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Morimoto K, Sato M. 
“Is Serotonin a Chemical Transmitter in the Frog Taste Organ?”. 
Life Sci.. 1977;21(11):1685-95.
The possibility that serotonin is a chemical transmitter in frog taste organ was investigated. - American bullfrogs (Rena catesbeiana,250-400g) were anesthetized with 200rethane (i.p.) and the lingual artery was perfused with various drug solutions. Gustatory receptors were stimulated by applying taste solutions over the surface of the tongue. The consequences of these procedures were assessed in terms of changes in glossopharyugeal nerve (GN) activity. Although perfusion of the lingual artery with serotonin (5-HT) creatinine sulfate (C) complex (10 4-lo 6 g) elicited dosedependent increases in GN acitivity' a similar effect was observed using C alone. Responses to C, usually occurred with a long latency and gradually increased, whereas responses to 5-HT exhibited an initial phasic component with a short latency. Perfusion with high concentrations of 5-HT, but not C, caused depression of responses in the GN to taste stimuli and contraction of the muscles in the tongue with rhythmic firing in mechanosensitive fibers. The neural responses to gustatory receptor stimulation by solutions of NaCl, KCl, (0.3 M) or CaCl2 (0.3 mM) and by deionized water were markedly depressed by lingual artery perfusion with LSD ( >10 6 g/ml). At an LSD concentration of 10-5 g/ml the stationary state of responses to quinine and HCl was significantly augmented. Responses to taste stimuli were generally unaffected by lingual artery perfusion with gramine, 5-hydroxytryptophan or phenformin. Taste and tactile responses were, however, somewhat reduced by gramine or phenformin (10 4 g/ml). Pretreatment of bullfrogs with daily p-chlorophenylalanine (100 mg/kg i.p.) and reserpine (10 mg/kg) during 5-10 days preceding the electrophysiological experiment did not result in significant alteration of the neural responses to taste stimuli. 5-HT is unlikely to be a chemical mediator from taste cells to nerve terminals in the frog taste organ.
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