Thermal and respiratory control in young rats exposed to cold during postnatal development

Growing underground: Development of thermogenesis in pups of the fossorial rodent Ctenomys talarum

In mammals, during the pup’s development and adult life, integrated requirements of all activities of the individual must conform to a sustained rate of metabolism. Thus, partitioning the available energy according to short-term priorities at a specific moment allows animals to survive and optimize long-term reproductive success. In altricial rodents, thermal balance is a key factor for survival. When no exogenous source of heat is present, altricial pups rapidly lose heat, reaching ambient temperature (Ta). Fossorial rodents showed a strong dependence on burrows, where Ta remains relatively stable within narrow ranges. Pups of the fossorial rodent Ctenomys talarum are altricial, making them an excellent model to evaluate the development of thermogenic capacity. In this study, the ontogeny of the thermogenic capacity of pups of C. talarum was evaluated. Using respirometry techniques, non-shivering thermogenesis (NST), total thermogenic capacity (cold-induced maximum metabolic rate, MMR), and resting metabolic rate (RMR) in pups until post-weaning age (day 60) were analyzed. No NST was present in pups until day 60 despite the presence of molecular markers for NST in brown adipose tissue deposits, which became functional in adults. Although pups are altricial at birth, they maintain their thermal balance behaviorally during lactation. Total thermogenic capacity became fixed at an early age, indicating an improvement in shivering thermogenesis (ST) efficiency after day 10, which might be related to the development of musculature related to digging. Before the aboveground dispersal period (~day 60), pups gradually reached adult Tb by improving ST and thermal isolation, allowing them to confront climatic fluctuations on the surface.

Developmental Nicotine Exposure Alters Synaptic Input to Hypoglossal Motoneurons and Is Associated with Altered Function of Upper Airway Muscles

Nicotine exposure during the fetal and neonatal periods [developmental nicotine exposure (DNE)] is associated with ineffective upper airway protective reflexes in infants. This could be explained by desensitized chemoreceptors and/or mechanoreceptors, diminished neuromuscular transmission or altered synaptic transmission among central neurons, as each of these systems depend in part on cholinergic signaling through nicotinic AChRs (nAChRs). Here, we showed that DNE blunts the response of the genioglossus (GG) muscle to nasal airway occlusion in lightly anesthetized rat pups. The GG muscle helps keep the upper airway open and is innervated by hypoglossal motoneurons (XIIMNs). Experiments using the phrenic nerve-diaphragm preparation showed that DNE does not alter transmission across the neuromuscular junction. Accordingly, we used whole cell recordings from XIIMNs in brainstem slices to examine the influence of DNE on glutamatergic synaptic transmission under baseline conditions and in response to an acute nicotine challenge. DNE did not alter excitatory transmission under baseline conditions. Analysis of cumulative probability distributions revealed that acute nicotine challenge of P1–P2 preparations resulted in an increase in the frequency of nicotine-induced glutamatergic inputs to XIIMNs in both control and DNE. By contrast, P3–P5 DNE pups showed a decrease, rather than an increase in frequency. We suggest that this, together with previous studies showing that DNE is associated with a compensatory increase in inhibitory synaptic input to XIIMNs, leads to an excitatory-inhibitory imbalance. This imbalance may contribute to the blunting of airway protective reflexes observed in nicotine exposed animals and human infants.

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.

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.

Ventilatory response to hypoxia of the 1-day old chicken hatchling after prenatal cold-induced hypometabolism

Sustained prenatal hypoxia decreases the growth and metabolic rate of the embryo and causes a blunted hypoxic ventilatory response (HVR) in the newborn. The most likely interpretation is that the sustained hypoxic stimulation may interfere with the normal prenatal development of the chemoreceptors. However, we wanted to consider the possibility that the prolonged hypoxic hypometabolism may be a contributing factor. Chicken embryos were incubated at 35°C (Cold group, N=14), which is known to lower the embryonic oxygen consumption (VO2) by ≈ 30% throughout incubation, or at 37.5°C (Controls, N=16). Cold incubation delayed hatching by ≈ 2 days. The 1-day old hatchlings had normal pulmonary ventilation (VE), measured by the barometric technique, and oxygen consumption (VO2), simultaneously measured by an open flow methodology. During acute hypoxia (≈ 15% or ≈ 11% O2) the hyperventilation (increase in VO2), the hyperpnea and the hypometabolism were almost identical between the two groups of hatchlings. We conclude that a sustained decrease in metabolic rate during the embryonic period by itself does not carry obvious consequences on the newborn's resting VE and HVR.

