Stereological assessment of the glial reaction to chronic deafferentation of the cochlear nuclei in the macaque monkey (Macaca fascicularis)
Insausti AM, Cruz-Orive LM, Jáuregui I, Manrique M, Insausti R.
Neurectomy of the auditory nerve produces a massive deafferentation of the cochlear nuclei (CN) in the brainstem. Degenerating primary afferents are removed in the acute phase, and this is followed by a synaptic reorganization 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 studied the chronic response of astrocytes in the CN to bilateral deafferentation of the VIIIth cranial nerve.
Four control and five deafferentated animals were employed. The treated animals had a bilateral extradural section of the VIIIth cranial nerve and a survival of 3 months. Animals were euthanized and perfused, and the brainstem was serially sectioned. The astrocyte population of the CN was studied by glial fibrillary acidic protein (GFAP) immunohistochemistry and quantified by unbiased stereological methods. The total length of astrocyte processes, L(proc), was estimated as the product of nuclear volume V(nuc), which was estimated by the Cavalieri method, times the ratio L(V)(proc, nuc) of process length to nuclear volume. Mean nuclear volume 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 indicates a decrease of the astrocytic reaction after the acute phase. No glial scar is present in the CN of the monkey after long-term deafferentation, so the usefulness of auditory brain implants to stimulate CN neurons directly as a means to overcome deafness resulting from direct damage to the VIIIth cranial nerve (i.e., acoustic neuromas) is plausible.
CITATION J Comp Neurol. 1999 Nov 29;414(4):485-94