GABA-A RECEPTORS IN AUDITORY BRAIN-STEM NUCLEI OF THE CHICK DURING DEVELOPMENT AND AFTER COCHLEA REMOVAL

The presence of GABA(A) receptors (GABARs) in auditory brainstem nuclei of the chick was determined by immunocytochemical (ICC) and receptor autoradiographic techniques. A monoclonal antibody to the GABAR/benzodiazepine/chloride channel complex and radiolabeled ligand binding using [H-3]-muscimol, a GABA agonist, revealed labeling in nucleus magnocellularis (NM), nucleus laminaris (NL), nucleus angularis (NA), and the superior olive (SO) in both posthatch and embryonic chicks. GABAR-immunoreactivity (GABAR-I), as well as [H-3]-muscimol binding, appear homogeneous throughout these nuclei at all ages studied. During development, GABAR-I is first observed in these nuclei around embryonic day 13 (E13). GABAR-I, which appears heavier in embryos than in posthatch chicks, becomes less intense with age in all 4 nuclei. Levels of receptor binding are also greater in embryos compared to posthatch chicks. [H-3]-Muscimol binding is consistently greatest in SO followed by that in NL. NM and NA exhibit the least amount of binding at all ages studied. [H-3]-Muscimol binding decreases in auditory brainstem nuclei as a function of age. Two days after unilateral cochlea removal, there is an apparent increase in GABAR-I in the ipsilateral NM compared to controls. This, however, may be the result of a decrease in the cross-sectional area of NM neurons as a result of de-afferentation (Born and Rubel, 1985). In contrast, there is a 28% decrease in [H-3]-muscimol binding in the ipsilateral NM compared to controls probably reflecting the 30% reduction in the number of NM neurons due to cochlea removal (Born and Rubel, 1985). Fourteen days after cochlea removal, there is still a small, but not significant, decrease in [H-3]-muscimol binding in the ipsilateral NM. In the contralateral NM, GABAR-I is less intense compared to that in the ipsilateral NM and controls. Additionally, there is a slight but insignificant decrease in [H-3]-muscimol binding compared to that in controls 2 days after cochlea removal. After 14 days survival, however, the average binding is similar to that in controls. Thus, cochlea removal appears to transiently decrease the number of GABARs in the ipsilateral NM and may have a similar, but not as dramatic, effect in the contralateral NM. These GABARs are most likely to be postsynaptic, that is, located on NM neurons.