Search tips
Search criteria 


Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Anesthesiology. Author manuscript; available in PMC 2011 November 1.
Published in final edited form as:
PMCID: PMC2962768

Disrupted Sleep and Delayed Recovery from Chronic Peripheral Neuropathy are Distinct Phenotypes in a Rat Model of Metabolic Syndrome

Aaron R. Muncey, M.D., Intern, Adam R. Saulles, B.S., Pharm D Student, Lauren G. Koch, Ph.D., Assistant Professor, Steven L. Britton, Ph.D., Professor, Helen A. Baghdoyan, Ph.D., Professor, and Ralph Lydic, Ph.D., Professor



Sleep apnea, hypertension, atherosclerosis, and obesity are features of metabolic syndrome associated with decreased restorative sleep and increased pain. These traits are relevant for anesthesiology because they confer increased risks of a negative anesthetic outcome. This study tested the one-tailed hypothesis that rats bred for low intrinsic aerobic capacity have enhanced nociception and disordered sleep.


Rats were from a breeding strategy that selected for low aerobic capacity runners (LCR) and high aerobic capacity runners (HCR). Four different phenotypes were quantified. Rats (n=12) underwent von Frey sensory testing, thermal nociceptive testing (n=12), electrographic recordings of sleep and wakefulness (n=16), and thermal nociceptive testing before and for six weeks after a unilateral chronic neuropathy of the sciatic nerve (n=14).


Paw withdrawal latency to a thermal nociceptive stimulus was significantly (P<0.01) less in LCR than HCR rats. There were significant differences in sleep. LCR rats spent significantly (P<0.01) more time awake (18%) and less time in non-rapid eye movement sleep (−19%) than HCR rats. Non-rapid eye movement sleep episodes were of shorter duration (−34%) in LCR than HCR rats. Rapid eye movement sleep of LCR rats was significantly more fragmented than Rapid eye movement sleep of HCR rats. LCR rats required two weeks longer than HCR rats to recover from peripheral neuropathy.


Rodents with low aerobic capacity exhibit features homologous to human metabolic syndrome. This rodent model offers a novel tool for characterizing the mechanisms through which low aerobic function and obesity might confer increased risks for anesthesia.


Metabolic syndrome and associated co-morbidities have special relevance for anesthesiology.13 Central obesity is a feature of metabolic syndrome which confers an increased risk of type II diabetes,4 and the combination of obesity and diabetes is associated with increased perioperative risks.57 Obesity increases risk for aspiration during induction,8 decreases perioperative tissue oxygenation,9 is a predictor for difficult mask ventilation,10 and is a risk factor for anesthesia-related maternal mortality.11 Metabolic syndrome also is associated with poor sleep,1220 and disrupted sleep can contribute to metabolic syndrome.13,1719,21,22 Metabolic syndrome increases risk for chronic neuropathic pain,23 and sleep disruption enhances pain perception.2428

The complexity and bi-directional nature of the foregoing pathologies emphasize the need for animal models that can help elucidate mechanisms underlying the interactions between metabolic syndrome, sleep, and pain. Two broad observations suggest that aerobic capacity and its attendant subordinate phenotypes are critical in the divide between health and disease, and motivated the development of rat models of disease risk. First, clinical studies reveal a strong statistical relationship between dysfunctional oxygen metabolism and all-cause morbidity and mortality. Indeed, for both healthy individuals and those with cardiovascular disorders low exercise capacity is a stronger predictor of decreased survival compared to other conventional risk factors such as body mass index, smoking, hypertension, dyslipidemia, or diabetes.29 Second, exercise is an effective prescription for a surprisingly wide range of chronic diseases.30

These ideas formed the basis for speculating that experimental models that divide for health and disease could be created by selection for low and high intrinsic aerobic endurance running capacity. Starting from a genetically heterogeneous population of rats developed at the National Institutes of Health, a divergent (two-way) artificial selection process was used to generate lines of low aerobic capacity runners (LCR) and high aerobic capacity runners (HCR).31 At 10 generations of selection the two lines divided widely for both the expected running capacity and the hypothesized differences for health risks including metabolic syndrome.32

Development of LCR and HCR lines generated a unique model system for dissection of aerobic endurance capacity and its correlated health-related phenotypes. The major hypothesis is that functional alleles at multiple interacting loci that affect intrinsic aerobic capacity have been enriched or fixed differentially between the LCR and HCR lines. It is critical that the models are maintained as genetically heterogeneous lines by using a mating paradigm that minimizes inbreeding.31 Compared to inbred strains, in which essentially all loci have been taken to fixation, outbred selected lines maintain genetic complexity which allows combinations of allelic variants at multiple interacting loci to be enriched by selection pressure.33 As a result, the LCR-HCR model is better suited to discover epistatic interactions, modifier genes, and synergistic actions.34 Importantly, the concurrent breeding of the LCR and HCR rats at every generation allows the lines to serve as reciprocal controls for unknown environmental changes.

The present study tested the hypothesis that LCR rats, in addition to a predisposition for metabolic syndrome,32,35 also have altered nocioception and disrupted sleep. The results are consistent with the notion that low intrinsic aerobic capacity and attendant correlated traits such as obesity can at least partly underlie pathology associated with altered nocioception and disrupted sleep.

