Differential expression of GABA(B)R1 and GABA(B)R2 receptor immunoreactivity in neurochemically identified neurons of the rat neostriatum

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the neostriatum. Functions of GABA are known to mediate GABA(A) and GABA(B) receptors. A functional GABA(B) receptor is known to compose of heteromeri c subunits, namely the GABA(B)R1 and GABA(B)R(2) subunits. Our previous rep ort (Yung et al. [1999] Brain Res. 830:345-352) has demonstrated that all m ajor subpopulations of striatal neurons express GABA(B)R1 immunoreactivity. The cellular localization of the second subunit of GABA(B) receptor protei n, i.e., GABA(B)R2 immunoreactivity, in tile rat neostriatum is not yet kno wn. By using a new commercially available specific antibody against GABA(B) R2, immunofluorescence was performed to investigate the cellular expression of GABA(B)R2 in neurochemically identified subpopulations of neurons in th e rat neostriatum. Immunoreactivity for GABA(B)R2 was primarily found in ti le neuropil of the rat neostriatum. Double labeling revealed that those per ikarya that expressed immunoreactivity for parvalbumin, choline acetyltrans ferase, nitric oxide synthase, glutamate receptor two, N-methyl-D-aspartate receptor one, or GABA(A)alpha1 receptor, respectively, did not express GAB A(B)R2 immunoreactivity. In addition, perikarya and most of the neuropilar elements in the neostriatum that expressed glutamic acid decarboxylase 67 i mmunoreactivity were found to be GABA(B)R2-negative. In contrast, immunorea ctivity for GABA(B)R1 was found to be expressed by all of the above neurona l subpopulations. Moreover, a vast number of SV2-immunoreactive profiles an d a number of tyrosine hydroxylase-immunoreactive profiles in the neuropil of the neostriatum were found to display GABA(B)R2 immunoreactivity. The pr esent results indicate that there is a differential expression of GABA(B)R2 and GABA(B)R1 immunoreactivity in different subpopulations of striatal neu rons that are identified by their specific neurochemical markers. Immunorea ctivity for GABA(B)R2 is likely to localize in neuropilar elements of the n eostriatum that may belong to non-GABAergic elements. These findings provid e anatomical evidence of GABA(B)R2 receptor localization in the neostriatum that may have an important functional implication of the GABA(B)-mediated functions in neurons of the neostriatum. (C) 2001 Wiley-Liss, Inc.