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Direct analysis of the release of methionine-enkephalin from guinea pig myenteric plexus: modulation by endogenous opioids and exogenous morphine.

The in vitro release of methionine-enkephalin (met-enkephalin) from two longitudinal muscle myenteric plexus strips from guinea pig ileum has been obtained during continuous superfusion and quantitated directly using a radioimmunoassay specific for this opioid peptide. Electrical stimulation (5-80 Hz) produced a significant increase in the rate of release of met-enkephalin the magnitude of which was not dependent on the frequency of stimulation. Analysis of the release of met-enkephalin per pulse as a function of the frequency of stimulation indicated that the release of this opioid peptide from the myenteric plexus is inversely proportional to the frequency of stimulation. Electrically evoked release (40 Hz) of met-enkephalin was reduced by greater than 80% by substituting CoCl2 for CaCl2 or by pretreatment with tetrodotoxin (1 microgram/ml for 15 min). Evoked release was also reduced substantially by pretreatment with morphine (1 microM, 1.5 min). Alternatively, pretreatment of naive longitudinal muscle myenteric plexus strips with the opiate antagonist (-)-naloxone caused a significant increase in the rate of met-enkephalin release in the absence of electrical stimulation. In contrast, (+)-naloxone was devoid of any activity. These data, in combination with indirect pharmacological experiments, strongly indicate that met-enkephalin functions as a neurotransmitter in the enteric nervous system. Moreover, the activity of neurons that transmit via met-enkephalin appears to be under opioid regulation.