Materials and Methods


Experiments were conducted in accordance with the Policy on Humane Care and Use of Laboratory Animals established by the National Institutes of Health (publication 80-23). All procedures were reviewed and approved by the University of Michigan Committee on Use and Care of Animals (Ann Arbor, Michigan). This study used a recently developed rat model selectively bred as low aerobic capacity runners and high aerobic capacity runners. These rats were derived from a genetically heterogeneous founder population developed at the National Institutes of Health.36 As described in detail previously,31 a large-scale rotational breeding strategy was started in 1996 to develop lines of rats that differ for intrinsic (i.e., untrained) aerobic capacity and retain wide heterogeneity. Each rat in the founder population (96 males and 96 females) was tested for maximal endurance running capacity on a speed-ramped treadmill. This test was patterned after clinical treadmill running tests and provided a reliable estimate of aerobic capacity (VO2 max) to segregate rats into the lowest and highest capacity runners. Twenty-six mating pairs were selected to create 13 base families of LCR and 13 base families of HCR. Offspring arising from these low and high breeder animals were tested at 11 weeks of age for intrinsic (i.e., untrained) running capacity. The highest performing male and female in each HCR family and the lowest performing male and female from each LCR family served as parents for each subsequent generation. After 11 generations of selective breeding, HCR and LCR rats differed by almost 350% for endurance running capacity and LCR rats exhibited several features consistent with metabolic syndrome.32

Rats used in the present study were males from generations 20 and greater. Differences in body weight between HCR and LCR rats are illustrated by Figs. 1A and B. Typical weights ranged from about 380 g (HCR) to 550 g (LCR). The difference in aerobic phenotype of distance run to exhaustion by HCR and LCR rats has been quantified for generation 26 and is illustrated by figure 1C. Two key points are that 1) these rats were never exercise trained and 2) this breeding procedure made it possible for the present study to evaluate the impact of genetically segregated intrinsic capacity for oxygen metabolism on nociception and sleep. Table 1 summarizes some of the previously documented physiological, behavioral, and metabolic differences between HCR and LCR rats.

Table 1
Physiological, metabolic, and behavioral characteristics of HCR and LCR rats

Experiment 1: Quantifying Sleep of High and Low Aerobic Capacity Runners

There is a convergence of evidence that sleep neurobiology can help elucidate some of the mechanisms by which anesthetics cause a loss of wakefulness (reviewed in3741). Therefore, the goal of experiment 1 was to phenotype states of sleep and wakefulness as a prelude to future studies designed to characterize responses of HCR and LCR rats to intravenous and volatile anesthetics. The procedures used to study sleep in rats have been described in detail.42,43 Briefly, eight male HCR rats and eight male LCR rats were anesthetized with 3% isoflurane in 100% O2 delivered at a flow rate of 1 l/min. Delivered isoflurane concentration was measured by spectrometry (Cardiocap™/5, Datex-Ohmeda, Louisville, CO). Once anesthetized, each rat was placed in a Kopf Model 962 small animal stereotaxic frame with a Model 906 rat anesthesia mask (David Kopf Instruments, Tujunga, CA) and delivered isoflurane concentration was decreased to 1.5%. Core body temperature was maintained at 37°C throughout the surgical procedure with the use of a re-circulating heat pump (Gaymar Industries, Orchard Park, NY). A midline scalp incision was made and electrodes were implanted for recording the electroencephalogram and electromyogram. The cortical electroencephalogram electrodes (8IE36320SPCE; Plastics One, Roanoke, VA) were screwed into the cranium at three different sites relative to bregma: 2.0 mm posterior, 1.3 mm lateral; 2.0 mm posterior, −1.5 mm lateral; and 1.0 mm anterior, 1.5 mm lateral. Two electromyogram electrodes were constructed from AS632 biomed wire (4.5 cm length, Cooner Wire Company, Chatsworth, CA) and were placed into the dorsal neck muscles. Another electrode was placed subcutaneously near the dorsal aspect of the skull and served as an indifferent electrode. Leads from all six electrodes were placed together in a six-pin multi-channel electrode pedestal (MS363, Plastics One) that was secured to the skull with three anchor screws and dental acrylic (Jet Acrylic Self Curing Resin and Liquid; Lang Dental Manufacturing Company, Inc., Wheeling, IL).

After surgery the rats were allowed to recover for 7 to 10 days while habituating to the sleep recording chambers. Habituation consisted of placing the rats in the recording chambers and connecting the electrode pedestal on the skull to a six-channel cable (363–441/six 80CM 6TCM, Plastics One) that led to amplifiers and a computer. Housing and sleep recordings occurred in a 12:12 hour light-dark cycle with transitions occurring at 8:00 a.m. and 8:00 p.m. States of sleep and wakefulness were recorded across 24 h in two 12-h, continuous segments that coincided with the light and dark cycle start times. The electrographic signals were amplified, filtered (electroencephalogram, 0.3–30 Hz; electromyogram, 10–100 Hz), and recorded at 128 Hz. After habituation, states of sleep and wakefulness were recorded for 24 h. For each 24 h digital recording, 10 s bins were scored as wakefulness, rapid eye movement (REM) sleep, or non-REM (NREM) sleep. During the recording sessions the rats could move freely and had ad libitum access to food and water.

Experiment 2: von Frey testing in High and Low Aerobic Capacity Runners

A second group of HCR (n=6) and LCR (n=6) rats was used to phenotype mechanoreceptor thresholds by measuring paw withdrawal away from 2, 8, and 15 g of pressure caused by von Frey hairs applied to the bottom of a hind paw (Touch Test Sensory Evaluator Kit, Stoelting Co., Wood Dale, IL). As described previously44,45 these nylon hairs were slowly pushed against the skin until the hair bent. Each of the three hairs was applied five times in ascending order. Paw withdrawal was recorded as a positive response and the percent response to each von Frey hair was tabulated and averaged for each animal.

Experiment 3: Measures of thermal nociception

As described previously46, rats were conditioned daily to being placed in a Model 336T, IITC chamber for nociceptive testing (Life Science, Inc.; Woodland Hills, CA). The Hargreaves device made it possible to quantify the latency in seconds to paw withdrawal (PWL) away from onset a light beam focused on the hind paw.47 On the day of testing, animals were placed in the Hargreaves chamber and allowed to habituate for 60 min before obtaining measures of PWL. The light beam and an electronic timer were activated simultaneously. When the rat lifted its hind paw the heat source and the timer were stopped and the latency to paw withdrawal was recorded.46,48,49 Five measurements were obtained and averaged over 5 min of testing.

