The paraventricular nucleus of the hypothalamus (PVN) is a heterogeneous structure within the hypothalamus of the brain. Research has shown this nucleus has various roles such as cardiovascular control, regulation of metabolism and is involved in the stress pathway [for review see 1]. Of particular interest to our group is the potential role of the PVN in osmoregulation [2]; water deprivation leads to increased expression of c-fos in parvocellular PVN neurones and hypertonic solutions have been shown to increase activity of PVN neurones [3,4]. However, the ionic mechanisms responsible for the role of the PVN in osmotic homeostasis have not yet been determined. More recent studies have implicated the transient receptor vanilloid channel (TRPV4) as a possible candidate for osmosensing [5]. Using a combination of in vitro techniques we have investigated a possible role for TRPV4 in osmosensing within the PVN. Cellular mechanisms were investigated by cell-attached patch-clamp recording in mouse PVN hypothalamic brain slices. Further investigation using isolated PVN neuronal cells from Wistar rats included whole-cell current clamp recordings and intracellular calcium measurements using the ratiometric dye Fura-2AM (5µM). In addition, we hypothesise that this mechanism may in part play a role in cardiovascular control in response to osmotic challenge using in vivo blood pressure recording. CD1 mice were anaesthetized with urethane-chloralose (1.4-2.2mg/kg-7-11µg/kg), administered intraperitoneally. BP was measured by arterial cannulae and compounds applied by intracerebroventricular (ICV) injection. Results are given as mean ± SEM; significances were assessed by two-way ANOVA and Student’s paired t-tests where appropriate. We initially investigated the effect of hypotonic challenge on the activity of neurones using action currents as an indication of action potential frequency in PVN brain slice experiments. Osmotic challenge decreased action current frequency by 79±10 % (n=10; p<0.0001; from 300 mOsm (control) to 270 mOsm (hypotonic)). The effect of hypotonic challenge in the reduction of action current frequency was mimicked using the TRPV4 agonist 4αPDD (1 µM) and the highly selective agonist GSK1016790A (100 nM) with reductions of 36±10 % (n=6; p<0.01) and 72 ± 8 % (n=6; p<0.05) respectively. The response to hypotonic solution was significantly reduced by the TRPV4 antagonists RN1734 (5 µM) and the highly selective HC067047 (300 nM): hypotonic challenge: 70±14 % reduction vs. hypotonic with RN1734: 45±15 % reduction (n=6; p<0.05), and a 10±13 % reduction (n=6; p<0.01) with HC067047 in action current frequency. These results provide further evidence TRPV4 has a role for osmosensing within neurones of the PVN. Interestingly, TRPV4 did not seem to have a role in temperature sensing in PVN neurones. In order to confirm this is a direct effect upon PVN neurones themselves, cell-attached electrophysiology was used to record single channel activity of PVN neurones in CD1 mouse brain slices. A population of ion channels was identified with a mean slope unitary conductance of 57±7 pS and reversal potential of -5±3 mV (n=6); indicative of a non-selective cation channel. This channel activity was seen in 50 % of patches (8/16), with a mean open probability (Po) of 0.1±0.0 at -40 mV. Po increased by 48±9 % (n=4) upon addition of the TRPV4 agonist 4αPDD. Furthermore, during whole-cell recordings from isolated Wistar rat PVN cells the addition of 4αPDD and GSK1016790A resulted in depolarisations of 11±2 mV (n=4; p<0.05) and 12±5 mV (n=5; p<0.05) respectively. In addition, during Ca2+ recordings using Fura-2AM an increase in intracellular Ca2+ concentration was seen with both hypotonic solution: 77±3 nM to 198±28 nM (n=8; p<0.001) and 4αPDD: 75±2 nM to 121±14 nM (n=6; p<0.005). In vivo studies uncovered a potential role for TRPV4 in cardiovascular control. ICV injection of hypotonic saline led to a rapid decreases in blood pressure, whereas isotonic saline had no effect (-9±2 mmHg vs. -2±1 mmHg; n=6; p<0.01). These decreases in BP were abolished by the TRPV4 antagonist RN1734 (-1±1 mmHg; n=6). These results suggest a role for TRPV4 channels in sensing central osmolality changes within the PVN. Furthermore, central osmolality changes can modulate PVN neuronal activity, which may subsequently have effects upon cardiovascular control.