Stress cardiovascular/autonomic interactions in mice

Performance on the mouse vibration actuating search task is compromised by sleep deprivation

As we go about our daily routines we are continuously bombarded with environmental feedback that requires appraisal and response. Sleep loss can compromise the efficiency by which these cognitive processes function. Operationally, poor performance caused by insufficient sleep translates to increased health and safety risks in settings where attention and timely and/or accurate decisions to respond are critical (e.g., at work, on the road, etc.). Current rodent tasks that assess altered cognition after sleep deprivation (SD) do not accurately model the continuous multisensory feedback that informs goal-oriented behavior in humans. Herein, we describe the vibration actuating search task (VAST), which consists of a vibrating open field with pseudo-randomly selected entrance and target destination points. To successfully complete a trial, mice use feedback from rotary motor-induced floor vibrations to navigate from the entrance point to the target destination. Sets of 20 trials were conducted on 3 consecutive days, and before testing on the third day control mice were undisturbed while other mice were sleep deprived for 10 h. On the first 2 days mice learned the task with high success rates. Alternatively, VAST performance was compromised following SD as measured by increased failures in task completion, time to target, time spent immobile, and decreased speed as compared with undisturbed mice. The VAST enables the analysis of continuous feedback via multiple sensory modalities in mice and is applicable to a variety of operational settings.NEW & NOTEWORTHY The vibration actuating search task (VAST) is a novel performance assay that uses continuous auditory and haptic feedback to motivate and direct search behaviors in mice. The VAST is rapidly acquired by mice and performance is disrupted by sleep deprivation. The VAST has practical application in occupational settings. The cognitive aspects of the sensorimotor integration in the VAST may prove useful for rodent models of neurodegenerative disease.

Upregulating Nrf2 in the RVLM ameliorates sympatho-excitation in mice with chronic heart failure

Nuclear factor E2-related factor 2 (Nrf2) is a key transcription factor that maintains redox homeostasis by governing a broad array of antioxidant genes in response to oxidant stress. We hypothesized that overexpression of Nrf2 in the rostral ventrolateral medulla (RVLM) ameliorates sympatho-excitation in mice with coronary artery ligation-induced chronic heart failure (CHF). To address this, we overexpressed Nrf2 in the RVLM using an HIV-CamKIIa-Nrf2 lenti virus in C57BL/6 mice. In addition, we used a Lenti-Cre virus in Keap1^flox/flox mice to upregulate Nrf2 non-selectively in the RVLM. Arterial blood pressure (AP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were recorded under conscious and anesthetized conditions, respectively. Protein expression was assayed using western blotting and immunofluorescence staining. We found that (1) Nrf2 and two target proteins, NQO1 and HO-1 in the RVLM were significantly lower in CHF compared to Sham mice. Nrf2 viral transfection of the RVLM upregulated Nrf2 protein. (2) Urinary NE excretion in CHF mice was markedly attenuated following Nrf2 upregulation (812 ± 133 vs 1120 ± 271 ng/24hr mean. ±SE, *p < 0.05, n = 8/group). (3) In the conscious state, CHF mice overexpressing Nrf2 exhibited an enhancement in spontaneous baroreflex gain and in phenylephrine-induced baroreflex control of HR. (4) Acute experiments under anesthetisa revealed a significant decrease in basal RSNA (44.0 ± 6.5 vs 64.7 ± 8.3% of Max. *P < 0.05 n = 8/group) and enhancement in baroreflex sensitivity (Maximal gain -1.8 ± 0.3 vs 1.1 ± 0.2 of mmHg. **p < 0.01. n = 6/group) in CHF mice that were virally transfected with Nrf2 compared with CHF mice transfected with Lenti-GFP. Finally, Lenti-Cre viral overexpression of Nrf2 in Keap1^flox/flox mice reduced Keap1 protein and increased Nrf2, NQO1, and HO-1 in the RVLM of Sham and CHF mice. CHF-Cre mice exhibited a significant decrease in baseline RSNA and plasma NE concentration (8.9 ± 1.1 vs 12.7 ± 0.9 ng/mL *P < 0.05 n = 6/group) as compared with CHF-GFP mice. Based on the above data, we conclude that upregulating Nrf2 selectively in the RVLM attenuates sympatho-excitation in CHF mice. Nrf2 may be an important central target for autonomic modulation in cardiovascular disease and during stress.

