Subpopulations of neurons in rat substantia nigra display GABA(B)R2 receptor immunoreactivity

A functional gamma-aminobutyric acid (GABA) B receptor is the first metabotropic receptor known to be composed of two heteromeric subunits, GABA(B)R1 and GABA(B)R2. Our previous report [Neuroscience 99 (2000) 65] has demonstrated that subpopulations of neurons in the rat substantia nigra display distinct patterns of distribution of GABA(B)R1 receptor immunoreactivity. A robust level of GABA(B)R1 receptor is only found in the dopaminergic neurons of the substantia nigra pars compacta (SNc). The objective of the present study was to determine the precise cellular localization of GABA(B)R2 subunit in the rat substantia nigra using double immunofluorescence. Neuropilar elements in the SNc and the substantia nigra pars reticulata (SNr) were found to display GABA(B)R2 immunoreactivity. In addition, the tyrosine hydroxylase-immunoreactive dopaminergic neurons and the parvalbumin-immunoreactive GABAergic neurons in the SNr were also found to display GABA(B)R2 immunoreactivity. The present results thus demonstrate that a functional GABA(B) receptor may be expressed by the dopaminergic neurons in the SNc. It is less clear whether neurons in the SNr express a functional GABA(B) receptor. The present findings have important functional implications in GABA neurotransmission in the substantia nigra.