Evidence for an interaction between CB1 cannabinoid and melanocortin MCR-4 receptors in regulating food intake.

Melanocortin receptor 4 (MCR4) and CB(1) cannabinoid receptors independently modulate food intake. Although an interaction between the cannabinoid and melanocortin systems has been found in recovery from hemorrhagic shock, the interaction between these systems in modulating food intake has not yet been examined. The present study had two primary purposes: 1) to examine whether the cannabinoid and melanocortin systems act independently or synergistically in suppressing food intake; and 2) to determine the relative position of the CB(1) receptors in the chain of control of food intake in relation to the melanocortin system. Rats were habituated to the test environment and injection procedure and then received intracerebroventicular injections of various combinations of the MCR4 receptor antagonist JKC-363, the CB(1) receptor agonist Delta(9)-tetrahydrocannabinol, the MCR4 receptor agonist alpha-MSH, or the cannabinoid CB(1) receptor antagonist SR 141716. Food intake and locomotor activity were then recorded for 120 min. When administrated alone, SR 141716 and alpha-MSH dose-dependently attenuated baseline feeding, whereas sub-anorectic doses of SR 141716 and alpha-MSH synergistically attenuated baseline feeding when combined. Delta(9)-Tetrahydrocannabinol-induced feeding was not blocked by alpha-MSH, whereas SR 141716 dose-dependently attenuated JKC-363-induced feeding. Locomotor activity was not significantly affected by any drug treatment, suggesting that the observed effects on feeding were not due to a nonspecific reduction in motivated behavior. These findings revealed a synergistic interaction between the cannabinoid and melanocortin systems in feeding behavior. These results further suggested that CB(1) receptors are located downstream from melanocortin receptors and CB(1) receptor signaling is necessary to prevent the melanocortin system from altering food intake.

Inverse agonist activity of agouti and agouti-related protein.

Agouti and agouti-related protein (AgRP) are endogenous antagonists of the melanocortin receptors (MCxR). Previous data showed that recombinant full-length agouti and a synthetic fragment of AgRP, AgRP (83-132), are inverse agonists at the MC1R and MC4R, respectively. This study demonstrates the smaller analogs AgRP (87-120) and ASIP [90-132 (L89Y)], and short peptides Yc[CRFFNAFC]Y and Qc[CRFFRSAC]S are also MC4R inverse agonists. Furthermore, the relative affinity of the series of MC4R ligands for displacement of radiolabeled antagonist 125I-AgRP (86-132) versus radiolabeled agonist 125I-NDP-MSH did not correlate with ligand efficacy, which is more consistent with an induced-fit model than a simple two-state model of MC4R activation. These data shed new light on the determinants and mechanism of inverse agonism at the MC4R.

Sustained orexigenic effect of Agouti related protein may be not mediated by the melanocortin 4 receptor.

Intracerebroventricular (ICV) injection of Agouti related protein (AgRP), an endogenous melanocortin 3 and 4 receptor (MC3/4-R) antagonist, produces a prolonged increase in food intake. To clarify the roles of the MC3-R and MC4-R in AgRP-induced hyperphagia, the feeding effect of AgRP (83-132) was compared with that of the selective MC4-R antagonist, JKC-363 (cyclic [Mpr11, D-Nal14, Cys18, Asp22-NH2]-beta-MSH11-22). Single ICV administration of AgRP (83-132) increased food intake for 48 h whilst ICV JKC-363 increased food intake for 8h. An increase in body weight at 24 and 48 h was observed following AgRP (83-132) but not JKC-363 treatment. These data suggest that the sustained orexigenic action of AgRP (83-132) may not be through MC4-R antagonism.