Oxygen consumption in new-born rats

A systematic review of immune-based interventions for perinatal neuroprotection: closing the gap between animal studies and human trials

BACKGROUND

Perinatal infection/inflammation is associated with a high risk for neurological injury and neurodevelopmental impairment after birth. Despite a growing preclinical evidence base, anti-inflammatory interventions have not been established in clinical practice, partly because of the range of potential targets. We therefore systematically reviewed preclinical studies of immunomodulation to improve neurological outcomes in the perinatal brain and assessed their therapeutic potential.

METHODS

We reviewed relevant studies published from January 2012 to July 2023 using PubMed, Medline (OvidSP) and EMBASE databases. Studies were assessed for risk of bias using the SYRCLE risk of bias assessment tool (PROSPERO; registration number CRD42023395690).

RESULTS

Forty preclinical publications using 12 models of perinatal neuroinflammation were identified and divided into 59 individual studies. Twenty-seven anti-inflammatory agents in 19 categories were investigated. Forty-five (76%) of 59 studies reported neuroprotection, from all 19 categories of therapeutics. Notably, 10/10 (100%) studies investigating anti-interleukin (IL)-1 therapies reported improved outcome, whereas half of the studies using corticosteroids (5/10; 50%) reported no improvement or worse outcomes with treatment. Most studies (49/59, 83%) did not control core body temperature (a known potential confounder), and 25 of 59 studies (42%) did not report the sex of subjects. Many studies did not clearly state whether they controlled for potential study bias.

CONCLUSION

Anti-inflammatory therapies are promising candidates for treatment or even prevention of perinatal brain injury. Our analysis highlights key knowledge gaps and opportunities to improve preclinical study design that must be addressed to support clinical translation.

Role of gut microbiota in the postnatal thermoregulation of Brandt's voles

Homeothermy is crucial for mammals. Postnatal growth is the key period for young offspring to acquire gut microbiota. Although gut microbiota may affect mammal thermogenesis, the impact of developmental regulation of gut microbiota on the ability of young pups to produce heat remains unclear. Antibiotics were used to interfere with the establishment of gut microbiota during the development of Brandt's voles, and their thermogenic development and regulatory pathways were determined. Deprivation of microbiota by antibiotics inhibits the development of thermogenesis in pups. Butyric acid and bile acid, as metabolites of gut microbiota, participated in the thermoregulation of pups. We propose that gut microbiota promote the development of thermoregulation through the butyric acid-free fatty acid receptor-2-uncoupling protein-1 or the deoxycholic acid-Takeda-G-protein-receptor-5-uncoupling protein-1 pathway in pups. These results show a relationship between gut microbiota and thermogenesis and expand the mechanism of postnatal development of thermogenesis in small mammals.

Rearing mice at 22°C programs increased capacity to respond to chronic exposure to cold but not high fat diet

OBJECTIVE

Rodent models raised at environmental temperatures of 21-22 °C are increasingly switched to thermoneutral housing conditions in adulthood to better capture human physiology. We quantified the developmental effects of rearing mice at an ambient temperature of 22 °C vs. 30 °C on metabolic responses to cold and high fat diet (HFD) in adulthood.

METHODS

Mice were reared from birth to 8 weeks of age at 22 °C or 30 °C, when they were acclimated to single housing at the same temperature for 2-3 weeks in indirect calorimetry cages. Energy expenditure attributable to basal metabolic rate, physical activity, thermic effect of food, and adaptive cold- or diet-induced thermogenesis was calculated. Responses to cooling were evaluated by decreasing the ambient temperature from 22 °C to 14 °C, while responses to HFD feeding were assessed at 30 °C. Influences of rearing temperature on thermogenic responses that emerge over hours, days and weeks were assessed by maintaining mice in the indirect calorimetry cages throughout the study.

RESULTS

At an ambient temperature of 22 °C, total energy expenditure (TEE) was 12-16% higher in mice reared at 22 °C as compared to 30 °C. Rearing temperature had no effect on responses in the first hours or week of the 14 °C challenge. Differences emerged in the third week, when TEE increased an additional 10% in mice reared at 22 °C, but mice reared at 30 °C could not sustain this level of cold-induced thermogenesis. Rearing temperature only affected responses to HFD during the first week, due to differences in the timing but not the strength of metabolic adaptations.

CONCLUSION

Rearing at 22 °C does not have a lasting effect on metabolic adaptations to HFD at thermoneutrality, but it programs an enhanced capacity to respond to chronic cold challenges in adulthood. These findings highlight the need to consider rearing temperature when using mice to model cold-induced thermogenesis.

Effects of hypoxia on the respiratory and metabolic responses to progressive cooling in newborn rodents that range in heterothermic expression

NEW FINDINGS

What is the central question of this study? Adult homeotherms and heterotherms differ in cold and hypoxia tolerance and in how they match O₂ supply and demand in response to these stressors. It has never been ascertained whether these differences reflect different developmental trajectories or whether they are already present at birth. What is the main finding and its importance? When exposed to cold and hypoxia, newborn rodents differed in how they matched O₂ supply and demand, with responses reflecting the degree of heterothermic expression and tolerance. Our findings indicate that elements of the adult phenotype are already present at birth.

