Stress and Addiction Part 3: Molecular Changes

Stress-BrainThis 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.

Paper #1 Sorg 1991. Title

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.

Figure 1.
Figure 1.

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.

Sorg 1991. Figure 2
Figure 2.

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?

Paper #2

Zhou 1996. Title

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.

Figure 1.
Figure 1.

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?

Figure 2.
Figure 2.

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.

Paper #3

Boyson 2014. Title

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.

Figure 1.
Figure 1.

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!

Figure 2.
Figure 2.

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!

Summary

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.

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Stress and Addiction Part 2: Early Life Stress and the Susceptibility to Addiction

Stress-Brain

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.

Paper #1

Title. Kosten et al. Brain Res. 2000

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.

Figure 1.
Figure 1.

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.

Figure 2.
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.

Paper #2

Title. Baarendse PJJ et al. 2011The 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.

Figure 1.
Figure 1.

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.

Figure 2.
Figure 2.

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.

Title. Sinha. 2001

Title. Enoch M. 2011

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!

The Science of Stress and Addiction: A Mini-review of the Research, Part 1

Stress-Brain

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.

The Hypothalamic-Pituitary-Adrenal (HPA) Axis
The Hypothalamic-Pituitary-Adrenal (HPA) Axis

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.

Paper #1

Piazza 1990. Title
Paper #1

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.

Figure 1: Behavioral sensitization to amphetamine. Locomotor activity test (left panel) and self-administration (right panel).
Figure 1: Behavioral sensitization to amphetamine. Locomotor activity test (left panel) and self-administration (right panel).

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).

Figure 2: Impact of stress on the behavioral effects of amphetamine. Locomotor activity (left panel) and self-administration (right panel).
Figure 2: Impact of stress on the behavioral effects of amphetamine. Locomotor activity (left panel) and self-administration (right panel).

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.

Figure 3: Comparison of prior exposure to drug (sensitization) to stress: impact on the behavioral effects of amphetamine. Locomotor activity (left panel) and self-administration (right panel).
Figure 3: Comparison of prior exposure to drug (sensitization) to stress: impact on the behavioral effects of amphetamine. Locomotor activity (left panel) and self-administration (right panel).

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.

Paper #2

Paper #2
Paper #2

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.

Haney 1995. Fig 1
Figure 1: Corticosterone levels in a novel environment in stresses and un-stressed male and female rats

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.

Figure 2: The effect of social stress on cocaine self-administration.
Figure 2: The effect of social stress on cocaine self-administration.

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  🙂