Amongst various neuropeptidergic systems, neuropeptide Y (NPY), hypocretin-1 and melanin-concentrating hormone (MCH) producing neurons have been shown to play an important role in the regulation of food intake and body weight. All of these neuropeptides are orexigenic signals and recent evidence suggests the existence of morphological connections between these neuronal systems in the hypothalamus. However, the functional interactions between these neuronal systems are not clearly understood. Therefore, in the present study, we examined whether there is a cooperative action on food intake between these neuropeptides after third intracerebroventricular (icv) injection in the rat. The icv administration of NPY (0.118, 0.588, 1.176 nmol), hypocretin-1 (1, 3 nmol) and MCH (0.42, 1.048, 2.096 nmol) stimulated food intake in a dose dependent manner. Coinjection with 0.118 nmol of NPY and hypocretin-1 (1, 3 nmol) or MCH (0.42, 1.048, 2.096 nmol) had no additive effect on food intake as compared to that of NPY alone. However, coinjection with lower dose of NPY (0.023 nmol) and hypocretin-1 (0.25 nmol), that did not have any effect alone, significantly induced food intake. In contrast, combination of a lower dose of NPY (0.023 nmol) or hypocretin-1 (0.25 nmol) with lower stimulatory dose (0.21, 0.42 nmol) of MCH did not result in further increase in food intake as compared to that of MCH alone. Also, combination of 0.25 nmol hypocretin-1 and a non-stimulatory dose of MCH (0.105 nmol) was ineffective in stimulating food intake. Finally, coinjection with of 0.023 nmol NPY and 0.105 nmol MCH significantly induced food intake as compared to saline control group but not as compared to NPY or MCH treated groups. In total, these results show (1) a synergistic action between NPY and hypocretin-1, (2) no interaction between hypocretin-1 and MCH and (3) very little interaction, if any, between NPY and MCH in inducing food intake. In conclusion, these results provide a physiological concomitant to the previous demonstration of morphological contacts between NPY and hypocretin producing neurons by suggesting an interaction between these two orexigenic signals in control of food intake, and further suggest that MCH's action on feeding may be independent of NPY and hypocretin-1 action.
