Tolerance to morphine-induced mydriasis in the rat pupil

Despite the commonly held view that tolerance does not develop to the pupillary effects of narcotics, recent studies have demonstrated tolerance to heroin-induced miosis in humans and to morphine-induced mydriasis in the mouse. Previous studies in this laboratory have shown that morphine produces a dose-related mydriasis and fluctuation (large amplitude, irregular oscillations in pupil diameter) in the rat pupil; the present study was designed to determine if tolerance develops to these effects following subcutaneous implantation of morphine pellets. We found that there is, indeed, tolerance to the morphine-induced mydriasis, but it is not complete. Furthermore, statistically significant tolerance does not develop to morphine-induced fluctuations in the rat pupil.