Reports Potentiation of the mydriatic effect of change.7 After topical application of the mono- norepinephrine in the rabbit after amine releaser Ro 4-1284 to the eyes of rabbits monoamine oxidase inhibition. BRENDA K. pretreated with these MAO inhibitors, however, COLASANTI AND ERNST H. BARANY. pupillary dilation ensued. As in the case of re- sponses of other peripheral tissues, the response Dose-response curves of pupillary dilation after topical of the iris to indirectly acting sympathomimetic administration of norepinephrine or methoxamine have agents has been shown to be potentiated in hu- been determined in rabbits after chronic inhibition of man subjects undergoing treatment with MAO in- ocular monoamine oxidase by treatment with pargyline hibitors.8 or pheniprazine. Eyes treated with either monoamine ox- In the present communication, we report a idase inhibitor showed an enhanced responsiveness to potentiation of the mydriatic response of the rab- the mydriatic effect of norepinephrine given either topi- bit eye to norepinephrine after prior treatment cally or intravenously. Increments in pupil size of the with MAO inhibitors. This increase in sensitivity treated and control eyes in response to methoxamine applied topically, on the other hand, were the same. of the iris contrasts with the unaltered responses of These results suggest that monoamine oxidase may play a other peripheral organs and tissues to directly act- 2 role in the iris as one factor influencing the concentration ing catecholamines after MAO inhibition. of norepinephrine at the receptors. Methods. Adult male albino rabbits weighing 2 to 3 kg were used in these experiments. Pheni- The principal mechanism for physiological inac- prazine (Draco, Lund and Merrell—National tivation of norepinephrine released from adren- Laboratories, Ohio) was applied topically by mi- ergic nerve endings for action on autonomic effec- crodrop to the right eyes of three groups of six tor organs involves reuptake into the prejunctional rabbits once daily for 7 days. For this purpose, a neuron and binding in the storage granules.' Some microsyringe with polyethylene 50 tubing at- released transmitter is also susceptible to destruc- tached to the needle was used. After proptosis of tion by the enzyme monoamine oxidase (MAO; each eye, 10 fi\ of phosphate buffer (pH 7.4) con- EC 1.4.3.4), which is localized both within the taining 400 /Ltg of the drug were delivered to the nerve ending and extraneuronally. Because the cornea by slow placement, over a period of 1 min, responses of a variety of peripheral organs and tis- of the microdrops formed after manipulation of the sues to directly acting catecholamines are not al- micrometer. To allow the solution to dry, the eye tered after inhibition of MAO,2 this route of inac- was left proptosed for 2 additional minutes. Phos- tivation is felt to exert little influence on the phate buffer vehicle (10 /xl) was applied similarly amount of transmitter available at the receptors. to contralateral eyes, which served as controls. In contrast, MAO apparently does play an impor- Pargyline hydrochloride (Saber Laboratories, Il- tant role in determining responses to indirectly linois), 2 mg dissolved in 10 fx\ of saline, was in- acting sympathomimetic amines, the effects of jected once intravitreally into the right eyes of four which are potentiated by MAO inhibition.3 groups of six rabbits. An equivalent volume of sa- Studies undertaken with homogenates of vari- line was delivered also by a single intravitreal in- ous ocular structures have demonstrated a high jection to the contralateral control eyes. level of MAO activity in the iris-ciliary body of In the pheniprazine experiments, pupillary di- rabbits.4' 5 Recent experiments on MAO obtained ameters in response to norepinephrine or methox- from rabbit iris-ciliary body preparations 2 to 4 amine were determined 8 days after initiation of weeks after superior cervical ganglionectomy have the chronic treatment. In the pargyline studies, indicated that this enzyme has a predominantly pupil responses to these agonists were determined extraneuronal localization." 