Enkephalins and the pupils

Morphine causes mydriasis in mice, which is mediated through activation of opiate receptors. We have now shown that this effect is mimicked by metenkephalin, leuenkephalin and BW-180C. The relative potency of these agents is similar to that which is found in analgetic experiments and it is similarly blocked by the specific narcotic antagonist naloxone. In order to discover whether endogenous enkephalins are involved in the regulation of pupillary size we have treated animals with D-phenylalanine, an inhibitor of carboxypeptidase, the enzyme which inactivates enkephalins. Doses of 4-400 mg/kg i.p. failed to affect pupillary size. Similarly naloxone (1-10 mg/kg i.p.) injected alone did not affect pupillary size. We conclude that enkephalins do not participate in pupillary regulation in mice, although specific opiate receptors exist in this system.