Evidence for GABA receptors on serotonergic afferent terminals in the substantia nigra and on nigral efferent projections to the caudal mesencephalon.

Changes in [3H]GABA binding in rat substantia nigra (SN) were examined after (1) hemitransections at various levels posterior to SN and (2) 5, 7-dihydroxytryptamine (5, 7-DHT) induced lesions of the dorsal or median raphe or SN. Hemitransections 2 mm caudal to SN resulted in a 52% decrease in specific high affinity [3H] GABA binding in SN when measured 2-3 weeks postoperatively. No significant change in nigral GABA binding was found in rats with hemitransections placed at a postcollicular level(through the rostral pons). 5, 7-DHT placements in median raphe, or directly into SN, caused a 27% decrease in specific [3H] GABA binding in SN. 5, 7-DHT lesions of the dorsal raphe did not significantly affect the binding of [3H]GABA in SN. These data suggest that a significant protion of GABA receptors in SN may be located presynaptically on axon terminals of serotonergic projections from the median raphe to SN. Other pathways between the SN and the caudal mesencephalon also appear to contain GABA receptors; these are likely to be efferent nigral projections to the mesencephalic reticular formation.