Effects of rearing temperature on sympathoadrenal activity in young adult rats

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.

Cold Exposure Drives Weight Gain and Adiposity following Chronic Suppression of Brown Adipose Tissue

Therapeutic activation of thermogenic brown adipose tissue (BAT) may be feasible to prevent, or treat, cardiometabolic disease. However, rodents are commonly housed below thermoneutrality (~20 °C) which can modulate their metabolism and physiology including the hyperactivation of brown (BAT) and beige white adipose tissue. We housed animals at thermoneutrality from weaning to chronically supress BAT, mimic human physiology and explore the efficacy of chronic, mild cold exposure (20 °C) and β3-adrenoreceptor agonism (YM-178) under these conditions. Using metabolic phenotyping and exploratory proteomics we show that transfer from 28 °C to 20 °C drives weight gain and a 125% increase in subcutaneous fat mass, an effect not seen with YM-178 administration, thus suggesting a direct effect of a cool ambient temperature in promoting weight gain and further adiposity in obese rats. Following chronic suppression of BAT, uncoupling protein 1 mRNA was undetectable in the subcutaneous inguinal white adipose tissue (IWAT) in all groups. Using exploratory adipose tissue proteomics, we reveal novel gene ontology terms associated with cold-induced weight gain in BAT and IWAT whilst Reactome pathway analysis highlights the regulation of mitotic (i.e., G2/M transition) and metabolism of amino acids and derivatives pathways. Conversely, YM-178 had minimal metabolic-related effects but modified pathways involved in proteolysis (i.e., eukaryotic translation initiation) and RNA surveillance across both tissues. Taken together these findings are indicative of a novel mechanism whereby animals increase body weight and fat mass following chronic suppression of adaptive thermogenesis from weaning. In addition, treatment with a B3-adrenoreceptor agonist did not improve metabolic health in obese animals raised at thermoneutrality.

Reducing Adiposity in a Critical Developmental Window Has Lasting Benefits in Mice

Although most adults can lose weight by dieting, a well-characterized compensatory decrease in energy expenditure promotes weight regain more than 90% of the time. Using mice with impaired hypothalamic leptin signaling as a model of early-onset hyperphagia and obesity, we explored whether this unfavorable response to weight loss could be circumvented by early intervention. Early-onset obesity was associated with impairments in the structure and function of brown adipose tissue mitochondria, which were ameliorated by weight loss at any age. Although decreased sympathetic tone in weight-reduced adults resulted in net reductions in brown adipose tissue thermogenesis and energy expenditure that promoted rapid weight regain, this was not the case when dietary interventions were initiated at weaning. Enhanced energy expenditure persisted even after mice were allowed to resume overeating, leading to lasting reductions in adiposity. These findings reveal a time window when dietary interventions can produce metabolic improvements that are stably maintained.

Developmental influences on circuits programming susceptibility to obesity

Suboptimal maternal nutrition exerts lasting impacts on obesity risk in offspring, but the direction of the effect is determined by the timing of exposure. While maternal undernutrition in early pregnancy is associated with increased body mass index, in later pregnancy it can be protective. The importance of the timing of maternal undernutrition is also observed in rodents, however, many of the processes that occur in the last trimester of human gestation are delayed to the postnatal period. Neonatal leptin administration exerts lasting impacts on susceptibility to obesity in rodents. Although leptin can influence the formation of hypothalamic circuits involved in homeostatic control of feeding during the postnatal period, these effects are too late to account for its ability to reverse adverse metabolic programming due to early gestational exposure to maternal undernutrition. This review presents an alternative framework for understanding the effects of neonatal leptin through influences on developing thermoregulatory circuits.

