Evidence of a peripheral and a central effect of oxytocin on pancreatic hormone release in rats.

The aim of the present study was to investigate how oxytocin given subcutaneously (SC) and intracerebroventricularly (ICV) influences the secretion of insulin, glucagon and glucose and to investigate whether the effect on these variables of suckling in lactating rats is mediated by oxytocinergic mechanisms. Male rats were given oxytocin in doses of 2 or 20 ng (SC) or 2 or 200 ng (ICV). Trunk blood was collected and hormone analysis performed by radioimmunoassay. Subcutaneous injections of oxytocin increased insulin, glucagon and glucose levels significantly. Two nanograms oxytocin given ICV had no effect on glucagon and glucose levels but caused a significant rise in insulin levels at this time point. This effect was abolished by atropine. The oxytocin antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin administered ICV increased insulin levels itself and therefore the effect on oxytocin-induced insulin secretion was difficult to evaluate. Intracerebroventricular injections of 200 ng oxytocin caused a significant rise not only of insulin but also of glucagon and glucose levels. Since this dose of oxytocin also caused a substantial rise of circulating oxytocin levels, these effects on glucose and glucagon may have been exerted at a peripheral site. Suckling in lactating rats was followed by a significant increase of glucose and glucagon levels. These effects were completely abolished by pretreatment with an oxytocin antagonist. In conclusion, oxytocin seems to influence pancreatic hormone secretion by two different mechanisms. Elevated circulating levels of oxytocin-e.g. as seen in response to suckling in lactating rats-are accompanied by a rise of glucagon and glucose levels which is blocked by the oxytocin antagonist. In contrast, nanogram amounts of oxytocin administered ICV cause a rise of insulin levels. Since this effect was blocked by atropine, it is likely to involve activation of vagal cholinergic neurons, innervating pancreatic beta-cells.