Expression of GABAB receptor in the avian auditory brainstem: Ontogeny, afferent deprivation, and ultrastructure

Abstract

Nucleus magnocellularis (NM), nucleus angularis (NA), and nucleus laminaris (NL), second- and third-order auditory neurons in the avian brainstem, receive GABAergic input primarily from the superior olivary nucleus (SON). Previous studies have demonstrated that both GABAA and GABAB receptors (GABABRs) influence physiological properties of NM neurons. We characterized the distribution of GABABR expression in these nuclei during development and after deafferentation of the excitatory auditory nerve (nVIII) inputs. We used a polyclonal antibody raised against rat GABA BRs in the auditory brainstem during developmental periods that are thought to precede and include synaptogenesis of GABAergic inputs. As early as embryonic day (E)14, dense labeling is observed in NA, NM, NL, and SON. At earlier ages immunoreactivity is present in somas as diffuse staining with few puncta. By E21, when the structure and function of the auditory nuclei are known to be mature, GABAB immunoreactivity is characterized by dense punctate labeling in NM, NL, and a subset of NA neurons, but label is sparse in the SON. Removal of the cochlea and nVIII neurons in posthatch chicks resulted in only a small decrease in immunoreactivity after survival times of 14 or 28 days, suggesting that a major proportion of GABABRs may be expressed postsynaptically or on GABAergic terminals. We confirmed this interpretation with immunogold TEM, where expression at postsynaptic membrane sites is clearly observed. The characterization of GABABR distribution enriches our understanding of the full complement of inhibitory influences on central auditory processing in this well-studied neuronal circuit. © 2005 Wiley-Liss, Inc.