Suppression of cold-induced thermogenesis in full-term pregnant rats

Neurophysiological and cognitive changes in pregnancy

The hormonal fluctuations in pregnancy drive a wide range of adaptive changes in the maternal brain. These range from specific neurophysiological changes in the patterns of activity of individual neuronal populations, through to complete modification of circuit characteristics leading to fundamental changes in behavior. From a neurologic perspective, the key hormone changes are those of the sex steroids, estradiol and progesterone, secreted first from the ovary and then from the placenta, the adrenal glucocorticoid cortisol, as well as the anterior pituitary peptide hormone prolactin and its pregnancy-specific homolog placental lactogen. All of these hormones are markedly elevated during pregnancy and cross the blood-brain barrier to exert actions on neuronal populations through receptors expressed in specific regions. Many of the hormone-induced changes are in autonomic or homeostatic systems. For example, patterns of oxytocin and prolactin secretion are dramatically altered to support novel physiological functions. Appetite is increased and feedback responses to metabolic hormones such as leptin and insulin are suppressed to promote a positive energy balance. Fundamental physiological systems such as glucose homeostasis and thermoregulation are modified to optimize conditions for fetal development. In addition to these largely autonomic changes, there are also changes in mood, behavior, and higher processes such as cognition. This chapter summarizes the hormonal changes associated with pregnancy and reviews how these changes impact on brain function, drawing on examples from animal research, as well as available information about human pregnancy.

Mammary alveolar epithelial cells convert to brown adipocytes in post-lactating mice

During pregnancy and lactation, subcutaneous white adipocytes in the mouse mammary gland transdifferentiate reversibly to milk-secreting epithelial cells. In this study, we demonstrate by transmission electron microscopy that in the post-lactating mammary gland interscapular multilocular adipocytes found close to the mammary alveoli contain milk protein granules. Use of the Cre-loxP recombination system allowed showing that the involuting mammary gland of whey acidic protein-Cre/R26R mice, whose secretory alveolar cells express the lacZ gene during pregnancy, contains some X-Gal-stained and uncoupling protein 1-positive interscapular multilocular adipocytes. These data suggest that during mammary gland involution some milk-secreting epithelial cells in the anterior subcutaneous depot may transdifferentiate to brown adipocytes, highlighting a hitherto unappreciated feature of mouse adipose organ plasticity.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

Sex steroid levels near the term of pregnancy do not alter lipopolysaccharide-induced fever in oophorectomized rats

NEW FINDINGS

What is the central question of this study? Do plasma concentrations of oestrogen and progesterone similar to those observed near the term of pregnancy alter basal core temperature or the core temperature response to bacterial pyrogen in oophorectomized rats? What is the main finding and its importance? Plasma concentrations of oestrogen and progesterone similar to those observed near the term of pregnancy do not alter basal core temperature or the overall febrile response to bacterial pyrogen in oophorectomized rats. Thus, our data do not support the hypothesis that sex steroids mediate the regulated decrease in basal core temperature or the attenuated/absent core temperature response to bacterial pyrogen observed in rats near the term of pregnancy. Fever, an important component of the host's defence response to infection, is absent or attenuated in rats near the term of pregnancy concurrent with major changes in blood concentrations of the sex steroids, oestrogen and progesterone. The present experiments were carried out to determine the potential role of oestrogen and progesterone in mediating the altered core temperature response to bacterial pyrogen. For the experiments, oestrogen and progesterone were administered alone or in combination to oophorectomized, non-pregnant rats in concentrations that mimicked plasma levels of these hormones measured in pregnant rats on days 17, 18, 19 and 20 of gestation. Treatment with oestrogen or progesterone alone or in combination did not alter basal core temperature or the overall febrile response (i.e. 12 h fever index) to an EC50 dose of Escherichia coli lipopolysaccharide (i.e. 20 μg kg(-1) ). Administration of oestrogen did, however, influence the early core temperature response and increase the latency to fever following administration of lipopolysaccharide. Thus, our data provide evidence that although oestrogen may influence the early core temperature response to lipopolysaccharide, sex steroids in concentrations similar to those observed late in gestation do not alter the overall febrile response and are therefore unlikely to mediate the attenuated or absent febrile response to bacterial pyrogen in rats near the term of pregnancy.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Rat brown adipose tissue thermogenic features are altered during mid-pregnancy

