This inhibits the release of GABA in the ventral tegmental area. Inhibiting GABA allows the dopaminergic neurons to fire more vigorously. The release of extra dopamine in the nucleus accumbens is intensely pleasurable. The dopamine transporter responsible for its reuptake is blocked by cocaine. So cocaine allows more dopamine to accumulate in the synapses.
Amphetamines act directly on the nerve endings and release extra dopamine too.
Chronic cocaine-use causes a decrease in the production of enkephalin, one of the brain's natural opioids. This in turn causes a compensatory increase in the number of mu-receptors. The number of unoccupied mu-receptors may be associated with the craving and abstinence syndrome. The mu receptors are crucial to the rewarding and addictive properties of opioids. Government researchers and pharmaceutical companies are urgently searching for powerful analgesics that won't make their users feel happy.