A Universal Scaling Relation for Defining Power Spectral Bands in Mammalian Heart Rate Variability Analysis

Background: Power spectral density (PSD) analysis of the heartbeat intervals in the three main frequency bands [very low frequency (VLF), low frequency (LF), and high frequency (HF)] provides a quantitative non-invasive tool for assessing the function of the cardiovascular control system. In humans, these frequency bands were standardized following years of empirical evidence. However, no quantitative approach has justified the frequency cutoffs of these bands and how they might be adapted to other mammals. Defining mammal-specific frequency bands is necessary if the PSD analysis of the HR is to be used as a proxy for measuring the autonomic nervous system activity in animal models. Methods: We first describe the distribution of prominent frequency peaks found in the normalized PSD of mammalian data using a Gaussian mixture model while assuming three components corresponding to the traditional VLF, LF and HF bands. We trained the algorithm on a database of human electrocardiogram recordings (n = 18) and validated it on databases of dogs (n = 17) and mice (n = 8). Finally, we tested it to predict the bands for rabbits (n = 4) for the first time. Results: Double-logarithmic analysis demonstrates a scaling law between the GMM-identified cutoff frequencies and the typical heart rate (HRm): fVLF-LF = 0.0037⋅ HR m 0.58 , fLF-HF = 0.0017⋅ HR m 1.01 and fHFup = 0.0128⋅ HR m 0.86 . We found that the band cutoff frequencies and Gaussian mean scale with a power law of 1/4 or 1/8 of the typical body mass (BMm), thus revealing allometric power laws. Conclusion: Our automated data-driven approach allowed us to define the frequency bands in PSD analysis of beat-to-beat time series from different mammals. The scaling law between the band frequency cutoffs and the HRm can be used to approximate the PSD bands in other mammals.

TRPA1 mediates the cardiac effects of acrolein through parasympathetic dominance but also sympathetic modulation in mice

Numerous studies have demonstrated that short-term air pollution exposure causes cardiac autonomic imbalance as measured by heart rate variability (HRV). We previously showed that a single exposure to acrolein, a ubiquitous gaseous component of air pollution, not only causes autonomic imbalance, but also increases arrhythmia through transient receptor potential A1 (TRPA1) cation channels. Thus, the goal of this study was to characterize acrolein-induced autonomic changes in both normal and TRPA1-knockout mice (KO). Conscious, unrestrained C57BL/6 (WT) and KO mice were exposed to 3 ppm acrolein for 3 h. Separate groups were treated with either atenolol (sympathetic blocker), atropine (parasympathetic blocker) or hexamethonium (autonomic neurotransmission blocker), immediately before exposure. Electrocardiogram (ECG) and heart rate (HR) were recorded continuously before, during and after exposure. Exposure to acrolein produced significant increases in standard deviation of normal-to-normal R-R intervals (SDNN), Root Mean Square of the Successive Differences (RMSSD) and Low-Frequency (LF), as well as an increase in arrhythmia in WT mice. Treatment with atenolol reduced this response while atropine enhanced it, and both drugs blocked the acrolein-induced increase in arrhythmia; hexamethonium had no effect. On the other hand, neither acrolein nor any drug had an effect in the KO mice. Thus, acrolein-induced HRV responses appear to be mediated by a combined parasympathetic and sympathetic modulation. KO mice did not demonstrate any increases in HRV with exposure to acrolein. These data demonstrate that the cardiac effects of irritant air pollutants likely involve disruption of homeostatic balance and altered regulation even in healthy animals.

