Journal of the Autonomic Nervous System, 3 (1981) 511--523 © Elsevier/North-Holland Biomedical Press 511 EFFECTS OF NORADRENERGIC AND SEROTONERGIC NEURONS ON BLOOD PRESSURE, HEART RATE AND BARORECEPTOR-HEART RATE REFLEX OF THE CONSCIOUS RABBIT P.I. KORNER and G.A. HEAD Baker Medical Research Institute, Melbourne 3181 (Australia) Keywords: baroreceptor reflex -- blood pressure -- circulatory control -- heart rate -- noradrenergic neurons -- bulbospinal pathway -- serotonergic neurons -- suprapontine pathway ABSTRACT The acute and chronic effects of intracisternal (i.c.) 6-hydroxydopamine (6~)HDA) and 5,6<lihydroxytryptamine (5,6-DHT) on mean arterial pres- sure (MAP), heart rate, and baroreceptor-heart rate reflex properties were examined in conscious rabbits. The reflex was studied by deriving MAP-- heart period (HP, pulse interval) curves. The acute effects occurring over the first few hours were due to release of transmitter (noradrenaline after 6-OHDA; serotonin after 5,6-DHT) from specific synaptic sites; the changes on day 14 were due to destruction of specific neurons. Noradrenergic bulbar neurons facilitate vagal heart rate motoneurons and serotonergic neurons have an inhibitory effect. Noradrenergic bulbospinal pathways have a depres- sor effect on MAP and serotonergic bulbospinal activity increases MAP. These effects on heart rate and MAP were mediated through independent parallel pathways. The acute circulatory changes due to each neurotoxin also included a long latency pressor component, which was mediated through a pathway where a serotonergic neuron was in series with a noradrenergic neu- ron which sends fibers to a suprapontine pressor site. There were no chronic changes in resting MAP or heart rate, but chronic changes in reflex HP range and gain were opposite to the acute effects of transmitter release. These findings indicate that noradrenergic and serotonergic neurons normally parti- cipate in the central baroreceptor--heart rate reflex pathways of the intact animal.