ROLE OF ALPHA-ADRENERGIC MECHANISMS ON RESPONSES TO AREA POSTREMA STIMULATION AND CIRCULATING VASOPRESSIN

This study evaluated the role of catecholamines in the nucleus tractus solitarius (NTS) on the inhibition of renal sympathetic nerve activit y (RSNA) due to direct electrical stimulation of the area postrema. In addition, the effects of catecholaminergic mechanisms in the NTS on t he ability of circulating arginine vasopressin (AVP) to modulate arter ial baroreflex control of RSNA were evaluated. Electrical stimulation of the area postrema (15 muA, 0.3 ms, 5-80 Hz) produced progressive de creases in RSNA. Responses to area postrema activation were not altere d by microinjection of the alpha1-adrenergic antagonist, prazosin, or vehicle bilaterally into the NTS. Microinjection of the alpha2-antagon ist, yohimbine (4 injections of 20-40 nl, 1 ng/nl), unilaterally into the NTS significantly attenuated the RSNA response to area postrema st imulation. Bilateral injection of yohimbine into the NTS abolished the response to area postrema activation (P < 0.05). Baroreflex inhibitio n of RSNA was significantly greater during infusion of AVP than during infusion of phenylephrine (slopes = -5.18 +/- 0.39 and -2.64 +/- 0.27 for AVP and phenylephrine, respectively). Microinjection of yohimbine bilaterally into the NTS did not alter the slope of baroreflex contro l of RSNA for phenylephrine but normalized the slope for AVP (-2.85 +/ - 0.54) to that of phenylephrine. Data are consistent with the hypothe sis that AVP effects on baroreflex inhibition of RSNA and area postrem a-mediated inhibition of RSNA require alpha2-adrenergic signaling with in the NTS.