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Christine F. Wildsoet, Lisa A. Ostrin, Mariana Garcia; Pilocarpine-Stimulated Pupil and Accommodative Changes in Guinea Pigs. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3461.
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The role of accommodation in myopia development is not well understood. While the guinea pig is being increasingly used as a model of human myopia, little is known about their accommodative ability. Here, topical pilocarpine, a nonselective muscarinic receptor agonist, is used to evaluate the effects on guinea pig anterior segment biometry as a proxy for accommodative ability.
Measurements were made on one eye of each of 4 pigmented guinea pigs at 4 ages (2, 6, 12 and 16 months); each animal underwent three recording sessions, separated by 1 week. For each recording session, animals were anesthetized with ketamine and xylazine and the eye was held open with a lid speculum. Following baseline measurements, 2 drops of 2% pilocarpine were instilled every 4 minutes for a total of 3 instillations to elicit a maximum response. Measurements began after the first instillation and continued for 20 minutes, at 2-minute intervals in the first session and minute intervals in the other 2 sessions. During the first session, high frequency A-scan ultrasonography was used to measure lens thickness. In the second session, anterior segment OCT imaging was used to evaluate changes in pupil size and anterior chamber depth (ACD). In the third session, corneal topography was assessed.
Instillation of pilocarpine resulted in a transient increase in lens thickness, coupled to decreases in ACD and pupil size. Lens thickness increased from a baseline value of 4.19±0.4 mm to 4.24±0.35 mm at t=10 min, but had returned to baseline at t=20 min (4.19±38 mm). ACD decreased from 1.12±0.11 mm before pilocarpine, to 1.07±0.13 mm at t=20 min (mean change: 0.054 mm; p<0.005, paired t-test). Pupil size decreased to approximately 63% of its baseline value of 4.03±0.50 mm, prior to 2.54±0.14mm at t=20 min. Corneal curvature was not significantly affected; corneal powers at baseline and t= 20 min were 83.7±7.9 D and 84.8±7.6 D (mean change: 1.1 D; p=0.12, paired t-test).
Pilocarpine elicited pupil constriction, a decrease in anterior chamber depth, and an increase in lens thickness, implying that there are muscarinic receptors on both the iris sphincter and ciliary muscle of guinea pigs. Pilocarpine did not alter corneal curvature. Together, these findings point to a lenticular mechanism of accommodation in the guinea pig.
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