Consequence of dopamine D2 receptor blockade on the hyperphagic effect induced by cannabinoid CB1 and CB2 receptors in layers.

1. Endocannabinoids (ECBs) and their receptors play a regulatory function on several physiological processes such as feed intake behaviour, mainly in the brain. This study was carried out in order to investigate the effects of the dopaminergic D1 and D2 receptors on CB1/CB2 ECB receptor induced hyperphagia in 3-h feed-deprived (FD3) neonatal layer chickens. 2. A total of 8 experiments were designed to explore the interplay of these two modulatory systems on feed intake in neonatal chickens. In Experiment 1, chickens were intracerebroventricular (ICV) injected with control solution, L-DOPA (levo-dihydroxyphenylalanine as precursor of dopamine; 125 nmol), 2-AG (2-arachidonoylglycerol as CB₁ receptor agonist; 2 µg) and co-adminstration of L-DOPA (125 nmol) plus 2-AG (2 µg). Experiments 2-4, were similar to Experiment 1 except birds were injected with either 6-OHDA (6-hydroxydopamine as dopamine synthesis inhibitor; 150 nmol), SCH23390 (D1 receptor antagonist; 5 nmol) and AMI-193 (D2 receptor antagonist; 5 nmol) instead of L-DOPA, respectively. Additionally, Experiments 5-8 followed the previous ones using the same dose of L-DOPA, 6-OHDA and dopamine antagonists except that birds were injected with CB65 (CB2 receptor agonist; 5 µg) instead of 2-AG. Co-administrations were at the same dose for each experiment. Cumulative feed intakes were measured until 120 minutes after each injection. 3. ICV administration of 6-OHDA and AMI-193 significantly attenuated 2-AG induced hyperphagia. Interestingly, the hyperphagic effect of CB65 was significantly attenuated by administration of L-DOPA whereas the administration of 6-OHDA and AMI-193 together amplified the hyperphagic effect of CB65.4. It was concluded that cannabinoid-induced feeding behaviour is probably modulated by dopamine receptors in neonatal layer-type chickens. It seems that their interaction may be mediated by the D2-dopamine receptor.