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1. Sex differences in head‐fixed voluntary running behavior in C57BL/6J mice

   https://doi.org/10.1111/ejn.14654  

   Warner, Emily J.; Padmanabhan, Krishnan (December 2019, European Journal of Neuroscience)

   Abstract Sex differences in running behaviors between female and male mice occur naturally in the wild. Recent experiments using head‐fixed mice on a voluntary running wheel have exploited analogous locomotor activity to gain insight into the neural underpinnings of a number of behaviors ranging from spatial navigation to decision‐making. It is however largely unknown if sex differences exist between females and males in a head‐fixed experimental paradigm. To address this, we characterized locomotor activity in head‐fixed female and male C57BL/6J mice on a voluntary running wheel. First, we found that over the initial 7‐day period, on average, animals increased both the velocity and the time spent running. Furthermore, we found that female mice habituated to running forward over the initial 2 days of encountering the wheel, while male mice took up to 4 days to habituate to running forward. Taken together, we characterized features of a sexually divergent behavior in head‐fixed running that should be considered in experiments employing female and male mice. 

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2. Overexpression of striatal D2 receptors reduces motivation thereby decreasing food anticipatory activity

   https://doi.org/10.1111/ejn.14219  

   LeSauter, Joseph; Balsam, Peter D.; Simpson, Eleanor H.; Silver, Rae (November 2018, European Journal of Neuroscience)

   Abstract Dopamine has been implicated in circadian timing underlying the food entrainable oscillator (FEO) circuitry and overexpression of the dopamine D2 receptor (D2R) in the striatum has been reported to reduce motivation to obtain food rewards in operant tasks. In the present study, we explored both of these mechanisms by examining food anticipatory activity (FAA) in dopamine D2 receptor‐overexpressing (D2R‐OE) mice under various durations of food availability. First, we noted that at baseline, there were no differences between D2R‐OEmice and their littermates in activity level, food intake, and body weight or in circadian activity. Under conditions of very restricted food availability (4 or 6 hr), both genotypes displayedFAA. In contrast, under 8‐hr food availability, control mice showedFAA, but D2R‐OEmice did not. Normalization of D2R by administration of doxycycline, a tetracycline analogue, rescuedFAAunder 8‐hr restricted food. We next tested for circadian regulation ofFAA. When given ad libitum access to food, neither D2R‐OEnor controls were active during the daytime. However, after an interval of food restriction, all mice showed elevated locomotor activity at the time of previous food availability in the day, indicating circadian timing of anticipatory activity. In summary, motivation is reduced in D2R‐OEmice but circadian timing behavior is not affected. We conclude that an increase in striatal D2R reducesFAAby modulating motivation and not by acting on a clock mechanism. 

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3. The Estrous Cycle Coordinates the Circadian Rhythm of Eating Behavior in Mice

   https://doi.org/10.1177/07487304241262356  

   Alvord, Victoria_M; Pendergast, Julie_S (July 2024, Journal of Biological Rhythms)

   The estrous cycle regulates rhythms of locomotor activity, body temperature, and circadian gene expression. In female mice, activity increases on the night of proestrus, when elevated estrogens cause ovulation. Exogenous estradiol regulates eating behavior rhythms in female mice fed a high-fat diet, but it is unknown whether endogenous estrogens regulate eating rhythms. In this study, we investigated whether diurnal and circadian eating behavior rhythms change systematically across the estrous cycle. We first studied diurnal eating behavior rhythms in female C57BL/6J mice in 12L:12D. Estrous cycle stages were determined by vaginal cytology while eating behavior and wheel revolutions were continuously measured. The mice had regular 4- to 5-day estrous cycles. Consistent with prior studies, the greatest number of wheel revolutions occurred on the night of proestrus into estrus when systemic levels of estrogens peak. The amplitude, or robustness, of the eating behavior rhythm also fluctuated with 4- to 5-day cycles and peaked primarily during proestrus or estrus. The phases of eating behavior rhythms fluctuated, but not at 4- or 5-day intervals, and phases did not correlate with estrous cycle stages. After ovariectomy, the eating behavior rhythm amplitude fluctuated at irregular intervals. In constant darkness, the amplitude of the circadian eating behavior rhythm peaked every 4 or 5 days and coincided with the circadian day that had the greatest number of wheel revolutions, a marker of proestrus. These data suggest that fluctuations of ovarian hormones across the estrous cycle temporally organize the robustness of circadian eating behavior rhythms so that it peaks during ovulation and sexual receptivity. 

