Inhibition by locus coeruleus on the baroreceptor reflex response in the rat

Abstract

We evaluated the modulation of baroreceptor reflex (BRR) response by locus coeruleus (LC) in adult, male Sprague-Dawley rats anesthetized with urethane (1.5 g/kg, i.p.). Under an electrical stimulation condition that did not appreciably alter the basal systemic arterial pressure and heart rate, the LC significantly suppressed the BRR response. Microinjection of l-glutamate (1 nmol, 50 nl) into the LC essentially duplicated this depressant effect. Intracerebroventricular (i.c.v.) administration of the α 1-adrenoceptor antagonist, prazosin (6.5 nmol), appreciably blunted the inhibition by LC on the BRR response. Yohimbine (6.5 nmol), the α 2-adrenoceptor blocker, however, was ineffective. Direct microinjection of prazosin (50 pmol), but not yohimbine (50 pmol), into the terminal site of baroreceptor afferents at the nucleus tractus solitarii (NTS) also significantly blunted the suppressive effect of LC on the BRR response. These results suggest that the LC may produce an inhibition on the BRR response by a process that involves the α 1-adrenoceptors located in the NTS.