Opioids increase potassium conductance in submucous neurones of guinea‐pig caecum by activating δ‐receptors

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Abstract
Intracellular records were made from neurones in the submucous plexus of the guinea‐pig caecum. [Met5]enkephalin, [Leu5]enkephalin, [D‐Ala2,D‐Leu5]enkephalin (DADLE) and [D‐Ser2,Leu5]enkephalin‐Thr (DSLET) hyperpolarized the membrane when applied in concentrations of 30 nM‐10 μM. Normorphine, [D‐Ala2, MePhe4,Gly5]enkephalin‐ol (DAGO), [D‐Ala2,MePhe4,Met(0)5]enkephalin‐ol (FK33824), dynorphin A and tifluadom had no effect at concentrations up to 10 μM. The hyperpolarization resulted from an increase in the membrane potassium conductance. Hyperpolarizations induced by [Met5]enkephalin were antagonized competitively by naloxone and by N‐bisallyl[aminoisobutyrate2'3, Leu5]enkephalin (ICI 174864). The Schild plots for these antagonisms had slopes not different from one, and the dissociation equilibrium constants among individual neurones were 5–50 nM for naloxone and 5–60 nM for ICI 174864. The results indicate that the opioid receptors on guinea‐pig submucous neurones which are coupled to potassium channels are of the δ‐type.