Isolation and drug addiction

We all know that adverse, early life experiences can affect normal development and the ability to lead a happy and healthy adult life.  A number of recent studies have shown that rodents which are mistreated as pups have long lasting changes to their gene expression (i.e. epigenetics).  They are more anxious and have a harder time forming new memories.  A paper this week in Neuron builds upon these results, by studying the effects of social isolation on the “reward pathway” in the brain.

Reward pathway

What is a reward pathway?  Deep in the brain is a region known as the ventral tegmental area (VTA), which makes connections to the nucleus accumbens and prefrontal cortex.  When we do something that is naturally good, like eating or sex, the neurons in the VTA release dopamine onto the nucleus accumbens and we interpret that as “feeling good”.  This is our reward for doing something that will help us survive and procreate. 

The Reward Pathway in a brain cross section (from brainfacts.org)

Many drugs of abuse like cocaine, amphetamines and alcohol increase the amount of dopamine signaling in this pathway; this is one reason why drugs produce a “high”.  When this pathway gets overstimulated by increased drug use, the brain will try to compensate by making the pathway less efficient.  This is why drug users feel depressed when not on drugs and why higher and higher concentrations of drugs are necessary to produce the same high feeling.  This is a neurological explanation for drug addiction.  Drug abusers also start to make connections in their lives, and in their brains, between environments (a certain room, certain people, etc) and the feeling of reward.  Getting sober is so difficult because the brain has to unlearn these connections and the reward system has to recover back to its normal level of activity.

Plasticity

Before we talk about the paper, I need to introduce one more concept.  Neurons become activated when channels in their membranes open and positive ions rush in.  They can then pass on this signal to another neuron by releasing neurotransmitters (like dopamine) onto the next neuron.  The activity in the neuron and the amount of transmitter it releases into the synapse can change over time, based on that neuron’s previous experiences.  This is known as synaptic plasticity.  There are short-term changes, like facilitation, and longer-term changes (we’re talking hours and days here).  One of the more famous types of long-term plasticity is called long term potentiation (LTP) and is thought to underlie learning and memory.  When drug users start to become addicted, these types of long term changes to neuronal activity are occurring throughout the reward pathway.

Social isolation and VTA neurons

The experiment begins when young male rats were either housed together in groups of 3 or alone.  The researchers then recorded neuronal activity of VTA neurons under various conditions.  They found that rats that were isolated for more than 3 weeks, specifically during the equivalent of the rats’ early adolescence, can more easily induce LTP in the VTA neurons.  In other words, rats that had no social interactions during a critical period had more sensitive VTA neurons.  That is to say, their reward pathway is primed to be overstimulated, just like during repeated drug use.

What are the behavioral manifestations of having a sensitive reward pathway?

The next set of experiments they did is called conditioned place preference.  The rats were placed in a cage that had two different compartments, with different wall colors and floor textures.  The rats were then injected with amphetamine in one of those particular compartments, so they learned to associate the drug high with that environment.  The rats were then given a choice between the two compartments and inevitably they went to the room that was associated with the drug.  The researchers found that isolated rats had a greater preference for the drug room and developed the preference sooner than the control rats.  Social isolation as an adolescent causes an increased rate of learning an association between drugs and environment.  This could make these rats more vulnerable to drug addiction. 

What about unlearning the drug association?

After the drug testing, the rats were exposed repeatedly to the drug room, but this time they didn’t receive any drugs.  This is called extinction of a memory and it is measured by the rats losing their preference for the former drug compartment.   Socially isolated animals had a significantly slower rate of unlearning the preference.  Their memory was more resistant to extinction.  If their VTA neurons are overly sensitive, then it may be harder to rewrite that connection in the brain between environment and reward.

In the context of drug addiction, these findings are big.  An adverse early adolescence can prime the brain to develop addiction more easily and make it harder to sober up.  If the VTA neurons start firing every time you go through an environment associated with drugs, you’re going to want to take a hit again.  The authors bring up an interesting point that social isolation generally causes a depression of neuronal activity in places like the hippocampus (the site of learning), so maybe the increased activity in the VTA is the way for the brain to maintain some sort of homeostasis – some areas increase, some decrease, but overall the brain may have normal amounts of activity.  This is an interesting way of looking at this problem.  I suspect that social isolation offers little in the way of rewards, so the reward pathway is trying to compensate by getting more sensitive.  It will be interesting to see if there is also a connection with changes in gene expression.  The authors explain how the VTA neurons get overactive, from a cellular point of view, but what actually initiates those changes?  And how can social interactions feed into the biology of the cell? 

