Physiology and pharmacology of centrally stimulated accommodation in rhesus monkeys

Purpose. Pharmacological agents have various roles in the study of accommodation. The goals of this research were to understand the relationship between ciliary process and lens edge movement and biometric changes in the anterior segment during Edinger-Westphal (EW) and pharmacologically stimulated accommodation, to determine the effects of adrenergic stimulation on resting refraction, pupil size, and accommodative amplitude and dynamics, to characterize the effects of pharmacological manipulation of accommodation, and to ascertain the effects of pirenzepine on accommodation.; Methods. Accommodation experiments were performed on rhesus monkeys with permanent indwelling electrodes in the EW nucleus of the midbrain. Pupil and resting refraction measurements were made on rhesus monkeys with normal eyes. Dynamic accommodation was recorded with infrared photorefraction. Ciliary process and lens edge mvmts were measured with goniovideography, and biometry was measured with continuous A-scan ultrasound. Time course and effects of phenylephrine and pirenzepine on pupil size, resting refraction and accommodative dynamics were evaluated. Pilocarpine and atropine were used to manipulate resting refraction and accommodative amplitude.; Results. During EW stimulated accommodation, ciliary processes and lens edge moved centripetally linearly with accommodative refractive changes. Ciliary process mvmt was greater than lens edge mvmt during supramaximal EW and pharmacological stimulation. During EW stimulated accommodation, lens thickness increased linearly with refraction. Peak velocity of accommodation and lens thickness increased with supramaximal stimulus currents. Carbachol stimulation resulted in an anterior shift of the lens. Accommodation was not affected by phenylephrine. Dynamics were not altered during the course of atropine cycloplegia, but were altered during pilocarpine stimulation as a function of the accommodative starting point. Pirenzepine resulted in an increase in pupil size, a hyperopic shift in resting refraction, and cycloplegia.; Conclusions. These studies have identified how various pharmacological manipulations influence accommodation and many differences between EW and pharmacologically stimulated accommodation in rhesus monkeys. Alpha adrenergic stimulation had no effect on the accommodative system, while pilocarpine, atropine and pirenzepine were effective in altering resting refraction and accommodative amplitude. Peak velocity of accommodation depends on amplitude, biomechanics of the accommodative plant, starting point of the response and contributions of neural and receptor activity.