Abstract
The magnocellular neurosecretory cells (MNCs) of the hypothalamus regulate body fluid balance by releasing the hormones vasopressin (VP) and oxytocin (OT) in an osmolality-dependent manner. Elevations of external osmolality increase MNC firing and hormone release. MNC osmosensitivity is largely due to activation of a mechanosensitive non-selective cation current that responds to osmotically-evoked changes in MNC volume and is mediated by an N-terminal variant of the TRPV1 channel (∆N TRPV1). We report a novel mechanism by which increases in osmolality may modulate ∆N TRPV1-mediated currents and thus influence MNC electrical behaviour. We showed previously that acute elevations of external osmolality activate the enzyme phospholipase C (PLC) in isolated MNCs. We now show that the osmotic activation of PLC has a time course and dose-dependence that is consistent with a role in MNC osmosensitivity and that it contributes to the osmotically-evoked increase in non-selective cation current in MNCs through a protein kinase C-dependent pathway. We furthermore show that the mechanism of osmotic activation of PLC requires an increase in internal Ca2+ that depends on influx through L-type Ca2+ channels. Our data therefore suggest that MNCs possess an osmotically-activated Ca2+-dependent PLC that contributes to the osmotic activation of ∆N TRPV1 and may therefore be important in MNC osmosensitivity and in central osmoregulation.