Cardiovascular Changes in Anaesthetised Rats after the Intra-hypothalamic Administration of Adrenaline

Abstract

Bilateral injections of adrenaline (0.01–10.0 μg) into the anterior hypothalamic (AH) region, in urethane-anaesthetised spontaneous hypertensive (SH) rats, elicited dose-dependant falls in blood pressure and heart rate. The bradycardia was immediate in onset while the hypotension was preceded by a short-lasting rise in blood pressure. Bilateral injections of adrenaline into the anterior preoptic area (POA) and areas surrounding the AH had little or no effect on blood pressure and heart rate, while injections into the posterior hypothalamus (PH) induced tachycardia and hypertension followed by a smaller fall in blood pressure. Pretreatment with dl-propranolol (25-100 üg bilat. AH) appeared to potentiate the hypotension induced by adrenaline (1 μg bilat AH) in a dose-dependant manner, but did not affect the falls in heart rate. On the other hand, pretreatment with metoprolol (25–100 μg bilat. AH) effected a dose-dependant antagonism of the adrenaline-induced hypotension and bradycardia. Pretreatment with 1-propranolol (25 μg bilat. AH) also antagonised the adrenaline-induced cardiovascular depressor effects, while pretreatment with d-propranolol (25 μg bilat. AH) abolished the initial hypertensive effect. Pretreatment with piperoxan (25–100 μg bilat. AH) antagonised adrenaline (1 μgu bilat. AH) induced hypotension and bradycardia only at the highest dose used. The results give further support to the concept that hypothalamic adrenaline receptors may be involved in the central regulation of blood pressure and heart rate. Furthermore, while an involvement of hypothalamic α-adrenoceptors cannot be ruled out, it is suggested that hypothalamic β-adrenoceptors may be involved in mediating the cardiovascular depressor effects of adrenaline injected into the AH.