In users who develop an addiction, a massive increase in serotonin is triggered. This acts as a brake on the over-excitation of the reward system caused by dopamine.
- Cocaine has a kind of natural brake that is effective four times out of five, causing about 20% of users to develop an addiction.
- Addiction occurs when an imbalance is created between the two neuroregulators and dopamine exceeds serotonin.
Between 15% and 20% of cocaine users develop an addiction. Swiss neuroscientists from the University of Geneva have sought to better understand what happens in their brains when they lose control of their consumption. In a study published on September 10 in the journal Sciencethey highlight a cerebral mechanism specific to cocaine, which has the particularity of triggering a massive increase in serotonin in addition to the increase in dopamine.
Differentiate between addiction and dependence
Addiction is defined as the compulsive search for a substance despite the negative consequences, while dependence is characterized by the occurrence of a withdrawal symptom. “The same principle applies to all potentially addictive products, says Christian Lüscher who led the research. Here in Switzerland, for example, almost all adults occasionally consume alcohol, which is a powerful stimulator of the reward system. However, only a small proportion of us will become alcoholics.”
To assess how cocaine addiction arises in the brain, the research team conducted a series of experiments. They first taught a large group of mice to voluntarily self-administer cocaine before adding a constraint: each time they self-administered cocaine, the mice received a slightly unpleasant – electric shock or air blast.
Triple addiction without serotonin
Among the mice, two groups formed. The first, the most important since it brought together 80% of rodents, stopped consuming. The second, made up of 20% of the mice, continued despite the inconvenience. “This compulsive behavior is precisely what defines addiction, which affects 20% of individuals, both in mice and in humans,” said Vincent Pascoli, co-author of this study.
The experiment was then repeated with mice in which cocaine was no longer bound to the serotonin transporter, so only dopamine increased when the substance was taken. Dependence was then observed in 60% of them. “If serotonin is administered to this last group, the dependency rate drops to 20%, asserts Christian Lüscher. Cocaine therefore has a kind of natural brake which is effective four times out of five.”
What about other drugs?
When cocaine is consumed, two forces are at work in the brain: dopamine on the one hand, the sudden increase of which leads to compulsion, and serotonin on the other, which acts as a brake on compulsion . Addiction occurs when an imbalance is created between these two neuroregulators and dopamine exceeds serotonin. “Dopamine triggers a phenomenon of synaptic plasticity, by strengthening the connections between the synapses of the cortex and those of the dorsal striatumsays Christian Lüscher. This intense stimulation of the reward system then triggers the compulsion. Serotonin has the opposite effect by inhibiting dopamine-induced reinforcement to keep the reward system in check.”
Researchers now want to focus on other drugs to understand the mechanisms of the brain that are triggered, in particular with opiates which are more addictive than cocaine and ketamine which is less so. The aim is to understand in detail how the brain reacts to these substances and why some people are much more vulnerable to their harmful effects than others.
.