ABSTRACT

There are physiological differences in how adult rodents regulate O₂ supply and O₂ demand when exposed to hypoxia in the cold. We examined whether these differences reflect divergent developmental trajectories of homeotherms and heterotherms or whether the differences are already present at birth. We exposed newborn rodents (0-4 days old) that ranged in heterothermic expression [a homeotherm, the rat (Rattus norvegicus); two facultative heterotherms, the mouse (Mus musculus) and the hamster (Mesocricetus auratus); and an obligate heterotherm, the ground squirrel (Ictidomys tridecemlineatus)] to either normoxia (21% O₂ ) or hypoxia (7% O₂ ) and measured their metabolic, thermoregulatory and ventilatory responses while progressively reducing the ambient temperature from 33 to 15°C. All newborns reduced their body temperature, O₂ consumption rate and ventilation during progressive cooling, both in normoxia and in hypoxia. When progressively cooled in hypoxia, however, the homeothermic rats exhibited the greatest thermogenic response, depressed their O₂ consumption rate the least and increased ventilation the most. In contrast, the obligate heterotherm, the ground squirrel, did not mount a thermogenic response, exhibited the greatest reduction in O₂ consumption rate and increased O₂ uptake not by increasing ventilation like the rat, but by extracting ≤80% of the O₂ from each breath. Facultative heterotherms (mice and hamsters) exhibited responses in between these two extreme phenotypes. We conclude that even as newborns, homeotherms and heterotherms diverge in how they match O₂ supply and O₂ demand when progressively cooled in hypoxia, with responses reflecting the degree of heterothermic expression, in addition to reported hypoxia and cold tolerance.

Influence of chronic hyperoxia on the developmental time course of the hypoxic ventilatory response relative to other traits in rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with postnatal age, but this transition occurs earlier in rat pups reared in moderate hyperoxia. This pattern is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the timing of developmental events. The present study investigated whether this plasticity is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60 % O₂ (Hyperoxia) exhibited a less biphasic ventilatory response to 12 % O₂ than pups reared in 21 % O₂ (Control) at 4 days of age (P4) and transitioned to a sustained HVR by P10-11; Control rats exhibited a biphasic HVR at both ages. However, the average ages at which pups attained other key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. Moreover, growth rates and maturation of the metabolic response to cooling were not accelerated, and may have been delayed slightly, relative to Control rats. For example, the capacity for pups to increase their metabolic rate at low ambient temperatures increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia pups at both P4 and P10-11 (i.e., lower CO₂ production rates below the lower critical temperature). Collectively, these data support the conclusion that hyperoxia specifically advances the age at which rat pups exhibit a sustained HVR, altering the relative timing of developmental events rather than compressing the entire period of development.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Development of behavioral responses to thermal challenges

Body temperature regulation involves the development of responses to cold and warm challenges. Matching our understanding of the development of body temperature regulation to warm challenges with that of cold challenges will enhance our understanding of the ontogeny of thermoregulation and reveal different adaptive specializations. Warm and cold thermoregulation are important processes, and they include direct thermal effects on offspring, as well as indirect effects on them, such as those imposed by thermally associated alterations of maternal behavior. The present paper is a selective review of the existing literature and a report of some new empirical data, aimed at processes of mammalian development, especially those affecting behavior. We briefly discuss the development of body temperature regulation in rats and mice, and thermal aspects of maternal behavior with emphasis on responses to high temperatures. The new data extend previous analyses of individual and group responses in developing rodents to warm and cool ambient temperatures. This literature not only reveals a variety of adaptive specializations during development, but it points to the earlier appearance in young mammals of abilities to combat heat loss, relative to protections from hyperthermia. These relative developmental delays in compensatory defenses to heating appear to render young mammals especially vulnerable to environmental warming. We describe cascading consequences of warming-effects that illustrate interactions across levels of physiological, neural, and behavioral development.

Environmental structure and energetic consequences in groups of young mice

Microenvironments can have considerable physiological consequences for the inhabitants by influencing the movements of individual members. The microenvironment can permit more diverse aggregation patterns or restrict movements to certain dimensions. Here, we tested whether aspects of the microenvironment that influenced aggregation patterns also influenced the energetics of groups of young animals. We tested the effects of enclosure configuration on the group temperature and respiration of infant mice (Mus musculus). We monitored the huddle temperature and respiration of groups in flat, concave and conical enclosures, which varied in shape and available space, and consequently the types of movements they permitted. We found that the amount of available space (or density) had a stronger effect on the group temperature than did the shape of the enclosure or types of permissible movements. We found no evidence that density or shape of the arena strongly affected the respiration rate of the group, with groups showing similar levels of oxygen consumption in all treatments. The lower density enclosures conveyed a considerable metabolic savings to groups in comparison to those tested in a higher density enclosure. These findings show density can have a large effect on the energetics of young mice, and provide insights on how simple features of the environment will influence physiology in a changing world.

Non-Mammalian Vertebrates: Distinct Models to Assess the Role of Ion Gradients in Energy Expenditure

Animals store metabolic energy as electrochemical gradients. At least 50% of mammalian energy is expended to maintain electrochemical gradients across the inner mitochondrial membrane (H⁺), the sarcoplasmic reticulum (Ca⁺⁺), and the plasma membrane (Na⁺/K⁺). The potential energy of these gradients can be used to perform work (e.g., transport molecules, stimulate contraction, and release hormones) or can be released as heat. Because ectothermic species adapt their body temperature to the environment, they are not constrained by energetic demands that are required to maintain a constant body temperature. In fact, ectothermic species expend seven to eight times less energy than similarly sized homeotherms. Accordingly, ectotherms adopt low metabolic rates to survive cold, hypoxia, and extreme bouts of fasting that would result in energy wasting, lactic acidosis and apoptosis, or starvation in homeotherms, respectively. Ectotherms have also evolved unique applications of ion gradients to allow for localized endothermy. Endothermic avian species, which lack brown adipose tissue, have been integral in assessing the role of H⁺ and Ca⁺⁺ cycling in skeletal muscle thermogenesis. Accordingly, the diversity of non-mammalian vertebrate species allows them to serve as unique models to better understand the role of ion gradients in heat production, metabolic flux, and adaptation to stressors, including obesity, starvation, cold, and hypoxia.

Ventilation and the Response to Hypercapnia after Morphine in Opioid-naive and Opioid-tolerant Rats

BACKGROUND

Opioid-related deaths are a leading cause of accidental death, with most occurring in patients receiving chronic pain therapy. Respiratory arrest is the usual cause of death, but mechanisms increasing that risk with increased length of treatment remain unclear. Repeated administration produces tolerance to opioid analgesia, prompting increased dosing, but depression of ventilation may not gain tolerance to the same degree. This study addresses differences in the degree to which chronic morphine (1) produces tolerance to ventilatory depression versus analgesia and (2) alters the magnitude and time course of ventilatory depression.