Brown adipose tissue (BAT) thermogenesis is inhibited during late-pregnancy and lactation in the rat. However, scarce information concerning BAT functionality during mid-pregnancy is available. The aim of this work was to investigate uncoupling proteins and leptin expression during placentation in rat BAT as well as other key parameters in the thermogenic function of the tissue. BAT mitochondrial content was found to be reduced 50% in 11 and 13 day pregnant rats as compared to nonpregnant controls, although uncoupling protein 1 (UCP1) content was not modified. Furthermore, UCP3 mRNA levels were found to be highly increased during this period. beta3-adrenergic receptor (beta3-AR) decreased expression resulted in a higher alpha2/beta3 ratio. Finally, leptin mRNA levels in BAT were found to be 3-fold up-regulated in pregnant animals. In conclusion, we show the existence of profound changes in thermogenic features in BAT during gestational days 11 and 13, pointing to the importance of this tissue during mid-pregnancy.

Attenuated fever in pregnant rats is associated with blunted syntheses of brain cyclooxygenase-2 and PGE2

Attenuation of fever occurs in pregnant animals. This study examined a hypothesis that brain production of PGE(2), the final mediator of fever, is suppressed in pregnant animals. Near-term pregnant rats and age-matched nonpregnant female rats were injected with lipopolysaccharide (100 microg/kg) intraperitoneally. Four hours later, colonic temperature was measured, their cerebrospinal fluid (CSF) was sampled for PGE(2) assay, and their brains were processed for immunohistochemistry of cyclooxygenase-2, an enzyme involved in PGE(2) biosynthesis. In the pregnant rats, lipopolysaccharide injection resulted in significantly smaller elevations in both colonic temperature and CSF-PGE(2) level than in nonpregnant rats. In the pregnant rats, lipopolysaccharide-induced cyclooxygenase-2 expression was blunted in terms of the number of positive cells. There was a significant correlation between PGE(2) level in CSF and the number of cyclooxygenase-2-positive endothelial cells. These results suggest that suppressed PGE(2) production in the brain is one cause for the attenuated fever response at near-term pregnancy and that this suppressed PGE(2) production is due to the suppressed induction of cyclooxygenase-2 in brain endothelial cells.

Arginine vasopressin does not mediate the attenuated febrile response to intravenous IL-1beta in pregnant rats

Rats have an attenuated febrile response to intravenous endogenous pyrogen [e.g., interleukin-1beta (IL-1beta)] near the term of pregnancy. The present experiments were carried out on 25 nonpregnant and 32 pregnant rats to test the hypothesis that arginine vasopressin functioning as an endogenous antipyretic substance in the central nervous system mediates this attenuated febrile response. An intravenous injection of recombinant rat IL-1beta (rrIL-1beta) after intracerebroventricular vehicle produced a significant increase in core temperature in both nonpregnant and pregnant animals, the magnitude and duration of which was greater in the nonpregnant rats. In nonpregnant rats, intravenous rrIL-1beta after intracerebroventricular vasopressin V1-receptor antagonist accentuated the core temperature response compared with that observed with intravenous rrIL-1beta after intracerebroventricular vehicle. In pregnant animals, however, intravenous rrIL-1beta after intracerebroventricular vasopressin V1-receptor antagonist produced a decrease in core temperature rather than an increase in core temperature, which was observed with intravenous rrIL-1beta after intracerebroventricular vehicle. Thus our data do not support the hypothesis that a pregnancy-related activation of arginine vasopressin as an endogenous antipyretic substance in the central nervous system attenuates the febrile response to intravenous rrIL-1beta near the term of pregnancy in rats.