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4. Circadian Synchrony: Sleep, Nutrition, and Physical Activity

   Healy, Kelly L.; Morris, Andrew R.; Liu, Andrew C. (October 2021, Frontiers in network physiology)

   The circadian clock in mammals regulates the sleep/wake cycle and many associated behavioral and physiological processes. The cellular clock mechanism involves a transcriptional negative feedback loop that gives rise to circadian rhythms in gene expression with an approximately 24-hour periodicity. To maintain system robustness, clocks throughout the body must be synchronized and their functions coordinated. In mammals, the master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is entrained to the light/dark cycle through photic signal transduction and subsequent induction of core clock gene expression. The SCN in turn relays the time-of-day information to clocks in peripheral tissues. While the SCN is highly responsive to photic cues, peripheral clocks are more sensitive to non-photic resetting cues such as nutrients, body temperature, and neuroendocrine hormones. For example, feeding/fasting and physical activity can entrain peripheral clocks through signaling pathways and subsequent regulation of core clock genes and proteins. As such, timing of food intake and physical activity matters. In an ideal world, the sleep/wake and feeding/fasting cycles are synchronized to the light/dark cycle. However, asynchronous environmental cues, such as those experienced by shift workers and frequent travelers, often lead to misalignment between the master and peripheral clocks. Emerging evidence suggests that the resulting circadian disruption is associated with various diseases and chronic conditions that further circadian desynchrony and accelerate disease progression. In this review, we discuss how sleep, nutrition, and physical activity synchronize circadian clocks and how chronomedicine may offer novel strategies for disease intervention. 

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5. Chronic environmental circadian disruption increases atherosclerosis and dyslipidemia in female, but not male, ApolipoproteinE-deficient mice

   https://doi.org/10.3389/fphys.2023.1167858  

   Chalfant, Jeffrey M.; Howatt, Deborah A.; Johnson, Victoria B.; Tannock, Lisa R.; Daugherty, Alan; Pendergast, Julie S. (March 2023, Frontiers in Physiology)

   Shift work chronically disrupts circadian rhythms and increases the risk of developing cardiovascular disease. However, the mechanisms linking shift work and cardiovascular disease are largely unknown. The goal of this study was to investigate the effects of chronically shifting the light-dark (LD) cycle, which models the disordered exposure to light that may occur during shift work, on atherosclerosis. Atherosclerosis is the progressive accumulation of lipid-filled lesions within the artery wall and is the leading cause of cardiovascular disease. We studied ApolipoproteinE -deficient ( ApoE −/− ) mice that are a well-established model of atherosclerosis. Male and female ApoE −/− mice were housed in control 12L:12D or chronic LD shift conditions for 12 weeks and fed low-fat diet. In the chronic LD shift condition, the light-dark cycle was advanced by 6 h every week. We found that chronic LD shifts exacerbated atherosclerosis in female, but not male, ApoE −/− mice. In females, chronic LD shifts increased total serum cholesterol concentrations with increased atherogenic VLDL/LDL particles. Chronic LD shifts did not affect food intake, activity, or body weight in male or female ApoE −/− mice. We also examined eating behavior in female ApoE −/− mice since aberrant meal timing has been linked to atherosclerosis. The phases of eating behavior rhythms, like locomotor activity rhythms, gradually shifted to the new LD cycle each week in the chronic LD shift group, but there was no effect of the LD shift on the amplitudes of the eating rhythms. Moreover, the duration of fasting intervals was not different in control 12L:12D compared to chronic LD shift conditions. Together these data demonstrate that female ApoE −/− mice have increased atherosclerosis when exposed to chronic LD shifts due to increased VLDL/LDL cholesterol, independent of changes in energy balance or feeding-fasting cycles. 

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