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.
|Original language||English (US)|
|Number of pages||1|
|State||Published - Jan 1 1977|
ASJC Scopus subject areas
- Pharmacology, Toxicology and Pharmaceutics(all)