Loperamide: evidence for a centrally mediated opioid effect on rumen motility in conscious goats and sheep

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Abstract
Loperamide inhibited the frequency and amplitude of cyclical contractions of the rumen in conscious goats (100 .mu.g/kg, i.v.) and sheep (250 .mu.g/kg, i.v.). In goats the inhibitory effect of loperamide could be prevented by pretreatment with the opiate antagonist naltrexone (.gtoreq. 12.5 .mu.g/kg, i.v.), but not be pretreatment with the dopaminergic antagonist domperidone (500 .mu.g/kg, i.v.). Intracerebroventricular administration of 1 .mu.g/kg loperamide in goats significantly depressed ruminal contraction frequency, whereas intravenous administration of 10 .mu.g/kg loperamide did not affect cyclical motility. Administered via the carotid artery, loperamide (4 .mu.g/kg) depressed both frequency and amplitude of cyclical contractions of reticulum and rumen, whereas the same dose was ineffective via the coeliac artery. In vitro, loperamide (10 nm-100 .mu.M) had no influence on spontaneous activity or tone of the reticular longitudinal muscle strips. It is concluded that loperamide inhibits cyclical ruminal contractions through a central opioid pathway.