Hormone therapy preserves ocular surface integrity following circadian alignment disturbance in menopausal rats

BACKGROUND: This study examines the impact of a light-cycle shift regimen on corneal and conjunctival tissues in menopausal rats and evaluates the protective role of combined hormone therapy. METHODS: Twenty-four menopausal female albino rats were randomly assigned to three groups (n = 8) following a 10-day acclimatization period. Group 1 (Control+Saline) was maintained under a 12:12 light/dark cycle. Light-cycle shift regimen was induced in Groups 2 and 3 using a rotating 7-day light-exposure sequence repeated over 21 days; this protocol consisted of 24 h of continuous light, 72 h of inverted dark-light timing, and 72 h of standard light-dark conditions. Groups 1 and 2 received saline, while Group 3 received 17β-Estradiol and drospirenone daily via oral gavage. After 31 days, eyes were enucleated for histological and immunohistochemical analyses of corneal, conjunctival, and palpebral tissues, including caspase-3 (Cas-3), tumor necrosis factor-alpha (TNF-α), and PERIOD-2 (PER2) expression. RESULTS: Light-cycle shift regimen (Group 2) significantly increased corneal thickness (p < 0.001), conjunctival inflammation, and vascular congestion, with marked upregulation of Cas-3 and TNF-α and downregulation of PER2. Hormone therapy (Group 3) attenuated these effects, showing reduced corneal edema, diminished inflammatory infiltration, and partial normalization of molecular markers. CONCLUSIONS: Shifting light-dark cycles may aggravate inflammatory and apoptotic changes in the ocular surface during menopause. Estrogen-progestin therapy attenuates these alterations by modulating the expression of the circadian-associated protein PER2 and maintaining structural integrity. These findings suggest that hormone therapy may offer potential benefits for preserving ocular surface homeostasis in menopausal women experiencing sleep or circadian rhythm disturbances.