THE JOURNAL OF COMPARATIVE NEUROLOGY 292:283-290 (1990) Synapsin I-Like Immunoreactivity zy in Nerve Fibers Associated With Lingual Taste Buds of the Rat THOW E. FINGER, MARY WOMBLE, JOHN C. KINNAMON, zyxwv AND TETSUFUMI UEDA Department of Cellular zyxwvuts & Structural Biology and Rocky Mountain Taste & Smell Center, University of Colorado School of Medicine, Denver, Colorado 80262 (T.E.F., M. W.); Department of Molecular, Cellular & Developmental Biology, University of Colorado, Boulder, Colorado 80309 (J.C.K.); Departments of Pharmacology and Psychiatry and Mental Health Research Institute, University of Michigan, Ann Arbor, Michigan 48109 (T.U.) ABSTRACT Immunoreactivity to synapsin I, a neuronal phosphoprotein, was local- ized in free-floating tissue sections prepared from lingual tissue of rats. Many nerve fibers within the tissue exhibited clear immunoreactivity including motor endplates on striated muscle, autonomic fibers innervating blood ves- sels or glands, and sensory fibers innervating muscles or the lingual epithelium including taste buds. Numerous immunoreactive fibers occurred within each taste bud, with fewer, fine fibers being dispersed in the epithelium between taste buds. The majority of the intragemmal immunoreactive fibers extended throughout the taste buds most of the distance outward from the basal lamina toward the surface of the epithelium. Fine, perigemmal fibers reached nearly to the epithelial surface. Ultrastructural analysis of the immunortactive sen- sory fibers revealed that synapsin I-immunoreactivity occurred diffusely throughout the cytoplasm, and heavily in association with microvesicles. The synaptic vesicles at the taste receptor cell-to-afferent fiber synapse were, how- ever, not immunoreactive for synapsin I, although these vesicles fall into the size class shown to be immunoreactive in other systems. This absence of synapsin I may be a common property of vesicles in axonless short receptor cells. Key words: circumvallate papilla, gustatory, microvesicles, primary receptor cells, fungiform papilla Taste buds are classically described as comprising modi- fied epithelial cells which serve as short, i.e., axonless, recep- tors synapsing onto intragemmal nerve fibers, i.e., those that penetrate the basal lamina to enter the confines of the taste bud (Kinnamon, zyxwvutsrqp '87). Until recently, all intragemmal nerve fibers were thought to receive synapses from the receptor cells. Recent studies utilizing antisera directed against substance P (Nishimoto et al., '82; Yamasaki et al., '84) and calcitonin gene-related peptide (CGRP) (Finger and Kinnamon, '84; Finger, '86) have shown that some pep- tidergic nerve fibers, possibly mediating common chemical sensibility, course through taste buds without receiving synapses from the taste receptor cells. Thus, at least two classes of intragemmal nerve fibers can be discerned, non- peptidergic fibers that are synapsed upon by taste receptor cells, and peptidergic fibers which do not receive synapses and which are located in the lingual epithelium both within and around taste buds. Thus, the detailed striicture of taste buds and the rela- tionships between taste receptor cells and the various nerve fibers is complex. Determination of the patterns of taste cell-nerve fiber synaptic interactions have required tedious, serial section reconstructions from thin or semithin sections (Kinnamon et al., '85). In an effort to expedite the process of reconstructing the locus and nature of synaptic sites within taste buds, we have attempted to use a variety of neuron- related immunochemical markers to label selectively dif- ferent elements of the peripheral gustatory system. The Acrepted September 8,1989. zyxwv 0 1990 WILEY-LISS, INC.