The neurobiology of cocaine-induced reinforcement

Cocaine has potent pharmacological actions on a number of monoaminergic systems in the brain, including those that use noradrenaline, dopamine and serotonin as neurotransmitters. There is growing evidence that cocaine's effects on dopaminergic neurons, particularly those that make up the mesolimbic system, are closely associated with its rewarding properties. For example, low doses of dopamine receptor antagonists reliably influence cocaine self-administration, whereas noradrenaline and serotonin receptor antagonists are without consistent effects. Similarly, selective lesions of dopaminergic terminals in the nucleus accumbens, a major target of the mesolimbic dopamine projection, disrupt cocaine self-administration in a manner that is consistent with loss of cocaine-induced reward. The introduction of in vivo brain microdialysis as a tool with which to investigate the neurochemical correlates of motivated behaviour has provided new opportunities for investigating the role of dopamine in the nucleus accumbens in the acquisition and maintenance of cocaine self-administration. Although the body of literature that has been generated by this approach appears to contain some important inconsistencies, these probably reflect the use of inappropriate microdialysis conditions by some investigators. A critical review of the literature suggests that microdialysis results are generally consistent with a role for mesolimbic dopamine in cocaine-induced reward, although it does not seem to be the case that animals will work to maintain consistent increases in extracellular concentrations of dopamine in the nucleus accumbens in all experimental conditions. Elucidation of the complete neural circuitry of cocaine-induced reward remains an important priority for future research.