Estrogen replacement attenuates stress-induced pressor responses through vasorelaxation via β2-adrenoceptors in peripheral arteries of ovariectomized rats

We examined whether chronic estrogen replacement has an inhibitory effect on stress-induced pressor responses via activation of β₂-adrenoceptor (AR) in peripheral arteries of ovariectomized rats. Female Wistar rats aged 9 wk were ovariectomized. After 4 wk, pellets containing either 17β-estradiol (E₂) or placebo (Pla) were subcutaneously implanted into the rats. After 4 wk of treatment, rats underwent cage-switch stress, and, in a separate experiment, a subset received an infusion of isoproterenol (ISO) with or without pretreatment with the β₁-AR blocker atenolol or the β₂-AR blocker butoxamine. In addition, the isolated mesenteric artery was used to assess the concentration-related relaxing responses to ISO and the β₁- or β₂-AR mRNA level. The cage-switch stress-induced pressor response was significantly attenuated in the E₂-treated group compared with the Pla-treated group. Pretreatment with atenolol reduced blood pressure responses in both groups. However, butoxamine enhanced the pressor response only in the E₂-treated group, resulting in no difference between the two groups. In addition, the intravenous ISO-induced depressor response was significantly enhanced in the E₂-treated group compared with the Pla-treated group. Furthermore, the difference in the depressor response was abolished by pretreatment with butoxamine but not by atenolol. In the isolated mesenteric artery, butoxamine caused a rightward shift in ISO-induced concentration-related relaxation in the E₂-treated group. The β₂-AR mRNA level in the mesenteric artery was higher in the E₂-treated group than in the Pla-treated group. These results suggest that estrogen replacement attenuated the stress-induced pressor response probably by suppressing vasoconstriction via activation of β₂-ARs in peripheral arteries of ovariectomized rats. NEW & NOTEWORTHY In this study, we show, for the first time, that estrogen replacement has an inhibitory effect on the psychological stress-induced pressor response through vasorelaxation via β₂-adrenoceptors, probably due to overexpression of β₂-adrenoceptor mRNA, in peripheral arteries of ovariectomized rats.

A Potential Link between the C5a Receptor 1 and the β1-Adrenoreceptor in the Mouse Heart

Purpose

Inflammation may contribute to the pathogenesis of specific cardiovascular diseases, but it is uncertain if mediators released during the inflammatory process will affect the continued efficacy of drugs used to treat clinical signs of the cardiac disease. We investigated the role of the complement 5a receptor 1 (C5aR1/CD88) in the cardiac response to inflammation or atenolol, and the effect of C5aR1 deletion in control of baseline heart rate in an anesthetized mouse model.

Methods

An initial study showed that PMX53, an antagonist of C5aR1 in normal C57BL6/J (wild type, WT) mice reduced heart rate (HR) and appeared to have a protective effect on the heart following induced sepsis. C5aR1 knockout (CD88-/-) mice had a lower HR than wild type mice, even during sham surgery. A model to assess heart rate variability (HRV) in anesthetized mice was developed to assess the effects of inhibiting the β₁-adrenoreceptor (β₁-AR) in a randomized crossover study design.

Results

HR and LF Norm were constitutively lower and SDNN and HF Norm constitutively higher in the CD88-/- compared with WT mice (P< 0.001 for all outcomes). Administration of atenolol (2.5 mg/kg) reduced the HR and increased HRV (P< 0.05, respectively) in the wild type but not in the CD88-/- mice. There was no shift of the sympathovagal balance post-atenolol in either strains of mice (P> 0.05), except for the reduced LF/HF (Lower frequency/High frequency) ratio (P< 0.05) at 60 min post-atenolol, suggesting increased parasympathetic tone of the heart due to the effect of atenolol administration. The HR of the WT mice were lower post atenolol compared to the CD88-/- mice (P = 0.001) but the HRV of CD88-/- mice were significantly increased (P< 0.05), compared with WT mice.

Conclusion

Knockout of the C5aR1 attenuated the effect of β₁-AR in the heart, suggesting an association between the β₁-AR and C5aR1, although further investigation is required to determine if this is a direct or causal association.