Thermoneutrality modifies the impact of hypoxia on lipid metabolism

Hypoxia has been shown to rapidly increase triglycerides in mice by decreasing plasma lipoprotein clearance. However, the usual temperature of hypoxic exposure is below thermoneutrality for mice, which may increase thermogenesis and energy requirements, resulting in higher tissue lipid uptake. We hypothesize that decreased lipid clearance and ensuing hyperlipidemia are caused by hypoxic suppression of metabolism at cold temperatures and, therefore, would not occur at thermoneutrality. Twelve-week-old, male C57BL6/J mice were exposed to 6 h of 10% O₂ at the usual temperature (22°C) or thermoneutrality (30°C). Acclimation to 22°C increased lipid uptake in the heart, lungs, and brown adipose tissue, resulting in lower plasma triglyceride and cholesterol levels. At this temperature, hypoxia attenuated lipid uptake in most tissues, thereby raising plasma triglycerides and LDL cholesterol. Thermoneutrality decreased tissue lipid uptake, and hypoxia did not cause a further reduction in lipid uptake in any organs. Consequently, hypoxia at thermoneutrality did not affect plasma triglyceride levels. Unexpectedly, plasma HDL cholesterol increased. The effect of hypoxia on white adipose tissue lipolysis was also modified by temperature. Independent of temperature, hypoxia increased heart rate and glucose and decreased activity, body temperature, and glucose sensitivity. Our study underscores the importance of ambient temperature for hypoxia research, especially in studies of lipid metabolism.

Age and sex differences in the ventilatory response to hypoxia and hypercapnia in awake neonatal, pre-pubertal and young adult rats

There is evidence for a "sensitive period" in respiratory development in rats around postnatal age (P) 12-13d. Little is known about sex differences during that time. The purpose of this study was to assess the effect of sex on breathing development, specifically around the "sensitive period". We used whole-body plethysmography to study breathing in normoxic, hypoxic and hypercapnic gases in non-anesthetized male and female neonatal rats from P10 to P15, juvenile (P30) and young adult (P90) rats. Compared to other neonatal ages, P12-13 male rats had significantly lower ventilation during normoxia, hypoxia, and hypercapnia. Compared to age-matched females, P12-13 male rats had lower ventilation in normoxia and hypoxia and a lower O(2) saturation during hypoxia. Circulating estradiol was greater in P12-13 male vs. female rats. Estradiol and ventilatory responses to hypoxia and hypercapnia were negatively correlated in neonatal male, but not female rats. Our results suggest that P10-15 includes a critical developmental period in male but not female rats.

Postnatal development of metabolic rate during normoxia and acute hypoxia in rats: implication for a sensitive period

Previously, we reported that the hypoxic ventilatory response (HVR) in rats was weakest at postnatal day (P) P13, concomitant with neurochemical changes in respiratory nuclei. A major determinant of minute ventilation (Ve) is reportedly the metabolic rate [O(2) consumption (Vo(2)) and CO(2) production (Vco(2))]. The present study aimed at testing our hypothesis that daily metabolic rates changed in parallel with ventilation during development and that a weak HVR at P13 was attributable mainly to an inadequate metabolic rate in hypoxia. Ventilation and metabolic rates were monitored daily in P0-P21 rats. We found that 1) ventilation and metabolic rates were not always correlated, and Ve/Vo(2) and Ve/Vco(2) ratios were not constant during development; 2) metabolic rate and Ve/Vo(2) and Ve/Vco(2) ratios at P0-P1 were significantly different from the remaining first postnatal week in normoxia and hypoxia; 3) at P13, metabolic rates and Ve/Vo(2) and Ve/Vco(2) ratios abruptly increased in normoxia and were compromised in acute hypoxia, unlike more stable trends during the remaining second and third postnatal weeks; and 4) the respiratory quotient (Vco(2)/Vo(2)) was quite stable in normoxia and fluctuated slightly in hypoxia from P0 to P21. Thus our data revealed heretofore unsuspected metabolic adjustments at P0-P1 and P13. At P0-P1, ventilation and metabolic rates were uncorrelated, whereas at P13, they were closely correlated under normoxia and hypoxia. The findings further strengthened the existence of a critical period of respiratory development around P13, when multiple physiological and neurochemical adjustments occur simultaneously.

