The Investigation of Latanoprost's Effects on the Intraocular Pressure of Healthy Male guinea Pigs Under Light and Dark Regimes.

Purpose: The current study aimed to investigate the effect of latanoprost on intraocular pressure (IOP) fluctuations of guinea pigs (Cavia porcellus) under different light and darkness regimes.

Methods: 16 healthy adult American pigmented male guinea pigs (C. porcellus) were used for this study. A single drop of 0.005% latanoprost ophthalmic solution (Lataprost® Sina Daru, Tehran, Iran) was administered in the right eye of guinea pigs at 5:30, whereas the left eye received a placebo. The guinea pigs were restrained gently by the experimenter while avoiding extra pressure on the neck and eyelids. Then, the animals were randomly divided into two groups (Groups A and B). Although Group A (n = 8) was exposed to 12 + 0.5 h of light, Group B (n = 8) was kept in a dark room throughout the whole experiment. IOP measurements were performed using rebound tonometry (TonoVet®, iCare; Helsinki, Finland) at 6:00, 7:00, 8:00 and 9:00, followed by measurements at 12:00, 15:00 and 18:00. Statistical analyses were conducted using repeated measures analysis of variance (ANOVA), but due to the small sample size and non-normal data distribution, non-parametric methods were employed, including the Wilcoxon Signed-Rank Test for pre- and post-treatment comparisons and the Mann-Whitney U Test for between-group differences (p-values <0.05 were considered significant).

Results: The IOP reduction in the treated eyes was greater in the guinea pigs in the dark after latanoprost instillation at 7:00 and 8:00 (-2.75 ± 1.49 and -1.37 ± 0.92 mmHg, respectively, p-value <0.05) compared to those in the light (-0.62 ± 2.13 and -0.37 ± 1.92 mmHg, p-value >0.05). Significant IOP reduction was also observed in the untreated eyes due to the systemic absorption of latanoprost.

Conclusion: The reduction in IOP caused by 0.005% latanoprost was more pronounced in guinea pigs kept in darkness compared to the animal exposed to light, suggesting that the diurnal and nocturnal patterns of IOP fluctuations are clinically significant in ocular hypertension therapy and, thus, should be considered when applying ophthalmic hypotensive agents. The study of IOP in guinea pigs provides valuable insights into circadian variations in IOP, which could help optimize treatment protocols by tailoring latanoprost administration to align with peak efficacy periods, potentially improving glaucoma management and therapeutic outcomes.