing the experimental data by the control data (this is called normalization). There appears to be no difference between prior exposure and stress on locomotor activity and self-administration.
The authors conclude that stress is as potent as prior exposure to enhance the properties of the drug; stress exposure may be a significant factor why some people become addicted while others do not.
So very cool, it looks like stress can cause rats to want to self-administer more amphetamine and enhance the physical effects of the drug. Many other studies have found similar effects of stress. Let’s take a look at one paper that uses a different stress and a different drug.
In this study, the drug studied is the psychostimulant cocaine and the stressor is social stress. There are many variations of the procedure used for social stress but many are similar. In this paper, the rat to be stressed (the intruder) is placed in the home cage of a different rat (the aggressor). Because rats are territorial, this provokes the aggressor to attack the intruder. The intruder is left in the aggressor’s cage until it is bitten 10 times by the aggressor. The intruder rat is then placed in a mesh cage and put back in the home cage of the aggressor for a period of time. This way the intruder can still see and smell its attacker but can’t be physically attacked. This is repeated for several days. Social stress has been shown to be a very potent stressor, probably more so than tail pinch.
Note: The other study looked only at males but this study is interested in both males and females but for what we are interested in, this is a minor detail.
First, activity of the HPA axis is measured by looking at corticosterone levels (this it the rodent equivalent of cortisol) when exposed to a novel environment (a novel environment is itself a type of mild stress). As you can see in Figure 1, rats that were previously exposed to social stress (black symbols) released higher amounts of coricosterone when placed in the novel environment compared to their unstressed counterparts (white symbols). This means the social stress has resulted in activation of the rat’s stress response, the HPA axis. Interestingly, female rats seemed to have a greater stress response overall.
Next, the effect of social stress on self-administration of cocaine is tested. As we saw with tail pinch stress and amphetamine, social stress caused an enhanced acquisition of cocaine self-administration whereas unstressed animals did not acquire cocaine self-administration. These data are presented in Figure 2, stressed rats (black symbols) and unstressed rats (white symbols).
In this paper, the authors also conclude that social stress—and activation of the HPA axis—makes it easier for a rat to acquire to cocaine self-administration; stress makes the rat want to self-administer cocaine.
To summarize: these studies have found that two different types of stress have a similar effect on two different kinds of drugs. The first study found that tail-pinch stress increases the amount of locomotor activity induced by amphetamine. This stress also increases the amount of drug that the animals will self-administer. The second paper found that a different kind of stress, social stress, caused an activation of the HPA axis and had the same effect on cocaine self-administration: animals exposed to stress acquired self-administration behavior.
Based on the self-administration data, we conclude that stress caused the drugs to have a greater reinforcing effect. This is measure of the amount of pleasure the animals get from the drug. Therefore, we interpret that the stress made the drugs more pleasurable to the animals because they wanted to self-administer more drug.
However, there are some caveats that need to be briefly discussed. Both of these studies only looked at short term self-administration experiments (5 days) and both used relatively low doses. Many studies have found the rats and mice will self-administer cocaine and amphetamine regardless of whether they were exposed to stress or not. Nevertheless, these two papers are examples of how exposure to stress can cause a drug to be more addictive (technically, more reinforcing).
Next, we’ll look at some more stress studies that try to identify the molecular mechanisms—what stress is actually doing to the brain—of stress and addiction.
If you made it this far, thanks so much for sticking with it!
Just as a last thought: both of these are old papers, from the 90s and both are not very extensive (compared to today). This may sound incredible but it’s just an example of how difficult and time consuming science really is!
Thanks for reading 🙂
One of the most important questions that every scientist learns to ask is “How do you know that…?” As scientists, we are trained to be skeptical. When we consider a bit of research done by a colleague, before we are inclined to believe the data, we need to be sure that they conducted the right experiments and that those experiments were done correctly. This doesn’t mean that scientists are stubborn or closed-minded. The reality is quite the opposite. Scientists are ready to incorporate new ideas and new results but first we need to know that the data are real. That’s what being a skeptic is all about: reserving judgment until you know all the facts.
