The magnocellular neurosecretory cells (MNCs) of the hypothalamus play a vital role in osmoregulation, but the mechanisms underlying MNC osmosensitivity are not fully understood. We showed previously that high osmolality activates phospholipase C (PLC) in rat MNCs in a Ca2+-dependent manner and that PLC activation is necessary for full osmotic activation of an N-terminal variant of the TRPV1 (ΔN-TRPV1) channel. We therefore hypothesized that the Ca2+-dependent δ1 isoform of PLC contributes to ΔN-TRPV1 activation and tested whether MNC function is defective in a transgenic PLCδ1 KO mouse. Water deprivation for 24 h caused greater increases in serum osmolality and losses in body weight in PLCδ1 KO mice than it did in control mice. Action potentials and ΔN-TRPV1 currents were measured in acutely isolated mouse MNCs using whole-cell patch clamp before and after exposure to hypertonic solutions. This treatment elicited a significant activation of ΔN-TRPV1 currents and an increase in firing rate in MNCs isolated...