Constant light and single housing alter novelty-induced locomotor activity and sociability in female Swiss Webster mice.

OBJECTIVE: Light exposure at night is known to produce behavioral aberrations in both human and animal models. One way to mimic light-at-night is through constant light exposure (LL), wherein animals are placed in an environment where a dark phase never occurs. Additionally, the type of housing condition for the rodents in experiments - grouped-housed vs singly-housed - can produce different behavioral responses, even in female mice. This study investigated whether LL produces alterations to emotionality and sociability, and whether group housing can alleviate some of those negative behavioral outcomes in female mice. METHODS: Female Swiss Webster mice were placed into group or single housing conditions and either into a standard 12:12 light:dark cycle or LL. Novelty-induced (open-field, light-dark box) and circadian locomotor activity, sociability, and serum oxytocin were measured during the middle of the day. RESULTS: LL and group-housing produced alterations to circadian home-cage activity and increases novelty-induced locomotor activity in the open-field and light-dark box. LL led to increased aggression in both group-housed and single-housed mice, while single-housed/LL mice showed reduced encounters towards the social mouse. Group-housed/LL mice exhibited increased interactions with the empty enclosure. Additionally, both LL and group-housing increased oxytocin levels. CONCLUSIONS: The increase in oxytocin may be a contributing factor to why female mice exhibit increased aggression and other impaired social behaviors in LL. Socialization via group housing was ineffective in reducing the negative sociability seen in mice under LL. These results indicate that aberrant light exposure and circadian misalignment are correlated with impaired social behaviors and emotionality.

Estradiol Regulates Circadian Responses to Acute and Constant Light Exposure in Female Mice.

Sex hormones are well known to modulate circadian timekeeping as well as the behavioral and physiological responses to circadian disruption. Gonadectomy, reducing the amount of circulating gonadal hormones, in males and females produces alterations to the free-running rhythm and the responses to light exposure by the central oscillator of the suprachiasmatic nucleus (SCN). In this study, we tested whether estradiol plays a role in regulating the circadian responses to acute (light pulses) and chronic light exposure (constant light [LL] vs standard light:dark [LD] cycle) in female C57BL6/NJ mice. Mice were either ovariectomized or given sham surgery and given a placebo (P) or estradiol (E) pellet for hormone replacement so that there were 6 groups: (1) LD/Sham, (2) LL/Sham, (3) LD/OVX + P, (4) LL/OVX + P, (5) LD/OVX + E, and (6) LL/OVX + E. After 65 days of light cycle exposure, blood and SCNs were removed and serum estradiol plus SCN estradiol receptor alpha (ERα) and estradiol receptor beta (ERß) were measured via ELISA. The OVX + P mice exhibited shorter circadian periods and were more likely to become arrhythmic in LL compared with mice with intact estradiol (sham or E replacement mice). The OVX + P mice exhibited reduced circadian robustness (power) and reduced circadian locomotor activity in both LD and LL compared with sham controls or OVX + E mice. The OVX + P mice also exhibited later activity onsets in LD and attenuated phase delays, but not advances, when given a 15-min light pulse compared with estradiol intact mice. LL led to reductions in ERß, but not ERα, regardless of the surgery type. These results indicate that estradiol can modulate the effects of light on the circadian timing system and that estradiol can enhance responses to light exposure and provide protection against a loss of circadian robustness.