SACCULE CONTRIBUTION TO IMMEDIATE-EARLY GENE INDUCTION IN THE GERBIL BRAIN-STEM WITH POSTERIOR CANAL GALVANIC OR HYPERGRAVITY STIMULATION

Immunolabeling patterns of the immediate early gene-related protein Fo s in the gerbil brainstem were studied following stimulation of the sa cculus by both hypergravity and galvanic stimulation. Head-restrained, alert animals were exposed to a prolonged (1 h) inertial vector of 2 G (19.6 m/s(2)) head acceleration directed in a dorso-ventral head axi s to maximally stimulate the sacculus. Fos-defined immunoreactivity wa s quantified, and the results compared to a control group. The hypergr avity stimulus produced Fos immunolabeling in the dorsomedial cell col umn (dmcc) of the inferior olive independently of other subnuclei. Sim ilar dmcc labeling was induced by a 30 min galvanic stimulus of up to -100 mu A applied through a stimulating electrode placed unilaterally on the bony labyrinth overlying the posterior canal (PC). The pattern of vestibular afferent firing activity induced by this galvanic stimul us was quantified in anesthetized gerbils by simultaneously recording from Scarpa's ganglion. Only saccular and PC afferent neurons exhibite d increases in average firing rates of 200-300%, suggesting a pattern of current spread involving only PC and saccular afferent neurons at t his level of stimulation. These results suggest that alteration in sac cular afferent firing rates are sufficient ro induce Fos-defined genom ic activation of the dmcc, and lend further evidence to the existence of a functional vestibulo-olivary-cerebellar pathway of adaptation to novel gravito-inertial environments. (C) 1997 Elsevier Science B.V.