METABOTROPIC GLUTAMATE RECEPTORS IN THE VENTROLATERAL MEDULLA OF RATS

We investigated the hypothesis that stimulation of metabotropic excita tory amino acid receptors in the ventrolateral medulla evokes cardiova scular responses. Thus, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic a cid [(1S,3R)-ACPD], a selective agonist of metabotropic excitatory ami no acid receptors, was microinjected into the rostral or caudal ventro lateral medulla of halothane-anesthetized Sprague-Dawley rats. Microin jections of (1S,3R)-ACPD (100 pmol-1 nmol) into the rostral ventrolate ral medulla produced dose-dependent increases in mean arterial pressur e (+20+/-4 mm Hg by 100 pmol and +35+/-2 mm Hg by 1 nmol, p<0.01 versu s artificial cerebrospinal fluid) and integrated splanchnic sympatheti c nerve activity (+17+/-3% and +46+/-4%, respectively, p<0.01), wherea s (1S,3R)-ACPD microinjected into the caudal ventrolateral medulla dec reased mean arterial pressure (-28+/-2 mm Hg by 100 pmol and -48+/-6 m m Hg by 1 nmol, p<0.01 versus artificial cerebrospinal fluid) and spla nchnic sympathetic nerve activity (-24+/-4% and -49+/-5%, p<0.01). The blockade of ionotropic excitatory amino acid receptors by the combine d injection of 2-amino-7-phosphonoheptanoic acid (200 pmol) and 6,7-di nitroquinoxaline-2,3-dione (200 pmol), which effectively blocked the r esponses elicited by either N-methyl-D-aspartate (20 pmol) or lpha-ami no-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol), failed to af fect the responses evoked by either (1S,3R)-ACPD (100 pmol) or L-gluta mate (2 nmol) microinjected in the rostral and caudal ventrolateral me dulla. These results suggest that metabotropic receptors are present a nd mediate cardiovascular responses evoked by L-glutamate injections i nto the rostral and caudal ventrolateral medulla.