Unmasking myenteric delta receptors

The population(s) of opioid receptor mediating responses of the ileum to mu and delta agonist were compared on the basis of their reactivity with the non-equilibrium opioid antagonist B-chlornaltrexamine (B-CNA), (q value). The q values for morphine and D-ala2-D-leu5-enkephalin (DADLE) calculated following pretreatment with B-CNA were 0.12 +/- 0.02 and 0.43 +/- 0.06, respectively. These results indicate that the above agonists do not act via identical populations of receptor in the ileum despite the similarity of their naloxone Ke values. In addition, the naloxone Ke for the delta agonist Tyr-D-Ser-Gly-Phe-Leu-Thre (DSLET) increased 10 fold following elimination of the mu subtype by exposure to the nonequilibrium selective antagonist B-funaltrexamine (B-FNA) suggesting the involvement of a second receptor subtype. It is suggested that this receptor is delta in nature.