Laminar compartmentalization of GABAA-receptor subtypes in the spinal cord: An immunohistochemical study

Abstract

To assess the significance of GABAA-receptor heterogeneity, which is based on a family of at least 15 subunits, the cellular localization and subunit composition of GABAA-receptor subtypes were analyzed immunohistochemically in the rat spinal cord. The distribution of subunits α1, α2, α3, α5, β2,3, and γ2 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 α3, β2,3, and γ2 in the spinal cord, whereas the three other α subunits displayed a more restricted, lamina-specific distribution. The α1 and α5 subunits were most abundant in the intermediate zone, whereas the α2 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 (α3/β2,3/γ2; α2/β2,3/γ2; α1/β2,3/ γ2; α5/β2,3/γ2; α1/α5/β2,3/γ2; α2/γ2; α2/α5/γ2) that correspond presumably to distinct receptor subtypes. Although most neurons expressed the subunit triplet αx/β2,3/γ2, the β2,3 subunits could not be detected in motoneurons that may thus possess "atypical" receptor subtypes (α2/γ2 and α2/α5/ γ2). On the subcellular level, aggregates of immunoreactivity, suggestive of postsynaptic GABAA 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 α2, α3, β2,3, and γ2, presumably localized on primary afferent terminals. The localization of GABAA-receptor subtypes in distinct laminar compartments of the spinal cord suggests that GABAA-receptor heterogeneity is of relevance for the modulation of sensory inputs, nociception, and motor control at segmental levels.