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SYNAPTIC INNERVATION IN RAT UTRICULAR MACULA.    A. Chu and A.­ Lysakowski*. Dept. of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago IL  60612.

                The afferent innervation pattern of the utricular macula has been examined in a previous study (Fernбndez et al., 1990). Afferents were found to be of three types. Calyx afferents constitute »6% of the total afferents, are restricted to the striola and are probably comparable to “M” fibers (Ross et al., 1986). Dimorphic afferents constitute »92% of the total and are found throughout the sensory epithelium. Bouton afferents comprise a relatively small »2% of the total population and are found in the extrastriolar region. Given this diversity of afferent innervation, we are interested in the regional pattern of normal synaptic innervation in the adult rat, to provide background data to interpret our hypergravity experiments. Hypergravity experiments are being done to study synaptic plasticity, and we  are using a variable linear force to prevent adaptation in the irregular afferents.

                Multiple samples were taken from each rat utricular macula.  Each sample spanned the entire sensory epithelium and included material from all three regions: striola, juxtastriola, and medial and lateral extrastriola. Dissector counts of synaptic ribbons and calyceal invaginations were made as described previously in a study of the chinchilla crista ampullaris (Lysakowski and Goldberg, 1997). Preliminary results from utricular macula indicate that there are slightly lower numbers of synaptic ribbons per hair cell in the rat and that these numbers do not vary by region, but that calyceal invaginations are more numerous in the striola, compared to the extrastriola. Synaptic ribbons in type II hair cells are larger and occur in clusters more often in the striola compared to the extrastriola, similar to our results in the crista.

                We will also present preliminary results of our hypergravity experiments.

(Supported by NASA NAG5-4593 and NIH R01 DC2521.)