In previous articles in this class, we have learned about some of the many mysteries around the workings of the human brain. In this article we will examine a slightly more well-understood change that can happen in the brain: addiction.

Addiction, when a behaviour becomes compulsive despite negative consequences, is a chronic disease which involves changes in the way the brain processes reward, motivation and memory. These changes take place across the whole brain, but perhaps the most well-understood part of this is the changes that take place in the basal ganglia, one of the most evolutionarily ancient parts of the brain that processes behaviour and reward, among other things.

The basal ganglia uses dopamine to encode when it detects a new reward, sending a bigger signal if the reward is more exciting, or if the reward is more unexpected. These signals create lasting changes in the brain – so if a behaviour generates a reward, the neural circuitry which triggered that behaviour is reinforced, so we are more likely to do it again. This applies to many behaviours: when eating nice food, a reward signal is generated, and we learn the behaviour of eating that food; when playing computer games, achievements trigger reward signals that make us want to continue playing. The reverse also applies – if we experience something negative, the dopamine signal in the basal ganglia is reduced, leading us to unlearn that behaviour – for example, if you eat a food you find disgusting. Many types of recreational drugs and alcohol also stimulate dopamine release in the basal ganglia directly (rather necessarily by triggering a sense of reward), meaning that we can learn these behaviours in the same way.

When we begin to learn these behaviours, they are impulsive – in that we consciously decide we want to do them, and then do them – however, they have the potential to become compulsive behaviours, which we perform without thinking about them. This occurs when the learning of the behaviour becomes too strong because too much dopamine signalling has occurred. This change is a result of many factors including rewiring of the circuitry of the brain and how it processes and prioritises information as a whole that can be seen on fMRI scans, right down to changes in cellular structure, and even changes in the epigenome which influences how our DNA is read to program our cells. The brain essentially undergoes changes from the very smallest levels to the very highest, and the brain’s processing becomes disproportionately focussed on these compulsive behaviours, as we become entirely addicted to that behaviour, be it drugs, alcohol, computer games, or even in some cases exercise! This effect is so strong that if you give rats a button they can press to release dopamine in their basal ganglia, they will press it without eating or sleeping until they die. We can even lose our ability to respond to other things which would normally be enjoyable, which can lead to some addicts starving to death because their brain no longer processes hunger and food reward response normally.

Another problem is that stopping of these behaviours results in negative effects, which means our basal ganglia interprets stopping addictive behaviours as a negative. Drug withdrawal, or boredom or anger when you cannot play your computer game are examples, which would reduce the dopamine levels in your basal ganglia, and would, therefore, program stopping computer games, or stopping taking drugs as negative behaviour which your brain tries to unlearn. This is one of the reasons that addictions are so hard to break.

Greg Howgego, 4th year Medicine student at St John’s College