Laminar compartmentalization of GABA(A)-receptor subtypes in the spinal cord: An immunohistochemical study

To assess the significance of GABA(A)-receptor heterogeneity, which is based on a family of at least 15 subunits, the cellular localization and subunit composition of GABA(A)-receptor subtypes were analyzed immunohistochemically in the rat spinal cord. The distribution of subunits alpha 1, alpha 2, alpha 3, alpha 5, beta 2,3, and gamma 2 was investigated with subunit-specific antibodies, and their colocalization within individual neurons was visualized by double-immunofluorescence staining. The results reveal a widespread expression of the subunits alpha 3, beta 2,3, and gamma 2 in the spinal cord, whereas the three other alpha subunits displayed a more restricted, lamina-specific distribution. The alpha 1 and alpha 5 subunits were most abundant in the intermediate zone, whereas the alpha 2 subunit was predominant in the superficial layers of the dorsal horn and in somatic and preganglionic motoneurons. From colocalization studies, seven subunit combinations could be identified (alpha 3/beta 2,3/gamma 2; alpha 2/beta 2,3/gamma 2; alpha 1/beta 2,3/gamma 2;alpha 5/beta 2,3/gamma 2; alpha 1/alpha 5/beta 2,3/gamma 2; alpha 2/gamma 2; alpha 2/alpha 5/gamma 2) that correspond presumably to distinct receptor subtypes. Although most neurons expressed the subunit triplet alpha x/beta 2,3/gamma 2, the beta 2,3 subunits could not be detected in motoneurons that may thus possess ''atypical'' receptor subtypes (alpha 2/gamma 2 and alpha 2/alpha 5/ gamma 2). On the subcellular level, aggregates of immunoreactivity, suggestive of postsynaptic GABA, receptors, typically were seen on the surface of neuronal somata and proximal dendrites. In addition, an intense, diffuse staining was observed in laminae I-III for the subunits alpha 2, alpha 3, beta 2,3, and gamma 2, presumably localized on primary afferent terminals. The localization of GABA(A)-receptor subtypes in distinct laminar compartments of the spinal cord suggests that GABA(A)-receptor heterogeneity is of relevance for the modulation of sensory inputs, nociception, and motor control at segmental levels.