Similarities in the regulatory patterns of pregnant and cycling rats

Rhythmic Three-Part Harmony: The Complex Interaction of Maternal, Placental and Fetal Circadian Systems

From the perspective of circadian biology, mammalian pregnancy presents an unusual biological scenario in which an entire circadian system (i.e., that of the fetus) is embodied within another (i.e., that of the mother). Moreover, both systems are likely to be influenced at their interface by a third player, the placenta. Successful pregnancy requires major adaptations in maternal physiology, many of which involve circadian changes that support the high metabolic demands of the growing fetus. A functional role for maternal circadian adaptations is implied by the effects of circadian disruption, which result in pregnancy complications including higher risks for miscarriage, preterm labor, and low birth weight. Various aspects of fetal physiology lead to circadian variation, at least in late gestation, but it remains unclear what drives this rhythmicity. It likely involves contributions from the maternal environment and possibly from the placenta and the developing intrinsic molecular clocks within fetal tissues. The role of the placenta is of particular significance because it serves not only to relay signals about the external environment (via the mother) but may also exhibit its own circadian rhythmicity. This review considers how the fetus may be influenced by dynamic circadian signals from the mother and the placenta during gestation, and how, in the face of these changing influences, a new fetal circadian system emerges. Particular emphasis is placed on the role of endocrine signals, most notably melatonin and glucocorticoids, as mediators of maternal-fetal circadian interactions, and on the expression of the clock gene in the 3 compartments. Further study is required to understand how the mother, placenta, and fetus interact across pregnancy to optimize circadian adaptations that support adequate growth and development of the fetus and its transition to postnatal life in a circadian environment.

Pregnancy-induced changes in the circadian expression of hepatic clock genes: implications for maternal glucose homeostasis

Adaptations in maternal carbohydrate metabolism are particularly important in pregnancy because glucose is the principal energy substrate used by the fetus. As metabolic homeostasis is intricately linked to the circadian system via the rhythmic expression of clock genes, it is likely that metabolic adaptations during pregnancy also involve shifts in maternal circadian function. We hypothesized that maternal adaptation in pregnancy involves changes in the hepatic expression of clock genes, which drive downstream shifts in circadian expression of glucoregulatory genes. Maternal liver and plasma (n = 6-8/group) were collected across 24-h periods (0800, 1200, 1600, 2000, 0000, 0400) from C57Bl/6J mice under isoflurane-nitrous oxide anesthesia prior to and on days 6, 10, 14 and 18 of pregnancy (term = day 19). Hepatic expression of clock genes and glucoregulatory genes was determined by RT-qPCR. Hepatic clock gene expression was substantially altered across pregnancy, most notably in late gestation when the circadian rhythmicity of several clock genes was attenuated (≤64% reduced amplitude on day 18). These changes were associated with a similar decline in rhythmicity of the key glucoregulatory genes Pck1, G6Pase, and Gk, and by day 18, Pck1 was no longer rhythmic. Overall, our data show marked adaptations in the liver clock during mouse pregnancy, changes that may contribute to the altered circadian variation in glucoregulatory genes near term. We propose that the observed reduction of daily oscillations in glucose metabolism ensure a sustained supply of glucose to meet the high demands of fetal growth.

Circadian variation in basal plasma corticosterone and adrenocorticotropin in the rat: sexual dimorphism and changes across the estrous cycle

Sexual dimorphism in the rat hypothalamic-pituitary-adrenal axis was investigated by determination of plasma corticosterone and immunoreactive (I-) ACTH in males and in females at each stage of the estrous cycle. A serial blood-sampling technique enabled assessment of covariation of the two hormones across the full circadian range of their concentrations within individual animals. Distinct diurnal rhythms in plasma corticosterone were evident in all rats, and the degree and timing of this rhythmicity, determined by cosinor analyses, did not vary with gender or cycle stage. There were, however, marked differences in absolute levels of corticosterone across the estrous cycle, with the average daily concentration (mesor) increasing progressively from a minimum at estrus (129 +/- 11 ng/ml) to a maximum 3 days later at proestrus (246 +/- 14 ng/ml). The mesor corticosterone value in male rats (102 +/- 21 ng/ml) was not different from that in estrous females, but was lower than that in females at all other stages of the cycle. In contrast, no gender- or cycle-related differences were detected in absolute levels of I-ACTH, although distinct diurnal rhythms, synchronous with those for corticosterone, were evident in all groups. Accordingly, a strong and positive within-rat relationship between plasma corticosterone and I-ACTH was observed in all groups, but there was a clear shift in the nature of this relationship across the estrous cycle, such that the slope (i.e. concentration of plasma corticosterone per unit concentration of I-ACTH) was minimal in males and estrous females and maximal in proestrous females. In conclusion, this study shows that the extent of sexual dimorphism in resting plasma corticosterone levels is dependent on estrous cycle stage, being absent at estrus and maximal at proestrus. Moreover, this variation in plasma corticosterone was not accompanied by corresponding changes in plasma I-ACTH, suggestive of cycle-related changes in responsiveness of the adrenal cortex to trophic stimulation.