NK1-receptor-expressing paraventricular nucleus neurones modulate daily variation in heart rate and stress-induced changes in heart rate variability

The paraventricular nucleus of the hypothalamus (PVN) is an established center of cardiovascular control, receiving projections from other nuclei of the hypothalamus such as the dorsomedial hypothalamus and the suprachiasmatic nucleus. The PVN contains a population of "pre-autonomic neurones" which project to the intermediolateralis of the spinal cord and increase sympathetic activity, blood pressure, and heart rate. These spinally projecting neurones express a number of membrane receptors including GABA and substance P NK1 receptors. Activation of NK1-expressing neurones increases heart rate, blood pressure, and sympathetic activity. However, their role in the pattern of overall cardiovascular control remains unknown. In this work, we use specific saporin lesion of NK1-expressing PVN rat neurones with SSP-SAP and telemetrically measure resting heart rate and heart rate variability (HRV) parameters in response to mild psychological stress. The HRV parameter "low frequency/high frequency ratio" is often used as an indicator of sympathetic activity and is significantly increased with psychological stress in control rats (0.84 ± 0.14 to 2.02 ± 0.15; P < 0.001; n = 3). We find the stress-induced increase in this parameter to be blunted in the SSP-SAP-lesioned rats (0.83 ± 0.09 to 0.93 ± 0.21; P > 0.05; n = 3). We also find a shift in daily variation of heart rate rhythm and conclude that NK1-expressing PVN neurones are involved with coupling of the cardiovascular system to daily heart rate variation and the sympathetic response to psychological stress.

Sexual dimorphism in autonomic changes and in the renin-angiotensin system in the hearts of mice subjected to thyroid hormone-induced cardiac hypertrophy

Based on the relevance of the renin-angiotensin system and the ongoing controversy regarding the role of the sympathetic nervous system in thyroid hormone-induced cardiac hypertrophy, the aim of the present study was to establish whether the putative difference in the degree of cardiac hypertrophy exhibited by males and females might be related to differences in the sympathetic-vagal balance and/or in the cardiac renin-angiotensin system in mice of different genders. Male and female mice (n = 117) were given 0.1 mg kg(-1) of triiodothyronine or normal saline each day for 10 days consecutively. At the end of that period, study of the heart rate variability, spectral analysis and histopathological examination were performed to assess the sympathetic-vagal balance and the diameter of cardiomyocytes. The cardiac levels of angiotensin I and II were also measured. Treatment with triiodothyronine induced a greater degree of cardiac hypertrophy in male (~73%) than in female mice (~42%). This difference was attributed to greater modulation of the sympathetic nervous system and higher levels of angiotensin I and II in male than in female mice. Our data indicate that thyroid hormone-induced cardiac hypertrophy was more intense in male mice due to the synergic effect of the sympathetic nervous system and the cardiac renin-angiotensin system.

Elevated blood pressure, heart rate and body temperature in mice lacking the XLαs protein of the Gnas locus is due to increased sympathetic tone

Imbalances of energy homeostasis are often associated with cardiovascular complications. Previous work has shown that Gnasxl-deficient mice have a lean and hypermetabolic phenotype, with increased sympathetic stimulation of adipose tissue. The Gnasxl transcript from the imprinted Gnas locus encodes the trimeric G-protein subunit XLαs, which is expressed in brain regions that regulate energy homeostasis and sympathetic nervous system (SNS) activity. To determine whether Gnasxl knock-out (KO) mice display additional SNS-related phenotypes, we have now investigated the cardiovascular system. The Gnasxl KO mice were ∼20 mmHg hypertensive in comparison to wild-type (WT) littermates (P ≤ 0.05) and hypersensitive to the sympatholytic drug reserpine. Using telemetry, we detected an increased waking heart rate in conscious KOs (630 ± 10 versus 584 ± 12 beats min(-1), KO versus WT, P ≤ 0.05). Body temperature was also elevated (38.1 ± 0.3 versus 36.9 ± 0.4°C, KO versus WT, P ≤ 0.05). To investigate autonomic nervous system influences, we used heart rate variability analyses. We empirically defined frequency power bands using atropine and reserpine and verified high-frequency (HF) power and low-frequency (LF) LF/HF power ratio to be indicators of parasympathetic and sympathetic activity, respectively. The LF/HF power ratio was greater in KOs and more sensitive to reserpine than in WTs, consistent with elevated SNS activity. In contrast, atropine and exendin-4, a centrally acting agonist of the glucagon-like peptide-1 receptor, which influences cardiovascular physiology and metabolism, reduced HF power equally in both genotypes. This was associated with a greater increase in heart rate in KOs. Mild stress had a blunted effect on the LF/HF ratio in KOs consistent with elevated basal sympathetic activity. We conclude that XLαs is required for the inhibition of sympathetic outflow towards cardiovascular and metabolically relevant tissues.

