Intravenous 6-hydroxydopamine attenuates vasopressin and oxytocin secretion stimulated by hemorrhage and hypotension but not hyperosmolality in rats

The present study sought to determine whether chemical destruction of peripheral catecholaminergic fibers with 6-hydroxydopamine (6OHDA) attenuates vasopressin (VP) and oxytocin (OT) secretion stimulated by hemorrhage, hypotension, and hyperosmolality. Rats received 6OHDA (100 mg/kg iv) or vehicle (1 ml/kg iv) on days 1 and 7, and experiments were performed on day 8. Serial hemorrhage (4 samples of 2 ml per 300 g body wt at 10-min intervals) increased plasma VP and OT levels in both groups; however, the increase in plasma VP and OT levels was significantly attenuated in 6OHDA-treated vs. control rats despite a significantly lower mean arterial blood pressure. Similarly, the increase in plasma VP and OT levels in response to hypotension produced by the selective arteriolar vasodilator diazoxide was significantly attenuated in 6OHDA-treated rats. In marked contrast to hemorrhage and hypotension, hyperosmolality produced by an infusion of 1 M NaCl (2 ml/h iv) stimulated increases in plasma VP and OT levels that were not different between 6OHDA-treated and control rats. In a parallel set of experiments, intravenous 6OHDA treatment reduced dopamine-β-hydroxylase immunoreactivity in the posterior pituitary but had no substantial effect in the hypothalamic paraventricular and supraoptic nuclei. In each experiment, the pressor response to tyramine (250 μg/kg iv) was significantly attenuated in 6OHDA-treated rats, thereby confirming that 6OHDA treatment destroyed sympathetic catecholaminergic fibers. Collectively, these findings suggest that catecholaminergic fibers located outside the blood-brain barrier contribute to VP and OT secretion during hemorrhage and arterial hypotension.