The leptin-regulated melanocortin (MC) system modulates energy homeostasis and hypothalamic MC neuronal circuits regulate insulin secretion. We therefore hypothesized that MC system components were present in the pancreas. In order to determine the veracity of the hypothesis, we examined c-Fos, melanocortin-4 receptor (Mc4r), and alpha-melanocyte-stimulating hormone (α-MSH) expression levels in nondiabetic (intact leptin receptor signaling) and Zucker diabetic fatty (ZDF; leptin receptor deficiency) rats. We infused rats via the third ventricle with the α-MSH analog Nle4, D-Phe7-α-MSH (NDP-MSH), a Mc4r agonist. Subsequently, both hypothalamic and pancreatic c-Fos and Mc4r mRNAs were upregulated. Likewise, immunohistochemical analysis showed that an increased Mc4r and α-MSH expression in nerves surrounding the pancreatic vasculature and islets. Increases in c-Fos, α-MSH, and Mc4r expression were independent of leptin receptor function. Conversely, serum insulin was significantly reduced by NDP-MSH treatment, an effect which was reversed by the Mc4r specific blocker HS014. Finally, proopiomelanocortin (POMC) mRNA, the precursor of α-MSH, was detected by RT-PCR in pancreatic tissue homogenates. These findings suggest that pancreatic Mc4r and autonomic neurons participate in a communication pathway between the central MC system and pancreatic islets to regulate insulin secretion.

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The hypothalamic melanocortin system (MCs) is part of a leptin regulated cascade that regulates feeding and energy expenditure. Recent observations suggest that hypothalamic MCs neuronal circuits' activation regulates pancreatic secretion of insulin. We hypothesized that hypothalamic infusion with the alpha MSH analog NDP‐MSH (a melanocortin receptor 4 [Mc4r] agonist) would modulate Mc4r expression and insulin levels via a leptin‐independent pathway. We infused the third brain ventricle of lean (with intact leptin signaling) and obese diabetic Zucker rats (with leptin receptor deficiency) with NDP‐MSH to test our hypothesis. NDP‐MSH infusion significantly decreased serum insulin and upregulated Mc4r mRNA message in the pancreas. Immunohistochemistry detected Mc4r expression in pancreatic islet‐like structures that also expressed neuron‐specific beta tubulin. This finding suggests that autonomic neurons are involved in a communication pathway between central MCs and peripheral pancreatic islets.