What is the nature of addiction?
Amy Winehouse was a terrific talent and one of the favorite recording artists of all time. She also struggled with
substance abuse. To what extent remains largely unknown but her death has fueled debate as to the
precise nature of drug addiction and how it should be addressed by society as a whole.
In other words, why do some people behave in the often
self-destructive manner in which they do? And why are people addicted to drugs in the first place? How, if at all, might they be helped?
Now, you might think that this is a spectacularly hard task to delve into the
neuroscience of addiction try to shed some light. Surely, drugs of abuse (such as amphetamines, cocaine, opiates, alcohol and nicotine) have a broad range of actions that are wholly dissimilar from each other? Well, that is partly true, but when it comes to their
addictive properties, they have
more in common than you might think.
What constitutes addiction?
Addiction, as most will know, develops as a result of
chronic exposure to a particular substance. People who are addicted to a drug show
similar patterns of behaviour. These include:
1.
A compulsion to seek the drug
2.
An inability to control the amount of drug consumption
3.
A negative emotional state when subjected to periods of withdrawal
Neuroscientists have sought to understand the
neural mechanisms underlying these behaviours for generations and emerging from these studies is the concept of the
‘reward circuit’ that seems to be involved in the
development and maintenance of
drug addiction. In fact, research on the
medial forebrain pleasure circuit may be entering a golden age of discovery.
Now, a lot of neuroscience (indeed any science) is about gradually building up a picture from fragments of information. In most cases, that information may not even come from human subjects but from animal models. That means that current ideas regarding
drug addiction are far from unequivocal, but from looking at the correlations between how the rodent brain and human brain are altered during the development of
drug addiction, we can make careful assumptions about what might be the general rule.
You see, the brain behaves a little bit like a
factory, with
different departments responsible for
producing a certain sort of feeling or behaviour. This means that there are
anatomically distinct areas that are concerned with
happiness, memory, movement and
so on. This is (as ever) an
oversimplification, as it is usually the
concerted action of
several brain regions that lead to
emergence of a particular thought or action. However, it is useful to think of the brain in this way, particularly when analysing how
brain circuits are altered in drug users.
The reward circuit
The
reward circuit consists of several
brain regions that are
heavily interconnected; each with distinct – but related – functionality. These include the
nucleus accumbens (a brain region that is extremely well documented for its involvement in
drug addiction),
the ventral tegmental area,
the amygdala, the
prefrontal cortex, the
hippocampus and the
insula, among others.
Dysfunction in each of these areas has been
implicated in the
onset and maintenance of drug-
addiction, each area being responsible for a slightly different facet of the drug-addicted mental state.
The nucleus accumbens: pleasure
Now, the
nucleus accumbens is known as the
reward center of the brain. Largely because in various different experimental paradigms, subjects that report that they are experiencing reward show
increased activity in this area. This can be whilst taking drugs of abuse or simply eating, engaging in sexual activity or indeed
anything associated with pleasurable reward.
All drugs of abuse
raise the level of a chemical known as
dopamine in the
nucleus accumbens and it is this elevation in dopamine (initiated in different ways by different drugs) that is
responsible for the feeling of euphoria associated with drug intake.
However,
persistent activation of the nucleus accumbens leads to the development of
drug addiction, primarily due to persistent changes in the level of dopamine, which leads to an alteration in how people respond to otherwise rewarding stimuli. Put simply, after disruption of the dopamine levels in the nucleus accumbens, people get
less satisfied with things that aren’t their drugs of choice. Furthermore, people derive progressively more pleasure from the drug in question, exacerbating the effect.
The amygdala: fear
On the flip side of the coin, the
amygdala is a region of the brain situated right next to the
nucleus accumbens but is often
associated with feelings of fear and anxiety.
As opposed to the positive reinforcement created by the nucleus accumbens, it is during
withdrawal that the
amygdala seems to have its
largest role. Basically, as access to the drug is taken away,
stress-related hormones are increased in the amygdala which can lead (at least partially) to the
negative feeling associated with withdrawal.
The hippocampus: memory
The
hippocampus is an area of the brain that is associated with
learning and memory as well as the ability to perform tasks requiring
spatial navigation, such as finding your way around a building. It is also important for
‘contextual-conditioning’, meaning that it enables the mind to
establish links between an
environmental cue (a nightclub, for example) and a
memory (such as taking a drug.)
Activation of the
hippocampus is implicated in
initiating cue-induced drug-seeking behaviour. Indeed, it has been shown
that cue-induced craving of drugs (eg. the feeling of needing a drink in a pub or a cigarette with a cup of coffee) is
associated with an
increase in activity in the hippocampus (as well as the amygdala) in humans, indicating that
normal memory function may be being
encroached upon by drugs of abuse.
The prefrontal cortex: decision
The
prefrontal cortex consists of several subdivisions including the
orbitofrontal cortex and
dorsolateral prefrontal cortex. These areas have a variety of different functions but are often described as being
imperative in carrying out decision-making processes in response to
internal/external-cues, a process known as
executive function.
It is believed that the activity of the
prefrontal cortex is disrupted in subjects who are addicted to drugs. This may go some way to explaining why drug users often make
poor decisions regarding drug intake (ie. pursuing a drug whilst fully aware of the negative impact drug intake will have on them and the people around them.)
What goes on in the brain of a drug addict
So…
As complicated as that may have sounded, it represents a
gross oversimplification of what actually goes on in the mind of a drug addict. I simply wanted to introduce you to some of the concepts that are emerging as a result of
neuroscientific research and give you a flavor of what is to come in future posts. As I move through my journey into
drug addiction, I will make constant references to the reward circuit and its components and so an introduction was necessary to familiarise you with the brain’s machinery. --
Dr. Paul
Dr. Paul is a graduate from the University of Bristol and the author of Dr Paul’s Brain Talk, a neuroscience-related magazine. Dr. Paul believes that over the coming decades, neuroscience research will begin to have a greater impact on our lives and he aims to bring the findings of this fascinating field to the general public, in words they can understand.
