Participation of delta opioid receptor subtypes in the stimulation of adenylyl cyclase activity in rat olfactory bulb.

A number of recent investigations have proposed the existence of two pharmacologically distinct delta opioid receptor subtypes, named delta 1 and delta 2. In the present study, we have investigated the involvement of the two delta receptors in the opioid stimulation of adenylyl cyclase activity in rat olfactory bulb. In addition, we have conducted a similar investigation in rat striatum, where delta agonists are known to inhibit cyclic AMP formation. Both (D-Ala2, Glu4) deltorphin (DELT), a delta 2 agonist, and [D-Pen2, D-Pen5] enkephalin (DPDPE), a delta 1 agonist, stimulated adenylyl cyclase activity in rat olfactory bulb in a concentration-dependent manner, DELT being 25-fold more potent than DPDPE. The selective delta 2 antagonist naltriben counteracted the stimulatory effects of both agonists with a potency about 10-fold higher than that of the selective delta 1 antagonist 7-benzylidenenaltrexone. Moreover, pretreatment of olfactory bulb membranes with the nonequilibrium antagonist naltrindole 5'-isothiocyanate, which irreversibly blocks the delta 2 subtype, reduced the stimulatory effects of both DELT and DPDPE, whereas pretreatment with [D-Ala2, Leu5, Cys6]enkephalin, which binds covalently to delta 1 receptors, failed to affect the response to the agonists. Similar results were obtained in rat striatum. These data indicate that delta opioid receptors coupled to either stimulation or inhibition of adenylyl cyclase in two different brain areas predominantly belong to the delta 2 subtype.