Cardiovascular responses of rats to intrahypothalamic injection of carbachol and noradrenaline

Carbachol (0.1-2 micrograms) and noradrenaline (5-20 micrograms) injected into the anterior hypothalamus of unanaesthetized unrestrained rats evoked dose-dependent falls in heart rate and increases in arterial blood pressure. When the above amine injections were repeated in rats anaesthetized with chloral hydrate, sodium pentobarbitone or halothane the evoked bradycardias were usually smaller and the changes in arterial blood pressure more variable. The cardiovascular responses to carbachol (1 microgram) and to noradrenaline (10 micrograms) were antagonized by intrahypothalamic injection of atropine (1 microgram) and phentolamine (10 micrograms), respectively. The bradycardias evoked by carbachol (1 microgram) and by noradrenaline (10 micrograms) were abolished by pretreatment with intra-arterial injection of methyl atropine (100 micrograms 100 g-1). The hypertensive responses to carbachol (1 microgram) and to noradrenaline (10 micrograms) were reduced by pretreatment with intra-arterial injection of thymoxamine (500 micrograms 100 g-1).