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.

Performing In Vivo and Ex Vivo Electrical Impedance Myography in Rodents.

Electrical impedance myography (EIM) is a convenient technique that can be used in preclinical and clinical studies to assess muscle tissue health and disease. EIM is obtained by applying a low-intensity, directionally focused, electrical current to a muscle of interest across a range of frequencies (i.e., from 1 kHz to 10 MHz) and recording the resulting voltages. From these, several standard impedance components, including the reactance, resistance, and phase, are obtained. When performing ex vivo measurements on excised muscle, the inherent passive electrical properties of the tissue, namely the conductivity and relative permittivity, can also be calculated. EIM has been used extensively in animals and humans to diagnose and track muscle alterations in a variety of diseases, in relation to simple disuse atrophy, or as a measure of therapeutic intervention. Clinically, EIM offers the potential to track disease progression over time and to assess the impact of therapeutic interventions, thus offering the opportunity to shorten the clinical trial duration and reduce sample size requirements. Because it can be performed noninvasively or minimally invasively in living animal models as well as humans, EIM offers the potential to serve as a novel translational tool enabling both preclinical and clinical development. This article provides step-by-step instructions on how to perform in vivo and ex vivo EIM measurements in mice and rats, including approaches to adapt the techniques to specific conditions, such as for use in pups or obese animals.

Strain specific behavioral and physiological responses to constant light in male CBA/J and CBA/CaJ mice.

OBJECTIVE: Being visually impaired increases the likelihood of sleep disorders and altered behavior. This study investigated physiological and behavioral differences in two similar mice substrains when exposed to constant light (LL) - CBA/J with retinal degeneration and CBA/CaJ mice (no retinal degeneration). MATERIAL AND METHODS: Male CBA/J and CBA/CaJ mice were placed into a 12:12 light:dark cycle or constant light (LL). Open field behavior, metabolic markers, and home-cage circadian activity were observed. RESULTS: CBA/CaJ mice have greater circadian period lengthening, increased weight gain, reduced glucose, and increased novelty-induced locomotor activity in LL, compared to CBA/J mice. LL reduced thyroid hormone and insulin in both substrains. DISCUSSION: While several baseline substrain differences were elucidated, CBA/CaJ mice were more effected by the exposure to LL than the blind CBA/J mice. These results illustrate that LL causes alterations in physiology and behavior and that circadian photoreceptivity might contribute to these effects.

Substrain specific behavioral responses in male C57BL/6N and C57BL/6J mice to a shortened 21-hour day and high-fat diet.

Altered circadian rhythms have negative consequences on health and behavior. Emerging evidence suggests genetics influences the physiological and behavioral responses to circadian disruption. We investigated the effects of a 21 h day (T = 21 cycle), with high-fat diet consumption, on locomotor activity, explorative behaviors, and health in male C57BL/6J and C57BL/6N mice. Mice were exposed to either a T = 24 or T = 21 cycle and given standard rodent chow (RC) or a 60% high-fat diet (HFD) followed by behavioral assays and physiological measures. We uncovered numerous strain differences within the behavioral and physiological assays, mainly that C57BL/6J mice exhibit reduced susceptibility to the obesogenic effects of (HFD) and anxiety-like behavior as well as increased circadian and novelty-induced locomotor activity compared to C57BL/6N mice. There were also substrain-specific differences in behavioral responses to the T = 21 cycle, including exploratory behaviors and circadian locomotor activity. Under the 21-h day, mice consuming RC displayed entrainment, while mice exposed to HFD exhibited a lengthening of activity rhythms. In the open-field and light-dark box, mice exposed to the T = 21 cycle had increased novelty-induced locomotor activity with no further effects of diet, suggesting daylength may affect mood-related behaviors. These results indicate that different circadian cycles impact metabolic and behavioral responses depending on genetic background, and despite circadian entrainment.

Male C57BL6/N and C57BL6/J Mice Respond Differently to Constant Light and Running-Wheel Access.

Previous studies have shown that exposure to circadian disruption produces negative effects on overall health and behavior. More recent studies illustrate that strain differences in the behavioral and physiological responses to circadian disruption exist, even if the strains have similar genetic backgrounds. As such, we investigated the effects of constant room-level light (LL) with running-wheel access on the behavior and physiology of male C57BL6/J from Jackson Laboratories and C57BL6/N from Charles River Laboratories mice. Mice were exposed to either a 12:12 light-dark (LD) cycle or LL and given either a standard home cage or a cage with a running-wheel. Following 6 weeks of LD or LL, their response to behavioral assays (open-field, light-dark box, novel object) and measures of metabolism were observed. Under standard LD, C57BL6/J mice exhibited increased locomotor activity and reduced exploratory behavior compared to C57BL6/N mice. In LL, C57BL6/J mice had greater period lengthening and increased anxiety, while C57BL6/N mice exhibited increased weight gain and no change in exploratory behavior. C57BL6/J mice also decreased exploration with running-wheel access while C57BL6/N mice did not. These results further demonstrate that C57BL/6 substrains exhibit different behavioral and physiological responses to circadian disruption and wheel-running access.