Central cholinergic and noradrenergic stimulation in spontaneously hypertensive rats

In order to test the integrity of central receptors, spontaneously hypertensive rats (SHR) of the Okamoto strain and weight-matched control rats of the Wistar Kyoto (WKY) strain were given intracerebroventricular (i.vt.) injections of carbachol and norepinephrine. The rats, in an unanesthetized, unrestrained state, were tested for drinking, antidiuretic and pressor responses. Antidiuretic hormone release was determined by using water loaded, diuresing rats as their own antidiuretic hormone bioassay. Blood pressure was measured directly from a femoral artery catheter. Drinking responses to i.vt. carbachol and antidiuretic responses to i.vt. carbachol and norepinephrine infusions were not different between SHR and WKY while pressor responses were potentiated in SHR. The potentiated pressor responses to central carbachol and norepinephrine injections were the result of increased vascular responsiveness to the antidiuretic hormone released by these drugs. A second, neurally mediated, factor was also apparent to i.vt. carbachol injections. This additional factor could be increased sympathetic outflow to central drug stimulation, increased vascular reactivity to sympathetic outflow, decreased baroreflex responses or a combination of the above.