METHODS

Juvenile rats received subcutaneous morphine for 3 days (n = 116) or vehicle control (n = 119) and were then tested on day 4 following one of a range of morphine doses for (a) analgesia by paw withdraw from heat or (b) respiratory parameters by plethysmography-respirometry.

RESULTS

Rats receiving chronic morphine showed significant tolerance to morphine sedation and analgesia (five times increased ED50). When sedation was achieved for all animals in a dose group (lowest effective doses: opioid-tolerant, 15 mg/kg; opioid-naive, 3 mg/kg), the opioid-tolerant showed similar magnitudes of depressed ventilation (-41.4 ± 7.0%, mean ± SD) and hypercapnic response (-80.9 ± 15.7%) as found for morphine-naive (-35.5 ± 16.9% and -67.7 ± 15.1%, respectively). Ventilation recovered due to tidal volume without recovery of respiratory rate or hypercapnic sensitivity and more slowly in morphine-tolerant.

CONCLUSIONS

In rats, gaining tolerance to morphine analgesia does not reduce ventilatory depression effects when sedated and may inhibit recovery of ventilation.

The hypometabolic response to repeated or prolonged hypoxic episodes in the chicken embryo

Hypoxia (hx) in embryos causes a drop in oxygen consumption ( [Formula: see text] ) that rapidly recovers upon return to normoxia. We asked whether or not this pattern varies with the embryo's hypoxic history. The [Formula: see text] of chicken embryos in the middle (E12) or at end-incubation (E19) was measured by an open-flow methodology during 15-min epochs of moderate (15% O2) or severe hx (10% O2). Each hx-epoch was repeated or alternated with air by various modalities (air-hx-air-hx-air-hx-air, air-2·hx-air-2·hx-air, air-5·hx-air), in randomized sequences. The hx drop in [Formula: see text] was larger with severe than with moderate hx; however, in either case, its magnitude was essentially independent of the preceding hx history. E19 embryos had hx drops in [Formula: see text] of the same magnitude whether their incubation was in air or in moderate hx from E4 to E19. A different protocol (air-12·hx-air) gave variable results; with moderate hx, the [Formula: see text] response was similar to that of the other hx regimes. Differently, with severe hx most embryos progressively decreased [Formula: see text] and eventually died. We interpret these data on the basis of what is known on the 'compensatory partitioning' between costs of growth and maintenance. With moderate hx presumably each episode caused an energy shortfall absorbed entirely by the blunted growth. Hypoxic events of this type, therefore, should have no long-term functional effects other than those related to the small birth weight. Differently, the aerobic energy shortfall with severe hypoxia probably impinged on some maintenance functions and became incompatible with survival.

Oxygen in demand: How oxygen has shaped vertebrate physiology

In response to varying environmental and physiological challenges, vertebrates have evolved complex and often overlapping systems. These systems detect changes in environmental oxygen availability and respond by increasing oxygen supply to the tissues and/or by decreasing oxygen demand at the cellular level. This suite of responses is termed the oxygen transport cascade and is comprised of several components. These components include 1) chemosensory detectors that sense changes in oxygen, carbon dioxide, and pH in the blood, and initiate changes in 2) ventilation and 3) cardiac work, thereby altering the rate of oxygen delivery to, and carbon dioxide clearance from, the tissues. In addition, changes in 4) cellular and systemic metabolism alters tissue-level metabolic demand. Thus the need for oxygen can be managed locally when increasing oxygen supply is not sufficient or possible. Together, these mechanisms provide a spectrum of responses that facilitate the maintenance of systemic oxygen homeostasis in the face of environmental hypoxia or physiological oxygen depletion (i.e. due to exercise or disease). Bill Milsom has dedicated his career to the study of these responses across phylogenies, repeatedly demonstrating the power of applying the comparative approach to physiological questions. The focus of this review is to discuss the anatomy, signalling pathways, and mechanics of each step of the oxygen transport cascade from the perspective of a Milsomite. That is, by taking into account the developmental, physiological, and evolutionary components of questions related to oxygen transport. We also highlight examples of some of the remarkable species that have captured Bill's attention through their unique adaptations in multiple components of the oxygen transport cascade, which allow them to achieve astounding physiological feats. Bill's research examining the oxygen transport cascade has provided important insight and leadership to the study of the diverse suite of adaptations that maintain cellular oxygen content across vertebrate taxa, which underscores the value of the comparative approach to the study of physiological systems.

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.

Group and individual regulation of physiology and behavior: a behavioral, thermographic, and acoustic study of mouse development

The traditional approach to the study of thermoregulation in young animals focuses on the regulatory capacities of individuals, which, for multiparous species, risks ignoring critical aspects of the early developmental niche. Here, we examined the ontogeny of regulatory behavior in C57BL/6 mice, employing simultaneous behavioral, thermographic, and acoustic measures of groups and individual pups. Litters of mice were placed in a chamber on Postnatal Day (PND) 2, 4, or 8, in which the ambient temperature (T(a)) gradually cycled (over 50 min) from warm (36.5°C) to cool (20°C) and back (to 36.5°C). Litters of all three ages displayed "group regulatory behavior," whereby group size varied with changes in T(a). This coupling, moreover, improved with age. Infrared thermography was used to monitor skin temperature of pups' interscapular (T(IS)) and rump (T(rump)) areas, and to estimate brown adipose tissue (BAT) thermogenesis (T(IS)-T(rump)) in PND4 and PND8 individuals and huddles. Huddling was found to significantly reduce heat loss in pups subject to thermal challenge as groups, compared to pups challenged as individuals. Additionally, females were found to display significantly warmer T(IS) and T(rump) values than male huddlemates. Huddling did not have a consistent effect on emissions of ultrasonic vocalizations, which were generally correlated with ambient temperature and BAT activation. Our results indicate that simultaneous measures of behavioral and physiological response to cooling may prove useful for a variety of applications, including the phenotyping of social dysfunction.

