Reproducible stimulation of ciliary muscle contraction in the cynomolgus monkey via a permanent indwelling midbrain electrode

Abstract

To better study the pivotal role of the ciliary muscle in ocular accommodation and aqueous humor outflow, a technique was developed to permit long-term reproducible stimulation of the muscle via the normal efferent neuronal pathway in cynomolgus monkeys. Following a small midline craniotomy, the stereotaxic coordinates of the Edinger-Westphal (E-W) nucleus were localized by contrast ventriculoradiography. A bipolar stimulating electrode was stereotaxically positioned to generate the maximum accommodative amplitude, and then permanently affixed to the surrounding cranium. Once experience was gained, the implant procedure required approximately 3 h and was well-tolerated by the animals, which subsequently exhibited normal behavior and no apparent discomfort or neurological deficits. Most animals retained the implant seemingly permanently (8-42 months follow-up), although a few animals extruded them after 4-8 months. Electrode placement in the E-W nucleus was verified histologically. The stimulation current and frequency-accommodative response relationships varied remarkably little between eyes, animals, or experimental sessions, although there was moderate inter-animal variation in the maximum accommodative amplitude. Using square-wave pulses of 0.5 ms duration, maximum accommodation, averaging approximately 15 diopters, was achieved at approximately 700 microA and approximately 70 Hz, and was greater under pentobarbital than under halothane anesthesia. Topical application of carbachol, eserine and echothiophate to the eye induced more accommodation than did central electrical stimulation. Permanent implantation of a stimulating electrode into the E-W nucleus may facilitate study of ocular physiological parameters dependent upon ciliary muscle contraction.