Pre- and post-weaning cold exposure does not lead to an obese phenotype in adult Brandt's voles (Lasiopodomys brandtii)

Evidence has shown that postnatal undernutrition, overnutrition and cold stress are associated with imbalanced metabolic regulation as rodents achieve adulthood. In this study, we used a breeding colony of Brandt's voles (Lasiopodomys brandtii), a wild rodent species from the Inner Mongolia grasslands in China, to examine the effects of pre- and post-weaning cold exposure on the adult body (fat) mass, serum hormones and hypothalamic neuropeptides. Unlike laboratory rodents, vole offspring exposed to pre-weaning cold did not exhibit overweight or obese phenotypes in adulthood compared with unexposed controls. Moreover, adult male voles that remained in colder conditions had less body mass and lower serum leptin levels despite having higher food intake compared to other groups. To understand the mechanism of this unexpected regulation, hypothalamic gene expression was assessed for pre- and post-weaning cold exposure. Voles exposed to cold before weaning increased hypothalamic, orexigenic agouti-related protein (AgRP) and decreased anorexigenic proopiomelanocortin (POMC) mRNA expression at weaning. These expression changes were associated with hyperphagia and catch-up growth after weaning. Interestingly, these changes in hypothalamic neuropeptides were short lasting because in adult voles these differences were no longer apparent, which might explain why the pre-weaning, cold-exposed voles did not become obese in adulthood. These data suggest that some species do not develop an obese phenotype in response to early life cold stress.

Reduced cardiac contractile force due to sympathovagal dysfunction mediates the additive hypotensive effects of limited-access regimens of ethanol and clonidine in spontaneously hypertensive rats

Our previous attempts to investigate the long-term hemodynamic interaction between ethanol and clonidine in telemetered spontaneously hypertensive rats (SHRs) were hampered by the lack of a sustained hypotensive response to continuous clonidine exposure. This limitation was circumvented when we adopted a limited-access clonidine (8:30 AM-4:30 PM) paradigm in a recent study. The latter paradigm was employed here to evaluate the ethanol-clonidine interaction and possible roles of myocardial function and autonomic control in this interaction. Changes in blood pressure (BP), heart rate, maximum rate of rise in BP wave (+dP/dt(max)), and spectral cardiovascular autonomic profiles were measured by radiotelemetry in pair-fed SHRs receiving clonidine (150 μg/kg/day), ethanol [2.5% (w/v)], or their combination during the day for 12 weeks. Ethanol or clonidine elicited long-term decreases in BP, and their combination caused additive hypotensive response. Significant reductions in +dP/dt(max) were observed upon concurrent treatment with ethanol and clonidine, in contrast to no effect for individual treatment. In addition, the combined treatment increased the high-frequency (HF) spectral band of interbeat interval (IBI-HF(nu), 0.75-3 Hz) and decreased low-frequency (IBI-LF(nu), 0.2-0.75 Hz) bands and IBI(LF/HF) ratios. Clonidine-evoked reductions in plasma and urine norepinephrine and BP-LF spectral power (measure of vasomotor sympathetic tone) were not affected by ethanol. In conclusion, concurrent treatment with ethanol and clonidine shifts the sympathovagal balance toward parasympathetic dominance and elicits exaggerated hypotension as a result of a reduction in cardiac contractile force.

Role of Myocardial Contractility and Autonomic Control in the Hypotensive Response to a Limited Access Ethanol Paradigm in SHRs

BACKGROUND

Previous experimental studies that evaluated the chronic hemodynamic effect of ethanol employed the continuous exposure protocol of ethanol, which does not mimic the pattern of alcohol consumption in humans. This study dealt with the long-term hemodynamic and cardiovascular autonomic effects of ethanol, in a limited-access regimen in telemetered spontaneously hypertensive rats (SHRs).

METHODS

Changes in blood pressure (BP), heart rate (HR), myocardial contractility (dP/dt(max)), and spectral cardiovascular autonomic profiles during the ethanol exposure period (2.5 or 5% w/v, 8 h/d, 8:30 am till 4:30 pm) were followed for 12 weeks.