Experiment 4: Quantifying Recovery from Chronic Peripheral Nerve Injury

A unilateral chronic peripheral mononeuropathy50 of the sciatic nerve was created in adult male HCR (n=7) and LCR (n=7) rats. Prior to surgery, each rat was anesthetized with 3% isoflurane until breathing rate decreased to 60 breaths per min. After loss of wakefulness the rat was fitted with a Kopf Model 906 rat anesthesia mask and isoflurane concentration was lowered to 1.5% for the remainder of the procedure. Delivered isoflurane concentration and core body temperature were measured. The sciatic nerve was accessed via hind limb incision followed by blunt dissection of the biceps femoris. At a level proximal to its trifurcation, 5–8 mm of nerve were separated from tissue with minimal damage to surrounding muscle and tissue.50 Three polyglycolic acid sutures were placed around the sciatic nerve at 1 mm intervals from each other loosely enough to prevent complete ligation but tightly enough to induce a leg twitch response.50 The wound margin was sutured closed and rats were allowed six days to recover from surgery.

Thermal nociceptive testing was performed prior to and after the chronic constriction injury. A beam of light, which served as a noxious thermal stimulus, was directed through a glass floor and onto the plantar aspect of the hind paw. This light remained on until the rat withdrew its paw from the thermal stimulus. PWL was defined as the time (s) between commencement of the light beam and withdrawal of the hind paw.47 PWL was measured for all rats the day before sciatic nerve ligation and on days 7, 14, 21, 28, 35, and 42 after nerve ligation. During this six-week testing period, rats were conditioned in the testing chamber for 2 h daily. At the beginning of each testing day, rats were habituated to the chambers for 1 h before five PWL measurements were obtained with the Hargreaves method applied to the hind paws of both the operated and non-operated limb. To prevent tissue damage during nociceptive testing, the thermal light stimulus was set to automatically terminate (cut-off time) if there was no paw withdrawal after 18 s.

Statistical Analyses

This study tested the one-tailed hypothesis that rats bred for low intrinsic aerobic capacity have enhanced nociception and disordered sleep. The 24-h recordings of sleep and wakefulness were scored for 138,240 epochs, each 10 s in duration. To avoid the problem of inflated degrees of freedom, the data were averaged for each of the 16 rats. Dependent measures included the percent of recording time spent in wakefulness, NREM sleep, and REM sleep; episode duration and number of episodes for each of the three states; and number of transitions between states. The data for percent of time spent in each state were normally distributed and fit the assumptions of two-way analysis of variance for repeated measures. The data describing recovery from chronic constriction nerve injury as a function of time were also analyzed using two-way analysis of variance for repeated measures. Post-hoc comparisons were made using Mann-Whitney and Bonferroni statistics. The von Frey data were not normally distributed and were analyzed using the Mann-Whitney statistic. Software used for statistical analyses included SAS v9.2 (SAS Institute Inc., Cary, NC), GBStat (Dynamic Microsystems, Inc., Silver Spring, MD) and Prism 5 (Graph Pad Software, Inc., La Jolla, CA).

Poincaré analyses are particularly useful for quantitatively evaluating the periodic nature of bi-stable rhythms such as the cardiac cycle,51 breathing,52,53 and sleep.54 Poincaré analyses were used in the present study to compare HCR and LCR animals for their ability to maintain stable episodes of REM sleep. Poincaré analysis made it possible to quantify two indices of variability. Short-term variability (SD1) expressed the variability in time from one REM sleep episode to the next REM sleep episode. Long-term variability (SD2) quantified the overall variability in duration of REM sleep epochs duration across the 24-h recording. Calculation of SD1 values for HCR and LCR rats was achieved with a four-step process: 1) The duration in s of each REM sleep interval was plotted (y) relative to (x) representing the duration of the previous REM sleep interval. 2) After all REM sleep epochs were plotted, the line of identity (x=y) was calculated and added to the Poincaré graph. 3) The Pythagorean theorem was used to calculate the perpendicular distance between each data point and the line of identity. 4) The standard deviation of these raw distances was calculated as the SD1 value for each HCR and each LCR rat, and these values were averaged across all HCR and LCR rats, respectively. SD2 values were obtained using step one above and then calculating the distance of each point from a line perpendicular to the line y=x and running through the mean duration of REM sleep intervals (i.e., the centroid of data points). As a final step, individual animal means of SD1 and SD2 for each HCR and LCR rat were used to evaluate rat strain differences by t-test. For all inferential statistics a P-value of less than 0.05 was considered significant.


Time Spent in States of Sleep and Wakefulness Varied as a Function of Aerobic Capacity

Figure 2 illustrates the temporal distribution of sleep and waking states recorded across a continuous 24-h period for one representative HCR rat and one LCR rat. Figure 3 summarizes the average of eight different 24-h recordings for each strain and quantifies the strain-specific differences in sleep architecture. During the 24-h recordings LCR rats spent significantly (P<0.009) more time in wakefulness and significantly (P<0.01) less time in NREM sleep than HCR rats (fig. 3A). These differences were not apparent in the dark portion of the light:dark cycle (fig. 3B) and resulted predominantly from significant differences in wakefulness (P<0.007) and NREM sleep (P<0.005) that occurred during the light phase of the recordings (fig. 3C). HCR and LCR rats did not differ in the percent of recording time spent in REM sleep (fig. 3).