The role of CO(2) and central chemoreception in the control of breathing in the fetus and the neonate

Central chemoreception is active early in development and likely drives fetal breathing movements, which are influenced by a combination of behavioral state and powerful inhibition. In the premature human infant and newborn rat ventilation increases in response to CO(2); in the rat the sensitivity of the response increases steadily after ∼P12. The premature human infant is more vulnerable to instability than the newborn rat and exhibits periodic breathing that is augmented by hypoxia and eliminated by breathing oxygen or CO(2) or the administration of respiratory stimulants. The sites of central chemoreception active in the fetus are not known, but may involve the parafacial respiratory group which may be a precursor to the adult RTN. The fetal and neonatal rat brainstem-spinal-cord preparations promise to provide important information about central chemoreception in the developing rodent and will increase our understanding of important clinical problems, including The Sudden Infant Death Syndrome, Congenital Central Hypoventilation Syndrome, and periodic breathing and apnea of prematurity.

Lung and metabolic development in mammals: contribution to the reconstruction of the marsupial and eutherian morphotype

Marsupials represent only 6% of all living mammals. Marsupialia and Placentalia are distinguished mainly by their modes of reproduction. In particular, the differences in the stage of development of the neonates may be one explanation for the divergent evolutionary success. In this respect one important question is whether the survivability of the neonate depends on the degree of maturation of the respiratory system relative to the metabolic capacity at the time of birth. Therefore, this review highlights the differences in lung morphology and metabolic development of extant Marsupialia and Placentalia. The Marsupial neonate is born with a low birth weight and is highly immature. The neonatal lung is characterized by large terminal sacs, a poorly developed bronchial system and late formation of alveoli. Marsupialia have a low metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity late in postnatal development. In contrast, the eutherian neonate is born with a relative high birth weight and is always more mature than marsupial neonates. The neonatal lung has small terminal sacs, the bronchial system is well developed and the formation of alveoli begins few days after birth. Placentalia have a high metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity early in postnatal development. The differences in the developmental degree of the newborn lung between Marsupialia and Placentalia have consequences for their metabolic and thermoregulatory capacity. These differences could be advantageous for Placentalia in the changing environments in which they evolved.

Electron microscopy and microcalorimetry of the postnatal rat heart (Rattus norvegicus)

The interplay of ultrastructure and tissue metabolism was examined in neonatal, infant and adult rat hearts by electron microscopy and microcalorimetry. Morphometry was used to determine parameters of oxygen diffusion capacity (distance between capillaries and mitochondria, capillary surface density) and oxidative metabolic capacity (mitochondrial volume fraction). Thin slices and large samples of living tissue were examined calorimetrically to quantify aerobic metabolism and ischemia tolerance, respectively. After birth, rat hearts grow in parallel to body mass and show characteristics of cellular hypertrophy. Capillary surface density increases from neonatal to infant rats, and decreases to an intermediate value in adult rats. The distance between capillaries and mitochondria shows no significant changes throughout postnatal development. Mitochondrial volume fraction increases continuously until adulthood. The specific aerobic tissue metabolic rate is higher in the neonatal than in the infant and adult rat. However, the ischemic decline in metabolic rate is much slower in the neonatal rat, reflecting an elevated hypoxia tolerance. In conclusion, the neonatal rat heart exhibits a high metabolic rate despite a low mitochondrial volume fraction. The subsequent structural rearrangements can be interpreted as long-term adaptations to the increased postnatal workload and may contribute to the progressive loss of hypoxia tolerance.

Implications of hypoxic hypometabolism during mammalian ontogenesis

During hypoxia, many newborn mammals, including the human infant, decrease metabolic rate, therefore adopting a strategy common to many living creatures of all classes, but usually not adopted by adult humans and other large mammals. In acute hypoxic conditions, hypometabolism largely consists in actively dropping mechanisms of thermoregulation. One implication is a decrease in body temperature. This is a safety mechanism, which favours hypoxic survival. Indeed, artificial warming during hypoxia can be counterproductive. Because carbon dioxide is an important stimulus for pulmonary ventilation, the drop in its metabolic production may tilt the balance of ventilatory control in favor of respiratory inhibition. Some experimental data support this view. In conditions of sustained hypoxia, the newborn's hypometabolism also results from a depression of tissue growth and differentiation. Some organs are affected more than others. To what extent the blunted organ growth will be compatible with survival depends not only on the severity and duration of hypoxia, but also on the timing of its occurrence during development. Upon termination of hypoxia, the newborn's metabolic rate recovers and growth resumes at higher rate. Even if body weight may be completely regained, alterations in the respiratory mechanical properties and in aspects of ventilatory control can persist into adulthood, a phenomenon not seen when the hypoxia was experienced at later stages of development. Some of the long-term respiratory effects of neonatal hypoxia are reminiscent of those observed in adult animals and humans native and living in high altitude regions.

