[Inhibitory role of the opioid system in the initiation of rumination in sheep].

Major movements of the reticulo-rumen occur approximately once per minute as a result of periodic vagal motor discharges from "gastric centres" located in the medulla oblongata. During rumination evoked by stroking the interior wall of the reticulum or caused by the injection (bolus) of catecholamines, an additional contraction of the reticulum occurs at the time of regurgitation before the normal biphasic contraction. In sheep, the intravenous (IV) and intracerebroventricular (ICV) administration of naloxone at the doses of 0.1 mg/kg and 10 micrograms/kg respectively, enhances the occurrence of rumination caused by adrenaline. Morphine has opposite effects. Rumination can also be induced by catecholamines in sheep under naloxone regardless of the coarseness of the diet and during eating. The results suggest the involvement of an inhibitory opioid system in the central regulation of rumination and then in the occurrence of reticular contractions linked to regurgitation.