Ocular action of an opioid peptide, DPDPE.

This study was performed to examine some ocular actions of an opioid agonist. Experiments were performed to evaluate the effects of a delta opioid agonist, DPDPE ([D-pen2, D-pen5]enkephalin (where pen = penicillamine)), on: 1) intraocular pressure (IOP) in rabbits; 2) cAMP accumulation in rabbit iris ciliary bodies (ICBs). Unilateral, topical administration of DPDPE caused bilateral depression of IOP. Intravitreal injection of DPDPE caused a greater IOP decrease than intravitreal injection of NaH2PO4 (vehicle). Topical administration of naloxone partially inhibited the effect of DPDPE on IOP in normal rabbits. In other experiments, DPDPE suppressed both basal and isoproterenol (ISO)-stimulated cAMP accumulation in ICBs. The presence of naltrindole (NTI), a delta receptor antagonist, did not prevent the suppression of cAMP levels by DPDPE. The conclusions drawn from the findings suggest that the lowering of IOP by DPDPE is mediated, in part, by actions at postjunctional (ciliary body) sites and may involve an atypical opioid receptor.