The question “How do you know that..?” is one of the intellectual tools we use when considering whether or not data are real or not. This question has two parts: 1) how do you measure the thing that you interested in and 2) how do you know the effect you are seeing is actually based on what you think it is? What type of comparisons do you need to make in order to test the effect you’re interested in?
The first point of the question relies on special tools, equipment/technology, and experimental setups that are used to take measurements. For example, if you want to know how much a mouse likes taking a drug, then you need a way to measure how much drug it takes and how often it takes the drug (more on this in a bit). Today, I’ll go over a few of the tools that we use in addiction research.
The second part is more important (and more difficult to explain) but is really at the heart of the scientific method. It is all about experimental design and making sure you make the proper comparisons and analyses. I won’t discuss these details any more right now but will save this discussion for a future post.
Instead, let’s take a look at a few of the tools a scientist studying drug addiction has in his/her toolbox.
Locomotor Activity Test
The psychostimulants amphetamine and cocaine act in very similar ways and have very similar effects on the brain. We know that stimulants sort of “amp you up” or make you feel like you have more energy. Think of how you feel after drinking too much coffee. And what do you do when you have more energy? You tend to move around more (maybe you feel a little twitchy/antsy after too much of that coffee…). The same thing happens to mice and rats.
We can measure the amount of movement using a locomotor activity test. This test uses a special piece of equipment that uses light beams and a light-sensitive detector. Whenever the animal moves around the test box, the light beams are broken and the detector records that information. One way to analyze the data is by simply plotting beam-breaks (photo-cell counts are the same thing) that occur over the time of the test period. This way you have a measure of how much the animal moves around in a certain amount of time (more beam-breaks/time unit equals greater movement). A more sophisticated analysis of this same data can actually give you information on where in the box the animal spends its time. Does is just pace back and forth in a small area of the box or does it explore the entire chamber? This type of exploratory behavior data is valuable information and can be useful to other fields that may or may not study drug addiction. The general test for this exploratory behavioral analysis, regardless of speed of the movement caused by drugs, is the open field test.
An interesting phenomenon has been identified with psychostimulants. If you give an animal an injection of cocaine it will move around more compared to regular animals. But if you give it another dose of cocaine the next day it will move around even more than it did on the first day. This is called locomotor sensitization and is an important property of psychostimulants like amphetamine and cocaine.
The graphs below are real data that I took from a figure from one of our lab’s papers so you can see what locomotor sensitization looks like.
It’s a little hard to read but there are two groups of animals: one that receives cocaine injections (the top line) and the other that receives saline injections (the bottom line). Saline is a saltwater solution that is a standard control solution that has no biological effects. Each data point represents an average of several animals from each group. The baseline graph shows the locomotor activity before injections (no differences). As you can see, at day 1 the cocaine animals are already moving more than the saline group. This increase in movement continues over the 14 days of the experiment, evidence of locomotor sensitization.
This video shows an analysis of locomotor activity using video tracking software instead of light-beam breaks.
Locomotor activity is all good and well but not all drugs of abuse cause locomotor sensitization. More directly related to addiction in humans, how do we even know if the animal likes the drug or wants to take the drug? Humans addicts crave the drug and compulsively use it, meaning the desire to do the of the drug overpowers the addict’s self-control. Is there a way we can study this type of drug-taking behavior in animals? The answer is yes!
Self-administration is a very versatile and powerful technique used throughout the addiction field. This technique allows the animal to control whenever it takes the drug and however much it wants. We can study many different aspect of drug taking using self-administration.
The basic idea is is simple: The rodent (mouse or rat) is placed in a chamber and presented with two levers. If the mouse the presses one lever (the active lever) it receives a dose of drug but if it presses the other lever (inactive lever) it does not. The self-administration sessions are run for a set period of time and the number of presses is recorded for each lever. Over the course of several days the animal steadily increases the amount of lever presses, thus the amount of drug it takes. Meaning the animal learns how to take drug and then takes more and more of it. Just like a human addict would do!
