title: Mechanism of morphine-induced mydriasis in the cat.
abstract: Neurons that increased their firing rate in response to light stimulation were recorded from the pretectal region and the anterior oculomotor nucleus of sedated immobilized cats while morphine (2 mg/kg iv) was administered. The dose produced mydriasis and an increase in spontaneous firing rate of all light-sensitive neurons recorded from the anterior oculomotor nucleus. This suggested that morphine did not produce mydriasis through inhibition of the third nerve. Phenoxybenzamine, iv or topically, antagonized the mydriasis, indicating a peripheral source of sympathetic input as the basis for morphine-induced mydriasis. Morphine (2 mg/kg iv), administered 1 h after adrenalectomy, produced miosis and increased the spontaneous firing rate of light-sensitive neurons recorded from the oculomotor nucleus. These observations suggest that, in the cat, although morphine activates the oculomotor neurons to produce miosis, the effect is masked by the morphine-induced release of catecholamines, mainly from the adrenal glands, which produced mydriasis.
mesh_terms: D000200:Action Potentials; D000311:Adrenal Glands / Q000502:physiology*; D000818:Animals; D002415:Cats / Q000502:physiology*; D005260:Female; D008027:Light; D008297:Male; D009020:Morphine / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D009802:Oculomotor Nerve / Q000502:physiology*; D010643:Phenoxybenzamine / Q000494:pharmacology; D011680:Pupil / Q000502:physiology*
chemical_list: D010643:Phenoxybenzamine; D009020:Morphine