EFFECTS OF BLOOD-PRESSURE CHANGES ON THE CATECHOLAMINE RELEASE IN THE LOCUS-CERULEUS OF CATS ANESTHETIZED WITH PENTOBARBITAL OR CHLORALOSE

Effects of carotid occlusion and drugs applied intravenously on the release of endogenous catecholamines in the locus coeruleus of cats anaesthetized with pentobarbital or chloralose were investigated. The locus coeruleus was superfused bilaterally with artificial cerebrospinal fluid through push-pull cannulae inserted stereotaxically. Dopamine, noradrenaline and in some experiments also adrenaline were determined radioenzymatically in the superfusate. Under pentobarbital anaesthesia, a bilateral carotid occlusion increased the release rate of noradrenaline in the locus coeruleus, while the release of dopamine was decreased. These changes were due to the fall of blood pressure in the carotid sinus caused by the occlusion. Loading of baroreceptors by elevating blood pressure with phenylephrine (10 mug . kg-1 . min-1, iv. infusion) was accompanied by a decreased release of noradrenaline in the locus coeruleus. This decrease in noradrenaline release was not detected in the caudal aspect of the locus coeruleus. Under chloralose anaesthesia, phenylephrine diminished the release rate of noradrenaline to about the same extent as under pentobarbital anaesthesia. The release rate of adrenaline was also decreased. A prolonged infusion of phenylephrine led to a prolonged pressor response associated with a sustained decrease in the noradrenaline release rate. Intravenous injection of chlorisondamine (3 mg . kg-1) did not change the release of noradrenaline, while dopamine release was reduced. It is concluded that the release of catecholamines in the locus coeruleus is influenced by signals originating from peripheral baroreceptors. The influences are similar under pentobarbital and chloralose anaesthesia. Noradrenergic neurons responding to haemodynamic signals are not uniformly distributed within the locus coeruleus. It is suggested that noradrenergic and possibly dopaminergic and adrenergic neurons of the locus coeruleus are involved in the baroreceptor reflex, thus contributing to central homeostasis of blood pressure.