Autonomic drugs and the accommodative system in rhesus monkeys

Abstract

Accommodation and pupil constriction result from parasympathetic stimulation from the Edinger-Westphal (EW) nucleus of the midbrain resulting in release of acetylcholine at the neuromuscular junctions of the ciliary muscle and iris. Cholinergic and adrenergic drugs can be applied topically to evaluate the effects on the pupil and accommodative system without input from the EW nucleus. This study is directed at characterizing how topical low dose echothiophate, an anti-cholinesterase inhibitor (i.e., an indirect cholinergic agonist), epinephrine, an adrenergic agonist, and timolol maleate, a beta adrenergic antagonist, affect pupil diameter, resting refraction and accommodative amplitude and dynamics in rhesus monkeys. The effects of 0.015% echothiophate, 2% epinephrine, 0.5% timolol maleate and saline on pupil diameter and resting refraction were measured in one eye each of four normal rhesus monkeys for 60-90 minutes following topical instillation. Pupil diameter was measured with infrared videography and refraction was measured with a Hartinger coincidence refractometer. Effects on static and dynamic EW stimulated accommodation were studied in three iridectomized monkeys (ages 5, 6 and 12 years) with permanent indwelling stimulating electrodes in the EW nucleus. Dynamic accommodative responses were measured with infrared photorefraction for increasing current amplitudes before and during the course of action of the pharmacological agents. Echothiophate caused a significant decrease in pupil diameter of 3.07 ± 0.65 mm (mean ± SEM, p < 0.01), and a myopic shift in resting refraction of 1.30 ± 0.39 D (p < 0.05) 90 minutes after instillation. Epinephrine caused a 2.76±0.38 mm (p < 0.01) increase in pupil diameter with no change in resting refraction 60 minutes after instillation. Timolol maleate resulted in no significant change in either pupil diameter or resting refraction 60 minutes after instillation. There was no significant change in maximum EW stimulated accommodative amplitude after any agent tested. The amplitude vs. peak velocity relationship for accommodation was significantly different after echothiophate and timolol maleate, and for disaccommodation after echothiophate, epinephrine and timolol maleate. In conclusion, when tested objectively in anesthetized monkeys, epinephrine and timolol maleate did not alter resting refraction or accommodative amplitude, but did have small, significant affects on accommodative dynamics. This suggests that there is an adrenergic component to the accommodative system. Low dose echothiophate had significant effects on pupil diameter and resting refraction, with only small effects on the dynamics of the accommodative response.

The Role of the Iris in Chick Accommodation

“…[42][43][44] In contrast to our findings that ID had no significant effect on nicotine-induced accommodation in the chicken, in vivo, total ID has been shown to cause a decrease in nicotine-induced accommodation in primates. 27 Similar to results reported here in chickens, the resting corneal curvatures were not significantly altered in iridectomized monkey eyes, while the anterior chambers were shallower.…”

The effect of pupil dilation on AL-Scan biometric parameters

“…Studies have shown that forward movement and thickening of the lens in accommodation and the thinning and backwards movement in relaxation cause several changes in the dimensions of ACD [9,10]. Application of cyclopentolate causes the lens reported a statistically significant change in ACD values but no significant change in AL or IOL power between undilated and dilated pupil biometry readings [8,11,12].…”

Autonomic Control of the Eye

“…126, 395, 396). A more recent study states that the sympathetic system only slightly affects the dynamics of accommodation and has little effect on the resting level or amplitude of accommodation (278). …”