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Veterinary Research Communications

, Volume 9, Issue 1, pp 89–113 | Cite as

Central and local actions of opioids upon reticulo-ruminal motility in sheep

  • C. L. Maas
  • B. F. Leek
Research Articles

Abstract

The effects of opioids and naloxone on cyclical forestomach motility were determined in anaesthetized and conscious sheep. To assess central or peripheral opioid actions, differential routes of administration were used. Possible dynamic effects along the innervating vagovagal reflex arc were investigated electrophysiologically at the cervical level of the vagus nerve. Further, direct influences on the smooth muscle were evaluatedin vitro on isolated longitudinal reticular strips. Additionally, the effects of some spasmogenic agents were studied for comparative purposes. In anaesthetized sheep, opioids depressed in an identical manner both the amplitude of spontaneous cyclical contractions and contractions evoked by electrical stimulation of the distal end of the cut cervical vagus. In conscious sheep, low doses of normorphine and loperamide inhibited frequency and amplitude centrally (20 μg/kg and 4 μg/kg via carotid artery respectively), whereas locally higher dose levels (200 μg/kg and 10 μg/kg via coeliac artery respectively) affected only the amplitude of cyclical contractions. Furthermore the opioid peptides Leu-, Met-enkephalin and [D-Ala2-Met5]-enkephalinamide preferentially depressed the amplitude of cyclical motility most efficiently if administrated via the coeliac artery. These results indicate the presence both of a central opioid action depressing frequency and amplitude and of a local opioid action depressing only the amplitude of cyclical reticulo-ruminal motility. Opioids did not alter the resting discharge of afferent tension units and similarly failed to modulate tone of reticular stripsin vitro, suggesting that the opioids act locally on the intramural neuronal plexus, possibly by diminishing the output of excitatory transmitter. Whether substance P could play a role as a vagal excitatory transmitter besides the classically implicated acetylcholine has been discussed. The central opioid mechanism is probably not situated within the gastric centres but elsewhere in the brain. Naloxone (≥ 100 μg/kg, jugular vein) stimulated the frequency of cyclical ruminal motility only in well-defined experimental conditions, probably via a central mechanism.

Keywords

Naloxone Loperamide Excitatory Transmitter Opioid Action Cyclical Contraction 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Elsevier Science Publishers B.V 1985

Authors and Affiliations

  • C. L. Maas
    • 1
  • B. F. Leek
    • 2
  1. 1.Institute of Veterinary Pharmacology, Pharmacy and Toxicology, Faculty of Veterinary MedicineUtrecht UniversityUtrecht(The Netherlands)
  2. 2.Department of Veterinary Physiology and BiochemistryUniversity College DublinDublin 4(Ireland)

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