# Time‐of‐Day Impacts Uterine Circadian Rhythms and Response to Oxytocin: Comparison of Uterine Function in Melatonin‐Deficient C57BL/6 Versus Melatonin Proficient CBA/B6 Hybrid Mice

## TL;DR

This study shows that the time of day affects how the uterus responds to oxytocin, with differences observed between two mouse strains based on their melatonin levels.

## Contribution

The study reveals that circadian rhythms influence uterine function and oxytocin responsiveness, with strain-specific differences in circadian period and baseline contractions.

## Key findings

- Oxytocin receptor (Oxtr) mRNA levels in the uterus vary by time-of-day and tissue layer.
- CBA/B6 mice have shorter PER2::Luciferase circadian periods and lower baseline uterine contractions compared to C57BL/6.
- Uterine response to oxytocin is modulated by circadian time, independent of mouse strain.

## Abstract

Reaching term gestation requires a complex interplay between the uterus and hormonal signals regulating its contractile profile. Most pregnancy‐associated hormones vary in their overall level of release throughout pregnancy, but also have a circadian release pattern, including progesterone, oxytocin, and melatonin. It remains poorly understood how the circadian release of hormones impacts uterine function. To determine how time‐of‐day, mouse strain, and melatonin proficiency were associated with the uterotonic efficacy of oxytocin, the primary hormone promoting uterine contractions, we used melatonin‐deficient C57BL/6 and melatonin‐proficient CBA/C57BL/6 (CBA/B6) female mice on gestation day 18. Through RNAscope, we found that oxytocin receptor (Oxtr) mRNA exhibited a time‐of‐day variation that differed between the uterine endometrium and myometrium. This uterine layer‐specific, time‐of‐day difference in Oxtr was associated with a shift in phase of the molecular clock reporter PER2::Luciferase. A strain‐specific effect of PER2::Luciferase rhythms were observed in the uterus, where CBA/B6 had a shorter PER2::Luciferase period than C57BL/6. In addition, CBA/B6 uteri had lower spontaneous uterine contraction force compared to C57BL/6. Despite the difference in spontaneous contractions and circadian period, the capacity of oxytocin to induce contractions varied by time‐of‐day, independent of mouse strain. Together, these findings reveal that uterine responsiveness to oxytocin is gated by circadian time, with Oxtr expression and uterine contractions showing diurnal variation. At the same time, mouse strain was associated with PER2::Luciferase period and baseline uterine contractility. These results underscore the relevance of circadian timing in uterine physiology and that strain differences impact basal uterine function.