Mechanisms of hemodynamic responses to electrical stimulation of subfornical organ

The rat subfornical organ (SFO) is involved in the pressor response to circulating angiotensin II, and recent evidence indicates that SFO electrical stimulation also produces a pressor response. In the present experiments we examined the hemodynamic, neural, and humoral mechanisms that underlie the pressor response to electrical stimulation of the SFO. Rats were anesthetized with urethan and instrumented with femoral arterial catheters and with pulsed Doppler flow probes on the superior mesenteric and renal arteries and on the abdominal aorta. Constant-current stimulation, delivered to the SFO via tungsten microelectrodes, resulted in stimulus-locked frequency-dependent pressor responses and vasoconstriction in all vascular beds tested. The stimulation-evoked increases in vascular resistance were greatest in the mesenteric circulation and least in the renal. Movement of the electrode away from the SFO produced significantly smaller responses. Ganglionic blockade abolished the responses to electrical stimulation, whereas vasopressin blockade significantly attenuated the responses. The responses of baroreceptor-denervated rats were qualitatively similar to but approximately double in magnitude of those of normal rats. We conclude that electrical stimulation of the SFO elicits widespread regional vasoconstriction that is most pronounced in the mesenteric circulation. The sympathetic nervous system appears responsible for these effects, but there may be facilitation of the responses by vasopressin.