Relative importance of rostral ventrolateral medulla in sympathoinhibitoryaction of rilmenidine in conscious and anesthetized rabbits

The presser region of the rostral ventrolateral medulla (RVLM) is a critica l site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to com pare the importance of the RVLM in mediating the inhibitory action of rilme nidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instru mented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressur e in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast. the decrease in resting RSNA evoked by rilmenidine was simila r in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmeni dine microinjection into the RVLM was equally effective in inhibiting the R SNA baroreflex in both groups of animals. The upper plateau of the RSNA bar oreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after r ilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of ri lmenidine on arterial pressure in conscious, chronically instrumented rabbi ts may relate to a lower level of sympathetic drive compared with anestheti zed animals.