Influence of rostral ventrolateral medulla on renal sympathetic baroreflexin conscious rabbits

Previous studies with anesthetized animals have shown that the pressor regi on of the rostral ventrolateral medulla (RVLM) is a critical site in vasomo tor control. The aim of this study was to develop, in conscious rabbits, a technique for microinjecting into the RVLM and to determine the influence o f this area on renal sympathetic nerve activity (RSNA) and arterial pressur e (AP) using local injections of glutamate, rilmenidine, ANG II and sarile. Rabbits were implanted with guide cannulas for bilateral microinjections i nto the RVLM (n = 7) or into the intermediate ventrolateral medulla (IVLM, n = 6) and an electrode for measuring RSNA. After 7 days of recovery, injec tions of glutamate (10 and 20 nmol) into the RVLM increased RSNA by 81 and 88% and AP by 17 and 25 mmHg, respectively. Infusion of glutamate (2 nmol/m in) into the RVLM increased AP by 15 mmHg and the RSNA baroreflex range by 38%. By contrast, injection of the imidazoline receptor agonist rilmenidine (4 nmol) into the RVLM decreased AP by 8 mmHg and the RSNA baroreflex rang e by 37%. Injections of rilmenidine into the IVLM did not alter AP or RSNA. Surprisingly, treatments with ANG II (4 pmol/min) or the ANG II receptor a ntagonist sarile (500 pmol) into the RVLM did not affect the resting or bar oreflex parameters. Infusion of ANG II (4 pmol/min) into the fourth ventric le increased AP and facilitated the RSNA baroreflex. Our results show that agents administered via a novel microinjecting system for conscious rabbits can selectively modulate neuronal activity in circumscribed regions of the ventrolateral medulla. We conclude that the RVLM plays a key role in circu latory control in conscious rabbits. However, we find no evidence for the r ole of ANG II receptors in the RVLM in the moment-to-moment regulation of A P and RSNA.