Long-term effects of the perinatal environment on respiratory control

The respiratory control system exhibits considerable plasticity, similar to other regions of the nervous system. Plasticity is a persistent change in system behavior triggered by experiences such as changes in neural activity, hypoxia, and/or disease/injury. Although plasticity is observed in animals of all ages, some forms of plasticity appear to be unique to development (i.e., “developmental plasticity”). Developmental plasticity is an alteration in respiratory control induced by experiences during “critical” developmental periods; similar experiences outside the critical period will have little or no lasting effect. Thus complementary experiments on both mature and developing animals are generally needed to verify that the observed plasticity is unique to development. Frequently studied models of developmental plasticity in respiratory control include developmental manipulations of respiratory gas concentrations (O2and CO2). Environmental factors not specifically associated with breathing may also trigger developmental plasticity, however, including psychological stress or chemicals associated with maternal habits (e.g., nicotine, cocaine). Despite rapid advances in describing models of developmental plasticity in breathing, our understanding of fundamental mechanisms giving rise to such plasticity is poor; mechanistic studies of developmental plasticity are of considerable importance. Developmental plasticity may enable organisms to “fine tune” their phenotype to optimize the performance of this critical homeostatic regulatory system. On the other hand, developmental plasticity could also increase the risk of disease later in life. Future directions for studies concerning the mechanisms and functional implications of developmental plasticity in respiratory motor control are discussed.

Hypoxic incubation blunts the development of thermogenesis in chicken embryos and hatchlings

We asked to what extent sustained hypoxia during embryonic growth might interfere with the normal development of thermogenesis. White Leghorn chicken eggs were incubated at 38°C either in normoxia (Nx, 21% O2) or in hypoxia [Hx, 15% O2, from embryonic day 5 (E5) until hatching]. The Hx embryos had lower body weight (W) throughout incubation, and hatching was delayed by about 10 h. For both groups, all measurements were conducted in normoxia. At embryonic day E11, the static temperature-oxygen consumption (ambient T-V̇o2) curve was typically ectothermic (Q10 = 1.92–1.94) and similar between Nx and Hx. Toward the end of incubation (E20), the Q10 averaged 1.41 ± 0.06 in Nx and 1.79 ± 0.08 in Hx ( P < 0.005), indicating that the onset of the thermogenic response in Hx lagged behind Nx. In the 1-day-old hatchlings (H1), body weight did not significantly differ between Nx and Hx. At H1, the T-V̇o2 curves were endothermic-type, and more so in the older (>8 h old) than in the newly hatched (<8 h old) chicks, whether examined statically or dynamically as a function of time. In either case, the thermogenic responses of Hx were lower than those of Nx. In a 43–31°C thermocline, the preferred T of the Hx hatchlings was around 37.3°C, and similar to Nx, suggesting a similar setpoint for thermoregulation. We conclude that hypoxic incubation blunted the development of thermogenesis. This could be interpreted as an example of epigenetic regulation, in which an environmental perturbation during early development alters the phenotypic expression of a regulatory system.

