Pharmacology of reticulo-ruminal motor function.

experimental studies of the pharmacological control of the reticulo-rumen reveal two major central inhibitory pathways: (i) an adrenergic system which inhibits the magnitude of reticulo-ruminal movements and involves alpha 2-receptors and (ii) an inhibitory opioid system, which modulates the rate of the contractions. In addition, the coarseness of the stomach contents and the masticatory movements represent the major excitatory inputs to the gastric centres, hence the hypomotility and stasis observed in disease processes which lead to anorexia. Finally, the amplitude of reticular contractions will tend to vary inversely with the rate of contractions following the administration of stimulatory drugs acting centrally. As a practical statement, a direct action on the gastric centres is possible for drugs such as alpha blockers and/or morphine antagonists. Reflex excitatory effects may be induced by stimulation of muscarinic receptors of the reticulorumen wall as long as it remains below the threshold of a neuromuscular blockade. The highly complex mechanisms whereby reticuloruminal movements are regulated provide for (i) drug interactions, like those of histamine antagonists and opioid agonists, (ii) reflex inhibition of both amplitude and frequency of contractions arising from an enhanced abomaso-duodenal motility, like that induced by serotonin, and (iii) reflex stimulation of the rate of contractions elicited by the i.v. injection of catecholamines, an effect limited to the sheep. The multifactorial nature of reticuloruminal function suggests that treatment is unlikely to effect a cure but should materially re-establish a more normal motility. Further studies, required in diseased animals to provide for a curative use of drugs and a more thorough understanding of drug effects in normal ruminants, represent only a first step toward rational therapy. In addition, the extrapolation of drug effects from sheep to calves or cattle could be hazardous, especially for drugs whose mechanisms of action are currently unknown.