Thermoregulation in rats during early postnatal maturation: importance of nitric oxide

Experiments were carried out to determine the role of nitric oxide in mediating autonomic and behavioral thermoregulatory control in rat pups on postnatal days 1-2, 5-6, and 10-11. For an experiment, each pup received a subcutaneous injection of vehicle, NG-nitro-d-arginine methyl ester (d-NAME; 100 mg/kg), or NG-nitro-l-arginine methyl ester (l-NAME; 100 mg/kg) before being placed in a metabolic chamber or in a thermocline with a linear temperature gradient of 23 to 43°C. In the metabolic chamber, oxygen consumption and core temperature were measured as ambient temperature was decreased from 40 to 15°C over a 60-min period. Decreasing ambient temperature elicited an increase in oxygen consumption in all age groups that received vehicle or d-NAME. The lower critical temperature and peak oxygen consumption upon exposure to cold after vehicle were 41 ± 10 ml · kg-1 · min-1 at 30°C, 43 ± 12 ml · kg-1 · min-1 at 28°C, and 55 ± 11 ml · kg-1 · min-1 at 25°C in the 1- to 2-, 5- to 6-, and 10- to 11-day-old pups, respectively. Administration of l-NAME abolished the oxygen consumption response to cold in the 1- to 2- and 5- to 6-day-old pups and significantly attenuated the oxygen consumption response to cold in the 10- to 11-day-old pups. Selected ambient temperature in the thermocline was not significantly affected by prior administration of d-NAME or l-NAME compared with vehicle. Thus our data provide evidence that the nitric oxide system plays a role in mediating autonomic but not behavioral thermoregulatory control in rat pups during early postnatal maturation.

Body temperature and heat production in suckling rat endotoxaemia: beneficial effects of glutamine

BACKGROUND/PURPOSE

Sepsis is an important cause of neonatal mortality. The aim of the study was to investigate the metabolism of endotoxic neonatal rats and the potential beneficial effect of glutamine.

METHODS

Suckling rats received intraperitoneal saline (control; C), endotoxin (300 microg/g LPS; E), saline+glutamine (2 mmol/g; CG), endotoxin+glutamine (EG), saline+leucine (2 mmol/g; CL) or endotoxin+leucine (EL). Sepsis score (0-8) and rectal temperature were monitored. Hypothermia was defined as rectal temperature less than 32 degrees C. Oxygen consumption (VO2, mL/kg/h), a determinant of heat production, was measured by indirect calorimetry. Data (mean +/- SEM) were compared by analysis of variance (ANOVA), paired t test or Fisher's Exact test.

RESULTS

Endotoxic (E) rats had significantly lower VO2 than C rats from 90 minutes postinjection to the end of the experiment, 210 minutes (VO2 from 150 to 210 minutes: C 671 +/- 45; E 429 +/- 36, P <.0004; n = 8; paired t test). VO2 of CL or CG rats was elevated between 90 and 210 minutes compared with control, but significantly (P <.01) only in the L group (C 706 +/- 31; CG 871 +/- 63; CL 984 +/- 31; n = 7-9, ANOVA). VO2 was significantly higher (P <.05) in EG rats than E rats (E 460 +/- 29; EG 654 +/- 68; n = 9-10). In the EL group, VO2 was raised but was not significantly different from E (E 460 +/- 29; EL 637 +/- 52; n = 8-10). EG rats were significantly less hypothermic between 90 and 210 minutes (58 of 132 measurements) compared with E (95 of 147; P =.0007, Fisher's Exact test), whereas the EL group were similarly hypothermic (74 of 120) to E (P =.7). Sepsis score was significantly lower in the EG group than both E and EL groups (E 4.9 +/- 0.3; EG 3.6 +/- 0.3; EL 5.0 +/- 0.3; n = 40; P <.01; ANOVA).

CONCLUSIONS

Neonatal endotoxaemia lowers VO2, heat production, and body temperature. Glutamine and leucine both cause nutrient-induced thermogenesis in control animals and restore VO2 of endotoxic animals. Glutamine additionally increases rectal temperature, reduces incidence of hypothermia, and improves clinical signs of endotoxic rats. This suggests that glutamine may be beneficial for nutrition in neonatal sepsis.

Do infant rats cry?

In the current revival of interest in the emotional and mental lives of animals, many investigators have focused attention on mammalian infants that emit distress vocalizations when separated from the home environment. Perhaps the most intensively studied distress vocalization is the ultrasonic vocalization of infant rats. Since its discovery, this vocalization has been interpreted both as a communicatory signal for the elicitation of maternal retrieval and as the manifestation of emotional distress. In contrast, the authors examined the cardiovascular causes and consequences of the vocalization, and on the basis of this work, they hypothesized that the vocalization is the acoustic by-product of the abdominal compression reaction (ACR), a maneuver that results in increased venous return to the heart. Therefore, the vocalization may be analogous to a sneeze, serving a physiological function while incidentally producing sound.

Postnatal age influences the ability of rats to autoresuscitate from hypoxic-induced apnea

Failure to autoresuscitate from apnea by gasping has been suggested to have a role in sudden infant death. Little is known, however, about the factors that influence the ability of gasping to sustain life during acute hypoxia in the newborn. The present experiments were carried out on 105 rat pups to investigate the influence of postnatal age on the time to last gasp during a single hypoxic exposure and on the ability to autoresuscitate from primary apnea during repeated hypoxic exposures. On days 1-2, 5-6, 10-11, 15-16, and 19-20 postpartum, each pup was placed into a temperature-controlled chamber regulated to 37 +/- 1 degrees C and was exposed either to a single period of hypoxia produced by breathing an anoxic gas mixture (97% N(2)-3% CO(2)), and the time to last gasp was determined, or repeated exposure to hypoxia was performed, and the ability to autoresuscitate from primary apnea was determined. Increases in postnatal age decreased the time to last gasp following a single hypoxic exposure and decreased the number of successful autoresuscitations following repeated hypoxic exposures. Thus our data provide evidence that postnatal age influences protective responses that may prevent death during hypoxia as may occur during episodes of prolonged sleep apnea.

