Pupillographic analysis of morphine action in the rabbit: role to the autonomic nervous system.

Pupillary response of albino rabbits were recorded using an infrared video pupillometer. The transient response to an i.v. injection of morphine (1-12 mg/kg) consists of three phases. Phase 1 is a brief mydriasis of 20 s duration. Phase 2 is a miosis that reaches a maximum within 2-4 min and is the only dose-related endpoint observed. Time-averaged pupil size is not decreased by morphine in a dose-related manner when measured 5 min after injection. Phase 3 is large amplitude fluctuation that reaches peak amplitude within 20 min and may last for several hours. Thus, dynamic pupillography reveals that the terms "miosis' and "mydriasis' may be insufficient to fully describe opiate-induced pupillary effects. All three phases are blocked by naloxone (0.1 mg/kg i.v.) and by topical pretreatment with scopolamine. These results are consistent with the theory that morphine alters pupil size by interaction with opiate receptors that are involved in the control of parasympathetic output to the iris.