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? 

Burning carbs in the Andes

I am finally back from a very busy semester.  I taught physiology classes at Mills College and UC Berkeley this semester, so I have been interested in new topics in human physiology.  This week’s paper by Schippers et al. came out recently in Current Biology and describes adaptations that mice must make in order to live at high altitude.  They compared the metabolism of mice that live at 4000m above sea level in the Andes where the oxygen content of air is about 13%, to mice at sea level, which contains 21% oxygen.  We all know from experience that we need oxygen to survive and it’s harder to exercise at high altitudes, but why do our bodies actually need oxygen?

Oxygen in cellular respiration

Most all physiology can be explained by the following equation, which describes the process of cellular respiration:

Glucose + 6 Oxygen (O2) --> 6 Carbon dioxide (CO2) + 6 Water (H2O) + 34 ATP

Glucose is a simple carbohydrate (sugar) that we use as a direct energy source.  Glucose, which is 6 carbons long, gets broken down step by step in a series of chemical reactions.  At each step, a little bit of energy is released by the reaction and that is stored in carrier molecules.  These carriers then donate the energy in the form of electrons, which is then harnessed to make another molecule called ATP.  ATP is cellular energy.  The chemical bonds in ATP store high energy and can be used to drive other cellular reactions, like pumping ions, or the process that causes muscle contraction.  Without ATP we die. 

But what does oxygen have to do with this?  Well, the electrons that are donated by the carrier molecules must hop from protein to protein in what is known as the “electron transport chain”.  The final electron acceptor is oxygen (O2), which forms water with that extra electron.  That’s it.  That’s why we breathe, that’s why our heart pumps blood— our tissues need energy (ATP) to perform cellular tasks and in order to get energy from glucose, we need oxygen to accept the final electron.  Carbon dioxide is produced as a byproduct and is removed from the body during exhalation.

Glycolysis is anaerobic respiration and does not use oxygen. Note that oxygen is used as the last step of the electron transport chain to make the majority of the ATP. (www.phschool.com/science/biology_place/biocoach/cellresp)

I should also mention here that the more our tissues are active and working, the more ATP they need and the more O2 needs to get to the cells.  When we exercise our muscles are very active, so that’s why the heart rate and breathing rate increase; our body needs to intake more oxygen and distribute it faster to our muscles.

You can see that oxygen plays a critical role in our cells, so the mice at high altitudes are going to have a harder time getting their cellular energy.  How do they manage to run around when there is so little oxygen?

Energy sources

As I mentioned above, we can make ATP directly from glucose (a carbohydrate).  We can also make ATP by using fats as an energy source.  There are two differences between these two energy sources:

1) When we use fats as an energy source, it always requires oxygen.  Glucose, on the other hand, can make a limited amount of ATP without oxygen, which is called anaerobic respiration.  This is useful during short vigorous activity, but we cannot make enough ATP by anaerobic respiration for sustained exercise.

2) For a given amount of oxygen, more ATP is produced from carbohydrates, like glucose, than from fats.  However, the amount of ATP created from a single fat molecule is greater than from a glucose molecule.  In other words, if you have plenty of oxygen, you should be burning fats.  But once oxygen becomes limiting, either because you’re working so hard, or because you’re at a high altitude, then carbohydrates should be used.

Given this information, the authors hypothesized that the mice at high altitudes will burn more carbohydrates than mice at sea level.  They have a limited amount of oxygen in the air, so they need to use it in the most efficient way to produce the energy they need.

High altitude mice burn more carbohydrates, but fatigue sooner

The authors did all their tests in the same experimental conditions, with the same amount of oxygen in the air for both sets of mice.  Under normal oxygen conditions and when there was low oxygen content, the high altitude mice burned more carbohydrates than the other mice during moderate exercise.  At rest, they also burned more carbohydrates under low oxygen conditions.  The authors found that the activity of enzymes associated with breaking down carbohydrates were greater in the high altitude mice, specifically in the heart muscles.  The heart has to work harder at high altitude to get enough oxygen to the tissues, so it makes sense that these muscles, in particular, would be burning carbohydrates preferentially. 