Poincaré Analysis Revealed Differences in REM Sleep as a Function of Aerobic Capacity

Figure 4 summarizes the variability in duration of REM sleep episodes characteristic of HCR and LCR animals. Comparison of the Poincaré distributions for the HCR (fig. 4A) and LCR (fig. 4B) rats indicates differences in the variability of REM sleep duration. Figure 4C illustrates the significantly (P<0.05) greater long-term variability in REM sleep episode duration characteristic of LCR rats. Short-term variability (SD1) in a Poincaré analysis quantifies the duration of one REM sleep episode (REMi) relative to the previous REM sleep episode (REMi-1). The differences in SD1 between HCR (fig. 4A) and LCR (fig. 4B) rats were not significant. Long-term variability (SD2) quantifies the variability in REM sleep episode duration as a function of rat strain. Figure 4C shows that SD2 was greater for LCR than HCR rats. Thus, LCR animals could initiate but not maintain stable episodes of REM sleep. Comparing the REM sleep data in figures 2, ,3,3, and and44 demonstrates how Poincaré analyses made it possible to quantify differences in the temporal organization of REM sleep.

The results of the Poincaré analyses encouraged a more detailed comparison of the temporal organization of sleep-wake states, and figure 5 summarizes those results. Averaging across 24 h, LCR animals had significantly (P<0.001) more transitions from wakefulness to NREM sleep and significantly (P<0.001) more transitions from NREM sleep to wakefulness than HCR rats (fig. 5A). Consistent with the figure 1 results, across the 24-h recordings LCR rats had a significantly (P <0.04) greater number of wakefulness episodes than HCR rats (fig. 5B). When LCR rats did have NREM sleep, the average duration of each NREM sleep episode was significantly (P<0.002) shorter than NREM sleep duration of HCR rats (fig. 5C).

There was no difference between HCR and LCR rats in mechano-sensory response as measured by von Frey test (fig. 6A). Power calculation indicated that 61 rats would be needed to demonstrate a difference in von Frey response at the P<0.05 level. As described previously,46 LCR rats revealed a modest but significantly shorter latency to paw withdrawal away from a thermal nociceptive stimulus (fig. 6B). Figure 6C shows the time course of PWL measured in HCR and LCR rats for one week before and for 42 days after chronic constriction injury of one sciatic nerve.50 Each of the seven points in figure 6C summarizes the mean and standard deviation latency in s of paw withdrawal away from a thermal stimulus to the hind paw. Before the peripheral neuropathy (pre-injury) there was no statistically significant difference in PWL between LCR and HCR animals. For the LCR rats, (fig. 6C) two-way ANOVA for repeated measures revealed a significant effect of peripheral neuropathy on PWL. Post-hoc test revealed significantly (P<0.05) enhanced nociception in LCR rats on days 7, 14, 21, and 28 after peripheral neuropathy. Measures of paw withdrawal in HCR rats (fig. 6C) revealed significantly (P<0.05) enhanced nociception for days 7 and 14 after chronic constriction injury of the sciatic nerve.


In 2000, the National Task Force on the Prevention and Treatment of Obesity reported delayed recognition of the fact that obesity is a significant health risk.55 Although relevance of obesity for anesthesiology was noted in 1975,56 only recently has the interaction between obesity, sleep, and pain become apparent.3,5,37,57 Obesity has replaced smoking as a major disease burden58 and projection data indicate that by 2020, half of all Americans may be obese.59 The results are discussed relative to emerging evidence that obesity is associated with disordered sleep, which can cause hyperalgesia and increase opioid requirement for effective pain management.

Aerobic Fitness, Metabolic Syndrome, and Sleep

The data from rats bred for low intrinsic aerobic capacity are consistent with human data documenting a relationship between metabolic syndrome, sleep, and nociception. For example, humans with poor physical fitness and a high body mass index exhibit disrupted and diminished sleep compared to those with a normal body mass index.18,20,60 Humans with metabolic syndrome and type II diabetes spend more time in bed and less time asleep.12,13,16,17

Human data also support the view that reduced sleep leads to increased body mass and decreased fitness by way of increasing cortisol and interleukin-6, increasing ghrelin, and decreasing leptin.1719 Thus, shortened sleep has been associated with decreased fitness and the present results show for the first time that diminished aerobic fitness in LCR rats is characterized by disordered sleep. Low intrinsic aerobic capacity was associated with decreased NREM sleep time and increased time awake (figs. 2 and and3).3). Sleep disruption in rats with low aerobic capacity also was characterized by greater variability in duration of REM sleep (fig. 4) and more transitions between wakefulness and NREM sleep (fig. 5) compared to rats bred for high intrinsic aerobic capacity. The decreased amount of sleep and inability to maintain sleep characterizing this rodent model of metabolic syndrome31,32,61 resemble features of human insomnia. In support of the potential clinical relevance of this rodent model, disordered sleep and short sleep duration characteristic of human insomnia are risk factors for type II diabetes.62 The present results encourage future studies designed to obtain long term recordings of sleep and wakefulness from HCR and LCR rats.

Aerobic Fitness, Disordered Sleep, and Nociception

Obesity and insulin resistance,6365 specific features of metabolic syndrome, are associated with painful, idiopathic neuropathy,23 enhanced nociception,66,67 and disordered sleep.6871 Although data clearly document an association between metabolic syndrome and pain, no previous studies have characterized the effect of metabolic syndrome on the resolution of experimentally imposed, chronic pain. Compared to HCR rats, the LCR phenotype revealed a delayed recovery from chronic peripheral mononeuropathy (fig. 6C). The fig. 6C results, and the fact that HCR and LCR rats have been bred for the intrinsic (i.e., untrained) component of aerobic capacity, implies that genetic and metabolic factors altered the resolution of the neuropathy. This implication fits with the finding that human obesity is associated with increased hospital admissions and longer hospital stays.72 The disordered sleep characteristic of LCR rats may also contribute to hyperalgesia. In normal, pain-free humans sleep loss causes hyperalgesia25,28 and non-restorative sleep impacts pain management.73 Opioids disrupt NREM sleep and inhibit the REM phase of sleep38 and REM sleep disruption consistently has been shown to diminish the analgesic properties of opioids.74

Models, Limitations, and Future Directions

There is good precedent for animal models contributing to clinical anesthesiology. Rodent studies have provided the background for spinal drug administration now routine in clinical care,75 porcine models helped elucidate the mechanisms of malignant hyperthermia,76 and unique features of goat brain vasculature helped differentiate brain versus spinal cord sites of anesthetic action.77 All models have a limited domain of applicability and the strengths and limitations of the LCR and HCR rats have been described in detail.61 A known limitation of sensory threshold testing is that the data are characterized by a high degree of variability.78,79 This variability may contribute to the lack of rat strain-specific difference in response to von Frey testing (fig. 6A), as well as differences between HCR and LCR rats during acute nociceptive testing (fig. 6B) but not during pre-injury nociceptive testing (fig. 6C). Likewise, the complexity of animal models of nociception is recognized80 and future nociceptive testing should include additional pain models and sensory modalities.

