The Role of Central Catecholamines in the Control of Blood Pressure through the Baroreceptor Reflex and the Nasopharyngeal Reflex in the Rabbit

There is good evidence that central monoaminergic nerves participate in baroreflex control of the circulation. In particular, there is evidence that bulbospinal catecholaminergic fibres terminating in the lateral horns of the spinal cord form an essential element in the baroreflex arc and mediate changes in efferent sympathetic activity, Catecholaminergic nerves also appear to play a part in a brain stem depressor mechanism involving the nucleus tractus solitarii, the site of primary synapse for afferent fibres from the arterial baroreceptors. We have recently examined the role of central catecholamines in another cardiovascular reflex - the “smoke reflex”. This is a trigeminal nerve reflex that produces a pronounced sympathetic vasoconstriction and a vagally mediated bradycardia in response to cigarette smoke stimulation of the nasopharynx of the rabbit. Following intracisternal 6-hydroxydopamine (6-OHDA), the vasoconstrictor component of the response was inactivated, but the bradycardia appeared to be unaffected. At the end of experiments, measurements were made of regional brain noradrenaline concentration, dopamine-beta-hydroxylase (DBH) activity and phenylethanolamine- N-methyl-transferase (PNMT) activity. In animals receiving 6-OHDA, noradrenaline concentration and DBH activity were reduced t o about 50% of control, but PNMT activity was unchanged. These data suggest that central pathways mediating vasoconstriction in response to nasopharyngeal stimulation, utilise noradrenaline rather than adrenaline as a neurotransmitter.