TONIC GLUTAMATE-MEDIATED CONTROL OF ROSTRAL VENTROLATERAL MEDULLA ANDSYMPATHETIC VASOMOTOR TONE

Sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM) play a key role in the tonic maintenance of resting arterial pressure . Removal of tonically active inhibitory inputs to the RVLM provided b y the caudal ventrolateral medulla (CVLM) elicits a large increase in arterial pressure. The present study addresses the hypothesis that exc itatory amino acids (EAA) provide the excitation of the RVLM responsib le for the increase in arterial pressure that occurs after withdrawal of CVLM-mediated inhibition of the RVLM. In rats anesthetized with eit her alpha-chloralose or urethan, inhibition of the CVLM by local injec tion of muscimol markedly elevated arterial pressure. Subsequent injec tion of the EAA receptor antagonist kynurenic acid into the RVLM cause d arterial pressure to fall to levels comparable to those that occur w ith total autonomic blockade. In contrast, injection of kynurenic acid into the RVLM of control rats had little effect on arterial pressure. These results indicate that the large increase in arterial pressure p roduced by inhibition of the CVLM is mediated by EAA excitation of RVL M neurons. Furthermore, these data suggest that EAA play a prominent r ole in the tonic excitation of RVLM neurons, but, in intact rats, inhi bition of EAA in the RVLM elicits no change in arterial pressure becau se of removal of inhibitory as well as excitatory drives of the RVLM.