Taste transduction involves the conversion of a chemical stimulus to an electrical response in a manner which encodes both the quality and intensity of the stimulus. These transduction processes take place at the level of the receptor cell, the receptor-afferent synapse, and possibly through the branching network of a single afferent fiber. To distinguish between electrolytes, the taste cell must possess different early transduction mechanisms which must ultimately converge at the receptor-afferent synapse. Study of these early transduction processes (those leading to receptor cell depolarization) has been impeded by the technical difficulty of intracellular recording from these small cells. Diverse theoretical models have appeared based largely on neural recordings. This proposal seeks to gain empirical evidence via intracellular recording of frog taste cell receptor potentials to be objectively interpreted in view of these existing theories. Single cell response profiles will be recorded to differing electrolytes (NaCl , KCl , CaCl2, MgCl2, and HCl). An example of two possible classes of HCl cells which might serve as a basis for HCl/NaCl distinctions is presented. Comparative use of iontophoretic and bulk flow stimulation methods, which have clearly different early transduction mechanisms for most stimuli but highly similar neural outputs, provides insight into ion channel, phase boundary, and synaptic activation. The use of iontophoretic stimuli also deepens our understanding of electrogustometry, a clinical diagnostic procedure used for human taste pathologies. Two inhibitors, amiloride and CoCl2, will be used to characterize differences in electrolyte transduction mechanism. Amiloride acts with less ion specificity in frog than in rodent. Intracellular records indicate that amiloride may operate by decreasing the cell input resistance. CoCl2, a well known inhibitor of calcium channels, specifically and reversibly inhibits the neural calcium response. Inhibition is nearly 100%. Finally, analysis of the membrane time constant prior to and during stimulation with either bulk flow or iontophoretic means should serve as first approximation of how capacitative and resistive elements subserve transduction mechanism. Thus far preliminary data tends to support the notion that existing theories may more accurately model a particular subclass of electrolyte rather than serve as a general model for all receptor potentials.
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