Dynamic, sex- and diet-specific pleiotropism in the PAC1 receptor-mediated regulation of arcuate proopiomelanocortin and Neuropeptide Y/Agouti related peptide neuronal excitability by anorexigenic ventromedial nucleus PACAP neurons

This study furthers the investigation of how pituitary adenylate cyclase activating polypeptide (PACAP) and the PAC1 receptor (PAC1R) regulate the homeostatic energy balance circuitry. We hypothesized that apoptotic ablation of PACAP neurones in the hypothalamic ventromedial nucleus (VMN) would affect both energy intake and energy expenditure. We also hypothesized that selective PAC1R knockdown would impair the PACAP-induced excitation in anorexigenic proopiomelanocortin (POMC) neurones and inhibition of orexigenic neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurones in the hypothalamic arcuate nucleus (ARC). The results show CASPASE-3-induced ablation of VMN PACAP neurones leads to increased energy intake and meal frequency as well as decreased energy expenditure in lean animals. The effects were more robust in obese males, whereas we saw the opposite effects in obese females. We then utilized visualized whole-cell patch clamp recordings in hypothalamic slices. PAC1R knockdown in POMC neurones diminishes the PACAP-induced depolarization, increase in firing, decreases in energy intake and meal size, as well as increases in CO2 production and O2 consumption. Similarly, the lack of expression of the PAC1R in NPY/AgRP neurones greatly attenuates the PACAP-induced hyperpolarization, suppression of firing, decreases in energy intake and meal frequency, as well as increases in energy expenditure. The PACAP response in NPY/AgRP neurones switched from predominantly inhibitory to excitatory in fasted animals. Finally, the anorexigenic effect of PACAP was potentiated when oestradiol was injected into the ARC in ovariectomized females. This study demonstrates the critical role of anorexigenic VMN PACAP neurones and the PAC1R in exciting POMC and inhibiting NPY/AgRP neurons to control homeostatic feeding.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.