Participation of Central Noradrenergic Neurons in Arterial Baroreceptor Reflexes in the Rabbit

Disappearance rates of intracisternally administered 3H-norepinephrine and activities of tyrosine hydroxylase were examined in the rabbit in five brain regions (telencephalon, hypothalamus, midbrain, medulla-pons, and cerebellum) and in three cord regions (cervical, thoracolumbar, and lumbosacral) 2 weeks after section of the carotid sinus and aortic nerves. Mean blood pressure rose by 29% and heart rate by 17% in the animals with neurogenic hypertension. Endogenous catecholamine concentrations in the eight regions examined were not altered by denervation. In the thoracolumbar region of the spinal cord, 3H-norepinephrine turnover and tyrosine hydroxylase activity were increased approximately twofold in hypertensive rabbits. We suggest that these changes reflect increased physiological activity of bulbospinal noradrenergic neurons and that this increase may mediate the rise in arterial pressure or heart rate that follows sinoaortic denervation. The turnover of 3H-norepinephrine increased in the hypothalamus of denervated animals; tyrosine hydroxylase activity remained unchanged in this region.