Dopamine-deficient mice are severely hypoactive, adipsic, and aphagic

Drugs of abuse are very powerful reinforcers, and even in conditions of limited access (where the organism is not dependent) these drugs will motivate high rates of operant responding. This presumed hedonic property and the drugs' neuropharmacological specificity provide a means of studying the neuropharmacology and neuroanatomy of brain reward. Three major brain systems appear to be involved in drug reward--dopamine, opioid and GABA. Evidence suggests a midbrain-forebrain-extrapyramidal circuit with its focus in the nucleus accumbens. Data implicating dopamine and opioid systems in indirect sympathomimetic and opiate reward include critical elements in both the nucleus accumbens and ventral tegmental areas. Ethanol reward appears to depend on an interaction with the GABAA receptor complex but may also involve common elements such as dopamine and opioid peptides in this midbrain-forebrain-extrapyramidal circuit. These results suggest that brain reward systems have a multidetermined neuropharmacological basis that may involve some common neuroanatomical elements.

The basal ganglia have become a focus for work on neurotransmitter interactions in the brain. These structures contain a remarkable diversity of neuroactive substances, organized into functional subsystems that have unique developmental histories and vulnerabilities in neurodegenerative diseases. A new view of the basal ganglia is emerging on the basis of this neurochemical heterogeneity, suggesting that dynamic regulation of transmitter expression may be a key to extrapyramidal function.