Localization of the GABAB receptor 1a/b subunit relative to glutamatergic synapses in the dorsal cochlear nucleus of the rat.

Abstract

Metabotropic gamma-aminobutyric acid receptors (GABA(B)) are involved in pre- and postsynaptic inhibitory effects upon auditory neurons and have been implicated in different aspects of acoustic information processing. To understand better the mechanisms by which GABA(B) receptors mediate their inhibitory effects, we used pre-embedding immunocytochemical techniques combined with quantification of immunogold particles to reveal the precise subcellular distribution of the GABA(B1) subunit in the rat dorsal cochlear nucleus. At the light microscopic level, GABA(B1) was detected in all divisions of the cochlear complex. The most intense immunoreactivity for GABA(B1) was found in the dorsal cochlear nucleus, whereas immunoreactivity in the anteroventral and posteroventral cochlear nuclei was very low. In the dorsal cochlear nucleus, a punctate labeling was observed in the superficial (molecular and fusiform cell) layers. At the electron microscopic level, GABA(B1) was found at both post- and presynaptic locations. Postsynaptically, GABA(B1) was localized mainly in the dendritic spines of presumed fusiform cells. Quantitative immunogold immunocytochemistry revealed that the highest concentration of GABA(B1) in the plasma membrane was in dendritic spines, followed by dendritic shafts and somata. Thus, the most intense immunoreactivity for GABA(B1) was observed in dendritic spines with a high density of immunogold particles at extrasynaptic sites, peaking around 300 nm from glutamatergic synapses. This is in contrast to GABAergic synapses, in which GABA(B1) was only occasionally found. Presynaptically, receptor immunoreactivity was detected primarily in axospinous endings, probably from granule cells, in both the active zone and extrasynaptic sites. The localization of GABA(B1) relative to synaptic sites in the DCN suggests a role for the receptor in the regulation of dendritic excitability and excitatory inputs.