The present study examined the participation of central µ-opioidergic receptors in the modulatory/inhibitory effect of D-Ala², N-Me-Phe⁴, Gly⁵-ol enkephalin (DAGO), a selective mu-opioid receptor agonist, on the spike burst activity of the rumen, reticulum and antrum prior to and 10 min after the intracerbroventriculary (i.c.v) - during 1 min - infusion of naloxonazine, a competitive mu-opioid receptor antagonist in sheep. DAGO was infused i.c.v. at a doses 0.02, 0.05, 0.1 and 1.0 µg × kg⁻¹, naloxonazine at doses 5 times higher. All the doses of DAGO administered i.c.v. significantly inhibited myoelectrical activity of the wall of the rumen, reticulum and abomasum. The effects of DAGO were prevented by naloxonazine previously infused at doses of 0.1, 0.5, 1.0 and 5.0 µg × kg⁻¹ (i.e. at a doses 5 times higher in comparison to doses of DAGO). The results of this present study indicate that central µ-opioid receptors participated in the inhibitory action of a specific opioid (DAGO) on myoelectrical activity of the forestomaches and antrum in sheep. Furthermore, this result suggests that it is a specific mu-receptor stimulation, because naloxonazine, a competitive µ-receptor antagonist, prevented the modulatory action of DAGO on the reticular, ruminal and antral motility in sheep.
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