Do you know the feeling that you get when eating your favourite meal, or of drinking your first beer on a Friday evening? Supposedly, this pleasure you get from these and many other experiences is because dopamine is being released in your brain at that moment. Thus, you may often see it being called the ‘happy’ or ‘feel-good hormone’. In this article, we’ll discover that this is far from the full picture and what may go wrong in the dopamine system during addiction.
First of all, dopamine is a hormone/neurotransmitter of your central nervous system and is mostly produced by neurons in the midbrain. Even though only about 1% of all neurons in your brain use it, dopamine is involved in many functions, like movement, learning and memory, as well as reward.
Dopamine is extremely relevant for movement: Parkinson’s disease patients have their typical severe motor deficits partly because of a lack of dopamine in their basal ganglia. In Parkinson’s the dopaminergic neurons of the substantia nigra die, and thus the basal ganglia which typically helps in movement decisions does not function properly anymore. This leads to the characteristic tremors, rigidity, and slowness of movement that patients experience.
So how come we think of dopamine as this pleasure hormone then? Several animal studies (mostly in mice) blocked dopamine action with drugs or destroyed dopaminergic neurons entirely. This led to the animals to stop looking for food, even though it was readily available, almost as if they didn’t find eating pleasurable anymore. Thus, the (an)hedonia hypothesis was formed, which states essentially that dopamine brings about the good feeling we get from ‘reinforcers’, which are rewards like food, drink and also sex. This also includes less obviously rewarding stimuli such as your favourite movie or video game, called ‘learned reinforcers’.
However, more research into this found that more dopamine was actually released not when the reward was given, but shortly before, while animals/humans were still approaching it. This suggests that the anhedonia account of dopamine is not quite correct! Many scientists now believe that dopamine is actually tied not so much to the ‘liking’ of a certain reinforcer, but rather to the ‘wanting’ of it. This is called the ‘incentive salience hypothesis’. Thus, here dopamine is still highly important for motivation to e.g., seek out food, but it doesn’t directly cause the good feeling of eating.
Another way to interpret the early release of dopamine is that it is important for learning about what behaviours are rewarding. Dopamine release is here linked to learning that a stimulus is associated with a good feeling and to predicting pleasure before a reward is given. This is the ‘reward learning hypothesis’.
Unfortunately, the reality is a bit more complicated still. Several studies do not find this early release of dopamine, which may show us that also the incentive salience and the reward learning hypotheses are not completely perfect. So, as you can see, it’s pretty difficult to say what dopamine does in the brain!
So far, so good. Now, how is dopamine related to addiction then? Firstly, addictive drugs in general seem to enhance the effect that dopamine has on the nucleus accumbens, a structure in the midbrain. According to the incentive sensitization hypothesis, addiction happens when repeated abuse of a substance, e.g., of alcohol or cocaine, leads to anatomic and functional changes in the brain. (This does not mean that repeated drug taking will lead to addiction for everyone necessarily as some people are more susceptible than others!). These changes to the brain happen especially in circuits that are involved in motivation,making them hypersensitive. As we know by now, dopamine plays a vital role in motivation. Thus, the brain is changed to overreact (with higher dopamine release) to the drug (e.g., heroin) and associated cues (e.g., the tools to take heroin, like a syringe). This results in the person craving the drug, even though they may not feel much pleasure anymore when taking it! (Here we can see the same differentiation between wanting and liking: the addict wants/craves the drug but does not like/enjoy the effect anymore.)
It's also important to note that you cannot only get addicted to drugs. Next to addictive substances, people can get addicted to certain behaviours too, e.g., gambling or internet addiction. These addictions seem to be biologically very similar to drug addiction. For example, brain imaging studies show the involvement of the same areas. Dopamine involvement is thought to be very similar too, with release heightened for stimuli associated with the addiction (e.g., the site of a casino table in gambling).
So, our main conclusions are:
1. Dopamine is not really a ‘happy hormone’. It is involved in many processes like movement and motivation.
2. Dopamine sensitivity is increased for addictive drugs and their associated stimuli.
3. This is also the case for addiction to behaviours.
So, I hope this small review was helpful and that you can be critical when anyone tells you about feel-good hormones in the future!
Author: Melanie Smekal