FUNCTIONAL CONNECTION BETWEEN TASTE RECEPTOR CELLS AND AFFERENT NERVES IN THE LOWER VERTEBRATE

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The morphological and neurophysiological basis to reveal the functional connection between the taste receptor cells and the afferent nerve were obtained in a novel animal model: the Mexican salamander, axolotl (Ambystoma mexicanum).The response property to four basic taste and other chemicals were surveyed with a whole nerve recording of the glossopharyngeal nerve (IX nerve). Salts were the most potent stimulant, but amino acids were the least even at 50 mM. The IX nerve contained mechanosensory fibers also, possibly with a larger diameter than chemosensory ones.The cell body of the IX nerve was labelled by cobaltic-lysine (Co-lys). The process, when successfully labelled, was pseudo-unipolar. The labelled cells (the average diameter = 23 mum) were uniformly distributed mainly in the rostral part of the combined IX-X nerve ganglion. However, the double labelling of Co- lys and HRP showed that the IX nerve was somewhat mixed up with the vagus nerve in their distributions. The structure of the IX nerve ganglion differed from that of mammals and frogs.The afferent and efferent components of the IX nerve in the brainstem were transganglionically labelled with Co-lys. Majority of the afferent fibers formed strong ascending and descending bundles in the solitary fasciculus. The rest of afferent fibers formed in the spinal tract of trigeminal nerve a weak bundle. The cell bodies of the efferent neurons had developed dendritic arbors, and formed a longitudinal cell column. In the brainstem, the topography of the IX nerve was basically similar to that in frogs.The peripheral branches of the IX nerve beneath a lingual epithelium were labelled with a fluorescent carbocyanine dye, DiI. To view the nerve terminals and the taste cells, the fluorescence and the differential interference optics are being combined. The recording from the labelled afferents will reveal their sensory function.