The present study examined the participation of central δ-opioidergic receptors in the modulatory/ inhibitory effect of [D-Pen²-D-Pen⁵-enkephalin] (DPDPE), a highly selective delta-opioid receptor agonist, on the spike burst activity of the rumen, reticulum and antrum prior to and 10 min after the i.c.v. - during 1 min - infusion of naltrindole, a competitive ä-opioid receptor antagonist in conscious sheep. DPDPE was infused i.c.v. at the doses of 0.25, 0.5 and 1.0 mg × kg⁻¹ B.W., whereas naltrindole - at doses 10 times higher. All the doses of DPDPE infused i.c.v. significantly inhibited myoelectrical activity of the wall of the rumen and reticulum. The effects of DPDPE were prevented by naltrindole previously infused at the doses of 2.5, 5.0 and 10.0 mg × kg⁻¹ B.W. (i.e. at doses 10 times higher than those of DPDPE). The results obtained indicate that central δ-opioid receptors participated in the inhibitory action of specific opioid peptide (DPDPE) on myoelectrical activity of the forestomachs in sheep. Furthermore, these results suggest that it is a specific delta-receptor stimulation, because naltrindole, a compeptitive δ-receptor antagonist, prevented the modulatory/inhibitory action of DPDPE on the reticular and ruminal motility in sheep.
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