Stereological assessment of the glial reaction to chronic deafferentation of the cochlear nuclei in the macaque monkey (Macaca fascicularis)

Neurectomy of the auditory nerve produces a massive deafferentation of the cochlear nuclei (CN) in the brainstem. Degenerating primary afferents are r emoved in the acute phase, and this is followed by a synaptic reorganizatio n in the CN. As part of an ongoing study on the effect and applicability of auditory brain implants in the CN of Macaca fascicularis monkeys, we studi ed the chronic response of astrocytes in the CN to bilateral deafferentatio n of the VIIIth cranial nerve. Four control and five deafferentated animals were employed. The treated animals had a bilateral extradural section of t he VIIIth cranial nerve and a survival of 3 months. Animals were euthanized and perfused, and the brainstem was serially sectioned. The astrocyte popu lation of the CN was studied by glial fibrillary acidic protein (GFAP) immu nohistochemistry and quantified by unbiased stereological methods. The tota l length of astrocyte processes, L(proc), was estimated as the product of n uclear volume V(nuc), which was estimated by the Cavalieri method, times th e ratio L-V(proc, nuc) of process length to nuclear volume. Mean nuclear vo lume was significantly lower in deafferented animals whereas the mean ratio L-V(proc, nuc) was higher (albeit no statistical significance was reached) . However, the mean total astrocytic process length was virtually the same in both groups. The absence of a length increase in the glial processes ind icates a decrease of the astrocytic reaction after the acute phase. No glia l scar is present in the CN of the monkey after long-term deafferentation, so the usefulness of auditory brain implants to stimulate CN neurons direct ly as a means to overcome deafness resulting from direct damage to the VIII th cranial nerve (i.e., acoustic neuromas) is plausible. (C) 1999 Wiley-Lis s, Inc.