Aortic baroreceptors exert a tonically active restraining influence on centrally mediated depressor responses

This study tested the hypothesis that aortic baroreceptors exert a central restraining influence on centrally mediated depressor responses and that this mechanism is tonically active and is independent of their modulation of basal arterial pressure. The effects of short-term (48-72 h) aortic baroreceptor deafferentation (ABD) on the acute hemodynamic (peripherally mediated pressor and centrally mediated depressor) effects of clonidine were investigated in conscious and anesthetized normotensive rats. ABD caused an immediate increase in basal arterial pressure and heart rate and a significant attenuation of the baroreceptor reflex control of heart rate. Since only arterial pressure subsided to control levels by 48-72 h, the data suggest that central reorganizational changes were potent enough to overcome the tonically active restraining influence of aortic baroreceptors on basal arterial pressure but not heart rate. Clonidine produced a similar pressor effect in conscious ABD and sham rats but its depressor effect was significantly greater in ABD rats whose baroreceptor reflex control of heart rate was significantly attenuated. Intracisternal (i.c.) administration of 0.1 microgram of clonidine, a dose that had no effect when administered i.v., produced a near-maximal depressor effect in conscious ABD rats vs. no effect in sham rats (-16.7 +/- 4.1 vs. -0.3 +/- 2 mm Hg; p less than 0.001). The depressor effect of clonidine was also enhanced in chloralose-anesthetized rats and coincided with an anesthesia-induced attenuation of the baroreceptor reflex control of heart rate. It is concluded that aortic baroreceptors exert a potent restraining influence on the centrally mediated depressor effect of clonidine. Since ABD had no significant effect on the depressor response to nitroprusside, but enhanced the depressor response to ganglion blockade by hexamethonium, the data suggest that a higher peripheral sympathetic neural activity existed in ABD rats. The reorganizational changes that occurred within 48-72 h after ABD were potent enough to overcome the tonically active restraining influence of aortic baroreceptors on basal arterial pressure but not on the centrally mediated depressor responses. Thus, the buffering influence of aortic baroreceptors and their central projections on centrally mediated depressor responses seems to be tonically involved in blood pressure control.