Vasopressin-induced antipyresis. Sex- and experience-dependent febrile responses

There is now good evidence that vasopressin (AVP) acts, in the male rat, as a neurotransmitter in the ventral septal area to reduce fever. In light of the well known sexual dimorphism in the AVP innervation of the brain, we asked if female rats would (a) display fevers different from those seen in male rats, (b) respond to AVP with antipyresis, (c) display evidence of endogenous AVP-induced antipyresis during fever, and (d) display altered fevers and AVP involvement as a function of hormonal status. Our experiments indicate that female rats display larger fevers to intracranial prostaglandin E2 (PGE2) but not to systemic lipopolysaccharide or interleukin-1 beta than do male rats. The larger fevers may be due, in part, to a lack of AVP-induced antipyresis, as an AVP antagonist elevates PGE2 fever in male but not in female rats and dialysates of the ventral septal area show increased AVP levels only in male rats during defervescence. Nonetheless, females respond to exogenous AVP with antipyresis. Throughout late pregnancy, parturition, and lactation, PGE2 fevers are reduced, but this appears to be due to a general suppression of autonomic output not involving enhanced AVP antipyresis. Fevers due to lipopolysaccharide and interleukin-1 beta are also suppressed at this time, and in some animals, fevers are dramatically suppressed at about the time of parturition. Our results indicate that female rats may utilize different strategies for antipyresis than do male rats and that hormonal status may influence both peripherally generated and centrally activated fevers.

AVP mediates the attenuated febrile response to administration of PGE1 in rats near term of pregnancy

Rats have an attenuated febrile response to intracerebroventricular injection of PGE1 near the term of pregnancy, the mechanism of which is unknown. The present experiments were carried out to test the hypothesis that arginine vasopressin (AVP), functioning as an endogenous antipyretic substance in the central nervous system, mediates this attenuated febrile response. The febrile response to intracerebroventricular injection of 0.2 microg PGE1 was determined in pregnant and nonpregnant rats after an intracerebroventricular injection of either vehicle or a vasopressin V1-receptor antagonist. After intracerebroventricular administration of vehicle, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature in both nonpregnant and pregnant animals. The increase in core temperature, however, was attenuated both in magnitude and duration in pregnant compared with nonpregnant animals. After intracerebroventricular administration of a vasopressin V1-receptor antagonist, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature that were similar in nonpregnant and pregnant animals. Our data support the hypothesis that a pregnancy-related activation of AVP as an endogenous antipyretic substance in the central nervous system attenuates the febrile response to intracerebroventricular administration of PGE1 near term of pregnancy in rats.

Thermoregulatory control during pregnancy and lactation in rats

Although the mechanisms remain unknown, maternal core temperature (Tc) decreases near term of pregnancy and is increased throughout lactation in rats. The purpose of our present experiments was to determine whether pregnancy and lactation shift the thermoneutral zone of rats and to investigate whether the changes in maternal Tc during pregnancy and lactation result from "forced" or "regulated" thermoregulatory responses. Conscious, chronically instrumented nonpregnant and pregnant and lactating rats were studied both in a thermocline (a chamber with a linear temperature gradient from 12 to 36 degrees C) and in a metabolic chamber to determine the influence of pregnancy and lactation on selected ambient temperature as well as the thermoregulatory response to changes in ambient temperature. We found that selected ambient temperature, oxygen consumption, and thermal conductance did not change in rats studied in a thermocline as Tc decreased near term of pregnancy. There was, however, a downward shift in the thermoneutral zone of rats studied in a metabolic chamber near term of pregnancy. During lactation, selected ambient temperature decreased in rats studied in a thermocline as oxygen consumption and Tc increased. The thermoneutral zone of lactating rats was not different from that of nonpregnant animals. Thus our data provide evidence that the decrease in Tc near term of pregnancy in rats results from a regulated thermoregulatory response, whereas the increase in Tc during lactation results from a forced thermoregulatory response.

Influence of pregnancy on the febrile response to ICV administration of PGE1 in rats studied in a thermocline

Rats near term of pregnancy have an attenuated febrile response to intracerebroventricular (ICV) injection of prostaglandin E1 (PGE1) when they are studied at an ambient temperature below their thermoneutral zone. Given that nonshivering thermogenesis in brown adipose tissue is impaired in rodents near term of pregnancy, it is possible that the attenuated febrile response is forced by impairment of this component of the autonomic thermoregulatory response. If this were the case, then near-term pregnant rats should develop a "normal" fever after PGE1 administration if they were studied in a thermocline where they could utilize behavioral as well as autonomic thermoregulatory effectors to increase their body core temperature (Tbc). Experiments were, therefore, carried out on 13 nonpregnant and 14 pregnant chronically instrumented rats in a thermocline (temperature gradient 10-40 degrees C) to investigate their Tbc responses to ICV injection of PGE1. ICV injection of 0.2 microgram PGE1 produced significant increases in Tbc and fever index in both nonpregnant and pregnant animals (day 19 of gestation); the increases, however, were significantly attenuated in the pregnant compared with the nonpregnant rats. Behavioral (e.g., selected ambient temperature) and autonomic (e.g., oxygen consumption) thermoregulatory effectors were activated to increase Tbc after ICV PGE1 in both groups of animals, but the duration of activation was shortened in pregnant compared with nonpregnant rats. The abbreviated thermoregulatory effector responses and the resulting attenuated febrile response to PGE1 in the pregnant rats may have resulted from a pregnancy-related activation of an endogenous antipyretic system.

