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


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!

Is a Lack of Bonding the Cause of Addiction?

A well-written criticism of Johann Hari’s popular yet woefully inaccurate book “Chasing the Scream”. The piece is written by fellow addiction blogger Katie Mac Bride for Thanks to Katie for quoting me in the piece!

katie macbride


In which I examine the important but fatally flawed book, “Chasing the Scream,” by Johann Hari [continuedhere].

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The Irrationality of Alcoholics Anonymous

Interesting article in the Atlantic dealing with another example of how addiction treatment does not use an evidence-based approach.

All Things Chronic

Nowhere in the field of medicine is treatment less grounded in modern science. A 2012 report by the National Center on Addiction and Substance Abuse at Columbia University compared the current state of addiction medicine to general medicine in the early 1900s, when quacks worked alongside graduates of leading medical schools. The American Medical Association estimates that out of nearly 1 million doctors in the United States, only 582 identify themselves as addiction specialists. (The Columbia report notes that there may be additional doctors who have a subspecialty in addiction.) Most treatment providers carry the credential of addiction counselor or substance-abuse counselor, for which many states require little more than a high-school diploma or a GED. Many counselors are in recovery themselves. The report stated: “The vast majority of people in need of addiction treatment do not receive anything that approximates evidence-based care.”

This begs the question:  Dr. Kolodny, are…

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


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  🙂

The Scientist’s Toolbox: Techniques in Addiction Research, Part 1

Lab Mice IMG_4102
(Image © Derek Simon 2015)

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.

Locomotor Activity Test Chamber with a mouse. Image from UC-Davis Mind Institute (
Locomotor Activity Test Chamber with a mouse. Image from UC-Davis Mind Institute (

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.


Multiple test boxes with a computer that collects the data. Image from Douglas Mental Health Institute (
Multiple test boxes with a computer that collects the data. Image from Douglas Mental Health Institute (

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.

Cocaine-induced locomotor sensitization. Unterwald EM et al. J. Pharmacol. Expt. Ther. 1994.
Cocaine-induced locomotor sensitization. (Unterwald EM et al. J. Pharmacol. Expt. Ther. 1994.)

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.

A diagram for a typical self-administration chamber. Image from Med Associates (
A diagram for a typical self-administration chamber. Image from Med Associates (


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.

Oxycodone self-administration by adult and adolescent mice. (Zhang Y et al. Neuropsychopharmacol. 2009.)
Oxycodone self-administration by adult and adolescent mice. (Zhang Y et al. Neuropsychopharmacol. 2009.)


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).

Schematic of a microdialysis probe. Image from Wikipedia.
Schematic of a microdialysis probe. Image from Wikipedia.

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.

Evidence of probe placement in the Caudate  Putamen. (Zhang Y et al. Brain Res. 2001.)
Evidence of probe placement in the caudate putamen. (Zhang Y et al. Brain Res. 2001.)


Cocaine-induced increase in DA. (Zhang Y et al. Brain Res. 2001.)
Cocaine-induced increase in DA. (Zhang Y et al. Brain Res. 2001.)


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 🙂

New Review Paper-The Prescription Opioid and Heroin Epidemic

(Image by Mark Weiss/Corbis)
(Image by Mark Weiss/Corbis)

A new paper published online in January 2015 by Kolodny et al. provides an overview of the epidemic of addiction to opioid prescription medications and heroin which is sweeping through the United States. Numerous news outlets from the Huffington Post to the New York Times have been covering this disturbing trend. This important review paper is being released at a critical time.

Kolodny et al TitleYou can find the complete article here.