The polygenic nature of complex diseases emphasizes the advantages of models created by the selective breeding for naturally occurring traits over transgenic models that manipulate a single gene or gene product.61 This advantage is particularly clear for efforts to model hypertension, metabolic syndrome, and disordered sleep, all of which complicate anesthesia care. The potential for LCR and HCR rats to provide a unique resource for anesthesiology research is supported by evidence emphasizing the biological centrality of oxygen uptake and delivery61 and the special relevance of oxygen for anesthesiology.81

The present findings suggest homology between obese human and rat for the traits of sleep fragmentation and nociception, and support the interpretation that LCR rats develop some features of metabolic syndrome.32 Sleep apnea is the second most common sleep disorder and a potential problem for anesthesia care.82 This is consistent with evidence that obstructive sleep apnea, similar to metabolic syndrome, is a systemic disease.83 The present results encourage future studies using whole body plethysmography53 to characterize the actions of intravenous and volatile anesthetics on the respiratory control of LCR and HCR rats. In view of the fact that adenosine is antinociceptive84,85 and that exercise increases brain adenosine,86 future studies also will determine the extent to which adenosinergic neurotransmission87 is one mechanism through which intrinsic aerobic capacity modulates sleep and nociception.


For expert assistance the authors thank Sha Jiang, B.S. (Research Associate), Mary A. Norat, B.S. (Senior Research Associate), Sarah L. Watson, B.S. (Senior Research Associate) and Elizabeth Gauthier, B.S. (Research Assistant) from the Department of Anesthesiology, University of Michigan, Ann Arbor, MI, and Kathy Welch, M.A., M.P.H. (Center for Statistical Consultation and Research, University of Michigan). Lori Gilligan, L.V.T (Animal Technician Senior) and Nathan Kanner, B.A. (Senior Research Laboratory Technician) from the Department of Anesthesiology, provided expert care of the rat colony.

Support: Supported by grants HL40881, HL65272 (RL), MH45361 (HAB), and NCRR R17718 (LGK and SLB) from the National Institutes of Health, Bethesda, MD, and by the Department of Anesthesiology, University of Michigan, Ann Arbor, MI


Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Conflicts of Interest Ralph Lydic and Helen A. Baghdoyan received past support from Sepracor Pharmaceutical (Marlborough, Massachusetts) for studies of the effects of eszopiclone on acetylcholine release in rat brain stem.

Meetings at which work was presented: Abstracts presented at the Experimental Biology Meeting, San Diego, CA, April 7, 2008; and Association of Professional Sleep Societies Meeting, Baltimore, MD, June 7, 2008.

Summary Statement: Rats selectively bred for low intrinsic aerobic capacity develop metabolic syndrome and have phenotypes of disordered sleep and enhanced nociception that are homologous to traits of obese humans.


