Brain reward circuitry: a case for separate systems.

Anatomical, lesion and pharmacological data pertaining to intracranial self-stimulation are reviewed in the context of multiple independent substrates of brain-stimulation reward. Subsystems related to chemosensitive pathways, motor control and mood are identified. Forebrain dopamine pathways are recognized to be important components of two separate subsystems of reward. Nigrostriatal DA neurons are linked to an extrapyramidal circuit with reinforcement derived from response initiation and it is conjectured that the meso-limbic DA pathway is related to the control and perception of positive affect. Other candidate systems include a trigeminal system in the hind brain that may function independently from forebrain substrates of reward.