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

Loperamide inhibited the frequency and amplitude of cyclical contractions of the rumen in conscious goats (100 micrograms/kg, i.v. and sheep (250 micrograms/kg, i.v.). In goats, the inhibitory effect of loperamide could be prevented by pretreatment with the opiate antagonist naltrexone (greater than or equal to 12.5 micrograms/kg, i.v.) but not by pretreatment with the dopaminergic antagonist domperidone (500 micrograms/kg, i.v.). Intracerebroventricular administration of 1 microgram/kg loperamide in goats significantly depressed ruminal contraction frequency, whereas intravenous administration of 10 micrograms/kg loperamide did not affect cyclical motility. Administered via the carotid artery, loperamide (4 micrograms/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 microM) 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.