Deafferentation-induced changes in protein kinase C expression in the rat cochlear nucleus

Abstract

Isoforms of the signal transducing molecule, protein kinase C (PKC), may play a role in neural plasticity following sensory deafferentation. To explore the role of PKC in central auditory plasticity, we studied the effect of auditory deafferentation on the expression of PKC βI, βII, γ, and δ in the rat dorsal (DCN) and ventral cochlear nucleus (VCN), using immunocytochemistry. Male rats were treated with kanamycin and furosemide to induce hair cell loss. At various intervals post-treatment, brains were perfusion-fixed and processed for immunocytochemistry. Following deafferentation, we observed a gradual increase in PKC βI immunoreactivity (ir) in the deepest layers of the DCN, possibly representing synapses of primary afferents or parallel fibers on unlabeled neurons. Correlated with this, we observed an increase in the number of neurons in the deep DCN that showed PKC δ ir. In controls, we observed PKC γ ir in small ovoid cells concentrated in the middle layer of the DCN. From days 4 through 14 after deafferentation, we found an increase in the intensity of staining of these cells, with a return toward control levels by day 28. Finally, Purkinje-like cells (PLC) in the VCN, which express only PKC δ in control rats, began to express PKC γ after deafferentation, correlated with increased expression of calbindin D28k in PLC. Thus PKC isoforms are differentially regulated in the CN following deafferentation, supporting a role for PKC in auditory plasticity.

Introduction

Neural plasticity has been demonstrated in the adult central auditory system. In a recent review by Moore (1993) on auditory plasticity in late-onset hearing loss, some important questions were posed, among which were the following. First, how much reorganization can the adult auditory system undergo? Second, which segments of the auditory pathway undergo reorganization? Third, once identified, can the mechanisms of reorganization be regulated pharmacologically? A better understanding of the neurochemical mediators of auditory plasticity may lead to the development of novel therapeutic strategies for modulation of plastic changes that occur in response to deafness.

Neural plasticity in sensory systems may be manifested by changes in synaptic efficacy, through alterations in neurotransmitter systems, or by changes in synaptic arrangement, through reorganization of afferent and efferent pathways. Candidate neurochemical mediators include the receptors for the excitatory neurotransmitter, glutamate, and the serine/threonine kinase, protein kinase C (PKC). Both PKC and glutamate have been implicated in the induction and maintenance of synaptic plasticity in the mammalian central nervous system (Colley and Routtenberg, 1993, Thomas et al., 1994, Kullman and Siegelbaum, 1995); however, their role in the plasticity of auditory pathways is not well understood. Studies have been performed to characterize the distribution of glutamate receptor subunits in the central auditory system (Hunter et al., 1993, Petralia et al., 1994a, Petralia et al., 1994b, Petralia et al., 1996, Bilak et al., 1996, Bilak and Morest, 1998), including preliminary reports on regulation of expression of glutamate receptor subunits following deafferentation (Hunter et al., 1995, Hunter et al., 1996, Altschuler and Sato, 1995, Sato and Altschuler, 1996, Altschuler et al., 1996). While the distribution of the several isoforms of PKC in auditory pathways has been described (Garcia et al., 1993, Garcia and Harlan, 1997a), little is known about the role of PKC in auditory plasticity. As altered expression of PKC and its substrates has been demonstrated in central visual and olfactory systems following deafferentation (Elkabes et al., 1993), we studied the effect of auditory deafferentation on expression of specific PKC isoforms in central auditory pathways. We found differential regulation of the pattern and intensity of PKC immunostaining in the cochlear nucleus of the deafferented rat brain, supporting the hypothesis that certain isoforms of PKC may play a role in auditory plasticity.