Pharmacological characterization of the nociceptin receptor mediating hyperphagia: identification of a selective antagonist.

RATIONALE: Central injections of nociceptin (NC) stimulate feeding in rats.

OBJECTIVE

The present study evaluated the effect of N-terminal partial sequences or analogues of NC on food intake in male Wistar rats, to characterize pharmacologically the NC receptor mediating the hyperphagic effect.

METHODS

NC and related peptides were injected into the lateral (LV) or third (3V) cerebroventricle in freely feeding rats.

RESULTS

In the LV, NC stimulated feeding. The N-terminal fragment NC(1-13)NH2 proved to be the least active sequence with hyperphagic activity; NC(1-12)NH2 and NC(1-9)NH2 were inactive. [Phe(1)psi(CH2-NH)Gly(2)]NC(1-13)NH2 ([F/G)]NC(1-13)NH2), an analogue of NC(1-13)NH2, markedly stimulated feeding and, coadministered in the LV with NC, never reduced the hyperphagic effect of the natural sequence. These findings suggest that [F/G)]NC(1-13)NH2, which has been reported to act as a NC receptor antagonist in peripheral tissues, behaves as a full agonist at the central NC receptors controlling feeding. The hyperphagic potencies of NC and [F/G)]NC(1-13)NH2 were much higher following injection into the 3V than in the LV. Another analogue of NC(1-13)NH2, namely [Nphe(1)]NC(1-13)NH2, injected into the 3V did not stimulate feeding, but reduced the effect of NC. [Nphe(1)]NC(1-13)NH2 at a dose of 16.8 nmol/rat significantly reduced, and at 168 nmol/rat almost completely abolished the effect of NC (1.68 nmol/rat). The latter dose of [Nphe(1)]NC(1-13)NH2 significantly reduced also feeding induced by food deprivation, but did not modify the hyperphagic effect of neuropeptide Y (0.3 nmol/rat).

CONCLUSIONS

The present results confirm the orexigenic effect of NC in freely feeding rats and indicate that [Nphe(1)]NC(1-13)NH2 may represent a selective NC receptor antagonist to study the physiological and pathophysiological role of NC in feeding behaviour.