Postnatal development of auditory nerve and cochlear nucleus neuronal responses in kittens.

Neurons located within the auditory periphery of kittens (i.e., primary auditory nerve fibers and neurons of the cochlear nucleus (CN) exhibit similar response properties throughout the early stages of postnatal development. Neural thresholds to acoustic stimuli are uniformly high, spontaneous and acoustically-evoked discharge rates are low, input/output slopes are shallow, and temporal discharge patterns are markedly immature. Phase-locking abilities are poor in developing mammals and all neurons exhibit broad bandpass tuning curves, with center frequencies clustering near 1.5 kHz. Throughout the first week, response thresholds, maximum discharge rates, rate-intensity slopes, dynamic ranges and other response indices remain essentially unchanged. Thereafter, between the 7th and 20th postnatal days, peripheral auditory development proceeds rapidly, such that thresholds, tuning properties, temporal discharge patterns, and input/output functions achieve maturity. The role of synaptogenesis in the development of adult response properties has been studied through microionophoresis of neuroactive molecules onto the surface of neurons in the caudal divisions of the cochlear nuclei of developing kittens. Results of preliminary experiments suggest that inhibitory postsynaptic receptor function precedes intrinsic excitatory neurotransmission. Furthermore, during the first two weeks of postnatal development in kittens, GABA microionophoresis onto immature caudal CN neurons, exhibiting sustained responses to acoustic stimuli, converts response patterns to the onset type in the majority of neurons encountered.