Light-dark differences in the effects of ambient temperature on gaseous metabolism in newborn rats

Body temperature (T(b)) of rat pups (7-9 days old) raised under a 12:12-h light-dark (L-D) regimen (L: 0700-1900, D: 1900-0700) was consistently higher in D than in L by approximately 1.1 degrees C. We tested the hypothesis that the L-D differences in T(b) were accompanied by differences in the set point of thermoregulation. Measurements were performed on rat pups at 7-9 days after birth. O(2) consumption (VO(2)) and CO(2) production (VCO(2)) were measured with an open-flow method during air breathing, as ambient temperature (T(a)) was decreased from 40 to 15 degrees C at the constant rate of 0.5 degrees C/min. At T(a) >/=33 degrees C, VO(2) was not significantly different between L and D, whereas VCO(2) was higher in L, suggesting a greater ventilation. Over the 33 to 15 degrees C range the VO(2) values in D exceeded those in L by approximately 30%. Specifically, the difference was contributed by differences in thermogenesis at T(a) = 30 to 20 degrees C. As T(a) was decreased, the critical temperature at which VO(2) began to rise was lower in L. We conclude that the higher T(b) of rat pups in D is accompanied by a higher set point for thermoregulation and a greater thermogenesis. These results are consistent with the idea that, in newborns, endogenous changes in the set point of thermoregulation contribute to the circadian oscillations of T(b).

Postnatal development of thermoregulation in the semiaquatic muskrat (Ondatra zibethicus)

Developmental changes in thermoregulatory ability were followed in 1- to 34-d-old muskrats (Ondatra zibethicus) tested individually and in groups composed of 5 littermates. During their first 10-11 d of life, 20- to 60-g nestlings displayed limited thermogenic ability and could not maintain a stable body temperatures (Tb) during 2 h exposure to an air temperature (Ta) of 10 or 25°C. At 25°C, a Ta approximating nest temperatures in the field, nestlings were homeothermic at ca. 10-11 d of age, when they were fully furred, capable of limited swimming, and within 1-2 d of opening their eyes. The onset of thermoregulation occurred at a body mass (60 g) that was considerably less than predicted on the basis of adult body size. Huddling with littermates reduced the cooling of 20- to 60-g young at a Ta of 10°C, but did not advance the age (mass) at which they became effective homeotherms. Huddling by 20- to 60-g nestlings raised the mean Tb, leading to a substantive (64%) gain in the rate of oxygen consumption (Vo2). Conversely, in older nestlings that were close to or fully homeothermic, huddling with littermates consistently lowered Vo2, the greatest metabolic saving (38%) being realized by 17- to 21-d-old (100-140 g) young. We suggest that the rapid development of thermoregulation may be linked to the semiaquatic habit of muskrats, and that thermoregulatory behavior contributes importantly to the growth and morphological development of this prolific microtine rodent.

Thermoregulatory and cardiac responses of infant spontaneously hypertensive and Wistar-Kyoto rats to cold exposure

Cardiovascular function during cold exposure is dependent on effective thermoregulation. This dependence is particularly apparent in infants. For example, we have previously demonstrated that in infant rats during cold exposure, cardiac rate is directly related to their ability to produce heat endogenously. The primary source of endogenous heat production for infant rats is brown adipose tissue (BAT). Because of the dependence of cardiac rate on effective thermoregulation in the cold and because hypertension in spontaneously hypertensive rats (SHR) is influenced by the preweanling environment, in this study we examined the thermoregulatory and cardiac rate responses of infant SHR and Wistar-Kyoto rats (WKY) to varying levels of cold exposure. In experiment 1, 7- to 8-day-old SHR and WKY were acclimated at a thermoneutral air temperature (35 degrees C) and then exposed to successive decreases in ambient temperature (30.5 degrees C, 26.5 degrees C, 23 degrees C, and 17 degrees C) while thermal and metabolic measures were recorded. Although both strains increased BAT thermogenesis and oxygen consumption in response to cold exposure, SHR cooled more than WKY and exhibited lower levels of oxygen consumption at the lowest air temperatures. Experiment 2 was identical to experiment 1 except that cardiac rate was also measured. Again, SHR exhibited substantial thermoregulatory deficits compared with WKY; in addition, they were less able than WKY to maintain cardiac rate at the 2 lowest air temperatures tested. Finally, in experiment 3, infant SHR exhibited diminished BAT thermogenesis in response to a range of doses of a selective beta3-adrenoceptor agonist. We hypothesize that long-term thermoregulatory deficits during the early postnatal period influence cardiovascular function and contribute to the development of hypertension in SHR.

Development of vasomotor control in lean (+/?) and genetically obese (fa/fa) rat pups

Using laser Doppler flowmetry, we determined whether vasomotor control is already functional in 7-, 11-, and 16-day-old rat pups, and whether differences in vasomotor responses between 16-day-old lean (+/?) and fatty (fa/fa) pups occur. For 3 h, at the end of their daily light phase, the pups were exposed to 30-min step changes of ambient temperature (Ta) alternating between thermoneutrality and cold loads resulting in a 50-100% increase of metabolic rate (MR) above its thermoneutral value, while core temperature (Tc), skin blood flux at the hindpaw, and MR were recorded. In all age groups, blood flux changed significantly in response to these Ta changes. The time needed for a significant change in blood flux was shorter in the 7-day-old than in the 16-day-old lean pups, while the Ta dependency of Tc was more pronounced in the younger pups. Comparison of the responses of the 16-day-old fatty and lean pups showed a parallel shift to lower values in the correlation of Ta with MR in the fa/fa pups, but no difference in the correlation of Ta with blood flux. This demonstrates that the vasomotor control system in the rat is already functional by the age of 7 days, and that the fa/fa lesion influences the vasomotor control less than the metabolic cold defense.

Neonatal thermotaxis improves reversal of a thermally reinforced operant response

One-, 5-, and 11-day-old rats in a cool environment (25 degrees C) acquired an operant response when rewarded with a 20-s-long warming of the platform (from 25 to 36 degrees C) on which they lay. In Experiment 1, the head-turning response was learned by pups at all ages. When the contingency was reversed so that pups were reinforced for turning to the side opposite that correct during training, the original response extinguished for 1-day-olds, but not for 5- or 11-day-olds. In Experiment 2, the rewarded side was randomly selected for each trial. One-day-olds perseverated in turning to the side correct on that trial while the reinforcer remained on, but 5- and 11-day-old rat pups did not. We conclude that 1-day-old pups were more responsive to the change in experimental contingency in Experiment 1 due to this thermotaxic behavior.