Alternatively, the mouse can poke its nose at a special hole that acts just like the active lever. I’ll use “lever press” and “nose poke” interchangeably because they essentially mean the same thing.
Here’s a little cartoon I found on YouTube of a rat that is self-administering nicotine.
Here’s another video that shows a real mouse self-administering a natural reward (meaning not a drug of abuse but food in this case).
There are several important variations to this basic idea that help scientists to not only make the experiments easier to control and data better/easier to analyze, but allow different aspects of drug taking to be studied.
For example if you are studying alcohol addiction, then when the mouse presses the lever a spout may appear that allows the animal to drink the alcohol (the inactive lever produces a bottle of water only). This is perfect for testing alcohol self-administration because both humans and mice drink alcohol. But what if you want to study heroin or cocaine self-administration? Humans (nor mice) drink or eat these drugs. So how does the drug get delivered to the mouse when it presses the lever?
The answer is intravenous self-administration. In this version, a small surgery is performed where a small tube (a cathether) is threaded into the jugular vein of the animal. This tube is fixed to the mouse back and attached to another tube that is part of the self-administration apparatus. This time when the mouse hits the lever, a dose of drug is pumped directly into its vein! See the diagram and videos above for more details.
Intravenous self-administration has several advantages.
- As explained above, it allows us to deliver drugs to animals that won’t take them orally.
- It allows the drug to act immediately on the animal because the drug is being delivered directly into the bloodstream.
- It allows us to control the dose of the drug. When the mouse hits the lever (or nose pokes) it receives a fixed amount of drug that the scientist decides on ahead of time. That way we know how much total drug the mouse takes during a single self-administration session.
- There is no variability in whether the animal is receiving the full dose or not. For example, if the lever press results in a food pellet, there is no guarantee the animal will eat the whole thing. But if you set the self-administration apparatus to deliver 0.5mg of heroin every time the lever is pressed, then there is no doubt if the full 0.5mg dose is delivered to mouse ever time.
Warning: not for the squeamish! This video shows you how to do the catheter implantation surgery on a mouse that will be used for intravenous self-administration!
Finally, best of all, self-ad can be used to address many different types of questions related to different stages in the addiction cycle. Here I briefly describe some of the more common experimental questions and applications that self-ad can help to address.
- Initial use and escalation of use. How much will the animal take when it is first exposed to the drug? Will the animal reach a ceiling in the amount of drug it will take in a single session?
- Maintenance of drug taking. One cool variation is you can make it more difficult for the animal to get the same dose of drug. This is called a progressive ratio self-administration. For example, the animal may need to press the lever 5 times before it receives a dose. You can keep increasing the number of presses during each session to see how hard the animal will work for a dose. One way this experiment can be interpreted is how badly does the animal want the drug? Some animals will press the lever many, many times just to get a small dose. This type of behavior is similar to the intense cravings that human addicts can experience.
- Extinction and Relapse. You can run a special type of experiment where you run a self-administration experiment like normal and then change it so that the active lever no longer gives the animal a dose of drug. Eventually, the animal presses the lever less and less as it learns that it will no longer get the drug. This is called extinction of self-administration. This is like being in a rehab clinic where you are prevented from taking the drug. However, after the extinction sessions, if the scientist gives the animal another does of drug this will causes animal to start pressing the lever at high rates again. This a called reinstatement of self-administration and is model of relapse. What other types of conditions or factors can cause reinstatement (relapse behavior)? This situation is just like an abstinent cocaine addict who may not be craving cocaine but if he/she takes even a single hit, this can be sufficient for that person to sink back into full-blown addiction.
Let’s take a look at some real data. The graph below is from a paper from our group that looks at oxycodone self-administration in mice.
