Influence of dopamine on rumino-reticular motility and rumination in sheep.

The effects of intracerebroventricular (i.c.v.) and intravenous (i.v.) administration of dopamine and its antagonists (domperidone and metoclopramide) on forestomach motility were investigated in four sheep fitted with Nichrome electrodes and strain gauges implanted on the reticulum and the caudo-dorsal sac of the rumen. Infused by the i.c.v. route at a rate of 2 micrograms . kg-1 . min-1, dopamine inhibited the phasic contractions of the reticulo-rumen. A similar effect was obtained following i.v. administration of doses which were 10 times higher, but the effect was associated with an increased tone of the rumen wall. Prior i.v. administration of domperidone (0.5 micrograms . kg-1) blocked these effects and a selective blockade of the dopamine-induced inhibition of phasic contractions was obtained with higher dose of domperidone (2.5 micrograms . kg-1) administered centrally. When dopamine was infused alone ventricularly or intravenously after metoclopramide (40 micrograms . kg-1), it induced a transient (6-8 min) period of rumination, which could be blocked by a prior i.c.v. administration of tolazoline. It was concluded that dopamine acts on rumino-reticular motility in sheep through specific dopamine receptors (i) centrally by inhibiting the frequency of reticular contraction and (ii) peripherally by increasing the muscular tone of the rumen, and its effects on rumination involved alpha 2 adrenergic receptors located in the central nervous system.