Development of single- and two-tone responses of anteroventral cochlear nucleus neurons in gerbil.

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.