Medium-frequency oscillations dominate the inspiratory nerve discharge of anesthetized newborn rats

In this study we examined the synchronization of the discharge of phrenic and recurrent laryngeal motoneurons in anesthetized rat pups 14 to 36 days of age and kittens, 14-15 days old. We found that the inspiratory nerve activity consisted of synchronized bursts separated by 20-35 ms, corresponding to medium-frequency oscillations (MFO). Accordingly, the autospectra of the neurograms had two peaks, one at the respiratory rate and the other between 22. 8-43.0 Hz. No significant coherence was found between MFOs in the discharges of different nerves. High-frequency oscillations (HFO) characteristic for the adult inspiratory nerve activity were not present in the newborn rats. These findings demonstrate that phrenic nerve discharge of rat pups, like that of kittens and piglets, is in the MFO range, and suggest that MFO activity is an index of an early developmental stage of the respiratory system.

Leptin disinhibits nonshivering thermogenesis in infants after maternal separation

Prolonged maternal separation inhibits endogenous heat production in infant mammals exposed to cold. This inhibition of thermogenesis occurs many hours before energy stores have been fully depleted. The need to protect energy resources during separation-induced starvation may be signaled by declining levels of leptin, a hormone that acts as a "fat signal" and a regulator of energy utilization; in fact, starvation reduces leptin levels in adult mice and infant rats. It is not known, however, whether leptin has a functional role during starvation in infants. Such a role may be found in the regulation of nonshivering thermogenesis by brown adipose tissue (BAT), a specialized organ that provides heat to infant mammals, including humans, during cold exposure. Heat produced by BAT allows the cold-exposed infant to prevent the detrimental effects of hypothermia on physiology and behavior and, ultimately, growth. Here we show that leptin disinhibits BAT thermogenesis during cold exposure in infant rats after 18 h of maternal separation. This finding demonstrates that leptin is more than simply an adipostat for the regulation of body weight; specifically, leptin modulates thermogenesis and energy utilization in the early postnatal period.

Association of chronic sublethal hypoxia with ventriculomegaly in the developing rat brain

Bronchopulmonary dysplasia remains a major cause of neurodevelopmental handicap in preterm infants. Because bronchopulmonary dysplasia may be associated with prolonged hypoxemia without obvious changes in systemic blood pressure, we developed an animal model of chronic sublethal hypoxia to test the hypothesis that this insult results in significant alterations in corticogenesis in the developing brain. Three groups of newborn rats were placed in a chamber with FIO2 9.5% on postnatal day 3 (P3). One group was sacrificed at P13; a second group was sacrificed at P33, and the third group was removed at P33 and reared in normoxia until sacrifice at P63. Control rats were those raised in room air for the corresponding periods of time. Rats were transcardially perfused and the brains were embedded in celloidin and prepared for morphometric analysis using standard stereology methods. Although experimental rat pups in the third group demonstrated 'catch-up' of body weight following return to normoxia, these studies demonstrated both failure of brain growth (p<0.01) and progressive cerebral ventriculomegaly (p<0.01). Decreased subcortical white matter (p<0. 05) and corpus callosum size (p<0.01) were noted at P63 in pups reared under conditions of chronic hypoxia. Decreases in cortical volume (p<0.05) were noted at all three experimental time points for hypoxic-reared pups when compared to control animals. These data suggest that chronic sublethal hypoxia may lead to severe impairments in corticogenesis in an animal model of developing brain.

Oxidative activity in mitochondria isolated from rat liver at different stages of development

The purpose of this study was to evaluate the oxidative capacities in hepatic mitochondria isolated from prepubertal, young adult and adult rats (40, 90 and 180 days of age, respectively). In these rats, mitochondrial respiratory rates using FAD- and NAD-linked substrates as well as mitochondrial protein mass were measured. The results show that only the oxidative capacity of FAD-linked pathways significantly declined in mitochondria from 180-day-old rats compared with those from younger animals. When we consider FAD-linked respiration expressed per g liver, no significant difference was found among rats of different ages because of an increased mitochondrial protein mass found in 180-day-old rats. However, when FAD-linked and lipid-dependent respiratory rates were expressed per 100 g body weight, significant decreases occurred in 180-day-old rats. Therefore, the decrease in liver weight expressed per 100 g body weight rather than an impaired hepatic cellular activity may be the cause of body energy deficit in 180-day-old rats.

Thermoregulatory competence and behavioral expression in the young of altricial species--revisited

The behavioral and physiological thermoregulatory capabilities of newborn and infant mammals have been studied for over half a century. Psychobiologists have noted that the infants of altricial species (e.g., rats) have physical and physiological limitations such that heat loss overwhelms heat production, thus forcing a reliance on behavioral thermoregulation for the maintenance of body temperature. Recent evidence, however, suggests that a modification of this view is justified. Specifically, throughout a range of moderately cold air temperatures, nonshivering thermogenesis by brown adipose tissue contributes significantly to the infant rat's behavioral and physiological adaptations to cold challenge. Given the prominent use of altricial species for the study of infant behavior, increased understanding of the infant's physiological responses to cold and the effect of thermal factors on behavior is warranted.