This study is interested in comparing oxycodone self-administration between adult mice and adolescent mice. As you can see, the number of nose pokes at the active hole (remember, same thing as a lever presses) increases during the course of the experiment (don’t worry about FR1 vs FR3) while the inactive hole is ignored, because it does not result in drug administration. Note that the nose pokes are plotted over the time of the administration sessions (2 hours) and that 9 sessions are run (one every day).
The types of experiments I’ve described so far are great ways of studies addictive behaviors but they don’t really tell you about what’s going on in the brain. These behavior experiments are useful in themselves but they are much more powerful if they can be combined with another type of experiment that gives you a window into what’s changing in the brain at the same time as the behaviors.
In my post Synapse to it, I described how neurotransmitters are released by the pre-synaptic neurons into the synaptic cleft so that they can act on receptors located on the post-synaptic neuron. Using microdialysis, you can sample the fluid that exists in the synaptic cleft and actually measure the amount of neurotransmitters being released!
This is an extremely difficult and very technically complicated technique and I will only go into the basics about it. First, the microdialysis probe is surgically placed into a region of the brain that you are interested in studying.
The microdialysis probe itself is like a very thin piece of tubing that allows the experimenter to flow fluid into it one side(inlet) and collect the fluid that flows out of the other side (outlet). At the tip of the probe (the part that’s actually inside the brain) is a special type of material that allows fluid from inside the brain to flow into the tubing (a semi-permeable membrane).
After the surgery, you run your behavioral experiment, and while you are doing that you start flowing fluid into the brain. The fluid that the microdialysis probe flows in is of a similar consistency to the fluid that exists naturally in the brain. As the fluid inside the probe moves through the tubing, it causes fluids in the brain to enter into the probe and through the tubing where it can be collected when it flows out of the tubing.
Let’s say you give an animal a drug that causes a neurotransmitter to be released in the brain region you are interested in. Then some of those released neurotransmitters will enter the microdialysis probe because some of the fluid that enters the probe is from the synaptic cleft.
You keep collecting fluid at different time points during your experiment. When the experiment is over, then you can use chemistry to determine what neurotransmitters are in the fluid you collected. Best of all, you can determine how much of those neurotransmitters you have! How you do actually use chemistry to do this is a very technical part of the procedure and is not important to this discussion.
And all that work gives you a nice graph of the neurotransmitters that are released at different times during your experiment.
Now for some real data. Below are figures from a paper that our lab produced that uses microdialysis to study release of the neurotransmitter dopamine.
In this study, the effect of cocaine on dopamine release in a region of the brain called the caudate putamen is being studied. The first image shows you that the microdialysis probe was placed in the right area of the brain (the white line that pierces through the dark area is the tract in the caudate putamen). The graph shows that injection of cocaine (the arrows) causes an increase in dopamine release in this brain region. Interestingly, the dopamine levels have returned to normal by the end of the experiment. Note: C57Bl/6J is the strain of mouse used in this study.
These are just three of the techniques that are used in addiction research. But we scientists have very big toolboxes! I’ll to explain some more in a later post.
Feel free to contact me or comment if you have questions!
Thanks for reading 🙂
An interesting new study published in Science used deep brain stimulation (DBS), a technique approved for treatment of some psychiatric disorders but with an unknown therapeutic mechanism, to reverse cocaine-induced changes of the reward circuitry in the mouse brain. Using a combination of optogenetics, electrophysiology, and pharmacology, the authors were able to improve DBS in order to eliminate the behavioral sensitization to cocaine in mice. A well known neurobiological change induced by by cocaine is a strengthening of excitatory neuronal inputs into the Nucleus Accumbens, a brain region at the core of the reward pathway. The authors showed that the DBS was able to reverse the cocaine-induced changes in the neural circuitry of the Nucleus Accumbens and this is the most likely mechanism for the effectiveness of DBS in reversing the behavioral changes caused by cocaine. The study suggests that DBS may represent a potential therapy for reversing addiction to cocaine.
You can find the link to the full paper here.
Apologies for not being able to share the pdf but I requested permission and will hopefully be able to upload it soon!