mu-Opioid receptors and alpha 2-adrenoceptors coexist on myenteric but not on submucous neurones.

Intracellular recordings were made from neurones in the myenteric and submucous plexuses of the guinea-pig ileum. All myenteric neurones that were hyperpolarized by [Met5]enkephalin (or normorphine) were also hyperpolarized by noradrenaline (or clonidine); neurones unaffected by opioids were unaffected by noradrenaline. The hyperpolarizations resulted from an increase in potassium conductance of the membrane and were blocked by the respective antagonists naloxone and idazoxan. Neurones of the submucous plexus were hyperpolarized by noradrenaline but not by normorphine. The results suggest that myenteric neurones possesses both mu-opioid receptors and alpha 2-adrenoceptors whereas submucous neurones have alpha 2-adrenoceptors but not mu-opioid receptors.