Evidence for a single functional opioid delta receptor subtype in the mouse isolated vas deferens

The identification of opioid δ receptor subtypes in mouse brain led to the investigation of the nature of the opioid δ receptors in the mouse isolated vas deferens in vitro. Noncumulative concentration-effect curves were constructed for DPDPE (δ₁ agonist) and [D-Ala², Glu⁴]deltorphin (δ₂ agonist) in control tissues, or in tissues which had been incubated with either [D-Ala², Leu⁵, Cys⁶] enkephalin (DALCE) (noncompetitive δ₁ antagonist) or 5'-naltrindole isothiocyanate (5'-NTII) (noncompetitive δ₂ antagonist). Incubation of the tissues with DALCE, under either oxygenated or nonoxygenated conditions, did not alter the concentration-effect curves for either agonist. In contrast, incubation of the tissues with 5'-NTII resulted in a significant rightward displacement of the concentration-effect curves of both DPDPE and [D-Ala², Glu⁴]deltorphin. Additionally, naltriben, a selective and competitive δ₂ antagonist, showed no significant difference in its ability to antagonize a fixed, submaximal concentration of either DPDPE or [D-Ala², Glu⁴]deltorphin. Furthermore, there was no significant difference in the affinity of naloxone (i.e., pA₂) at the receptor(s) acted upon by either DPDPE or [D-Ala², Glu⁴]deltorphin. Tolerance to DPDPE or [D- Ala², Glu⁴]deltorphin was produced by incubation of the tissues with these agonists; construction of the [D-Ala², Glu⁴]deltorphin concentration- effect curve in DPDPE-tolerant tissues demonstrated cross-tolerance between these agonists and, conversely, construction of DPDPE concentration-effect curves in [D-Ala², Glu⁴]deltorphin-tolerant tissues revealed cross- tolerance between these agonists. Thus, the present data provide support for one subtype of opioid δ receptor in the mouse isolated vas deferens based on 1) the lack of antagonism of the effects of both agonists selective for δ₁ and δ₂ receptor subtypes by DALCE, a δ₁ antagonist, 2) the antagonism of δ₁ and δ₂ agonists by 5'-NTII or naltriben (δ₂ antagonists), 3) the similar antagonist potency of NTB against either DPDPE or [D-Ala², Glu⁴]deltorphin, 4) the lack of significant difference in the naloxone pA₂ against either δ agonist and 5) the demonstration of two-way cross-tolerance between the effects of DPDPE and [D-Ala², Glu⁴]deltorphin in this tissue.