Pharmacological and light-driven pupillary accommodation in guinea pigs: comparative effects of muscarinic modulators and illumination across developmental stages

To investigate the effects of different light intensities and various mydriatic and miotic drugs on pupil accommodation in guinea pigs.

Forty-two-week-old guinea pigs were randomly divided into four groups to assess pupillary responses under varying light intensities (100, 250, 500 lx) and pharmacological interventions (1% atropine, 1% cyclopentolate, 1% tropicamide, or 2% pilocarpine). Baseline pupil size and eccentricity were recorded using a non-contact Python-based imaging system integrating edge detection and pixel-to-distance conversion. Direct illumination effects were measured at sequential time points, followed by drug administration and longitudinal tracking of pupillary changes. The protocol was repeated at 12wk of age for developmental comparisons. Post-experiment, enucleated eyes were analyzed to evaluate in vitro vs in vivo differences.

Significant age-dependent differences in pupil dynamics were observed. Both 2- and 12-week-old guinea pigs exhibited marked pupil constriction under direct illumination (P<0.001), with decreased eccentricity post-constriction (P<0.001). Indirect illumination caused inconsistent pupil size changes (2-week: P=0.68; 12-week: P=0.49). Pharmacologically, atropine, cyclopentolate, and tropicamide induced pupil dilation (P<0.001), whereas pilocarpine caused constriction (P<0.001). All drug groups showed reduced eccentricity (P<0.001). In vivo/in vitro comparisons revealed significant structural differences.

This study investigates pupillary responses in developing guinea pigs, revealing a direct pupillary light reflex (PLR) with light intensity-dependent responses, while indirect PLR was undetectable. The differential effects of muscarinic modulators on pupillary responses underscore the critical role of cholinergic signaling in ocular accommodation, with age-related variations in sensitivity. Additionally, a novel non-contact measurement methodology achieved a precision of 0.01 mm for pupillary quantification, enhancing accuracy in ocular studies.