Thermogenesis in newborn rats after prenatal or postnatal hypoxia

Oxygen consumption (VO2) was measured in normoxia as ambient temperature (Ta) was lowered from 40 to 15 degrees C, at the rate of 0.5 degrees C/min (thermoneutrality approximately 33 degrees C). In 2-day-old rats born in hypoxia after hypoxic gestation, the Ta-VO2 relationship was as in controls; their interscapular brown adipose tissue (IBAT) was hypoplastic (less proteins and DNA), with lower concentration of the mitochondrial uncoupling protein thermogenin. In 8-day-old rats exposed to hypoxia postnatally (day 2 to day 8), at any Ta below thermoneutrality VO2 was higher than in controls; also, in this group IBAT was hypoplastic with decreased thermogenin. Additional measurements under various experimental conditions indicated that the increased thermogenic capacity was not explained by the smaller body mass and increased blood oxygen content or by the eventuality of intermittent cold stimuli during the chronic hypoxia. On the other hand, chronic hypercapnia (3% CO2 in normoxia, from day 2 to day 8) also resulted in increased normoxic thermogenesis. We conclude that chronic hypoxia in the perinatal period 1) reduces IBAT mass and thermogenin concentration and 2) can increase the newborn's thermogenic capacity because of stress-related mechanisms not specific to hypoxia.

Acute, early thermal experience alters weaning onset in rats

We hypothesized that first ingestion of solid food (weaning onset) would be accelerated in young rats with advanced thermoregulatory development. To manipulate the pups' thermoregulatory development, we exposed rat pups, but not their dams, to a Cold (10 degrees C), Moderate (21 degrees C), or Warm (31 degrees C) ambience for 2 h/day from postnatal Day 2-14, expecting that early exposure to cooler temperatures would accelerate development of thermoregulatory capabilities and thus accelerate nest egression as well as onset of feeding. Contrary to expectation, cold exposure was associated with a profile of developmental delays in both growth and maturation. Pups in the Cold condition began feeding later than pups with Moderate or Warm thermal experiences. We then evaluated thermoregulatory status (mechanisms for heat production and temperature conservation) on Day 15-16 (just prior to weaning onset). Thermogenesis, measured by oxygen consumption, was unaltered by the thermal manipulation. In contrast, pelage development (insulation) was altered. Pups in the Warm condition had greater fur density and an increased frequency of longer hairs relative to pups in the Cold condition. Although the developmental response to early cold exposure was in the direction opposite to our predictions, the hypothesized relation of thermoregulatory development to weaning onset was supported: Thermoregulatory status correlated with weaning onset.

Effects of temperature on sleep in the developing rat

In altricial species, such as humans and rats, much of the development of autonomic systems occurs postnatally. Consequently, vulnerabilities exist early in postnatal development when immature autonomic functions are challenged by external factors such as variations in ambient temperature (Ta). Ta profoundly influences sleep/wake state structure in adult animals and humans, and exposure to excessive warmth has been implicated as a risk factor in sudden infant death syndrome. To better understand the relationship between temperature and sleep during development, we investigated the effect of Ta variation on sleep/wake state structure and sleep intensity in developing rats. In this experiment, sleep intensity was measured by the intensity of slow-wave activity during slow-wave sleep. Neonatal Long-Evans hooded rat pups were surgically prepared for chronic sleep/wake state and brain temperature (Tbr) recording. Two-hour recordings of sleep/wake state and Tbr were obtained from rats on postnatal day 12 (P12), P14, P16, P18, and P20 at a Ta of either 28.0-30.0, 33.0-35.0, or 38.0-40.0 degrees C. Ta significantly influenced sleep/wake state structure but had little, if any, effect on sleep intensity in developing rats.

Oxygen transport in conscious newborn dogs during hypoxic hypometabolism

We questioned whether the decrease in O2 consumption (VO2) during hypoxia in newborns is a regulated response or reflects a limitation in O2 availability. Experiments were conducted on previously instrumented conscious newborn dogs. VO2 was measured at a warm ambient temperature (30 degrees C, n = 7) or in the cold (20 degrees C, n = 6), while the animals breathed air or were sequentially exposed to 15 min of fractional inspired O2 (FIO2): 21, 18, 15, 12, 10, 8, and 6%. In normoxia, VO2 averaged 15 +/- 1 (SE) and 25 +/- 1 ml . kg-1 . min-1 in warm and cold conditions, respectively. In the warm condition, hypometabolism (i.e., hypoxic VO2 < normoxic VO2) occurred at FIO2 </=10%, whereas in the cold condition, hypometabolism occurred at FIO2 </=12%. The same results were obtained in a separate group (n = 14) of noninstrumented puppies. For all levels of FIO2 with hypometabolism, the relationships between measures of O2 availability (arterial O2 saturation or content, venous PO2 or saturation, x-axis) vs. VO2 (y-axis) had lower slopes in warm than in cold conditions. Hence, VO2 during hypometabolism in the warm condition was not the maximal attainable for the level of oxygenation. The results do not support the possibility that the hypoxic drop in VO2 in the newborn reflects a limitation in O2 availability. The results are compatible with the idea that the phenomenon is one of "regulated conformism" to hypoxia.

The development of the ventilatory response to cold in very young rats

To assess the range of functional responses of the ventilatory apparatus of developing rats and the degree to which ventilatory function is developed in advance of other functional characteristics, rat pups at five ages (between 4 and 20 days old) were exposed to temperatures of 28, 32 and 36 degrees C while in a flow through metabolic chamber modified to serve as a whole body plethysmograph. Ventilatory frequency, tidal volume and oxygen extraction 'efficiency' (EO2 = VO2/FEO2 x VI) were measured at each age and temperature. Mean breathing frequency at 4 days old was 2.56 breaths per second, decreasing to 1.99 at 20 days old. There was insignificant modification of breathing frequency with temperature. Four day old rat pups at 28 degrees C had mass specific tidal volumes of 0.017 ml/g, 142% of the value at 36 degrees C (0.012 ml/g). Twenty day old pups at 28 degrees C had mass specific tidal volumes of 0.027 ml/g, also 142% of the thermoneutral value (0.019 ml/g at 32 degrees C). At all ages, increases in tidal volumes were similar and increases in tidal volume were the only response to increased metabolic demand. Oxygen extraction 'efficiency' was about half that previously observed in adult rodents. These observations of ventilation during a cold challenge suggest that although structural development is not complete until much later, functional development is sufficient, either at birth or shortly thereafter.