Role of monoamines in afferent synaptic transmission in frog taste organ.

To examine physiological and pharmacological characteristics of synaptic transmission from taste cells to afferent nerve terminals in frog taste organ, changes in the glossopharyngeal nerve discharges and responses to chemical stimulation of the tongue were examined by intra-arterially perfusing the tongue with Ringer solution containing monoamines and their related substances. Noradrenaline (NA), dopamine (DA), and adrenaline (AD) produced an increase in the spontaneous glossopharyngeal nerve discharges as well as an enhancement of taste responses to various kinds of chemical stimuli, but they did not affect tactile response. NA was the most effective of the three. Monoamine-releasing agents, tyramine and 6OHDA, enhanced spontaneous nerve discharges but depressed taste responses. Antimonoaminergic agents, phentolamine and phenoxybenzamine (alpha-blockers), increased nerve discharges and depressed taste responses, but did not affect tactile response. DCI (beta-blocker), except at high concentrations, affected neither nerve discharges nor taste responses. Pre-treatment of frogs with monoamine-depleting agents, alpha-MPT and 6OHDA, depressed taste responses. Reserpine enhanced the effect of alpha-MPT. The depressed taste responses recovered after perfusion of the tongue with NA. From the results obtained, the possibility of NA being a transmitter at the afferent synapse in the frog taste organ was discussed.