Taste-responsive neurons of the glossopharyngeal nerve of the rat

Abstract

1. Taste sensibilities of neurons in mammalian glossopharyngeal nerves have been inadequately studied, although they innervate the majority of taste buds and may provide unique taste information. 2. Extracellular responses of glossopharyngeal neural units to taste stimuli infused into foliate or vallate papillae were recorded in anesthetized rats. A 0.3-ml/min infusion of stimuli into papillae resulted in short-latency, 5-s nerve-impulse rates that approached 10 times the response rates observed using less invasive means of stimulation. 3. Sucrose, Na saccharin, NaCl, NH4Cl, KCl, HCl, citric acid, acetic acid, MgSO4, and quinine.HCl were effective stimuli for glossopharyngeal neurons at concentrations that have behavioral significance. 4. Response spectra for individual neural units with either foliate or vallate receptive fields fell into three clusters. Forty-six percent were A units that responded most strongly to acids and chloride salts, NH4Cl being the most effective; neither quinine nor sucrose was effective. Twenty-three percent were S units that responded to sugars and saccharin; quinine, salts, and acids were not effective. Thirty-one percent were Q units that responded to quinine; neither NaCl, HCl, nor sucrose was effective stimulus for these fragile units. 5. Glossopharyngeal A neural units were more sensitive to 1 mM HCl than were electrolyte-sensitive H units of the chorda tympani, although both respond generally to salts and acids. Units relatively specific for sodium salts (N units), which are common in the chorda tympani nerve, were not found in the glossopharyngeal nerve, which explains losses in sodium-specific behavior after cutting only the chorda tympani nerve. 6. Q units were the only glossopharyngeal neural units that responded significantly to quinine, and units with similar response spectra do not occur in the chorda tympani nerve. Q units probably mediate aversive reflexes to quinine that are eliminated by cutting only the glossopharyngeal nerve. Glossopharyngeal S neural units were more sensitive to sucrose and are more common than their counterparts in the chorda tympani, although it is not known how they might compare with sugar-sensitive units in the greater superficial petrosal nerve. 7. These data strongly suggest that posterior taste bud fields innervated by the glossopharyngeal nerve are specialized for functions different from those of anterior taste bud fields innervated by the facial nerve.