Rapid growth of astrocytic processes in N. magnocellularis following cochlea removal.

Removal of the cochlea or pharmacological blockade of eighth nerve activity in young postnatal chickens results in rapid transneuronal cell death and atrophy in neurons of n. magnocellularis. The present experiments were designed to examine the influence of afferent input on astrocyte structure in n. magnocellularis. Young chickens were subjected to unilateral cochlea removal. At times ranging from 5 minutes to 72 hours later, the brainstems were histologically processed with a polyclonal antibody against glial fibrillary acidic protein (GFAP). A second group of chick brainstems was impregnated by a Golgi method 6 hours after unilateral cochlea removal and impregnated three-dimensional reconstructions were made of glial cells in n. magnocellularis (NM). Analyses of GFAP positive processes in NM revealed an observable increase in the number of astrocytic processes at the borders of the nucleus within 30 minutes of cochlea removal and a twofold increase in GFAP + glial processes by 6 hours. A secondary increase in the number and density of GFAP + processes occurred between 24 and 72 hours following cochlea removal, during the period of axonal degeneration, and transneuronal cell atrophy and death. Analyses of astrocytes impregnated by the Golgi method revealed that individual glial cells had increased their total process length and the number of processes by approximately twofold by 6 hours after cochlea removal. These results suggest that the structure of astrocytes is rapidly and dramatically influenced by the level of excitatory activity in a neuronal system. Furthermore, the similarity of results obtained with GFAP immunohistochemistry and three-dimensional reconstruction of astrocytes provides evidence that the short-term changes observed following cochlea removal represent the actual growth of glial processes. We speculate that modulations in glial processes as a function of afferent activity may act to influence synaptic efficacy.