Effects of dopamine and serotonin on eructation rate and ruminal motility in sheep

The relationships between forestomach motility and eructation rate were studied during dopamine infusion (2 sheep) and serotonin infusion (2 sheep). The sheep were chronically fitted with strain gauges on the reticulorumen and with a cannula in the dorsal sac of the rumen. A tracheotomy was performed to intercept the eructated gases and to permit measurement of their volume. To maintain a regular rate of eructation during the control periods, experiments were performed with a moderately increased intraruminal pressure obtained by continuous ruminal insufflation of nitrogen. Dopamine and serotonin were infused (IV for 10 minutes) at rates of 25, 50, and 100 micrograms/kg/min and 4, 8, and 16 micrograms/kg/min, respectively. Dopamine and serotonin both decreased the frequency of primary contractions. Dopamine reduced the amplitude of secondary contractions, whereas serotonin increased the forestomach tone and suppressed secondary contractions which were replaced by unpropagated eructative contractions. These motility changes were associated with a decrease in eructation rate during dopamine infusion and an increase in the eructation rate during serotonin infusion. After dopamine infusion was stopped, rebounds of eructation rate and rumen motility were observed which disappeared when a constant intraruminal pressure was maintained, indicating that the rumen reacts to its own distention. Sulpiride, but not phentolamine or propranolol, blocked the effects of dopamine. The effects of serotonin were abolished by methysergide, but were unaffected by imipramine. Therefore, seemingly dopamine acts through specific dopaminergic receptors and serotonin impringes on smooth muscle serotoninergic receptors. Finally, the use of dopamine and serotonin revealed that close relationships exist between the eructation rate and the pattern of ruminoreticular motility which may be preponderant against the cardia tone in the elimination of ruminal gases.