Subpopulations of neurons in the rat neostriatum display GABA(B)R1 receptor immunoreactivity

Immunoreactivity for gamma aminobutyric acid BR1 receptor (GABA(B)R1) was detected in the neuropilar elements as well as in the perikarya of neurons in the neostriatum. Many of the GABA(B)R1-immunoreactive perikarya were medium-sized with a thin rim of cytoplasm. They resembled the morphology of medium spiny neurons, the projection neurons of the neostriatum. In addition, some GABA(B)R1-immunoreactive neurons were densely labeled and were of medium to large in size. These neurons were characterized by double immunofluorescence using their neurochemicals as markers. Over 90% of the parvalbumin- and choline acetyltransferase-immunoreactive neurons and about 80% of the nitric oxide synthase-immunoreactive neurons displayed GABA(B)R1 immunoreactivity. The present results show for the first time that the major four subpopulations of striatal neurons express GABA(B)R1 receptor and may have a functional implication in the GABA neurotransmission in the microcircuitry of the neostriatum.