Dose related morphine mydriasis in the rat

In contrast to findings of miosis in most animal species sedated by opiates, we now report that morphine (M) causes mydriasis in the rat. Using a simple pupillographic technique we have found that M induces a dose-related increase in pupil size. The procedure consists of tightly restraining adult male, albino SD rats and photographing the eye at 30- or 60- sec intervals under dim red illumination over a period of at least 2 hr following s.c. administration of M. Within 20 min, the pupil begins to dilate and reaches a peak of 3-4X control area about 1 hr after injection. Although significant effects are seen with doses as low as 1 mg/kg, the optimal change is noted with 32 or 64 mg/kg. Mydriasis lasts for over 2 hr, but rather than a relatively constant alteration in pupil size, a rapid and marked fluctuation occurs throughout the period of M action. This fluctuation is similar to that seen in the rabbit, where the dominant effect is miosis (Tallarida et al, JPET, in press). Further exploration into the action of M on the rat pupil will allow this effect to be used as an endpoint for the study of opiate receptor mechanisms.