Control of water intake by a pathway from the nucleus of the solitary tract to the paraventricular hypothalamic nucleus

Several brain areas have been shown to participate in thirst and control of fluid intake. An understanding of how these circuits interact, and their roles in the activation, maintenance, and termination of fluid intake remains incomplete. Central glucagon-like peptide-1 (GLP-1) receptor activation appears to be an important part of the termination of drinking, but the site(s) of action for this suppression has not yet been determined. In an attempt to use GLP-1 responsiveness as a means to screen targets of hindbrain cells that participate in the termination of thirst and the resultant water intake, we injected the GLP-1 receptor agonist exendin-4 (Ex-4) into three brain areas known to express GLP-1 receptors, and measured subsequent water intake. Ex-4 reduced water consumption when injected into the paraventricular hypothalamic nucleus (PVH) and nucleus of the solitary tract (NTS), but not when injected into the nucleus accumbens (NAc). Using the effective response after injection into the PVH as a guide, we examined the connection between the NTS – the site of endogenous central GLP-1 production – and the PVH. Retrograde tracing combined with Fos immunohistochemistry suggested intake-induced activity in PVH-projecting NTS cells. To test the hypothesis that this pathway is important in the termination of drinking, we chemogenetically activated PVH-projecting hindbrain cells. Interestingly, activation of this population of cells increased water intake, calling into question the heterogeneity of the pathway with respect to the control of fluid intake. Taken together, we conclude that the PVH is a site of action for GLP-1 receptor activation in the inhibition of water intake, but suspect that endogenous GLP-1 in NTS-to-PVH projections may be counterbalanced by a parallel pathway that either activates or maintains already activated water intake.