Morphine-induced pupillary fluctuation: physiological evidence against selective action on the Edinger-Westphal nucleus.

In the rat, the predominant pupillary effect of morphine sulfate (30 mg/kg, i.p.) is mydriasis, interrupted periodically by miotic excursions. Using infrared video pupillometry, respirometry and EEG recording, we had previously reported that miotic excursions are always correlated with the onset of EEG bursting, while mydriatic excursions are preceded by respiratory slowing and correlated with EEG burst cessation. In the current study, we found that during morphine-induced EEG bursting and related miosis, delivery of an alerting stimulus to the rat caused an immediate conversion sequence composed of respiratory slowing, burst cessation and mydriatic excursion. We also simulated morphine-induced EEG bursting through non-opioid means by delivering kindling electrical stimuli to the amygdala. When the stimulus was given during morphine-induced mydriasis, the stimulus-induced bursting resulted in a miotic excursion identical to spontaneous morphine-induced miotic excursions. These results indicate 1) that the reticular activating system is involved in morphine-induced mydriatic excursions and 2) that EEG bursting is not simply correlated with morphine-induced miotic excursions, but causes them. Collectively, these results strengthen our hypothesis that the Edinger-Westphal nucleus is not the site in which the selective, receptor-mediated pupillary effects of morphine are initiated.