1. Lemmens HJM, Saidman LJ, Eger IE, Laster MJ. Obesity modestly affects inhaled anesthetic kinetics in humans. Anesth Analg. 2008;107:1864–1870. [PubMed]
2. McKay RE, Malhotra A, Cakmakkaya OS, Hall KT, McKay WR, Apfel CC. Effect of increased body mass index and anaesthetic duration on recovery of protective airway reflexes after sevoflurane vs desflurane. Br J Anaesth. 2010;104:175–182. [PubMed]
3. Alvarez A, Brodsky JB, Lemmens HJM, Morton JM. Morbid Obesity: Peri-operative Management. 2nd Edition. Cambridge, UK: Cambridge University Press; 2010. pp. 1–246.
4. Li C, Ford ES. Definition of the metabolic syndrome: What's new and what predicts risk? Metab Syndr Relat Disord. 2006;4:237–251. [PubMed]
5. Neligan PJ, Fleisher LA. Obesity and diabetes: Evidence of increased perioperative risk? Anesthesiology. 2006;104:398–400. [PubMed]
6. Bagry HS, Raghavendran S, Carli F. Metabolic syndrome and insulin resistance. Anesthesiology. 2008;108:506–523. [PubMed]
7. Martyn JAJ, Kaneki M, Yasuhara S. Obesity-induced insulin resistance and hyperglycemia: Etiologic factors and molecular mechanisms. Anesthesiology. 2008;109:137–148. [PMC free article] [PubMed]
8. Lam AM, Grace DM, Penny FJ, Vezina WC. Prophylactic intravenous cimetidine reduces the risk of acid aspiration in morbidly obese patients. Anesthesiology. 1986;65:684–687. [PubMed]
9. Kabon B, Nagele A, Reddy D, Eagon C, Fleshman JW, Sessler DI, Kurz A. Obesity decreases perioperative tissue oxygenation. Anesthesiology. 2004;100:274–280. [PMC free article] [PubMed]
10. Kheterpal S, Han R, Tremper KK, Shanks A, Tait AR, O'Reilly M, Ludwig TA. Incidence and predictors of difficult and impossible mask ventilation. Anesthesiology. 2006;105:885–891. [PubMed]
11. Mhyre JM, Riesner MN, Polley LS, Naughton NN. A series of anesthesia-related maternal mortalities in Michigan, 1985–2003. Anesthesiology. 2007;106:1096–1104. [PubMed]
12. Gangwisch JE, Heymsfield SB, Boden-Albala B, Buijs RM, Kreier F, Pickering TG, Rundle AG, Zammit GK, Malaspina D. Short sleep duration as a risk factor for hypertension: Analyses of the first National Health and Nutrition Examination Survey. Hypertension. 2006;47:833–839. [PubMed]
13. Gangwisch JE, Heymsfield SB, Boden-Albala B, Buijs RM, Kreier F, Pickering TG, Rundle AG, Zammit GK, Malaspina D. Sleep duration as a risk factor for diabetes incidence in a large U.S. sample. Sleep. 2007;30:1667–1673. [PubMed]
14. Gangwisch JE, Malaspina D, Boden-Albala B, Heymsfield SB. Inadequate sleep as a risk factor for obesity: Analyses of the NHANES I. Sleep. 2005;28:1289–1296. [PubMed]
15. Moreno CR, Louzada FM, Teixeira LR, Borges F, Lorenzi-Filho G. Short sleep is associated with obesity among truck drivers. Chronobiol Int. 2006;23:1295–1303. [PubMed]
16. Singh M, Drake CL, Roehrs T, Hudgel DW, Roth T. The association between obesity and short sleep duration: A population-based study. J Clin Sleep Med. 2005;1:357–363. [PubMed]
17. Spiegel K, Knutson K, Leproult R, Tasali E, Van Cauter E. Sleep loss: A novel risk factor for insulin resistance and Type 2 diabetes. J Appl Physiol. 2005;99:2008–2019. [PubMed]
18. Taheri S, Lin L, Austin D, Young T, Mignot E. Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS Med. 2004;1:e62. [PMC free article] [PubMed]
19. Van Cauter E, Holmback U, Knutson K, Leproult R, Miller A, Nedeltcheva A, Pannain S, Penev P, Tasali E, Spiegel K. Impact of sleep and sleep loss on neuroendocrine and metabolic function. Horm Res. 2007;67 Suppl 1:2–9. [PubMed]
20. Vorona RD, Winn MP, Babineau TW, Eng BP, Feldman HR, Ware JC. Overweight and obese patients in a primary care population report less sleep than patients with a normal body mass index. Arch Intern Med. 2005;165:25–30. [PubMed]
21. Cizza G, Skarulis M, Mignot E. A link between short sleep and obesity: Building the evidence for causation. Sleep. 2005;28:1217–1220. [PubMed]
22. Knutson KL, Spiegel K, Penev P, Van Cauter E. The metabolic consequences of sleep deprivation. Sleep Med Rev. 2007;11:163–178. [PMC free article] [PubMed]
23. Smith AG, Robinson JS. Idiopathic neuropathy, prediabetes and the metabolic syndrome. J Neurol Sci. 2006;242:9–14. [PubMed]
24. Onen SH, Alloui D, Jourdan D, Eschalier A, Dubray C. Effects of rapid eye movement (REM) sleep deprivation on pain sensitivity in the rat. Brain Res. 2001;900:261–267. [PubMed]
25. Roehrs T, Hyde M, Blaisdell B, Greenwald M, Roth T. Sleep loss and REM sleep loss are hyperalgesic. Sleep. 2006;29:145–151. [PubMed]
26. Smith MT, Edwards RR, McCann UD, Haythornthwaite JA. The efffects of sleep deprivation on pain inhibition and spontaneous pain in women. Sleep. 2007;30:494–505. [PubMed]
27. Lavigne G, Sessle BJ, Choinière M, Soja PJ. Sleep and Pain. Seattle, WA: International Association for the Study of Pain; 2007. pp. 1–473.
28. Chhangani BS, Roehrs TA, Harris EJ, Hyde M, Drake C, Hudgel DW, Roth T. Pain sensitivity in sleepy pain-free normals. Sleep. 2009;32:1011–1017. [PubMed]
29. Myers J, Prakash M, Froelicher V, Do D, Attwood J. Exercise capacity and mortality among men referred for exercise testing. New Eng J Med. 2002;346:793–801. [PubMed]
30. Pedersen BK, Saltin B. Evidence for prescribing exercise as therapy in chronic diesease. Scand J Med Sci Sports. 2006;16 Suppl:3–63. [PubMed]
31. Koch LG, Britton SL. Artificial selection for intrinsic aerobic endurance running capacity in rats. Physiol Genomics. 2001;5:45–52. [PubMed]