7 days after intravitreal injection of the inhibitor. Only a limited number of studies concerning Progressively increasing doses of each sympa- responses of the iris to drugs after MAO inhibition thomimetic were applied topically to both right have been undertaken. When MAO was inhibited and left eyes at a constant volume of 25 fx\. A by topical administration of agents affecting either period of 90 min intervened between successive type A or B of the enzyme as well as those affecting doses. Pupillary diameter was determined every both forms, pupil size of albino rabbits did not 30 min. Immediately prior to measurement, each 200 0146-0404/79/020200 +04$00.40/0 © 1979 Assoc. for Res. in Vis. and Ophthal., Inc. Downloaded from iovs.arvojournals.org on 09/30/2021 Volume 18 Number 2 Reports 201 Control Intravitreal Pargyline 0.5 I 2 4 8 16 NOREPINEPHRINE (% SOLUTION) Fig. 1A. Changes in pupil size after topical application of norepinephrine to pargyline-treated and contralateral control eyes, Each value represents the mean ± S.E.M. for six rabbits. Asterisk, p < 0.05. Control Pheniprazine by Microdrop < -J 3 0.5 I 2 4 NOREPINEPHRINE (% SOLUTION) Fig. IB. Effect of norepinephrine applied topically on the pupil size of pheniprazine-treated and contralateral control eyes. Each value represents the mean ± S.E.M. for six rabbits. Asterisk, p < 0.05. animal was placed on a wire net encased in a cannulated. After 10 minutes, a norepinephrine wooden frame with the eye positioned at a con- solution prepared to deliver 10 //.g/kg/min of the stant distance of 55 cm from a fluorescent light drug was infused at a rate of 0.5 ml/min over a source. Pupil size was measured with a transpar- period of 30 min. Pupil size was measured at the ent millimeter ruler. All measurements were termination of the infusion. taken in the horizontal meridian and recorded to Calculations of the effective dose 50 (ED50; i.e., the nearest half millimeter. the dose required to produce a response 50% of In one series of experiments, pupil size of MAO the maximum) and its 95% confidence intervals for inhibitor-treated and contralateral eyes was de- norepinephrine in the pheniprazine experiment termined after intravenous administration of nor- were made in accordance with the method of epinephrine. The rabbits were placed in a com- Fleming et al.9 Because it was not always possible mercial restrainer and the marginal ear vein was to obtain maximal responses to norepinephrine in Downloaded from iovs.arvojournals.org on 09/30/2021 Invest, Ophthalmol. Visual Sri. 202 Reports February 1979 1 Control Pheniprozine by Microdrop 2 3 E 2 E D Control (Contralaterol) • Introvitreol Porgyline E2 Pheniprazine by Microdrop Fig. 2. Changes in pupil size after intravenous in- O.O5 0.1 0.2 0.4 0.8 fusion of norepinephrine (10 /Ag/kg/min for 30 METHOXAMINE (% SOLUTION) min) into rabbits after prior treatment of right eyes with either pargyline or pheniprazine. Each verti- Fig. 3B. Effect of methoxamine applied topically cal bar represents the mean ± S.E.M. for six on pupil size of pheniprazine-treated and con- animals. tralateral control eyes. Each value represents the mean ± S.E.M. for six rabbits. Control within 15 to 30 mins after application of this MAO 1 Intravitreal Pargyline inhibitor. However, 24 hr later, pupil size of the treated eyes did not significantly differ from that of the contralateral controls. After topical application of norepinephrine at progressively increasing doses, pupillary dilation of eyes treated with pargyline was significantly greater than that of the contralateral control eyes at all but the lowest doses (Fig. 1A). Calculation of the ED 1 mm for norepinephrine revealed that O.I 0.2 0.4 0.8 there was a 2.8-fold increase in sensitivity' of the METHOXAMINE (% SOLUTION) treated eyes over that for the controls [geometric Fig. 3A. Changes in pupil size after topical appli- mean (95% confidence interval) of 1.5% solution cation of methoxamine to pargyline-treated and (1.3 to 1.7) for treated eyes vs. 4.3% (4.0 to 4.7) for contralateral control eyes of rabbits. Each value controls; p < 0.05]. Eyes treated chronically with represents the mean ± S.E.M. for six animals. pheniprazine likewise showed an enhanced re- sponsiveness to norepinephrine applied topically (Fig. IB), with a threefold shift of the dose- the pargyline experiment, the effective dose 1 mm response curve to the left [ED50 of 1.2% solution (ED 1 mm; i.e., the dose required to produce an (1.1 to 1.3) for treated eyes vs. 3.5% (3.4 to 3.6) for increase in pupil size of 1 mm) was calculated in a controls; p < 0.05]. similar fashion. Statistical comparisons of all re- Changes in pupil size in response to norepi- sults for the treated and the contralateral control nephrine given intravenously (10 ^tg/kg/min for 30 eyes were made with the use of Student's paried t min) after unilateral inhibition of ocular MAO are test. shown in Fig. 2. Norepinephrine significantly in- Results. After delivery of pargyline to the right creased the pupillary diameter of eyes treated eyes of rabbits by a single intravitreal injection, with either pargyline (p < 0.01) or pheniprazine pupil size of the treated and contralateral eyes re- (p < 0.05) but not that of the contralateral control mained equal. In contrast, within 30 min after eyes. Differences in pupil size between the application of pheniprazine to right eyes of rabbits treated and control eyes were also highly sig- by the microdrop technique, all treated pupils be- nificant (p < 0.01) in the case of both inhibitors.