Alterations in the spontaneous discharge patterns of single units in the dorsal cochlear nucleus following intense sound exposure

Abstract

Electrophysiological recordings in the dorsal cochlear nucleus (DCN) were conducted to determine the nature of changes in single unit activity following intense sound exposure and how they relate to changes in multiunit activity. Single and multiunit spontaneous discharge rates and auditory response properties were recorded from the left DCN of tone exposed and control hamsters. The exposure condition consisted of a 10 kHz tone presented in the free-field at a level of 115 dB for 4 h. Recordings conducted at 5–6 days post-exposure revealed several important changes. Increases in multiunit spontaneous neural activity were observed at surface and subsurface levels of the DCN of exposed animals, reaching a peak at intermediate depths corresponding to the fusiform cell layer and upper level of the deep layer. Extracellular spikes from single units in the DCN of both control and exposed animals characteristically displayed either M- or W-shaped waveforms, although the proportion of units with M-shaped spikes was higher in exposed animals than in controls. W-shaped spikes showed significant increases in the duration of their major peaks after exposure, suggestive of changes in the intrinsic membrane properties of neurons. Spike amplitudes were not found to be significantly increased in exposed animals. Spontaneous discharge rates of single units increased significantly from 8.7 spikes/s in controls to 15.9 spikes/s after exposure. Units with the highest activity in exposed animals displayed type III electrophysiological responses patterns, properties usually attributed to fusiform cells. Increases in spontaneous discharge rate were significantly larger when the comparison was limited to a subset of units having type III frequency response patterns. There was an increase in the incidence of simple spiking activity as well as in the incidence of spontaneous bursting activity, although the incidence of spikes occurring in bursts was low in both animal groups (i.e., <30%). Despite this low incidence, approximately half of the increase in spontaneous activity in exposed animals was accounted for by an increase in bursting activity. Finally, we found no evidence of an increase in the mean number of spontaneously active units in electrode penetrations of exposed animals compared to those in controls. Overall our results indicate that the increase in multiunit activity observed at the DCN surface reflects primarily an increase in the spontaneous discharge rates of single units below the DCN surface, of which approximately half was contributed by spikes in bursts. The highest level of hyperactivity was observed among units having the response properties most commonly attributed to fusiform cells.

Introduction

Increased spontaneous neural activity is a major consequence of intense sound exposure that has been observed at several levels of the mammalian auditory system, including the cochlear nucleus, inferior colliculus and auditory cortex (Kaltenbach and Afman, 2000, Kaltenbach et al., 1998, Kaltenbach et al., 2000, Zhang and Kaltenbach, 1998, Ma et al., 2006, Dong et al., 2009, Brozoski et al., 2002, Komiya and Eggermont, 2000, Seki and Eggermont, 2003, Norena and Eggermont, 2003, Wallhausser-Franke et al., 2003). This alteration has been implicated as a possible factor contributing to the induction of tinnitus (Kaltenbach et al., 2004, Kaltenbach et al., 2005, Brozoski et al., 2002, Shore et al., 2008, Imig and Durham, 2005, Eggermont and Roberts, 2004, Ma et al., 2006, Wallhausser-Franke et al., 2003) and could play an important role in other auditory deficits. For example, excessive levels of activity may induce plastic changes in or cause excitotoxic injury to post-synaptic neurons (Kim et al., 1997). Such changes are potentially disruptive to normal signal detection and information processing in neurons further downstream from hyperactive cell populations. Excitotoxic damage to post-synaptic neurons could result in loss of inhibitory interneurons, which could enhance responses to suprathreshold stimuli, leading to recruitment, hyper-responsiveness and hyperacusis. Knowledge of the mechanisms underlying the induction of hyperactivity would thus seem likely to have important implications for the understanding and treatment of tinnitus and other related hearing disorders.

Much work in our laboratory has focused on hyperactivity in the dorsal cochlear nucleus (DCN) as a possible factor contributing to tinnitus (Kaltenbach and McCaslin, 1996, Kaltenbach et al., 1998, Kaltenbach et al., 2000, Zhang and Kaltenbach, 1998). These studies have been performed primarily at the multiunit level in order to provide details concerning the distribution pattern of hyperactivity in the different layers of the DCN (Kaltenbach and Falzarno, 2002), the profile of this activity along the tonotopic axis (Kaltenbach and Afman, 2000, Zhang and Kaltenbach, 1998), the relationship of activity changes to the type and distribution of hair cell injury along the cochlear partition (Kaltenbach et al., 2002), and the time course of its progression (Kaltenbach et al., 2000). However, the multiunit data is limited in what it can reveal about the nature of the changes in neuronal activity leading to hyperactivity. Increases in multiunit activity might be assumed to reflect increases in single cell spontaneous discharge rates, but in fact, other factors such as increases in the number of active neurons, average spike amplitude, or the incidence of bursting discharges, could also play important roles. Which of these changes occurs after intense sound exposure is critical for an understanding of underlying mechanisms. For example, increases in the discharge rates of individual neurons or in the density of spontaneously active units could indicate perturbations in neuronal transmission or network connections due to loss/weakening of inhibition or a net gain of excitation. In contrast, increases in spike amplitude or duration could signify changes in excitability due to changes in intrinsic membrane properties or loss of the intercellular insulation (e.g., demyelination). Finally, increases in bursting activity could point to changes in the spike generation mechanism.

The present study was conducted to determine which of the above changes contribute to the large increases in activity observed at the multiunit level. We performed multiunit and single unit recordings in the DCNs of animals previously exposed to intense sound as well as unexposed control animals. Multiunit activity was first mapped in a region of the tonotopic range where hyperactivity has previously been shown to be robust. Single unit recordings were then conducted at various depths within this same region. In these areas, we recorded the activity of well-isolated single units in the DCN. From these recordings we analyzed the mean discharge rates, mean spike amplitudes, the incidence of bursting activity and the numbers of units/penetration. Finally, we recorded the frequency-dependent and temporal responses of neurons in an effort to obtain clues concerning the categories of cells displaying abnormal activity.