The Paraventricular Nucleus of the Hypothalamus and its Role in Energy Homeostasis

Abstract

Obesity and its comorbidities are a significant threat to public health, necessitating the development of new therapies to treat and prevent obesity. Body weight is maintained by balancing food intake and energy expenditure, a process mediated by the central nervous system (CNS). The paraventricular nucleus of the hypothalamus (PVH) is a crucial component of the CNS maintenance of energy balance; lesioning the PVH or disrupting its development produces hyperphagia and energy expenditure deficits in rodents and humans. The PVH is a heterogenous nucleus with many neuronal populations that may regulate energy homeostasis differently. To interrogate these neuronal populations, we used Cre-dependent viral tools to chemogenetically activate or to silence neuronal populations of the PVH. We used genetically engineered mice and immunohistochemistry to identify the overlap of subpopulations of the PVH as well as retrograde and anterograde tracing to determine the inputs and outputs of the PVH. Insulin receptor substrate-4 (IRS4) and insulin receptor substrate-2 proteins act synergistically in the hypothalamus to maintain energy balance. Our in situ hybridization studies determined that IRS4 is expressed in the PVH, prompting us to investigate IRS4PVH neurons and their role in energy balance. The IRS4PVH neuronal population is distinct from oxytocin (OXT), nitric oxide synthase (NOS1), melanocortin 4-receptor (MC4R) and prodynorphin (PDYN) neurons, all of which regulate energy balance. We found that activating IRS4PVH neurons suppresses feeding and increases energy expenditure, while silencing IRS4PVH neurons results in hyperphagic obesity. IRS4PVH neurons project to regions of the hindbrain necessary for energy homeostasis such as the parabrachial nucleus (PBN), solitary nucleus (NTS), median eminence and the intermediolateral column of the spinal cord. The IRS4PVH neurons are innervated by brain regions such as the lateral hypothalamic area, ventromedial hypothalamus, amygdala, supraoptic nucleus, bed nucleus of the stria terminalis, preoptic area, arcuate nucleus and PBN. We therefore determined that PVH and peri-PVH neurons expressing IRS4 regulate energy homeostasis. Although we determined that calcitonin receptor (CalcR) expression in the PVH is not required for energy balance or the anorectic response to the salmon calcitonin ligand, we did find that CalcRPVH neuron activity is important in energy homeostasis. Using chemogenetic activation and tetanus toxin mediated silencing of CalcRPVH neurons, we determined that activating CalcRPVH neurons suppresses feeding and increases ambulatory activity, while silencing CalcRPVH neurons results in hyperphagic weight gain. CalcRPVH neurons project to regions involved in regulating energy homeostasis such as the PBN, NTS and median eminence. CalcRPVH neurons are distinct from many of the neuronal populations of the PVH that regulate energy balance such as those that express NOS1, arginine vasopressin, OXT and thyrotropin-releasing hormone with some overlap with corticotrophin-releasing hormone expressing PVH neurons. There is a notable overlap between the MC4R expressing PVH neurons and CalcRPVH neurons, which may account for the hyperphagic obesity observed when CalcRPVH neurons are silenced. In addition, deleting MC4R from CalcR cells produced hyperphagic obesity in male and female mice. Lastly, we aimed to elucidate the roles that specific projection fields of PVH neurons play in energy homeostasis. Using a retrograde AAV-Cre and chemogenetics, we have begun to parse the energy balance roles of PVH neurons that project to the PBN and NTS specifically. Overall, our studies have improved our understanding of the roles specific PVH neurons play in energy homeostasis and the neuronal circuits in which they exert their effects.