Administration of AVP to the area postrema alters response of NTS neurons to afferent inputs

This study was designed to determine if arginine vasopressin (AVP) facilitates the response of nucleus of the solitary tract (NTS) neurons to baroreceptor input. In anesthetized sinoaortic-denervated vagotomized rabbits, AVP was intravenously infused 115 μg · kg^-1^min^-1 min) or microinjected into the area postrema(AP; 1 ng/nl, 10 nl). Extracellular recordings of evoked NTS neuronal responses to electrical stimulation of the aortic depressor nerve (ADN) or vagus nerve (1 Hz, 2-20 V, 0.05-0.6 ms) were evaluated before and after AVP administration. In neurons receiving input from the ADN (n = 19), 58% of them increased their responses after AVP (40.3 ± 5.0 to 71.5 ± 4,8%, P < 0.001). Similarly, in neurons activated by vagal stimulation (n = 22), 55% of them were facilitated during AVP administration (59.7 ± 12.8 to 90.8 ± 10.7%, P < 0.01). This action of AVP was independent of the mode of AVP administration, since either microinjection or venous infusion was effective in augmenting responses of NTS neurons to aortic/vagal stimulation. In an additional 37 spontaneous NTS neurons, AVP showed no effect on the mean baseline firing rate (8.9 ± 1.3 vs. 9.6 ± 1.3 spikes/s, P > 0.05), but increased neuronal activity in 54% of neurons (6.9 ± 1.3 vs. 13.1 ± 1.7 spikes/s, P < 0.01). In two rabbits pretreated with vasopressin antagonist (15 pg/kg iv), AVP failed to produce facilitatory effects (n = 8). The results of this study provide evidence in support of the hypothesis that circulating peptides modulate the arterial baroreflex via activation of neurons in the AP.