Regulation of hypothalamic endocannabinoid levels by neuropeptides and hormones involved in food intake and metabolism: Insulin and melanocortins

Endocannabinoids are paracrine/autocrine lipid mediators with several biological functions. One of these, i.e. the capability to stimulate food intake via cannabinoid CB1 receptors, has been particularly studied, thus leading to the development of the first CB1 receptor blocker, rimonabant, as a therapeutic tool against obesity and related metabolic disorders. Hypothalamic endocannabinoids stimulate appetite by regulating the expression and release of anorexic and orexigenic neuropeptides via CB1 receptors. In turn, the tone of the latter receptors is regulated by hormones, including leptin, glucocorticoids and possibly ghrelin and neuropeptide Y, by modulating the biosynthesis of the endocannabinoids in various areas of the hypothalamus. CB1 receptor stimulation is also known to increase blood glucose during an oral glucose tolerance test in rats. Here we investigated in the rat if insulin, which is known to exert fundamental actions at the level of the mediobasal hypothalamus (MBH), and the melanocortin system, namely α-melanocyte stimulating hormone (α-MSH) and melanocortin receptor-4 (MCR-4), also regulate hypothalamic endocannabinoid levels, measured by isotope-dilution liquid chromatography coupled to mass spectrometry. No effect on anandamide and 2-arachidonoylglycerol levels was observed after 2 h infusion of insulin in the MBH, i.e. under conditions in which the hormone reduces blood glucose, nor with intra-cerebroventricular injection of α-MSH, under conditions in which the neuropeptide reduces food intake. Conversely, blockade of MCR-4 receptors with HS014 produced a late (6 h after systemic administration) stimulatory effect on endocannabinoid levels as opposed to a rapid and prolonged stimulation of food-intake (observable 2 and 6 h after administration). These data suggest that inhibition of endocannabinoid levels does not mediate the effect of insulin on hepatic glucose production nor the food intake-inhibitory effect of α-MSH, although stimulation of endocannabinoid levels might underlie part of the late stimulatory effects of MCR-4 blockade on food intake.