Pharmacological characterisation of the alpha-adrenoceptors responsible for a decrease of blood pressure in the nucleus tractus solitarii of the rat.

The effects of various alpha-adrenoceptor agonists, microinjected into the area of the nucleus tractus solitarii (NTS) at the level of the obex, on blood pressure, and the interaction between noradrenaline and some alpha-antagonists in the area of the NTS were investigated in anaesthetized male rats. A dose-dependent decrease in blood pressure was induced by noradrenaline. The relative potencies of the substances were adrenaline greater than noradrenaline greater than alpha-methylnoradrenaline greater than clonidine greater than tyramine. In contrast, phenylephrine was ineffective. The hypotensive effect of noradrenaline, alpha-methylnoradrenaline and clonidine was not affected by pretreatment with 6-hydroxydopamine given intraventricularly, while the effect of tyramine was blocked by the pretreatment. Prior application of phentolamine at the same site antagonized the hypotensive response to noradrenaline. Prazosin was about 10 times less potent than yohimbine in antagonizing the noradrenaline-induced hypotension. The present data suggest that the alpha-adrenoceptors in the area of the NTS responsible for the decrease in blood pressure are the same type as the peripheral presynaptic alpha 2-adrenoceptors but may be located postsynaptically. It appears that the noradrenergic neurons in the NTS can play a role in blood pressure regulation.