RESULTS

Compared with control pair-fed SHRs, body weight and urine output, osmolality, and potassium levels were decreased in SHRs receiving 5% but not 2.5% ethanol. Blood pressure showed progressive falls during ethanol-feeding periods with a maximum effect observed at week 5. The peak hypotensive effect was maintained thereafter in SHRs receiving 5% ethanol in contrast to steady rises in BP in the 2.5% ethanol group to near-control levels by the conclusion of the study. Heart rate was slightly but significantly increased by ethanol 5% whereas dP/dt(max) showed persistent reductions. Power spectral analysis showed that ethanol attenuated the baroreflex gain of HR as suggested by the reductions in index alpha, the spectral index of spontaneous baroreflex sensitivity (BRS).

CONCLUSIONS

It is concluded that limited access ethanol drinking in SHRs elicited hypotension that was concentration dependent and mediated, at least partly, through reductions in myocardial contractility. Baroreflex sensitivity attenuation by ethanol appeared to have limited the tachycardic response to ethanol and perhaps its capacity to offset the evoked hypotension.

Intermittent Clonidine Regimen Abolishes Tolerance to Its Antihypertensive Effect: A Spectral Study

The development of tolerance to the antihypertensive effect of clonidine and related imidazolines is clinically recognized. Here, we employed a restricted daytime (8:30 AM until 4:30 PM) clonidine regimen to establish a model of sustained hypotension in spontaneously hypertensive rats (SHRs). Blood pressure (BP), heart rate (HR), and myocardial contractility (dP/dt(max)) were measured by radiotelemetry in pair-fed SHRs receiving liquid diets with or without clonidine (150 microg/kg per day) for 12 weeks. The cardiovascular autonomic control was assessed by power spectral analysis [fast Fourier transformations (FFT)] of hemodynamic variability. Clonidine had no effect on dP/dt(max) and significantly decreased BP and HR during the 8 hour exposure periods throughout the study duration. BP returned to control levels during overnight periods, with no signs of rebound hypertension. FFT analysis of interbeat intervals (IBI) showed pronounced decreases and increases of spectral powers in low-frequency (IBI-LF, 0.20-0.75 Hz) and high-frequency (IBI-HF, 0.75-3 Hz) bands, respectively, in clonidine-treated rats. The IBI(LF/HF) ratio was significantly reduced by clonidine, suggesting cardiac parasympathetic dominance. Clonidine also decreased the vasomotor sympathetic tone, as reflected by the reduced BP-LF spectral density. The sympathoinhibitory effect of clonidine is further confirmed by the significant reductions in urinary norepinephrine levels. Clonidine increased urine output during the 8 hour treatment period but not during the 24 hour period. Plasma and urine osmolality and electrolytes were not altered by clonidine. It is concluded that by adopting the limited-access paradigm, tolerance to the hypotensive and sympathoinhibitory actions of clonidine and, possibly, its side effects, could be minimized.

Individual differences in glucose homeostasis: do our early life interactions with bacteria matter?

Exposure to endotoxin during the neonatal period in the rat has been shown to alter the development of the hypothalamic-pituitary-adrenal axis, inducing hyper-responsivity and increased glucocorticoid production in later-life. Glucocorticoids are known to have major metabolic effects, therefore, early life endotoxin exposure may have potentially serious consequences for metabolic homeostasis in the exposed animal. The aims of this study were therefore to assess the effect of neonatal bacterial endotoxin exposure on subsequent glucose homeostasis, insulin action and corticosterone production from puberty through to senescence. Male Fischer-344 rat pups were treated with bacterial endotoxin (0.05 mg/kg Salmonella enteritidis i.p.) or vehicle (sterile pyrogen free saline) on days 3 and 5 postnatally. Insulin and glucose levels were assessed before and during and intraperitoneal glucose tolerance test (IPGTT) together with body mass on postnatal days 40, 80, and 400. In addition, circulating levels of corticosterone were measured at 0, 30, and 90 min following a 30-min restraint challenge at these ages. Neonatal endotoxin challenge did not alter fasting plasma glucose or insulin, but impaired glucose tolerance at puberty (p<.05), improved glucose tolerance in adulthood (p<.05) and had no effect at senescence. During the IPGTT insulin was reduced at all ages (p<.05) following neonatal endotoxin challenge, but insulin sensitivity was unaltered, except for an increase in adulthood (p<.05), which is consistent with the observed improvement in glucose tolerance at this age. Neonatal endotoxin challenge reduced body mass during puberty and senescence (p<.05) but did not alter basal or stressed plasma corticosterone levels at any of the three developmental time points examined. These findings suggest that variations in an individual's early life bacterial environment may contribute to differences in glucose homeostasis, insulin action and disease susceptibility later in life.

