AJP - Regulatory, Integrative and Comparative Physiology, Vol 236, Issue 5 292-R296, Copyright © 1979 by American Physiological Society
Mechanism of morphine-induced mydriasis in the cat
M. C. Wallenstein and S. C. Wang
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.
