Other brain changes that occur through the process of becoming addicted include changes to the prefrontal cortex (PFC) and the way it sends messages back to the limbic brain.
Think of the limbic brain regions as the brain’s ‘GO” system (or green light) and think of the PFC as the brake (or red light).
The PFC’s job is to be the brain’s executive decision maker and exhibit good judgement and sound reasoning as to which behaviors are safe to engage in and which SHOULD BE STOPPED. In people who are actively addicted this process becomes impaired and they are increasingly unable to control their impulses OR STOP THEIR BEHAVIOR. Interestingly, this is only in relation to drugs and their cues, not in relation to other areas of life where the individual can often continue to display perfectly sound reasoning. This is because LTP has strengthened the neural connections in relation to taking rewards like drugs but not to anything else.
Another reason is that dopamine receptor sites which receive dopamine from the VTA start to die off with heavy drug use. This phenomenon is known as dopamine depletion (see fig. 2). Dopamine depletion (death of dopamine receptor sites) occurs because unnatural rewards like drugs produce much bigger bursts of brain dopamine than natural rewards like food. If we imagine rewards as sounds, then we can imagine that the dopamine signal coming from heroin or alcohol is much louder than the signal coming from other ‘normal’ rewards like eating or socializing. In response, the reward circuit begins to produce fewer brain cells that can receive dopamine (receptor sites) in an effort to “turn down the volume”. This results in the person having less receptor sites, and therefore, a lowered feeling of reward. They will then need bigger and bigger doses of dopamine to get the same effect and become caught in a vicous cycle.
Fig. 2 Dopamine depletion in an active drug user.
So dopamine depletion (a lower density of dopamine receptor sites) leads to the phenomena of tolerance. Tolerance is a form of neuro-adaptation (brain change) where you need more and more of a drug to feel its effects. In reality, addicted people are becoming dopamine tolerant as well as tolerant to their specific drug of choice.
So this is where we see the ancient brain areas (limbic system) that control reward, gaining the upper hand over newer brain area (the cortex) that controls reasoning. This and this is how our brain evolved to work. It’s just that in this situation (addiction) the natural evolved function of the brain is working against us.
So perhaps we should be asking another question. The question is “why do some people prioritize drugs as a survival need more than others?