EFFECT OF ALTERED NEURONAL-ACTIVITY ON CELL-SIZE IN THE MEDIAL NUCLEUS OF THE TRAPEZOID BODY AND VENTRAL COCHLEAR NUCLEUS OF THE GERBIL

Activity-dependent transneuronal regulation of neuronal soma size has been studied in the medial nucleus of the trapezoid body and ventral c ochlear nucleus of adolescent gerbils. Cochlear ablation or tetrodotox in has been used to eliminate afferent electrical activity in auditory nerve fibers permanently or for 24 or 48 hours. Previous studies have shown that the cross-sectional area of spherical cell somata in the i psilateral anteroventral cochlear nucleus decreases within 24 hours of electrical activity blockade with tetrodotoxin, which is fully revers ible when activity is restored. The present findings extend this work by directly comparing the results of unilateral blockade of auditory n erve action potentials or unilateral cochlear ablation on the size of spherical and globular cell bodies in the ventral cochlear nucleus wit h changes produced by the same manipulations in third-order cells, pri ncipal neurons in the medial nucleus of the trapezoid body. Soma size in both ventral cochlear nucleus cell types decreases reliably by 24 h ours after cochlear removal or eighth nerve activity blockade by tetro dotoxin. Soma size of neurons in the contralateral medial nucleus of t he trapezoid body decreases 48 hours, but not 24 hours, after either m anipulation. When activity in auditory nerve fibers is allowed to resu me for 7 days following a 48-hour activity blockade, soma size fully r ecovers in the medial nucleus of the trapezoid body as well as in vent ral cochlear nucleus neurons. We also report that the cross-sectional area of neuronal soma in the medial nucleus of the trapezoid body is l arger in lateral regions of medial nucleus of the trapezoid body (low- frequency representation) than in the medial regions of the nucleus (h igh-frequency representation). We conclude that cell body size changes in brainstem auditory neurons are reversible and that the signals ass ociated with the loss and subsequent recovery of soma size are activit y related. However, the delayed effect of activity deprivation in the medial nucleus of the trapezoid body suggests that trophic substances released by afferent axons may contribute to the maintenance of anatom ical characteristics. (C) 1994 Wiley-Liss, Inc.