Distinct cellular distribution of GABA BR1 and GABA Aα1 receptor immunoreactivity in the rat substantia nigra

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GABA is one of the most important inhibitory neurotransmitters in the substantia nigra. Functions of GABA are mediated by two major types of GABA receptors, namely the GABA A and GABA B receptors. Subunits of both the GABA A and GABA B receptors have been cloned and functional characteristics of the receptors depend on their subunit compositions. In order to characterize the cellular localization of GABA BR1 and GABA Aα1 subunit immunoreactivity in subpopulations of neurons in the rat substantia nigra, double and triple immunofluorescence was employed. Over 90% of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra pars compacta were found to display immunoreactivity for GABA BR1. In contrast, immunoreactivity for GABA Aα1 was found to be primarily displayed by neurons in the substantia nigra pars reticulata. Around 85% of the GABA Aα1-immunoreactive reticulata neurons were found to display parvalbumin immunoreactivity and some GABA Aα1-positive reticulata neurons were found to be parvalbumin negative. In addition, triple-labeling experiments revealed that at the single cell level, the tyrosine hydroxylase-positive, i.e. the dopaminergic neurons in the compacta displayed intense immunoreactivity for GABA BR1 but not GABA Aα1 receptors. The parvalbumin-positive neurons in the reticulata displayed intense immunoreactivity for GABA Aα1 but not GABA BR1 receptors. The present results demonstrate in the same sections that there is a distinct pattern of localization of GABA BR1 and GABA Aα1 receptor immunoreactivity in different subpopulations of the rat substantia nigra and provide anatomical evidence for GABA neurotransmission in the subpopulations of nigral neurons.