Synergistic effects of L-dopa and dopamine with melanocortin receptors (MC3/MC4) agonist on food intake in chickens

Appetite modulation includes a set of complex physiological mechanisms that affect different areas of the central nervous system. Previously, the role of dopaminergic and melanocortinergic systems in the central control of food intake in birds has been proven. The current study was conducted with the aim of investigating the synergistic effects of dopaminergic system and melanocortinergic system on feeding behavior in layer chickens. In this study, three experiments were designed, so that each experiment included one control group and three treatment groups(n=12 in each group). In all groups, birds received intracerebroventricular injection of diluent solution or drug solution after 3 hours of food deprivation. In the first experiment, in order to determine the sub-effective dose of dopamine, normal saline and dopamine were injected with doses of 10, 20 and 40 nmol respectively in the test groups. In the second experiment, normal saline, L-dopa (dopamine precursor, 250 nmol), MTII (2.45 pmol, MC3/4 receptor agonist) and L-dopa + MTII were injected, and the third experiment was conducted as second experiment, but instead of L-dopa, dopamine (10 nmol) was injected either alone or in combination with MTII. After the injection, water and food were freely available to the birds and food consumption (gr) was measured based on the percentage of body weight. Based on the obtained results, the dose of 10 nmol of dopamine was determined as the sub-effective dose. It was also observed that the injection of sub-effective doses of dopamine, L-dopa and MTII alone had no effect on the food intake of chickens (p > 0.05), and only co-injections of L-dopa + MTII and dopamine + MTII caused a significant decrease in food intake in chickens (p<0.05). Based on the findings, it seems that there is probably a synergistic effect between the dopaminergic system and the melanocortinergic system in the control of food intake in chickens.