Evidence for opioid-induced release of glutamate in guinea pig longitudinal muscle-myenteric plexus strip

Abstract

By the use of longitudinal muscle-myenteric plexus (LMMP) strips of the guinea pig ileum it should be investigated whether opioids can contribute to an excitatory component of the intestinal smooth muscle contractions. LMMP preparations were set up in Tyrode solution with 1 micromol/l naloxone present or without naloxone from the beginning of tissue preparation. After a 30-min equilibration period the twitch contractions evoked by the first and the fifth electrical 3-s-stimulus in an 80-s-sequence were significantly higher in the tissues prepared and tested without naloxone present. When after continuous electrical twitch responses (+) MK-801, an NMDA receptor antagonist (10-100 micromol/l) was added to the Tyrode solution, the electrically evoked contractions were reduced. The inhibition of the twitch response by (+) MK-801 was less pronounced in the preparations treated continuously with naloxone as compared to the otherwise untreated LMMP preparations. Morphine, (1 and 5 micromol/l), evoked a release of glutamate from this nerve-muscle-preparation. In conclusion these experiments provide evidence that endogenously released opioids or exogenously applied morphine can release glutamate as an excitatory component within the LMMP preparation.