Pituitary Adenylate Cylase-Activating Polypeptide Receptor: Multiple Signaling Pathways Involved in Energy Homeostasis
Date of Award
Doctor of Philosophy (PhD)
Pituitary adenylate cyclase activating polypeptide (PACAP) exerts pleiotropic effects on ventromedial nuclei (VMN) of the hypothalamus and its control of feeding and energy expenditure through the Type I PAC1 receptor (PAC1R). However, the endogenous role of PAC1R’s in the VMN and the downstream signaling responsible for PACAP’s effects are unknown.To determine the endogenous role of PAC1Rs and signaling that may explain PACAP’s pleiotropic effects, we knocked down VMN PAC1Rs and pharmacologically inhibited PKA, PKC and PAC1R trafficking. Knocking down PAC1Rs increased meal sizes, reduced total number of meals, and induced body weight gain. Inhibition of either PKA or PKC alone in awake male Sprague Dawley rats, attenuated PACAP’s anorectic effects during the dark phase. However, PKA or PKC inhibition resulted in a potentiation of PACAP’s hypophagic effects during the light phase. Additionally, PKA or PKC inhibition augmented PACAP’s thermogenic effects during the light phase, whereas only PKA inhibition augmented PACAP’s locomotor effects given that PKC inhibition had no effect. PACAP administration in the VMN induces PAC1R surface trafficking into the cytosol which was blocked by endocytosis inhibitors. Subsequently, inhibition of PAC1R trafficking into the cytosol attenuated PACAP-induced hypophagia. PACAP signaling replicates the effects of leptin administration in the VMN and appears to enable leptin regulation of energy homeostasis. However, the manner in which PACAP influences leptin signaling is unknown. We used co-immunoprecipitation to show that VMN PAC1 and leptin receptors are found in the same cell, and they form an immunocomplex. Inhibiting downstream effectors of PACAP signaling, such as PKA and PKC, enhanced or prevented leptin signaling respectively. The current findings revealed that endogenous PACAP signaling in the VMN has a potent regulatory influence over both energy intake in the form of feeding, and energy output via thermogenesis and locomotor activity. Moreover, PACAP actions in the VMN share a nearly identical sequelae to leptin administration in the same brain region suggesting that these two neuropeptides could functionally intersect. These experiments explored VMN PAC1Rs dependence on PKA, PKC, and receptor trafficking to mediate PACAP’s pleiotropic effects on feeding and metabolism as well as potential intersecting points with leptin receptor signaling.