Self-stimulation and circling reveal functional differences between medial and lateral substantia nigra

Functional differences between medial and lateral substantia nigra (SN) sites mediating intracranial self-stimulation (ICSS) and circling behaviours were explored. It was found that, for medial SN stimulation sites, D- and L-amphetamine (1 mg/kg) had equipotent, slightly facilitatory effects on ICSS rates while for lateral SN stimulation sites, D-amphetamine increased and L-amphetamine decreased ICSS rates. It is suggested that these effects may be due to medial and lateral dopamine cells having different sensitivities to amphetamine isomers. Imposed continuous stimulation of the ICSS sites induced contralateral circling from the medial SN placements and ipsilateral circling from the lateral SN placements. In rats receiving lesions through the ICSS electrode, D-amphetamine (2 mg/kg) induced ipsilateral circling following medial SN damage and contralateral circling after lateral SN damage. These results suggest that lateral and medial SN mechanisms of ICSS and circling differ, and that the lateral SN may be antagonistic to medial SN mechanisms involved in circling behaviour.