32. Wisloff U, Najjar SM, Ellingsen O, Haram PM, Swoap S, Al-Share Q, Fernstrom M, Rezaei K, Lee SJ, Koch LG, Britton SL. Cardiovascular risk factors emerge after artificial selection for low aerobic capacity. Science. 2005;307:418–420. [PubMed]
33. Carborg O, Jacobsson L, Ahgren P, Siegel P, Andersson L. Epistasis and the release of genetic variation during long-term selection. Nat Genet. 2006;38:418–420. [PubMed]
34. Pickrell JK, Coop G, Novembre J, Kudaravalli S, Absher D, Srinivasan BS, Barsh GS, Myers RM, Feldman MW, Pritchard JK. Signals of recent positive selection in a worldwide sample of human populations. Genome Res. 2009;19:826–837. [PubMed]
35. Noland RC, Thyfault JP, Henes ST, Whitfield BR, Woodlief TL, Evans JR, Lust JA, Britton S, Koch L, Dudek RW, Dohm GL, Cortright RN, Lust RM. Artificial selection for high-capacity endurance running is protective against high-fat diet-induced insulin resistance. Am J Physiol Endocrinol Metab. 2007;293:E31–E41. [PubMed]
36. Hansen C, Spuhler K. Development of the National Institutes of Health genetically heterogeneous rat stock. Alcohol Clin Exp Res. 1984;8:477–479. [PubMed]
37. Lydic R, Baghdoyan HA. Sleep, anesthesiology, and the neurobiology of arousal state control. Anesthesiology. 2005;103:1268–1295. [PubMed]
38. Lydic R, Baghdoyan HA. In: Neurochemical mechanisms mediating opioid-induced REM sleep disruption, Sleep and Pain. Lavigne G, Sessle BJ, Choinière M, Soja PJ, editors. Seattle: International Assoc. for the Study of Pain; 2007. pp. 99–122.
39. Lydic R, Biebuyck JF. Sleep neurobiology: Relevance for mechanistic studies of anaesthesia. Br J Anaesth. 1994;72:506–508. [PubMed]
40. Vanini G, Baghdoyan HA, Lydic R. In: Relevance of sleep neurobiology for cognitive neuroscience and anesthesia, Consciousness, Awarness, and Anesthesia. Mashour GA, editor. Cambridge: Cambridge University Press; 2010. pp. 1–23.
41. Watson CJ, Baghdoyan HA, Lydic R. A neurochemical perspective on states of consciousness. In: Hudetz AG, Pearce RA, editors. Supressing the Mind: Anesthetic Modulation of Memory and Consciousness. New York: Springer/Humana; 2010. pp. 33–80.
42. Osman NI, Baghdoyan HA, Lydic R. Morphine inhibits acetylcholine release in rat prefrontal cortex when delivered systemically or by microdialysis to basal forebrain. Anesthesiology. 2005;103:779–787. [PubMed]
43. Watson CJ, Soto-Calderon H, Lydic R, Baghdoyan HA. Pontine reticular formation (PnO) administration of hypocretin-1 increases PnO GABA levels and wakefulness. Sleep. 2008;31:453–464. [PubMed]
44. Leem JW, Willis WD, Chung JM. Cutaneous sensory receptors in the rat foot. J Neurophys. 1993;69:1684–1699. [PubMed]
45. Tal M, Bennett GJ. Extra-terrritorial pain in rats with a peripherial mononeuropathy: Mechano-hyperalgesia and mechano-allodynia in the territory of an uninjured nerve. Pain. 1994;57:375–382. [PubMed]
46. Geisser ME, Wang W, Smuck M, Koch LG, Britton SL, Lydic R. Nociception before and after exercise in rats bred for high and low aerobic capacity. Neurosci Letts. 2008;443:37–40. [PMC free article] [PubMed]
47. Hargreaves K, Dubner R, Brown F, Flores C, Joris J. A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia. Pain. 1988;32:77–88. [PubMed]
48. Brummett CM, Norat MA, Palmisano JM, Lydic R. Perineural administration of dexmedetomidine in combination with bupivacaine enhances sensory and motor blockade in sciatic nerve block in rat. Anesthesiology. 2008;109:502–511. [PMC free article] [PubMed]
49. Brummett CM, Padda AK, Amodeo FS, Welch R, Lydic R. Perineural dexmedetomidine added to ropivacaine causes a dose-dependent increase in the duration of thermal antinociception in sciatic nerve block in rat. Anesthesiology. 2009;111:1111–1119. [PMC free article] [PubMed]
50. Bennett GJ, Xie YK. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain. 1988;33:87–107. [PubMed]
51. Laitio T, Jalonen J, Kuusela T, Scheinin H. The role of heart rate variability in risk stratification for adverse postoperative cardiac events. Anesth Analg. 2007;105:1548–1560. [PubMed]
52. Gonsenhauser I, Wilson CG, Han F, Strohl KP, Dick TE. Strain differences in murine ventilatory behavior persist after urethane anesthesia. J Appl Physiol. 2004;97:888–894. [PubMed]
53. Icaza EE, Huang X, Fu Y, Neubig RR, Baghdoyan HA, Lydic R. Isoflurane-induced changes in righting response and breathing are modulated by RGS proteins. Anesth Analg. 2009;109:1500–1505. [PMC free article] [PubMed]
54. Pivik RT, Busby KA, Gill E, Hunter P, Nevins R. Heart rate variations during sleep in preadolescents. Sleep. 1996;19:117–135. [PubMed]
55. National Task Force Report. Overweight, obesity, and health risks. Arch Intern Med. 2000;160:898–904. [PubMed]
56. Fisher A, Waterhouse TD, Adams AP. Obesity: It relation to anaesthesia. Anaesthesia. 1975;30:633–647. [PubMed]
57. Manson JE, Skerrett PJ, Greenland P, VanItallie TB. The escalating pandemics of obesity and sedentary lifestyle. Arch Intern Med. 2004;164:249–258. [PubMed]
58. Jia H, Lubetkin EI. Trends in quality-adjusted life-years lost contributed by smoking and obesity. Am J Prev Med. 2010;38:138–144. [PubMed]
59. Wang Y, Beydoun MA, Liang L, Caballero B, Kumanyika SK. Will all Americans become overweight or obese? Estimating the progression and cost of the US obesit epidemic. Obesity. 2008;16:2323–2330. [PubMed]
