Functional Dissection of PVN MC4R Neural Pathway in Autonomic and Cardiovascular Functions

Abstract

Central melanocortin pathways, mainly via acting on melanocortin-4 receptor (MC4R), play an important role in both energy homeostasis and sympathetic control of cardiovascular function. The paraventricular nucleus of hypothalamus (PVN) is one of hypothalamic nuclei with highest MC4R expression and is well known for its role in metabolic and autonomic regulations; however, the role of PVN-MC4R neural pathways in this regard remains incompletely understood. In this study, we first tested whether chemogenetic activation of PVN-MC4R neurons affects cardiovascular function. Adeno-associated virus (AAV) expressing excitatory (hM3Dq) Designer Receptors Exclusively Activated by Designer Drugs (DREADD) receptors were directly injected into the PVN of MC4R-Cre knock-in mice that are implanted with radio-telemetry for chronic monitoring of blood pressure. DREADD activation of PVN-MC4R neurons significantly elevated mean arterial pressure and heart rate. Next, we prepared a separate group of mice and subjected them to a battery of tests to gain insight into the role of these neurons in other behavioral and physiological regulations. Whole-body infrared thermal imaging upon DREADD activation of PVN-MC4R neurons revealed significant increase of temperature in near neck region, lower back, and tail. Behavioral monitoring in PhenoTyper cage further showed that DREADD activation of PVN-MC4R neurons leads to hypoactivity, suppressed feeding, and trend toward increased drinking behavior. In order to gain further insight into the neuroanatomical organization of PVN-MC4R neurons, we comprehensively mapped afferent projections of PVN-MC4R neurons by stereotaxically delivering Cre-dependent AAV driving expression of eYFP into the PVN of MC4R-Cre mice, which revealed a broad innervation of PVN-MC4R neurons to the brain regions that are known to be important for metabolic homeostasis and autonomic-cardiovascular control, including but not limited to parabrachial nucleus, nucleus of solitary tract, ventrolateral medulla, and spinal cord. Additionally, using a combination of sophisticated viral tools, we also mapped the brain regions where neurons provide monosynaptic inputs to PVN-MC4R neurons. In addition to the brain regions in which neurons are known to send monosynaptic inputs to PVN-MC4R neurons, such as arcuate nucleus, few discreate presynaptic neurons were also observed in organum vasculosum of the lamina terminalis, bed nucleus of the stria terminalis, supraoptic nucleus (SON) and ventral subiculum (vSub). These results provide an important insight into the neuroanatomical organization and multifaceted role of PVN-MC4R neural pathways in behavioral and physiological regulations.