No intercellular communication in the frog taste discs for bitter taste: analysis of quinine-sensitive fibers in the glossopharyngeal nerve.

Abstract

Vertebrate taste buds contain diverse types of cells. Recent studies have shown that type II cells are likely to be the sensory receptor cells for bitter taste stimuli. However, type III cells form synapses with nerve fibers, whereas type II cells do not form morphologically identifiable synapses. Hence, it has been proposed that type II cells communicate with type III cells via gap junctions or paracrine secretions. If so, bitter responses from single taste fibers would be modulated by other taste stimulation. In the frog glosopharyngeal nerve, there are quinine-sensitive fibers (Q-fibers) and calcium-sensitive fibers (Ca²⁺-fibers). In this study, we examined whether quinine (Q) responses are altered by Ca²⁺ taste stimulation. Bullfrogs (Rana catesbeiana) were used. Antidromic unitary impulses of Q-fibers or of Ca²⁺-fibers were recorded from a single fungiform papilla drawn into a suction electrode. Q at 0.5 mM elicited a phasic response, whereas CaCl₂ at 1 mM elicited a sustained response. Q responses (the latency between onset of stimulation and appearance of the first impulse, and the frequency of impulses) were not altered by appearance of neural response from Ca²⁺-fibers. The present results suggest that Q and Ca²⁺ taste reception do not use a common pathway. In another experiments, we have shown that Q excites type II cells in the frog taste disc, whereas Ca²⁺ excites type III cells. Therefore, it is likely that bitter taste information from type II cells is directly communicated to the Q-fibers. [J Physiol Sci. 2008;58 Suppl:S168]