Role of circumventricular organs (CVO) in neuroendocrine responses: Interactions of CVO and the magnocellular neuroendocrine system in different reproductive states

1. Magnocellular neurons, which release oxytocin (OT) or vasopressin (VP) into the circulation in response to hyperosmolality, hypovolaemia and cholecystokinin (CCK), undergo a resetting during pregnancy and lactation that involves circumventricular organs. 2. During gestation, there is a similar lowering of the osmotic thresholds for stimulation of OT and VP release, whereas the responsiveness of both neuroendocrine systems to hyperosmolality is attenuated by lactation. These osmotic changes in the magnocellular system are mediated by depletion of hormone (OT) stores in the neurohypophysis (lactation), as well as by alterations in afferent stimulation via pathways involving the subfornical organ, organum vasculosum lamina terminalis and median preoptic nucleus. 3. During gestation, both the VP and OT systems are reset such that the expanded blood volume is maintained and defended as 'normal' in response to hypovlaemia. Thus, in virgin animals, only a 0-5% reduction in blood volume is needed to activate VP release, whereas 23-25% is required for stimulation of the oxytocinergic system. Thereafter, incremental changes in plasma levels of both hormones with increasing loss in blood volume are similar in virgin and pregnant animals. However, during lactation, the apparent hypovolaemic threshold for both hormones becomes significantly elevated to 20.4% (VP) and > 25% (OT) blood volume depletion, resulting in a decreased responsiveness of the magnocellular system to hypovolaemia. 4. The OT response to CCK is attenuated in lactating animals and circulating VP is unaffected in pregnant rats given the peptide.