Neuroanatomical organization and functional roles of PVN MC4R pathways in physiological and behavioral regulations

ABSTRACT Objective The paraventricular nucleus of hypothalamus (PVN) is an integrative center in the brain orchestrating a wide range of physiological and behavioral responses. While the PVN melanocortin 4 receptor (MC4R) signaling (PVNMC4R+) is undoubtedly involved in feeding regulation, the neuroanatomical organization of PVNMC4R+ connectivity and its role in other physiological regulations are not fully understood. Here we aimed to better characterize the input-output organization of PVNMC4R+ neurons and further test their physiological functions beyond feeding. Methods Using a combination of viral tools, we mapped PVNMC4R+ circuits and tested the effects of chemogenetic activation of PVNMC4R+ neurons on thermoregulation, cardiovascular control, and other behavioral responses beyond feeding. Results We found that PVNMC4R+ neurons innervate many different brain regions known to be important not only for feeding but also for neuroendocrine and autonomic control of thermoregulation and cardiovascular function, including but not limited to the preoptic area, median eminence, parabrachial nucleus, pre-locus coeruleus, nucleus of solitary tract, ventrolateral medulla, and thoracic spinal cord. Contrary to these broad efferent projections, PVNMC4R+ neurons receive monosynaptic inputs mainly from other hypothalamic nuclei (preoptic area, arcuate and dorsomedial hypothalamic nuclei, supraoptic nucleus, and premammillary nucleus), the circumventricular organs (subfornical organ and vascular organ of lamina terminalis), the bed nucleus of stria terminalis, and the parabrachial nucleus. Consistent with their broad efferent projections, chemogenetic activation of PVNMC4R+ neurons not only suppressed feeding but also led to an apparent increase in heart rate, blood pressure and brown adipose tissue temperature. These physiological changes accompanied an acute transient hyperactivity followed by hypoactivity and resting-like behavior. Conclusions Our results clarify the neuroanatomical organization of PVNMC4R+ circuits and shed new light on the roles of PVNMC4R+ pathways in autonomic control of thermoregulation, cardiovascular function and biphasic behavioral activation.