60. Hasler G, Buysse DJ, Klaghofer R, Gamma A, Ajdacic V, Eich D, Rossler W, Angst J. The association between short sleep duration and obesity in young adults: A 13-year prospective study. Sleep. 2004;27:661–666. [PubMed]
61. Koch L, Britton S. Aerobic metabolism underlies complexity and capacity. J Physiol. 2008:83–95. 586.1. [PubMed]
62. Vgontzas AN, Liao D, Pejovic S, Calhoun S, Karataraki M, Bixler EO. Insomnia with objective short sleep duration is associated with type 2 diabetes: A population based study. Diabetes Care. 2009;32:1980–1985. [PMC free article] [PubMed]
63. Singleton JR, Smith AG. Neuropathy associated with prediabetes: What is new in 2007? Curr Diab Rep. 2007;7:420–424. [PubMed]
64. Singleton JR, Smith AG, Russell J, Feldman EL. Implications for diagnosis and therapy. Curr Treat Options Neurol. 2005;7:33–42. [PubMed]
65. Sumner CJ, Sheth S, Griffin JW, Cornblath DR, Polydefkis M. The spectrum of neuropathy in diabetes and impaired glucose tolerance. Neurology. 2003;60:108–111. [PubMed]
66. Marcus DA. Obesity and the impact of chronic pain. Clin J Pain. 2004;20:186–191. [PubMed]
67. Hitt HC, McMillen RC, Thornton-Neaves T, Koch K, Cosby AG. Comorbidity of obesity and pain in a general population: Results from the Southern Pain Prevalence Study. J Pain. 2007;8:430–436. [PubMed]
68. Marty M, Rozenberg S, Duplan B, Thomas P, Duquesnoy B, Allaert F. Quality of sleep in patients with chronic low back pain: A case-control study. Eur Spine J. 2008;17:839–844. [PMC free article] [PubMed]
69. Menefee LA, Frank ED, Doghramji K, Picarello K, Park JJ, Jalali S, Perez-Schwartz L. Self-reported sleep quality and quality of life for individuals with chronic pain conditions. Clin J Pain. 2000;16:290–297. [PubMed]
70. Palermo TM, Toliver-Sokol M, Fonareva I, Koh JL. Objective and subjective assessment of sleep in adolescents with chronic pain compared to healthy adolescents. Clin J Pain. 2007;23:812–820. [PMC free article] [PubMed]
71. Sayar K, Arikan M, Yontem T. Sleep quality in chronic pain patients. Can J Psychiatry. 2002;47:844–848. [PubMed]
72. Schafer MH, Ferraro KF. Obesity and hospitalization over the adult life course: Does duration of exposure increase use? J Health Soc Behav. 2007;48:434–449. [PMC free article] [PubMed]
73. Moldofsky H. Sleep and pain. Sleep Med Rev. 2001;5:387–398.
74. Lautenbacher S, Kundermann B, Krieg J-C. Sleep deprivation and pain perception. Sleep Med Rev. 2006;10:357–369. [PubMed]
75. Saidman LJ. ASA award: Tony L. Yaksh. Anesthesiology. 1994;81:797–798. [PubMed]
76. Harrison GG, Saunders SJ, Biebuyck JF, Hickman R, Dent DM, Weaver V, Terblanche J. Anaesthetic-induced malignant hyperpyrexia and a method for its prediction. Br J Anaesth. 1969;41:844–855. [PubMed]
77. Antognini JF, Schwartz K. Exaggerated anesthetic requirments in the preferrentially anesthetized brain. Anesthesiology. 1993;79:1244–1249. [PubMed]
78. Bove G. Mechanical sensory threshold testing using nylon monofilaments: The pain field's "Tin Standard". Pain. 2006;124:13–17. [PubMed]
79. Lambert GA, Mallos G, Zagami AS. von Frey's hairs - a review of their technology and use - a novel automated von Frey device for improved testing of hyperalgesia. J Neurosci Methods. 2009;177:420–426. [PubMed]
80. Le Bars D, Gozariu M, Cadden SW. Animal models of nociception. Pharm Rev. 2001;53:597–652. [PubMed]
81. Lindahl S. Oxygen and life on earth. Anesthesiology. 2008;109:7–13. [PubMed]
82. Ramachandran S, Josephs L. A meta-analysis of clinical screening tests for obstructive sleep apnea. Anesthesiology. 2009;110:928–939. [PubMed]
83. Zamarron C, Paz VG, Riveiro A. Obstructive sleep apnea syndrome is a systemic disease. Current evidence. Eur J Intern Med. 2008;19:390–398. [PubMed]
84. Eisenach JC, Rauck RL, Curry R. Intrathecal, but not intravenous adenosine reduces allodynia in patients with neuropathic pain. Pain. 2003;105:65–70. [PubMed]
85. Gan TJ, Habib AS. Adenosine as a non-opioid analgesic in the perioperative setting. Anesth Analg. 2007;105:487–494. [PubMed]
86. Dworak M, Diel P, Voss S, Hollmann W, Struder H. Intense exercise increases adenosine concentration in rat brain: Implications for homeostatic sleep drive. Neuroscience. 2007;150:789–795. [PubMed]
87. Nelson AM, Battersby AS, Baghdoyan HA, Lydic R. Opioid induced decreases in rat brain adenosine are reversed by inhibiting adenosine deaminase. Anesthesiology. 2009;111:1327–1333. [PMC free article] [PubMed]
88. Novak CM, Escande C, Gerber SM, Chini EN, Zhang M, Britton SL, Koch LG, Levine JA. Endurance capacity, not body size, determines physical activity levels: Role of skeletal muscle PEPCK. PLoS One. 2009;4:e5869. [PMC free article] [PubMed]
89. Howlett RA, Kirkton SD, Gonzalez NC, Wagner HE, Britton SL, Koch LG, Wagner PD. Peripheral oxygen transport and utilization in rats following continued selective breeding for endurance running capacity. J Appl Physiol. 2009;106:1819–1825. [PubMed]
90. Thyfault JP, Rector RS, Uptergrove GM, Borengasser SJ, Morris EM, Wei Y, Laye MJ, Burant CF, Qi NR, Ridenhour SE, Koch LG, Britton SL, Ibdah JA. Rats selectively bred for low aerobic capacity have reduced hepatic mitochondrial oxidative capacity and susceptibility to hepatic steatosis and injury. J Physiol. 2009;587:1805–1816. [PubMed]
91. Lujan HL, Britton SL, Koch LG, DiCarlo SE. Reduced susceptibility to ventricular tachyarrhythmias in rats selectively bred for high aerobic capacity. Am J Physiol. 2006;291:E31–E41. [PubMed]