The regional, cellular and subcellular distribution of GABAA/benzodiazepine receptors was investigated by light and electron microscopy in the rat substantia nigra. The regional distribution and density of GABAA/benzodiazepine receptor subtypes (Type I and II) was studied using quantitative receptor autoradiography following in vitro labelling of cryostat sections with tritiated ligands. This was followed by a detailed study of the cellular and subcellular distribution and localization of GABAA/benzodiazepine receptors by light and electron microscopy using immunohistochemical techniques with a monoclonal antibody (bd-17) to the beta 2,3 subunits of the GABAA/benzodiazepine receptor complex. Finally, in situ hybridization histochemistry using 35S-labelled oligonucleotide probes was used to demonstrate the cellular distribution of mRNA for the alpha 1 and alpha 2 GABAA receptor subunits in the substantia nigra. The results of the autoradiographic and immunohistochemical studies showed a close correspondence in the regional distribution of GABAA/benzodiazepine receptors in the substantia nigra. A moderate-to-high density of receptors was present throughout the full extent of the substantia nigra pars reticulata with a very low density of receptors in the substantia nigra pars compacta. Quantitative autoradiographic studies showed that: (i) the pars reticulata contained mainly central Type I receptors; (ii) the highest density of receptors was present in the caudal pars reticulata (200 +/- 38 fmol/mg) with successively lower densities of receptors in the middle (176 +/- 31 fmol/mg) and rostral (150 +/- 26 fmol/mg) levels of the pars reticulata; and (iii) the density of receptors in the pars reticulata was reduced by 34% following 6-hydroxydopamine-induced degeneration of dopaminergic pars compacta neurons. At the cellular level, GABAA/benzodiazepine receptor immunoreactivity was localized in a punctate fashion on dendrites and neuronal cell bodies in the pars reticulata. At the subcellular level, GABAA/benzodiazepine receptor immunoreactivity was associated with the pre- and postsynaptic membranes of axodendritic synaptic complexes along the length of small-to-large sized smooth dendrites in the pars reticulata. Two types of immunoreactive axodendritic synaptic complexes were identified: most (about 80%) immunopositive synapses showed equal staining of the pre- and postsynaptic membranes and were associated with small (less than 1.0 micron) axon terminals containing few mitochondria and small, round-to-pleomorphic vesicles in synaptic contact with small, peripheral dendrites; less frequently (about 20%) immunopositive synapses showed a marked immunoreactive thickening of the postsynaptic membrane and were associated with large (greater than 1.0 micron) axon terminals containing numerous mitochondria and mainly pleomorphic vesicles in synaptic contact with large mainstem dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)