Glutamate Receptor Antagonists Block Gustatory Afferent Input to the Nucleus of the Solitary Tract

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Abstract
Li, Cheng-Shu and David V. Smith. Glutamate receptor antagonists block gustatory afferent input to the nucleus of the solitary tract. J. Neurophysiol. 77: 1514–1525, 1997. The effects of excitatory amino acid (EAA) receptor antagonists in blocking the synaptic transmission between gustatory fibers of the chorda tympani (CT) nerve and taste-responsive neurons within the nucleus of the solitary tract (NST) were examined electrophysiologically in urethan-anesthetized hamsters. Single neurons in the NST were recorded extracellularly and drugs were microinjected into the vicinity of the cell with the use of a multibarrel pipette assembly. The activity of each cell was recorded in response to lingual stimulation with 0.032 M NaCl, 0.032 M sucrose, 0.0032 M citric acid, 0.032 M quinine hydrochloride, and/or 25-μA anodal current pulses. Once a cell was identified as a taste-responsive neuron, one or more EAA receptor antagonists were administered by microinjection. Approximately 27 nl of 50 mM kynurenic acid (KYN), a broad-spectrum EAA receptor antagonist; 0.5 or 2.0 mM dl-2-amino-5-phosphonovalerate (APV), an N-methyl-d-aspartate (NMDA) receptor antagonist; 0.05 or 0.5 mM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist; or phosphate-buffered physiological saline was applied to the neuron. Responses to chemical stimulation of the anterior tongue were obtained before and after drug administration and again after recovery; responses to anodal current stimulation (0.1 Hz) were obtained continually throughout the drug administration protocol. Microinjection of KYN completely and reversibly abolished responses elicited by both anodal current and chemical stimulation of the anterior tongue. The excitatory responses of cells in the NST to chemical and electrical stimulation of the anterior tongue were also completely and reversibly blocked by CNQX, implicating the involvement of an AMPA/kainate receptor. Microinjection of APV was generally less effective and partially reduced the responses of some taste-responsive NST cells to chemical stimulation of the anterior tongue. There were no effects following microinjection of a 27-nl bolus of phosphate-buffered saline. None of these EAA receptor antagonists had a differential effect on responses to different taste stimuli. The responses to all tastants were completely blocked by both KYN and CNQX; there was no apparent relationship between the response to any particular tastant and the limited effects of APV. These data implicate glutamate as an excitatory neurotransmitter between CT gustatory fibers and taste-responsive NST cells and suggest that it acts primarily on AMPA/kainate receptors, with some contribution from NMDA receptors. This conclusion is strengthened by other data obtained from in vitro slice preparations, which show that responses of cells in the rostral NST to solitary tract stimulation are blocked by both NMDA and AMPA/kainate receptor antagonists.