Intracellular responses of onset neurones in the ventral cochlear nucleus to acoustic stimulation [Abstract]

Abstract

The ventral cochlear nucleus (VCN) contains a heterogeneous collection of cell types reflecting the multiple processing tasks undertaken by this nucleus. This in vivo study used intracellular recordings and dye-filling to examine membrane potential changes and firing characteristics of onset neurones to acoustic stimulation (50 ms pure tones, 5 ms r/f time, 0.2 Hz repetition). Using rats anaesthetised with urethane (1.3g/kg i.p), microelectrodes containing 1M KAc and 4% neurobiotin, were inserted into VCN. Stable impalements were made from 11 onset neurones, seven identified as multipolar cells. Neurones responded to characteristic frequency (CF) tones with sustained depolarisation at below spike threshold. Increasing stimulus intensity increased and sharpened the depolarisation occurring in the initial 5 ms of the response from which an onset spike was generated. With the exception of tones presented at the high frequency edge of the cells response area, which resulted in depolarisation and spike at both the onset and offset of the stimulus, off CF tones resulted in a broadening of the initial depolarisation with high stimulus intensities required to initiate an onset spike. The onset spike latency in response to a given frequency decreased with increasing intensity. Presentation of tones off CF resulted in longer latencies. The spike onset latency depended on the rise time of the depolarisation with its latency remaining constant across tones which is consistent with monosynaptic excitation from auditory nerve. Depolarisation rise times decreased with increasing stimulus intensity and increased off CP. These results suggest that multipolar cells receive convergent input from auditory nerve enabling intensity and frequency to be coded through changes in membrane responsiveness.