Metabolic response to cooling temperatures in chicken embryos and hatchlings after cold incubation

We asked to what extent cold exposure during embryonic growth, and the accompanying hypometabolism, may interfere with the normal development of thermogenesis. White Leghorn chicken eggs were incubated in control conditions (38 degrees C) or at 36 or 35 degrees C. Embryos incubated at a lower temperature (34 degrees C) failed to hatch. The cold-incubated embryos had lower oxygen consumption (VO2) and body weight (W) throughout incubation, and hatching was delayed by about, respectively, 1 and 2 days. The W-VO2 relationship of the cold-incubated embryos was as in controls, indicating that cold-induced hypometabolism was at the expense of the growth, not the maintenance, component of VO2. At embryonic day E11, the metabolic response to changes in ambient temperature (T) over the 30-39 degrees C range was typically poikilothermic, with Q10 = 1.8-1.9, and similar among all sets of embryos. Toward the end of incubation (E20), the thermogenic responses of the cold-incubated embryos were significantly lower than in controls. This difference occurred also in the few-hour old hatchlings (H1), even though, at this time, W was similar among groups. Exposure to cold during only the last 3 days of incubation (from E18 to H1), i.e. during the developmental onset of the endothermic mechanisms, did not lower the thermogenic capacity of the hatchlings. In conclusion, sustained cold-induced hypometabolism throughout incubation blunted the rate of embryonic growth and the development of thermogenesis. This latter phenomenon could be an example of epigenetic regulation, i.e. of environmental factors exerting a long-term effect on gene expression.

Late development of homoeothermy in mink (Mustela vison) kits - a strategy for maximum survival rate*

The objective of this study was to establish the age at which mink kits develop functional homoeothermy. The investigation was based on the hypothesis that in this species with very immature neonates, late development of homoeothermy may be an adaptation to economize with energy. Measurements of heat production (HE) by means of indirect calorimetry lasting 3 h were performed on neonatal kits and kits from 1 to 54 days of age. Both single kits and groups of 4-5 huddling kits were kept at 15 degrees C (L) or 30 degrees C (H) [from 35 days onwards at 25 degrees C (H)]. Animals were weighed before and after the experiments and evaporative water losses (EWL) were calculated. When exposed to L temperature, single kits responded with a very low HE until 29 days of age, and groups of kits until 14 days of age. It was not until they reached an age of approximately 6 weeks that single kits showed a clear thermoregulatory response to the L temperature by increased HE, whereas groups of kits showed increased HE from 29th day onwards. When kept at H temperature, HE was low initially, but all kits showed elevated HE at 8 days of age, and the metabolic rate was similar for single kits and kits huddling in groups. Evaporative water losses was higher among single than among groups of kits and slightly lower but more variable for animals at L than at H temperature. It was concluded that mink kits develop functional homoeothermy at an age of close to 6 weeks and that the failure of very young kits to thermoregulate is an adaptation mechanism in order to economise with their very limited body energy reserves.

Influence of temperature on metabolism and breathing during mammalian ontogenesis

A literature survey of the ventilatory responses to changes in ambient temperature (T) in neonatal mammals reveals that, as in adults, the metabolic response to T is the major determining factor. In fact, the newborn's metabolic response to changes in T determines not only the pulmonary ventilation and the breathing pattern, but also the magnitude of the ventilatory responses to chemical stimuli and the intensity of the pulmonary reflexes at different T. The important difference from the adult is that in many neonatal mammals the control of body temperature (T(b)) is poorly developed. Hence, the metabolic response can be more similar to that of an ectothermic, rather than endothermic, animal, and T(b) can vary substantially with T. When hypoxia occurs in cold, T(b) can decrease greatly, because of the hypoxic drop in the thermoregulatory set-point, and this lowers pulmonary ventilation. Hence, in addition to the metabolic response, also the changes in T(b) are a factor modulating the ventilatory responses to T. Artificial warming of the newborn during hypoxia causes heat-dissipation responses that can be counterproductive. During ontogenesis, with prolonged cold conditions, the sustained alterations in metabolic rate and body growth do not modify the postnatal development of the respiratory control mechanisms. Presumably, this indicates that respiratory regulation develops independently from the individual's metabolic history.

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.