Morphine-induced pupillary fluctuations in the rat: correlations with EEG and respiratory changes

Pupil area in rats was continuously monitored by infrared pupillometry during morphine-induced (30 mg/kg i.p.) mydriasis and pupillary fluctuations. EEG records from all visual system areas as well as frontal cortex, amygdala and mesencephalic reticular formation show intermittent electrographic bursting to be simultaneous with miosis during the period of fluctuations. Mydriasis occurs at the cessation of each burst, but both of these events occur 1-3 seconds after an effect on respiration composed of a slight change in tidal volume and a 10% decrease in minute respiratory rate. The reason for the essential synchrony of the respiratory change, the cessation of global bursting and the change in parasympathetic input to the pupil is unknown.