Catecholamine depletion of the diagonal band reduces baroreflex inhibition of supraoptic neurons

In the rat, neurons in the diagonal band of Broca (DBB) participate in baroreceptor-induced depression of spontaneous activity of vasopressin neurons in the supraoptic nucleus (SON). The present study examined the role of the catecholaminergic innervation of the DBB in this response. Male rats were anesthetized with pentobarbital (50 mg/kg ip) and stereotaxically injected in the DBB with either vehicle (2 μl), 6-hydroxydopamine (6-OHDA; 4 μg/2 μl), or 6-OHDA preceded 20 min earlier by desimipramine (25 mg/kg ip), a norepinephrine uptake inhibitor. Two weeks later, the rats were reanesthetized and a transpharyngeal approach was used for extracellular recording from SON neurons. In vehicle-injected controls, baroreceptor stimulation produced by brief increases in blood pressure from metaraminol injections (10 μg/10 μl iv) transiently arrested the spontaneous activity of 24 of 24 phasically active neurons tested. Sixty-three percent of the vasopressin neurons were not affected by comparable increases in blood pressure in 6-OHDA-treated rats, and the norepinephrine content of the DBB was significantly reduced. In experiments with desimipramine-pretreated rats, 92% of the vasopressin neurons were silenced by increases in blood pressure while the norepinephrine content of the DBB was not affected. Thus the noradrenergic innervation of DBB appears to participate in the baroreceptor sensitivity of SON vasopressinergic neurons.