Opiate receptors in the rabbit iris.

Evidence pointing to the presence of opiate receptors in the rabbit iris was obtained in an in vivo study of the effects on the pupil of the intraocular injection of morphine (10--100 micrograms) and D-Ala2-met-enkephalinamide (D-Ala-E) (5--50 micrograms). Both opiates induced a significant dose-dependent decrease in pupillary size and an appreciable fluctuation of pupillary size. I.v. administration of naloxone (0.5 mg/kg) before or after injection of opiates prevented the miotic effect and the fluctuation of pupil size. The parallelism of the dose-response curves of opiates in the presence and absence of naloxone indicated competitive reversible antagonism.