EXCITATORY AMINO-ACID RECEPTORS IN COMMISSURAL NUCLEUS OF THE NTS MEDIATE CAROTID CHEMORECEPTOR RESPONSES

Stimulation of carotid body chemoreceptors by saline saturated with 100% CO2 elicited an increase in mean arterial pressure, respiratory rate, tidal volume, and minute ventilation (V(E)). Microinjections of L-glutamate into a midline area 0.50. 75 mm caudal and 0.3-0.5 mm deep with respect to the calamus scriptorius increased V(E). Histological examination showed that the site was located in the commissural nucleus of the nucleus tractus solitarii (NTS). The presence of excitatory amino acid receptors [N-methyl-D-aspartic acid (NMDA); kainate, quisqualate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and trans 1-amino-cyclopentane-trans1, 3-dicarboxylic acid (ACPD)] in this area was demonstrated by microinjections of appropriate agonists. Simultaneous blockade of NMDA and non-NMDA receptors by combined injections Of DL-2-aminophosphonoheptanoate (AP-7; 1 nmol) and 6,7-dinitro-quinoxaline-2,3-dione (DNQX; 1 nmol) abolished the responses to stimulation of carotid body on either side. Combined injections of AP-7 and DNQX did not produce a nonspecific depression of neurons because the responses to another agonist, carbachol, remained unaltered. Inhibition of the neurons in the aforementioned area with microinjections of muscimol (which hyperpolarizes neuronal cell bodies but not fibers of passage) also abolished the responses to subsequent carotid body stimulation on either side. These results indicate that 1) the carotid body chemoreceptors project to a midline area adjacent to the calamus scriptorius (0-0.75 mm caudal and 0.3-0.5 mm deep); this site was tentatively designated as 'chemoreceptor projection site''; and 2) excitatory amino acid receptors (NMDA and non-NMDA subtype) located on the neurons in the chemoreceptor projection site mediate the carotid body chemoreceptor responses.