The authors do an excellent job of outlining the epidemic from a public health perspective. I just wanted to summarize some of the paper’s main points and findings:

  • Abuse of prescription opioid pain relievers (OPR) and heroin is reaching epidemic levels
    • From 1999-2011, oxycodone (a common OPR) use has increased by 500%
    • From 1997-2011, there has been a 900% increase in individuals seeking treatment to for opioid addiction
    • From 2004-2011, there has been a doubling in ER visits due to non-medical use of OPR
    • The author’s highlight that there is a disturbing correlation between the rise in opioid sales, opioid overdose deaths, and opioid addiction (See the figure below)
(Figure 1 from Kolodny et al. 2015)
(Figure 1 from Kolodny et al. 2015)
  • The authors contend that the cause of our current epidemic is rooted in:
    • The development of new opioid medications such as OxyContin (an extended release form of oxycodone introduced in 1995)
    • The over-prescription of OPR coupled with a shift in medical attitudes towards the treatment of chronic pain
    • A series of studies suggesting that long-term opioid use does not result in addiction. We now know this to be false.
      • According to a recent study, 25% of chronic pain patients treated with OPR fit criteria for opioid addiction and 35% for opioid abuse disorder
  • The public health issues related to non-medical use of OPR are significant
    • Heroin use has drastically increased over the same period as OPR abuse
    • 4 out of 5 current heroin users report that their addiction began with abuse of OPR (See here for more information).
    • Overdose deaths and hospitalizations as a result of OPR have been strikingly high since 2002. See the graphs below.
(Figure 4 from Kolodny et al. 2015)
(Figure 4 from Kolodny et al. 2015)
  • Using an epidemiologic approach, the authors outline a prevention strategy for opioid addiction broken down into primary, secondary, and tertiary interventions.
    • Primary prevention
      • Reduce the incidence of the disease condition: opioid addiction (ie prevent new addiction cases)
      • Education of prescribers regarding OPR use
        • The risks of chronic OPR use, such as addiction and respiratory depression (difficulty breathing), are high
        • Little data exists for the effectiveness of long-term OPR use in helping chronic pain patients
      • Substitution of OPR for non-opioid pain relievers must be strongly encouraged
      • Prevention of OPR use amongst adolescents
        • Caution in OPR prescribing
          • Most youths that experiment with them get OPR from family or friends who have an OPR prescription
        • Change the perception that OPR use is less risky than heroin use
          • In reality the risk of addiction to OPR is as high as it is for heroin
    • Secondary Prevention
      • Identify and treat opioid addicts early in their disease
        • Identify users of OPR that are detected by prior to more significant health problems or transition to heroin use
        • Difficulty in diagnosing opioid addiction
          • Urine toxicology screens in some cases
          • Use of prescription drug monitoring programs (PDMPs) to identify patients who seek prescriptions from multiple doctors
    • Tertiary prevention
      • Treatment and rehabilitation of opioid addiction
        • The National Survey on Drug Use and Health (NSDUH) estimates 2.1 million Americans are addicted to OPR and 467,000 to heroin.
        • Combination of pharmacologic and psychosocial treatments
          • Psychosocial therapies (residential treatment centers, mutual-help programs, 12-step programs) can be effective for some patients but should be use in combination with pharmacologic treaments
        • Pharmacologic treatments such as methadone and buprenorphine (Suboxone) are safe and highly effective
          • They work by effectively blocking cravings without causing the “high” the OPR and heroin cause
          • However, fewer than 1 million addicts are receiving these treatments
          • Significant federal limitations exist to buprenorphine prescription
            • See my Post on this topic, which links to an important Huffington Post article on the topic
        • Harm-reduction approaches
          • Needle-exchange programs to reduce HIV transmission
          • Naloxone for treatment of overdose deaths
    • Conclusions
      • Prescription opioid and heroin addiction are reaching epidemic levels in the United States
      • A coordinated public health effort of federal and state agencies, health care providers and insurers, treatment/recovery initiatives and the research community is required to deal with this crisis.

For more statistical information, consult the National Survey on Drug Use and Health.

Also, see the data section of the Substance Abuse and Mental Health Services (SAMSHA) for statistics related to non-medical use of OPR and heroin.