Pregnancy alters body-core temperature response to a simulated open field in rats

Exposure of a rat to a novel environment (e.g., a simulated open field) induces a transient increase in body-core temperature, which is often called stress-induced hyperthermia. Although pregnancy is known to influence thermoregulatory control, its effect on stress-induced hyperthermia is unknown. Therefore, 24 Sprague-Dawley rats (8 nonpregnant and 16 pregnant) were studied to test the hypothesis that pregnancy would alter the development of stress-induced hyperthermia after exposure to a simulated open field. Body-core temperature index increased significantly after exposure to a simulated open field in nonpregnant and gestation day-10 rats but not in gestation day-15 and day-20 rats. Thus our data provide evidence that pregnancy influences the body-core temperature response of rats exposed to a simulated open field in a gestation-dependent fashion. The functional consequences as well as the mechanisms involved remain to be determined.

Prostaglandin fever in rats throughout the estrous cycle late pregnancy and post parturition

We have examined the influence of natural variations in endocrine status on the ability to generate a prostaglandin-induced fever in virgin female, pregnant and lactating rats and compared responses to those in male rats. Endocrine status of virgin female rats was assessed from examination of vaginal smears and time of parturition noted to enable accurate dating of pre- and postparturient fevers. Unanesthetized rats, previously prepared with intraventricular guide cannulas and intraperitoneal telemetry thermistors, were given intraventricular injections of prostaglandin E1 (2-100 ng/5 microliters) and temperatures monitored for 3 h after injection. Virgin females developed significantly larger fevers than did males at higher doses. There were no significant alterations in either fever height or duration as a function of the phase of the reproductive cycle in the females. Both pregnant and postparturient rats within the several days around birth displayed significantly lower fevers than did virgin females, but there was no further reduction in the immediate periparturient period. These data indicate that there are sex-, and possibly hormone-dependent differences in the central mechanisms involved in fever generation and antipyresis.

Body temperature regulation in rats near term of pregnancy

Experiments were carried out to define the effects of pregnancy on body temperature (Tb) regulation in rats. Tb was measured by biotelemetry in six animals from day 10 of pregnancy (term day 21) to postpartum day 10. Average 24-h Tb decreased from day 15 of gestation to the time of parturition. Furthermore, there was a loss of the normal circadian variation of Tb late in gestation, which was again present by postpartum day 2. The decrease in 24-h Tb on day 15 of gestation resulted from this loss of circadian variation, as Tb did not increase during the dark period. The further decrease in Tb on day 20 of gestation resulted from an overall decrease in Tb during the light and dark periods as well as from a loss of the circadian variation in Tb. Tb increased dramatically within 4 h of birth of the first pup, which always occurred on day 21 during the light period. The mechanisms responsible for these dramatic changes in thermoregulation during late gestation and around the time of parturition are presently unknown.

Metabolic adaptations to exercise in the cold. An update

Metabolic adaptations to exercise in a cold environment include the liberation of heat by vigorous physical activity, shivering and various forms of nonshivering thermogenesis. During a single exposure to cold the main metabolic fuel is glycogen; however, repeated bouts of exercise in the cold also result in an increase in fat metabolism. Potential contributors to fat loss induced by exercise in the cold include: the energy cost of synthesising lean tissue; cold-induced excretion of ketones; stimulation of resting metabolism; and the high energy cost of movement in a cold environment (walking over snow, the weight of heavy boots, hobbling by winter clothing, and decreased mechanical efficiency of dehydrated muscles). Biochemical explanations of fat mobilisation include increased secretion of catecholamines, increased sensitivity of peripheral catecholamine receptors and a decrease in circulating insulin levels. Such fat loss may be helpful in treating moderate obesity, although the response seems less well developed in women than in men. Metabolic changes must be taken into consideration in preparing winter athletes for competition. Glycogen depletion has a negative effect on the performance of endurance competitors, but this can be countered by a combination of diet, training and cold acclimation.