Spatial coding properties of peripheral and central otolith neurons in postnatal rats

Abstract

The firing properties of primary otolith afferents and central otolith neurons that were responsive to off-vertical axis rotations (OVAR) were examined in young (7 – 21 days) and adult Sprague-Dawley rats. All animals were decerebrated under halothane anesthesia. Extracellular recordings were performed in the superior Scarpa’s ganglion or the lateral / descending vestibular nuclei. In P7-12 rats, a majority of primary otolith afferents exhibited full-cycle modulation to 360°OVAR while 75% of central otolith neurons showed clipped response pattern. Clipped response pattern was not observed in primary afferents after P12 while its proportion in central neurons progressively decreased from P12 to adulthood. During OVAR, 80-90% of otolith afferents in each age group showed one-dimensional response sensitivity. In contrast, 60-70% of central otolith neurons in young rats exhibited two-dimensional response sensitivity while only 20% in adults showed such a pattern. In rats >P13, the best vectors of both peripheral and central otolith neurons were homogeneously distributed on the plane of rotation. In P7-12 rats, however, the best response vectors of either peripheral or central otolith neurons were distributed predominantly along the roll direction. This finding indicates that the neonate’s ability to code head orientations with respect to gravity is more restricted than the adult. Our results also suggest that the otolith system gradually achieved its capability in coding horizontal head movement during postnatal development. Supported by HK RGC