Fig. ED10. A model for α-MSH and AgRP signaling at PVN MC4R neurons.

Data presented here supports a model in which MC4R may couple to both Gαs signaling and regulation of Kir7.1 activity in PVN MC4R neurons. α-MSH results in elevation of intracellular cAMP through activation of Gαs, and inhibition of K⁺ efflux through Kir7.1, both of which are depolarizing. AgRP lowers the constitutive activity of the MC4R and blocks α-MSH binding, but data here shows that AgRP also acts as an agonist to increase K⁺ efflux through Kir7.1, producing a strong hyperpolarizing signal. The relative distribution and composition of the MC4R signaling complex in different subcellular compartments of PVN MC4R neurons has not been directly determined. Earlier models of α-MSH and AgRP action suggested competitive binding of these peptides to individual MC4R sites (orange box). Existing neuroanatomical data characterizing POMC and AgRP neuronal projections shows that α-MSH may act independently of AgRP at many sites in the CNS, since AgRP immunoreactive fibers are only observed in a subset of MC4R expressing nuclei containing POMC-immunoreative fibers (right circle, for review see ²⁵). The ability of AgRP to act independently of α-MSH as a potent hyperpolarizing agonist, via regulation of Kir7.1, suggests the likely existence of independent AgRP sites of action (left circle). Recent reconstruction of EM images of the PVN in which POMC and AgRP containing synaptic vesicles have been specifically labeled with a genetically encoded marker provides preliminary anatomical support for this new model²⁶. This study demonstrates that 52% of AgRP boutons in the PVN are not found in synapses, potentially supporting volume transmission of AgRP that may lead to competition with α-MSH at synaptic and/or non-synaptic sites. Additionally, the study found the vast majority of AgRP and POMC synaptic sites localized to different subcellular compartments of PVN neurons, supporting the independent action of both peptides. Synaptic release sites on soma were almost exclusively AgRP-containing, while POMC release sites were concentrated on distal dendrites. Another MC4R signaling pathway, involving cAMP/PKA-dependent activation of K_ATP channels and α-MSH induced hyperpolarization, has been demonstrated in MC4R neurons in the dorsal motor nucleus of the vagus in the brainstem (bottom right)²¹. Thus, while Kir7.1 signaling appears to be essential for depolarization of PVN MC4R neurons by α-MSH, Gαs signaling and elevation of cAMP may be depolarizing or hyperpolarizing, depending on the cellular context.