Both mu and delta opiate receptors exist on the same neuron.

Abstract

Low concentrations of the relatively selective opiate receptor agonists dihydromorphine and normorphine (mu receptor agonists) and D-Ala 2-D-Leu 5-enkephalin (a delta receptor agonist) were applied to single enteric neurons while the frequency of action potential firing was recorded. Most neurons that were inhibited by the mu agonists were also inhibited by the delta agonist, but the two receptors could be distinguished by the higher concentration of naloxone required to antagonize the delta agonist. The results indicate that enteric neurons bear both mu and delta receptors and that cell firing is inhibited if either receptor type is activated.