Differential expression of GABABR1 and GABABR2 receptor immunoreactivity in neurochemically identified neurons of the rat neostriatum

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Abstract
Gamma‐aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the neostriatum. Functions of GABA are known to mediate GABAA and GABAB receptors. A functional GABAB receptor is known to compose of heteromeric subunits, namely the GABABR1 and GABABR2 subunits. Our previous report (Yung et al. [1999] Brain Res. 830:345–352) has demonstrated that all major subpopulations of striatal neurons express GABABR1 immunoreactivity. The cellular localization of the second subunit of GABAB receptor protein, i.e., GABABR2 immunoreactivity, in the rat neostriatum is not yet known. By using a new commercially available specific antibody against GABABR2, immunofluorescence was performed to investigate the cellular expression of GABABR2 in neurochemically identified subpopulations of neurons in the rat neostriatum. Immunoreactivity for GABABR2 was primarily found in the neuropil of the rat neostriatum. Double labeling revealed that those perikarya that expressed immunoreactivity for parvalbumin, choline acetyltransferase, nitric oxide synthase, glutamate receptor two, N‐methyl‐D‐aspartate receptor one, or GABAAα1 receptor, respectively, did not express GABABR2 immunoreactivity. In addition, perikarya and most of the neuropilar elements in the neostriatum that expressed glutamic acid decarboxylase 67 immunoreactivity were found to be GABABR2‐negative. In contrast, immunoreactivity for GABABR1 was found to be expressed by all of the above neuronal subpopulations. Moreover, a vast number of SV2‐immunoreactive profiles and a number of tyrosine hydroxylase‐immunoreactive profiles in the neuropil of the neostriatum were found to display GABABR2 immunoreactivity. The present results indicate that there is a differential expression of GABABR2 and GABABR1 immunoreactivity in different subpopulations of striatal neurons that are identified by their specific neurochemical markers. Immunoreactivity for GABABR2 is likely to localize in neuropilar elements of the neostriatum that may belong to non‐GABAergic elements. These findings provide anatomical evidence of GABABR2 receptor localization in the neostriatum that may have an important functional implication of the GABAB‐mediated functions in neurons of the neostriatum. J. Comp. Neurol. 433:458–470, 2001.