Preferences for foods with varying levels of salt and fat differ as a function of dietary restraint and exercise but not menstrual cycle

Women commonly report increased cravings for foods high in sugar, fat, and/or salt premenstrually relative to other times during the menstrual cycle. To determine if elevated cravings for foods high in salt and/or fat were related to alterations in food preferences across the menstrual cycle, preference and sensory ratings for air-popped popcorn with varying levels of salt (0.0, 1.5, and 4.0 g) and butter (3.3, 10, and 30 g) added to 30 g of popcorn were assessed in 34 normal-weight, college-aged women for 4 consecutive weeks. Additionally, using the Profile of Mood Scale (POMS), mood states were determined across the menstrual cycle. Dietary restraint, disinhibition, and hunger were assessed using the Three Factor Eating Questionnaire (TFEQ). Neither preference ratings nor ratings of the saltiness or fatness of the popcorn samples varied as a function of the menstrual cycle. Moreover, no differences in mood states were observed across the menstrual cycle. However, preference ratings for the popcorn samples were significantly greater for restrained than unrestrained eaters. Restrained eaters also rated the samples as significantly more salty, and had significantly higher scores on the tension-anxiety and depression-dejection subscale of the POMS than unrestrained eaters. Additionally, preference ratings of women who reported exercising more than 3 h a week were significantly greater than those of women who reported exercising less than 3 h a week. It is hypothesized that the variations in preference ratings observed as a function of dietary restraint and exercise are the result of differences in cognitive beliefs about food, rather than differences in physiological factors.

Effects of prenatal alcohol exposure on rat pups' ability to elicit retrieval behavior from dams

Research on fetal alcohol syndrome (FAS) has historically held that postnatal deficits result directly from prenatal alcohol exposure. Such exposure may alter infant behavior, and this in turn may affect maternal responsiveness and consequently increase the infant's risk for postnatal deficits. This study examined the effect of prenatal alcohol exposure on postnatal blood ethanol concentrations, pup development, and the ability of pups to elicit retrieval behaviors from the dam. Dams given access to a 15% ethanol solution throughout gestation and lactation showed blood ethanol concentrations of 0.1%, whereas their pups had concentrations below the sensitivity of the test. Prenatal alcohol exposure was shown to have pharmacological effects on the pup's righting reflex and nutritional effects on its rate of weight gain. Control pups were better able to elicit retrieval behavior from control and alcoholic dams than were fetally alcoholic pups. These results indicate that the condition of the pup can influence maternal responsiveness which, in turn, can influence pup development.

Possible explanations for excess weight gains in pregnancy: an animal model

An animal study was designed to address the reasons for excess pregnancy-associated weight gains. Forty-eight adult female rats were divided into two diet groups: a bland chow group and a chow plus glucose solution group. Half of each group was impregnated. Body weight, consumption of both foods, and water consumption were measured daily before, during, and after pregnancy. During pregnancy and after delivery, animals from each food-by-pregnancy group were killed, and adipose tissue analyses were performed. During pregnancy and during lactation, pregnant rats with two foods consumed more glucose solution and gained more adipose tissue with larger cells than did any other group. Taking both studies together, pregnancy is accompanied by sweet food consumption increases and can lead to adipose tissue gains in subjects not influenced by psychological variables.

Mating-induced alterations in the regulatory mechanisms of male rats

The effect of mating behavior on the regulatory mechanisms of male rats was investigated. Moderate levels of copulatory behavior (two ejaculations/week for 6 weeks) reduced body weight gain, and decreased food efficiency. Mating also affected carcass composition: it caused a decrease in fat content and a rise in nitrogen and water levels. The altered regulatory functions were observed in two age groups: 14-17-week- old male rats and 27-week- old male rats. Controls for the non-copulatory components of mating demonstrated the specificity of the copulation effects. The regulatory functions of the control groups (Social-Interaction, Voluntary-Exercise, and Novel-Cage) were similar to those of the sexually rested, Home-Cage group. These results demonstrate that a naturally occurring behavior, mating, significantly affects regulatory mechanisms in the male rat. Since growth and longevity have been shown to be interrelated, these findings support the hypothesis that copulation can affect aging.

Relationship among reproductive variables, sucrose taste reactivity and feeding behavior in humans

Magnitude-estimation of sucrose pleasantness and sweetness, and feeding behavior, were investigated in female and male college students. The subjects were individually tested five times over a five-week period. Female test intervals were scheduled to include the menses, pre-ovulatory, and luteal phases of the menstrual cycle. The data were analyzed by gender, length of menses, and phase of the menstrual cycle. The results of these analyses were consistent for both the sucrose taste reactivity tests and feeding tests. Male and long-menses females exhibited similar response patterns; and their pattern was significantly different from that of the short-menses females. The mean log pleasantness ratings of the males and long-menses females were significantly smaller than those of the short-menses females. Furthermore, both long-menses females and males behaved similarly in a time-limited surreptitious feeding test. They consumed significantly more food than did short-menses females. Phase of the menstrual cycle did not alter pleasantness reponse patterns, but there was one phase effect in the feeding tests. Luteal phase intake of short-menses females was elevated relative to that of the menses phase. These findings demonstrate that reproductive variables participate in the control of human regulatory behaviors.