No 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 hrs. Recordings conducted at 5-6 days postexposure 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.

1. The amino acid neurotransmitters gamma-aminobutyric acid (GABA) and glycine function as inhibitory neurotransmitters associated with nonprimary inputs onto spherical bushy and stellate cells, two principal cell types located in the anteroventral cochlear nucleus (AVCN). These neurons are characterized by primary-like (including phase-locked) and chopper temporal response patterns, respectively. 2. Inhibition directly adjacent to the excitatory response area has been hypothesized to sharpen or limit the breadth of the tonal frequency receptive field. This study was undertaken to test whether GABA and glycine circuits function primarily to sharpen the lateral edges of the tonal excitatory response area or to modulate discharge rate within central portions of the excitatory response area of AVCN neurons. 3. To test this, iontophoretic application of the glycineI antagonist, strychnine, or the GABAA antagonist, bicuculline, was used to block inhibitory inputs after obtaining control families of isointensity contours (response areas) from extracellularly recorded AVCN neurons. 4. Blockade of GABA and/or glycine inputs was found to increase discharge rate primarily within the excitatory response area of neurons displaying chopper and primary-like temporal responses with little or no change in bandwidth or in off-characteristic frequency (CF) discharge rate. 5. The principal sources of inhibitory inputs onto AVCN neurons are cells located in the dorsal cochlear nucleus and superior olivary complex, which appear to be tonotopically matched to their targets. In agreement with these morphological studies, the data presented in this paper suggest that most GABA and/or glycine inhibition is tonotopically aligned with excitatory inputs. 6. These findings support models that suggest that GABA and/or glycine inputs onto AVCN neurons are involved in circuits that adjust gain to enable the detection of signals in noise by enhancing signal relative to background.

1. Responses of low characteristic frequency (CF) neurons in the lateral limb of the lateral superior olive (LSO) of chinchilla and rat to binaural stimuli at various interaural phase and intensity differences were examined and compared to responses from previous studies of high CF neurons. 2. Ninety-six LSO neurons from chinchillas and 10 LSO neurons from rats with CFs less than 1,200 Hz were characterized. The majority of these neurons displayed phase-locked tone-evoked temporal discharge patterns to ipsilateral CF stimuli. 3. Similar to high-CF LSO neurons, low-CF LSO neurons were excited by ipsilateral stimuli and inhibited by contralateral stimuli, with discharge rate sensitive to interaural intensity differences (IID). Discharge rate increased as ipsilateral intensity was increased and decreased as contralateral stimulus intensity was increased. 4. Binaural inhibition, inhibition of ipsilaterally evoked activity by contralateral stimuli, was dependent on interaural phase differences (IPD) in the majority of low-CF LSO neurons. Responses of phase-sensitive neurons to binaural stimuli often varied with 90 or 180 degrees changes in IPD from total inhibition to a facilitated response when compared to responses to control ipsilateral stimuli alone. 5. In summary, like high-CF LSO neurons, LSO neurons with low CFs (less than 1,200 Hz) were ipsilaterally excited and contralaterally inhibited (EI) and were sensitive to IID. Unlike most high-CF EI LSO neurons, which are not responsive when the azimuth of the stimulus is directly in front of or directly behind the animal, many low-CF LSO neurons are responsive to these stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)