Spontaneous discharge and response characteristics of central otolith neurons of rats during postnatal development

To study the developmental profile of otolith-related vestibular nuclear ne urons, their spontaneous activities and response dynamics were examined in decerebrate rats aged seven, 14, 21 and 84 (adult) days. Extracellular reco rdings were performed in the lateral and descending vestibular nucleus of a nimals held at the stationary position in the earth-horizontal or subjected to constant velocity off-vertical axis rotation, which selectively stimula tes the otolith receptors. All neurons displayed sinusoidal position-depend ent modulation in discharge rate, indicating their capability in coding spa tial information during low-frequency head movement. Some neurons showed a full-cycle response to off-vertical axis rotation (nonclipped), while other neurons were silenced in discharge during parts of each rotary cycle (clip ped). In seven-day-old rats, three-quarters of the responsive neurons sampl ed were clipped and the proportion progressively decreased to less than one -quarter in adult rats. In each age group, the clipped neurons discharged i n similar to 60% of the stimulus cycle. Response gains of the neurons incre ased with age, reaching a plateau from 21 days of age for clipped neurons a nd 14 days for non-clipped neurons. The clipped neurons demonstrated higher response gains than the non-clipped neurons at or beyond 21 days of age. S pontaneous activities of the neurons at the stationary and earth-horizontal positions were analysed in relation to their response gains: a positive co rrelation was observed from 14 days of age onwards. Both types of neurons s howed progressive increase in spontaneous activity as the rats matured, tho ugh the clipped neurons exhibited significantly lower resting rates than th e non-clipped neurons at each of the age groups studied. Some neurons that responded to off-vertical axis rotation were not spontaneously active at th e stationary position, but the proportion of these decreased significantly with age. The coefficient of variation of each age group showed a bimodal d istribution, thereby allowing spontaneously active neurons to be assigned a s regular or irregular. Though the vast majority of both the dipped and non -clipped neurons showed irregular discharge patterns at seven days of age, the overall population became more regular as the rats matured. Irregular n eurons of young rats exhibited phase-stable and phase-shift responses, whil e those of older rats showed only the phase-stable response. This distincti on was not observed amongst regular neurons over the ages studied. Our results reveal features of central otolith neurons that can be taken as signs of maturation during the first three postnatal weeks. These neuronal features provide the framework for the analysis of behaviours mediated by the otolith system during postnatal maturation. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.