Spontaneous activity of the rat primary otolith afferents during postnatal development

Abstract

To investigate the developmental profile of primary vestibular afferents, their spontaneous activities at the horizontal stationary position were recorded from the superior Scarpa's ganglion in rats (aged 8 days-adult) decerebrated under halothane anesthesia. These activities of vestibular afferents could be divided into regular, irregular and burst-firing patterns. From P8 to P9, a majority of vestibular afferents showed irregular discharge pattern while the remaining exhibited rhythmic burst-firing with an average interburst interval of 150 ms. The number of spikes per burst was small (range, 3 to 5), and the mean interspike interval within the bursts was apprx10 ms. Such a burst-firing pattern was also found in P10-P13 rats, but rarely observed after P14. From P10 to adult, afferents showing either regular or irregular patterns were observed. These regular and irregular neurons were characterized as otolith afferents because all displayed sinusoidal position-dependent modulation in discharge during constant velocity off-vertical axis rotations (OVAR). The proportion of regular afferents increased steadily with age, from 11% in P10 to 54% in adult. The mean spontaneous discharge rate of the regular afferents was significantly higher than that of the irregularly discharging afferents. The mean spontaneous firing rate of both regular and irregular neurons also increased with age. These postnatal changes in spontaneous activity were paralleled by an increase in the response gain of these afferents to OVAR. Our results suggest that the primary vestibular afferents undergo progressive change in their discharge patterns over the postnatal period studied.