Ontogeny of neural discharge patterns in the ventral cochlear nucleus of the Mongolian gerbil

Abstract

Discharge patterns were recorded extracellularly from single neurons in the ventral cochlear nucleus (VCN) of Mongolian gerbils ranging in age from 10 days after birth (DAB) to adult, a period which includes the onset of responsiveness to acoustic stimulation. At 10 DAB none of the neurons encountered within the VCN responded to acoustic stimulation. At 12 DAB approximately 15% of the neurons isolated in VCN were responsive. This coincided with the earliest cochlear microphonic potentials and preceded the appearance of the cochlear compound action potential (AP) by two days. At 14 DAB, or older, the great majority of neurons isolated in VCN responded to acoustic stimulation. Most parameters of VCN neural function exhibited significant changes between 12 and 18 DAB: neural thresholds improved approximately 100 dB; mean spontaneous discharge rate increased; the high-frequency range of characteristic frequency (CF) values increased from 10.0 to 24.0 kHz; the upper limit for phase locking increased from 0.8 kHz to 3.0 kHz; dynamic range increased from 16 dB to 44 dB, and the proportion of units with well-defined initial onset peaks in their post-stimulus-time (PST) response patterns increased from 40% to 100% of units. Most of the neural parameters examined achieved adult characteristics by 18 DAB.

Frequency tuning (Q10dB) matured earlier for high-CF units. The most sharply tuned neurons with high CFs (>4 kHz) at 12 DAB had Q10dB values equal to those for adults. None of the neurons with low CFs (<4 kHz) had Q10dB values greater than 1.2 at this age. Classical ‘on’ PST response patterns were not seen at 12 and 14 DAB. A unique PST response type, characterized by very long latency phasic discharge, was observed only at 12 DAB. None of the VCN neurons recorded from 12 DAB subjects displayed rhythmic ‘bursting’ or ‘pulsing’ PST response patterns, as has been reported at the earliest stages of functional development in the VCN of the cat. Most units were capable of sustained discharge, even with long stimulus durations. Units with ‘primary-like’ PST response patterns at 12 exhibited greater variability in first spike latency and less pronounced initial rates of firing than was characteristic in adults, resulting in poorly defined onset peaks. In contrast, units with chopper PST response patterns showed well-defined onset peaks.