Objective: To evaluate the circadian rhythms of intraocular pressure (IOP) and tear production in dog exposed to a natural photoperiod.
Animals studied: We used 12 clinically healthy Beagles dog housed under natural photoperiod at indoor temperature and humidity.
Procedure: Intraocular pressure and Schirmer tear test (STT) I were measured every 4 h over a 48-h period in both eyes in each animal. Statistical analysis of the data was performed by one-way repeated-measures anova, Student's t-test, and single cosinor method.
Results: On each day, there was a highly significant effect of time on both parameters. A statistically significant difference of STT I values was observed comparing left and right eyes (P < 0.0001). Robust daily rhythms were observed for both parameters, IOP values showed diurnal acrophase (left eye: 09:33 +/- 00:50 h; right eye: 09:25 +/- 00:22 h), while STT I values showed nocturnal acrophase (left eye: 20:27 +/- 00:46 h; right eye: 20:00 +/- 00:05 h).
Conclusion: This study has demonstrated circadian rhythms in both IOP and STT I.