Hypernatraemic stimulation of oxytocin secretion : effects of opioids and pregnancy

• Main Author: Bull, Philip Mark
• Published: University of Edinburgh 1994
• Subjects: 615.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.642206

The thesis describes investigations into the responsiveness of the neuroendocrine oxytocin system to hypernatraemic stimulation during pregnancy in the rat. The effects of acute intraperitoneal and intravenous hyperosmotic saline were investigated in virgin rats and in pregnant rats after 16 and 21 days of gestation. Plasma oxytocin concentration was measured by radioimmunoassay, and the inhibitory effects of endogenous opioids and exogenous opiates on the osmotic stimulation of oxytocin secretion were also investigated. The response of the oxytocin system to intraperitoneal hyperosmotic saline was strikingly attenuated at day 21 of pregnancy. These results reveal a reduced influence of the osmoregulatory input to oxytocin neurones in pregnancy. Experiments to test whether endogenous opioids, angiotensin II or acute ovarian hormone effects were involved in this reduced oxytocin activity during pregnancy indicated that it was independent of these factors. However, the oxytocin response of 21 day pregnant rats to the intravenous administration of hyperosmotic saline after the opioid antagonist naloxone was significantly elevated. This may be due to an increased sensitivity of oxytocin neurones to changes in plasma volume produced by the intravenous infusion of hyperosmotic saline that is absent when administered by the intraperitoneal route. The sites of the osmoreceptors regulating the response of oxytocin neurones to changes in plasma osmolality were investigated in virgin rats. This involved the discrete application of hyperosmotic saline into the brain using infusion and microdialysis techniques. The results of these experiments indicate that the osmoreceptors are partly in the lamina terminalis but through their direct osmosensitivity the magnocellular oxytocin neurones themselves function as osmoreceptors.