Distribution of Immunoreactivity for the β2 and β3 Subunits of the GABAA Receptor in the Mammalian Spinal Cord

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The localization of GABAA receptors in cat and rat spinal cord was analyzed using two monoclonal antibodies specific for an epitope shared by the β2 and β3 subunits of the receptor. β23-subunit immunoreactivity was the most intense in inner lamina II, lamina III, and lamina X, and it was the least intense in lamina IX. In laminae I–III, generally, the staining had a rather diffuse appearance, but the surfaces of small cell bodies in these laminae were outlined clearly by discrete labeling, as were many cell bodies and dendrites in deeper laminae. Rhizotomy experiments and ultrastructural observations indicated that β23-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 overlapped with the terminal fields of different types of primary afferents and was also detected on the membranes of dorsal root ganglion neurons. With few exceptions (most notably, a highly immunoreactive group of dorsolaterally located cells in the cat lumbar ventral horn), motoneurons expressed low levels of β23-subunit immunoreactivity.

Labeling of neuronal membranes was fairly continuous, but focal accumulations of β23-subunit immunoreactivity were also detected using immunofluorescence. Focal “hot spots” correlated ultrastructurally with the presence of synaptic junctions. Dual-color immunofluorescence revealed that focal accumulations of β23-subunit immunoreactivity were frequently apposed by glutamic acid decarboxylase (GAD)-immunoreactive terminals. However, the density of continuous-membrane β23 immunolabeling and GAD terminal density were not correlated in many individual neurons. The results suggest the existence of “classical” (synaptic) and “nonclassical” (paracrine) actions mediated via spinal cord GABAA receptors. The study also revealed the relative paucity of β23-subunit immunoreactivity postsynaptic to certain GABAergic terminals, particularly those presynaptic to motoneurons or primary afferent terminals. © 1996 Wiley-Liss, Inc.



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