THE PLEASURE CENTRES AFFECTED BY DRUGS

The nucleus accumbens definitely plays a central role in the reward circuit. Its operation is based chiefly on two essential neurotransmitters: dopamine, which promotes desire, and serotonin, whose effects include satiety and inhibition. Many animal studies have shown that all drugs increase the production of dopamine in the nucleus accumbens, while reducing that of serotonin.

But the nucleus accumbens does not work in isolation. It maintains close relations with other centres involved in the mechanisms of pleasure, and in particular, with the ventral tegmental area (VTA).

Located in the midbrain, at the top of the brainstem, the VTA is one of the most primitive parts of the brain. It is the neurons of the VTA that synthesize dopamine, which their axons then send to the nucleus accumbens. The VTA is also influenced by endorphins whose receptors are targeted by opiate drugs such as heroin and morphine. Another structure involved in pleasure mechanisms is the prefrontal cortex, whose role in planning and motivating action is well established. The prefrontal cortex is a significant relay in the reward circuit and also is modulated by dopamine.

The locus coeruleus, an alarm centre of the brain and packed with norepinephrine, is another brain structure that plays an important role in drug addiction. When stimulated by a lack of the drug in question, the locus coeruleus drives the addict to do anything necessary to obtain a fix.

Three structures in the limbic system also play an active part in the pleasure circuit and, consequently, in drug dependency. The first is the amygdala, which imparts agreeable or disagreeable affective colorations to perceptions.


The second is the hippocampus, the foundation of memory, which preserves the agreeable memories associated with taking the drug and, by association, all of the details of the environment in which it is taken. Sometime in the future, these details may reawaken the desire to take the drug and perhaps contribute to recidivism in the patient.

The third structure, the most anterior portion of the insular cortex, or insula, is regarded as part of the limbic system and is thought to possibly play a role in the active pleasure-seeking associated both with food and with psychoactive substances. Damasio proposed that this part of the cortex tells us about the bodily states associated with our emotional experiences. In this way, the insula might also contribute to the conscious aspect of our needs and desires.

But the various drugs also act specifically on other areas of the brain.