Dose dependent morphine-induced pupillary fluctuations in the rat: frequency analysis.

Pupillary fluctuation induced by s.c. injection of morphine sulfate was studied in the rat using an infrared video pupillometer. The frequency and amplitude of the induced fluctuations throughout 47 min after drug injection was quantified using fourier analysis. Pupil fluctuation frequency was for the most part less than 0.25 Hz. The frequency profile showed a systematic increase in fluctuation amplitude with decreasing frequency. Aside from this systematic increase, no other consistent frequency trend was observed. The maximum drug effect occurred at approximately 15 min postinjection and remained constant for the remaining 30 min of the experiment. When fluctuation amplitude for all frequencies was pooled, a statistically significant, dose-dependent increase in fluctuation was found, with a peak effect between 10 and 40 mg/kg. Using frequency analysis techniques, we have demonstrated that pupil fluctuation induced by s.c. morphine administration in the rat is, like the concomitant mydriasis, a dose-related phenomenon.