Chemical Senses Advance Access originally published online on February 9, 2006
Chemical Senses 2006 31(4):307-313; doi:10.1093/chemse/bjj034
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Tonic Activity of Parasympathetic Efferent Nerve Fibers Hyperpolarizes the Resting Membrane Potential of Frog Taste Cells
1 Divisions of Integrative Sensory Physiology and 2 Oral Molecular Pharmacology, Nagasaki University Graduate School of Biomedical Sciences, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan
Correspondence to be sent to: Toshihide Sato, Division of Integrative Sensory Physiology, Nagasaki University Graduate School of Biomedical Sciences, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan. e-mail: toshi{at}net.nagasaki-u.ac.jp
We investigated the relationship between the membrane potential of frog taste cells in the fungiform papillae and the tonic discharge of parasympathetic efferent fibers in the glossopharyngeal (GP) nerve. When the parasympathetic preganglionic fibers in the GP nerve were kept intact, the mean membrane potential of Ringer-adapted taste cells was –40 mV but decreased to –31 mV after transecting the preganglionic fibers in the GP nerve and crushing the postganglionic fibers in the papillary nerve. The same result occurred after blocking the nicotinic acetylcholine receptors on parasympathetic ganglion cells in the tongue and blocking the substance P neurokinin-1 (NK-1) receptors in the gustatory efferent synapses. This indicates that the parasympathetic nerve (PSN) hyperpolarizes the membrane potential of frog taste cells by –9 mV. Repetitive stimulation of a transected GP nerve revealed that a –9-mV hyperpolarization of taste cells maintained under the intact GP nerve derives from an 
10-Hz discharge of the PSN efferent fibers. The mean frequency of tonic discharges extracellularly recorded from PSN efferent fibers of the taste disks was 9.1 impulses/s. We conclude that the resting membrane potential of frog taste cells is continuously hyperpolarized by on average –9 mV by an 10-Hz tonic discharge from the parasympathetic preganglionic neurons in the medulla oblongata.
Key words: gustatory efferent synapse, membrane potential, parasympathetic nerve, slow hyperpolarizing potential, taste receptor cell
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