Central cardiovascular effects of clonidine and another imidazoline derivative ST [600] in adult fowls.

In adult fowls (Gallus domesticus) with cannulae stereotaxically implanted into the III cerebral ventricle and into the hypothalamus, the effects of clonidine and another imidazoline derivative (St 600) were studied on arterial blood pressure and heart rate. It has been shown that clonidine and St 600 administration into the III ventricle or into the hypothalamus produced an immediate profound and long lasting decrease in blood pressure and heart rate. Contrary to the intravenous administration, this response was not preceded by an initial rise in blood pressure. On an equimolar basis St 600 appeared to be at least one-tenth as potent as clonidine. Prior intraventricular administration of alpha-adrenoceptor blocking agents, phentolamine and phenoxybenzamine, prevented changes in cardiovascular effects following intrahypothalamic infusion of clonidine and St 600. In conclusion, present experiments indicate that in fowls the hypothalamus represents the site or sites through which hypotension and bradycardia evoked by clonidine and related compounds are mediated.