The rewards of sex and alcohol

There’s a new fruit fly article making the rounds this week.  The public loves a good fruit-flies-acting-like-us story.  The take home message is that male fruit flies that are deprived of sex are more attracted to alcohol (which affects them in a way similar to humans).  Fruit flies like sex!  Fruit flies like alcohol!  Fruit flies are just like us!  Besides the fact that this is funny, who cares, right?  Well, these findings have important implications for the study of addiction.  Let’s dive into the science of the article to understand the connections to addiction.

Sexual deprivation and alcohol intake

The first thing the authors did was establish a behavioral test, which measured attraction towards alcohol.  Flies were put into a jar and in the lid were little tubes filled with fly food (sugar and yeast) or filled with fly food spiked with 15% ethanol.  The flies could choose to eat as much as they wanted of either type of food.  The researchers could measure how much of the food was consumed and create a ratio, which they called the preference index.  The more ethanol-spiked food was consumed, the higher the number for the preference index (in other words, the flies prefer ethanol consumption).  It’s previously been shown that flies will consume more of the ethanol-spiked food, even if the food tastes bad to them.

Next the authors tested two different sets of males.  In one set, the males were put into a container with many virgin females and allowed to mate for 6 hours (!)  In the other set, individual males were introduced to females who had already mated and were, therefore, not receptive.  In other words, these males never mated and were rejected day after day.  These two groups of males were then put in the jar with the food and ethanol and allowed to choose what to consume.  The males who were rejected consumed much more ethanol containing food, whereas the males who had mated ate the normal food.   (The New York Times has a nice video of flies mating or being rejecting, which is associated with their article about this research.)

Is this like drowning your frustrations in a beer?  Sort of.  The males who had been rejected, experienced two negative things: (1) They tried to mate and were rejected. (2) They didn’t have sex.  Is it the rejection or the lack of sex that drives them to drink?   

The authors tested this by putting males in with decapitated virgin females.  Yes, that’s right—these females were dead and lacked heads.  Males will still try to mate with these females, who obviously can’t reciprocate.  However, since the females are dead, they do not actively reject the males.  These males who couldn’t mate also showed a preference for ethanol, so it’s not the “anger” of being rejected that drives them to ethanol, it’s just not having sex.

Neuropeptide F

The authors measured levels of neuropeptide F (NPF) in their two sets of male flies, and found that males who had not mated had lower levels of NPF RNA and protein.  What is NPF?  Neurons communicate with each other by using chemical signals.  Some of these signals are small proteins, called neuropeptides, which act as modulators of neuronal activity and are often involved in behaviors.  The authors wondered if they could control the flies’ attraction to ethanol by experimentally altering the NPF levels.  Males who had no sex had lower NPF and higher preference for ethanol.  Therefore, one might expect that a mated male who normally does not want ethanol, could develop a preference for it, if his NPF concentration was artificially decreased.  

The authors decrease NPF levels by expressing inhibitory RNA (RNAi) for that gene.  RNAi is a way of decreasing expression of a gene of interest, without actually mutating the gene.  Mated males who expressed RNAi for NPF (their NPF levels were decreased), had a greater preference for ethanol than the controls. 

In the complimentary experiment virgin males were used who have not had sex, have decreased NPF levels and a greater preference for ethanol.  In order to change their ethanol preference, the authors would need to somehow artificially increase NPF levels.  Instead of doing this directly, what they did was force the neurons that would be activated by NPF into a state where they were always activated regardless of NPF levels.  In other words, they bypassed the need for NPF and just activated the output cell.  Virgin males with their cells artificially activated (as if there were a lot of NPF around) lost their preference for ethanol.

To summarize, mating increases NPF expression and increased NPF expression makes the flies less attracted to ethanol. 

The authors go on to show that mating is rewarding to flies, as is ethanol consumption and having high levels of NPF.  The NPF signal in the brain acts as a “reward” center.  When fruit flies have sex, this area of the brain is activated and they feel “good” and “satisfied”.  However, when the fly is unable to have sex, the reward center is under-activated and the fly fills in this lack of signals by consuming alcohol.  Ethanol, as well as other drugs, also activates this reward center and brings the signaling back to normal levels.  The flies which had sex don’t need ethanol, because their reward center has already been activated sufficiently.  Drug abuse then might be viewed as the brain trying to get that reward signal, which for whatever reason is not being satisfied by natural stimulants such as food, sex or other social interactions.