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SYNAPTIC PLASTICITY IN RAT UTRICULAR MACULA UNDER CONTROLLED HYPERGRAVITY CONDITIONS.   M. Ahmad and A. Lysakowski*.  Dept. of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago, IL  60612.

    The synaptic innervation of the crista ampullaris was examined in previous studies (Lysakowski, 1996; Lysakowski and Goldberg, 1997; Lysakowski, 2000).  Regional variations in cytoarchitecture and synaptic innervation were found, which included variations in afferent bouton number per type II hair cell, in synaptic ribbon size and number per hair cell (especially in type II hair cells), and in the numbers and sizes of calyceal invaginations. The latter may be related to synaptic activity since they are greatest in the central zone, where activity is highest.  Given this diversity of synaptic innervation, we were interested in the regional pattern of normal synaptic innervation in the adult rat, and in how this varies under controlled conditions of hypergravity stimulation.  We chose the utricular macula for these experiments because of its morphology and response to linear forces.  We are using a variable linear force paradigm 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 extrastriola.  Dissector counts of synaptic ribbons and calyceal invaginations were made as described previously (Lysakowski & Goldberg, 1997).  Results are still preliminary, based upon a total of 118.5 hair cell equivalents (and only 75 in the invaginations data) from four samples of rat utricular macula.  In particular, our numbers of type II cells are very low (striola = 7.5; extrastriola = 32), given the overall type I : type II hair cell ratio in rat (striola = 1.73, extrastriola = 1.19).  Overall, synaptic ribbons are increased on the excited side of the striola and extrastriola, especially for type IC and type II hair cells, while calyceal invaginations are decreased on the excited side of the striola only, compared to the inhibited side.  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.

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

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