Morphine-induced mydriasis and inhibition of pupillary light reflex and fluctuations in the cat

Morphine has species-characteristic effects on pupillary size. The effects of morphine on pupillary size, fluctuations and the light reflex were tested with an infrared video pupillometer in the gallamine-paralyzed cat. Compared with saline or base-line responses, i.v. morphine (0.06-1.5 mg/kg) caused a dose-related decrease in the light reflex and fluctuations but increased pupil size. Naloxone (1-100 μg/kg i.v.), injected 1 h after morphine, reversed all pupillary effects. Levorphanol (0.5 mg/kg i.v.) had pupillary actions like those of morphine, but dextrophan (0.5 mg/kg i.v.) was inactive. Sympathectomy did not alter the morphine response. It was concluded that morphine disrupts parasympathetic innervation of the iris through interactions with opiate receptors, some of which are in the brain. The morphine-induced changes on the light reflex and fluctuations in the cat are opposite those reported in the rat and rabbit. These results enlarge on the familiar species-dependent effects of opiates on pupillary size.