Blood pressure is maintained during dehydration by hypothalamic paraventricular nucleus-driven tonic sympathetic nerve activity.

Holbein WW, Bardgett ME, Toney GM. J Physiol. 2014 Jun 27. In this paper, they wanted to look at how what happens during dehydration that allows the blood pressure to be maintained. To do this they looked at the PVN , because previous papers suggested that it might be the region controlling th effect. Their first experiment was to look at the differences in SSNA between dehydrated (DH) and euhydrated (EH) rats. They found that MAP and phrenic nerve activity (PNA) that wasn't different between groups, but DH rats had a much higher baseline SSNA. When they gave injections of muscimol in to the PVN to block its activity, they saw almost no effect in the EH rats, but in the DH rats they a strong and rapid reduction MAP and SSNA. For PNA, both groups showed only a slight decrease in burst frequency, but not amplitude, after the injection of GABA. They also noted that, even though comparison of SSNA between animals is touchy, the post-inhibition SSNA was not different between EH and DH rats, suggesting that the entire process may be mediated by the PVN. This also suggests that the splanchnic nerve is critical to maintaining blood pressure during dehydration, though they have yet to examine nerves leading to other vascular beds. -DH