Potentiation of the mydriatic effect of norepinephrine in the rabbit after monoamine oxidase inhibition.

Dose-response curves of pupillary dilation after topical administration of norepinephrine or methoxamine have been determined in rabbits after chronic inhibition of ocular monoamine oxidase by treatment with pargyline or pheniprazine. Eyes treated with either monoamine oxidase inhibitor showed an enhanced responsiveness to the mydriatic effect of norepinephrine given either topically or intravenously. Increments in pupil size of the treated and control eyes in response to methoxamine applied topically, on the other hand, were the same. These results suggest that monoamine oxidase may play a role in the iris as one factor influencing the concentration of norepinephrine at the receptors.