The high altitude mice, therefore, have adapted to the low oxygen environment by having more active enzymes to break down carbohydrates rather than fats.  One problem with that, though, is that the carbohydrate storage is less extensive than fat storage.  The researchers found that high altitude mice fatigued more quickly than the sea level mice, which burned more fats.  They suggest that the fast fatigue is a result of using up all the carbohydrate stores.  These mice, though, don’t travel long distances and just need short bursts of speed to escape predators.

I really like how so many physiological processes can be explained through understanding cellular respiration.  It’s so logical that animals at high altitude need to use oxygen more efficiently, so they use carbohydrates more for energy.  It’s simple.

Cellways on temporary hiatus

I will not have time to update this blog this semester since I am teaching four classes plus research.  I'll catch up with you again in the winter.

In the meantime, enjoy this hilarious chemistry comic:

A biological reason for aging weight gain

There is a growing epidemic of obesity in the aging population.  Of course a lot of this has to do with our cultural lifestyle, but could there also be a biological explanation related to the way our bodies age?  One clue comes from the fact that older lab mice have a tendency to become obese, without the added influence of fast food restaurants.  Although there certainly are differences in metabolism as you age, the older mice also intake more food; it’s as if their body isn’t telling them the “I’m full” signal.  Yang et al examine the biological mechanism underlying this age-dependent obesity in the newest edition of Neuron. 

The Hypothalamus

There is a region towards the interior of the brain called the hypothalamus which controls all sorts of basic physiological parameters.  For instance, it sets the body temperature, monitors blood pressure and the water content of the blood, and initiates the feeling of thirst and hunger.  There are a group of neurons in the hypothalamus called POMC neurons, which release a hormone, called a-MSH, which decreases appetite (the feeling of “I’m done eating”).  Could it be that these neurons don’t function properly in older mice, so they aren’t getting enough a-MSH to signal them to stop eating?

Problems with the POMC neurons

The authors find that as the mice get older, their POMC neurons get more negative inside.  Remember that active neurons fire action potentials, which are basically short bursts of positive ions rushing into the cell.  If the POMC neurons are more negative than usual, they will have further to go to fire an action potential and will be less active.  The older POMC neurons are in fact much less active and therefore release less a-MSH.

What makes the older POMC neurons more negative?  The authors find that the neurons are overexpressing a potassium channel (K channel), which will mean there are more open pores in the membrane for K to escape the cell.  As the positive K ions leave the neuron, it will make the inside more negative.  Okay, but why are K channels overexpressed in older neurons?  Turns out this whole cascade is initiated by a key signaling protein called TOR.  Increased TOR levels have been associated with various aspects of aging before, and an inhibitor of TOR (called rapamycin) can increase the life span of mice and other animals.  Check out the diagram below, which puts all these steps together.

Summary: For whatever reason, POMC neurons overexpress TOR, which makes these neurons less active.  They release less of the a-MSH hormone, so the mice don’t get the “stop eating” signal and continue to intake food, leading to obesity.

Two complimentary experiments

To test that this pathway is actually correct, the authors did two complimentary experiments.

1) If TOR is artificially increased in young mice (to mimic older mice), will they intake more food and gain weight?

2) If TOR levels are decreased in older mice (to mimic younger mice), will they lose weight?

For experiment #1, they raised TOR levels in young mice by knocking out an upstream inhibitor of TOR.  TOR levels are normally controlled within a certain range by inhibitors, so if you get rid of that inhibition, there will be more TOR present.  Over many weeks, these mutant mice did in fact get fatter than the controls and their POMC neurons were too negative and didn’t function properly, just like older mice.

For experiment #2, they wanted to decrease TOR in older mice.  This is actually pretty easy to do by injecting the older mice with the drug rapamycin, which inhibits TOR (TOR actually stands for Target Of Rapamycin).  Rapamycin is made by bacteria, which were first discovered in soil samples from Easter Island.  It is currently approved for human use, as an immunosuppressant for organ transplant patients.  When the older mice were injected with rapamycin for a few weeks, the POMC neurons came back to life and fired many action potentials.  The cells weren’t so negative because there were less K channels being expressed.  And yes, rapamycin caused the older mice to eat less and lose a considerable amount of weight.

So there you have it: rapamycin is the wonder drug—it will make you live longer and healthier, but there will be a price to pay with a lowered immune system.  There may be other ways to tap into this dysfunction in the older POMC neurons to help prevent midlife obesity.