Change in Ucp1 mRNA Expression Following Long-Term Cold Exposure under Normal or High-Fat Diet Regimes in the Cold-Intolerant Mammal, Suncus murinus

The house musk shrew (Suncus murinus), or suncus, is a unique experimental mammal that is cold intolerant. However, even basic knowledge of brown adipose tissue (BAT), which is important for non-shivering thermogenesis (NST), is minimal. Therefore, we exposed suncus for 18 days to mild cold temperatures (8-14 degrees C) and/or a high-fat diet, which are factors that increase NST, and measured two mRNAs that are critical for NST in BAT, uncoupling protein 1 (Ucp1) and type II 5'-deiodinase (D2). Neither mild cold exposure nor a high-fat diet alone induced up-regulation of the mRNAs. However, combinations of cold exposure and high-fat diet significantly increased both mRNAs. Therefore, cold intolerance in suncus may be partly caused by dietary components.

Effect of a high or low ambient perinatal temperature on adult obesity in Osborne-Mendel and S5B/Pl rats

Perinatal environment is an important determinant of health status of adults. We tested the hypothesis that perinatal ambient temperature alters sympathetic activity and affects body composition in adult life and that this effect differs between S5B/Pl (S5B) and Osborne-Mendel (OM) strains of rat that were resistant (S5B) or susceptible (OM) to dietary obesity. From 1 wk before birth, rat litters were raised at either 18 or 30 degrees C until 2 mo of age while consuming a chow diet. Rats were then housed at normal housing temperature (22 degrees C) and provided either high-fat or low-fat diet. OM rats initially reared at 18 degrees C gained more weight on both diets than those reared at 30 degrees C. Perinatal temperature had no effect on body weight gain of the S5B rats on either diet. At 12 wk of age, OM and S5B rats reared at 18 degrees C had higher intakes of the high-fat diet than those reared at 30 degrees C but lower beta3-adrenergic receptor (beta3-AR) and uncoupling protein-1 (UCP1) mRNA levels in brown adipose tissue (BAT). The increase in metabolic rate in response to the beta3-agonist CL-316243, was greater in both OM and S5B rats reared at 18 degrees C than in those reared at 30 degrees C. Perinatal temperature differentially affects body weight in OM and S5B rats while having similar effects on food intake, response to a beta3-agonist, and BAT beta3-AR and UCP-1. The data suggest that OM rats are more susceptible to epigenetic programming than S5B rats.

Effects of dietary monosaccharides on sympathetic nervous system activity in adipose tissues of male rats

Dietary carbohydrate activates the sympathetic nervous system (SNS). As the mechanisms underlying this response are not fully characterized, studies were undertaken to compare SNS responses to ingestion of glucose, fructose, and galactose. SNS activity was examined using techniques of [(3)H]norepinephrine ([(3)H]NE) turnover in brown and white fat. In addition, gene expression for several sympathetically related proteins was also analyzed in these tissues. [(3)H]NE turnover in interscapular brown adipose tissue (IBAT) and retroperitoneal fat increased in response to glucose and fructose in the diet, whereas [(3)H]NE turnover in epididymal fat did not respond to either monosaccharide. Galactose feeding, by contrast, decreased [(3)H]NE turnover in IBAT, but increased it in epididymal, though not retroperitoneal, fat. Expression of GLUT4 was more abundant in IBAT and retroperitoneal fat from glucose- and fructose-fed animals than from diet- or galactose-fed rats. Chemical sympathectomy abolished the GLUT4 response in retroperitoneal fat, but was without effect on GLUT4 in epididymal fat. These studies are consistent with activation of a neural pathway by oral glucose or fructose, leading to SNS activation in IBAT and retroperitoneal fat and enhanced GLUT4 expression.