Peripheral and central mechanisms of the pressor response elicited by stimulation of the locus coeruleus in the rat.

Electrical stimulation of the pontine nucleus locus coeruleus (LC) caused an increase of the arterial blood pressure in anesthetized rats, and elevated plasma noradrenaline (NA) and adrenaline (A) levels. The stimulation-induced pressor response was characteristically biphasic and consisted of a sharp rise in arterial pressure at the onset of the stimulation, followed by a second elevation at the end of the stimulus. Bilateral adrenalectomy or adrenal demedullation completely blocked the secondary phase of the pressor response elicited by stimulation, but did not affect the primary phase. The latter was specifically eliminated by the destruction of the peripheral sympathetic vasomotor axons with intravenous 6-hydroxydopamine (6-OHDA). The active sites eliciting the secondary adrenomedullary pressor component appeared to be restricted to the nucleus LC, whereas the primary sympathetic vasomotor response could be elicited from sites in and around the nucleus. After brain transection at the midbrain level, stimulation of LC failed to evoke the adrenomedullary pressor response, while the sympathetic vasomotor component was not affected. Similarly, destruction of brain NA neurons by intraventricular administration of 6-OHDA did not change the sympathetic vasomotor response, but virtually abolished the adrenal response. The results demonstrate that the pressor response to stimulation of LC in the rat is due to both increased sympathetic vasomotor activity and CA released from the adrenal medulla. The study also provides evidence suggesting that the noradrenergic LC cell group play an important role in the activation of the adrenal medulla, but is not essential for the activation of the sympathetic vasoconstrictor fiber system.