Central nervous system opiate and 5-hydroxytryptamine influences on baroreflex control of the coronary circulation

The effect of intravenous infusion of fentanyl (an opiate receptor agonist, 0.55 microgram kg-1 min-1) on the control of the circumflex coronary circulation was examined in unsedated dogs at rest and during baroreceptor stimulation evoked by acute rises in aortic pressure (balloon inflation in thoracic aorta). Circumflex flow was measured using Doppler flow transducers in dogs with experimental complete heart block and with ventricles paced at a constant rate. Studies were also performed before and one week after intracisternal injection of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), to examine the role of CNS 5-hydroxytryptamine (5-HT) in any sympathetic vasoconstrictive effects. Fentanyl infusion caused after a few minutes a progressive rise in resting aortic pressure and a significant fall in circumflex conductance; circumflex flow usually fell. Atrial rate also fell. The gain of the baroreflex control of circumflex conductance was enhanced by fentanyl. One week after intracisternal 5,7-DHT, the gain of the baroreflex in each dog was diminished. When fentanyl was infused into these preparations, no consistent changes in resting atrial rate, aortic pressure and circumflex conductance could be observed, but all dogs showed a recovery of the coronary baroreflex gain towards values observed before intracisternal 5,7-DHT. These data suggest that the gain control of coronary baroreflexes is influenced by CNS opiate and 5-HT dependent mechanisms.