Miosis and fluctuation in the rabbit pupil: effects of morphine and naloxone.

In the unanesthetized rabbit, morphine (12-16 mg/kg i.v.) produced a miosis that was not sustained. The response to the narcotic was a pupillary constriction that became maximal within 2 minutes following i.v. administration, after which time the pupil size fluctuated appreciably. The magnitude of the fluctuation, typically as high as 40% of control diameter, was much larger than can be attributed to "noise." The fluctuations occurred within a frequency of approximately 0.5 to 2/min. The miosis and subsequent fluctuation seen with 12 and 16 mg/kg of morphine was blocked by naloxone (0.5 mg/kg i.v.), but 128 mg/kg of morphine overcame the block. Cumulative dose-response curves for morphine and for morphine after naloxone yielded the value pA2=6.8 which is in agreement with the values obtained using other effects and species. Administration of morphine after exposure to high intensity light produced less miosis than that seen under normal illumination.