DISTRIBUTION OF IMMUNOREACTIVITY FOR THE BETA(2) AND BETA(3) SUBUNITSOF THE GABA(A) RECEPTOR THE MAMMALIAN SPINAL-CORD

The localization of GABAA receptors in cat and rat spinal cord was ana lyzed using two monoclonal antibodies specific for an epitope shared b y the beta(2) and beta(3) subunits of the receptor. beta(2)/beta(3)-su bunit immunoreactivity was the most intense in inner lamina II, lamina III, and lamina X, and it was the least intense in lamina IX. In lami nae I-III, generally, the staining had a rather diffuse appearance, bu t the surfaces of small cell bodies in these laminae were outlined cle arly by discrete labeling, as were many cell bodies and dendrites in d eeper laminae. Rhizotomy experiments and ultrastructural observations indicated that beta(2)/beta(3)-subunit immunoreactivity in the dorsal horn was largely localized in intrinsic neuropil elements rather than in the terminals of primary afferent fibers, even though labeling over lapped with the terminal fields of different types of primary afferent s and was also detected on the membranes of dorsal root ganglion neuro ns. With few exceptions (most notably, a highly immunoreactive group o f dorsolaterally located cells in the cat lumbar ventral horn), motone urons expressed low levels of beta(2)/beta(3)-subunit immunoreactivity . Labeling of neuronal membranes was fairly continuous, but focal accu mulations of beta(2)/beta(3)-subunit immunoreactivity were also detect ed using immunofluorescence. Focal ''hot spots'' correlated ultrastruc turally with the presence of synaptic junctions. Dual-color immunofluo rescence revealed that focal accumulations of beta(2)/beta(3)-subunit immunoreactivity were frequently apposed by glutamic acid decarboxylas e (GAD)-immunoreactive terminals. However, the density of continuous-m embrane beta(2)/beta(3) immunolabeling and GAD terminal density were n ot correlated in many individual neurons. The results suggest the exis tence of ''classical'' (synaptic) and ''nonclassical'' (paracrine) act ions mediated via spinal cord GABA(A) receptors. The study also reveal ed the relative paucity of beta(2)/beta(3)-subunit immunoreactivity po stsynaptic to certain GABAergic terminals, particularly those presynap tic to motoneurons or primary afferent terminals. (C) 1996 Wiley-Liss, Inc.