Identification and detection of the isolated sinus venosus from the Asian toad

The pacemaker activity of mammalian sinoatrial node (SAN) of the heart plays a fundamental role in the integration of vital functions. Studying factors such as drugs that influence pacemaker activity of SAN has its significance. In this study, we isolated sinus venosus, SAN from toads (Bufo gargarizans), and analysed its electronic signal, histological characteristics and the influence of acetylcholine (ACh) and ivabradine on its pacemaker activity using PowerLab® and Chart® 5.0 software. We found that when isolated sinus venosus was treated with ACh, its histological distribution was disorganized and inter-beat (RR) interval was also broadened. The high frequency normalized unit (HFnu) and Poincaré plot of heart rate variability (HRV) of the isolated sinus venosus was also altered upon ACh treatment in a time-dependent and dose-dependent manner. When treated with ivabradine, these parameters of HRV such as square root of the mean of the squared differences between adjacent NN intervals (RMSSD) and HFnu were in the upward tendency, but low frequency normalized unit and low frequency/high frequency were in the opposite tendency. Taken together, we have developed a new model for studying the influences of drugs on autorhythmicity using isolated sinus venosus of the toad. With this model, we showed that ACh and ivabradine may affect the pacemaker activity by stimulating muscarinic receptor or inhibiting If current, respectively.

Effect of age and gender on the progression of adult vascular dysfunction in a mouse model of fetal programming lacking endothelial nitric oxide synthase

The objective of this study was to investigate vascular function at different ages in a transgenic murine model of fetal vascular programming using a model of uteroplacental insufficiency induced by lack of endothelial nitric oxide synthase. Homozygous NOS3 knockout (KO) and wild-type (WT) mice were cross bred to produce WT, KO, and heterozygous that developed in WT (KOP) or KO (KOM) mothers. Male/female offspring from the four groups were killed at 7, 14, and 21 wk of age (n = 5-10/group), and carotid arteries were used for in vitro vascular studies. Responses to phenylephrine (PE), with/without N(G)-nitro-L-arginine methyl ester (L-NAME), angiotensin (ANG), acetylcholine (ACh), sodium nitroprusside, and isoproterenol (ISO) were studied. At 7 wk, only KO offspring showed higher contractile response to PE, whereas, at 14 and 21 wk, both KO and KOM had a higher response. Incubation with L-NAME abolished these differences. ANG contraction was higher in male KO in all age groups and in 21-wk-old females. Relaxation to ACh and ISO was absent in KO, and significantly decreased in KOM offspring in all age groups compared with KOP and WT, independent of gender. Sodium nitroprusside was not different between groups. The effect of the altered intrauterine environment on the development of abnormal vascular function was limited at 7 wk of age and most evident at 14 wk; further deterioration was limited to ANG-mediated vascular contractility in KO offspring. Our findings provide some hope that at least the first seven postnatal weeks may be an appropriate therapeutic window to prevent cardiovascular disease later in life.

Environmental influences on the production of pre-implantation embryos

Generation and cryopreservation of transgenic mice depend on reliable and continuous production of pre-implantation embryos. To suppress circannual and circadian rhythms driving the physiological and sexual behaviour of free living animals, laboratory animals are housed under standardized conditions. It remains to be elucidated if the artificial climate can cover all environmental effects. Here, we report that the humidity in an animal facility affects the embryo yield. The weather at the location of the facility, especially the temperature, influences the climate within an animal facility; weather peaks are obviously covered in part only, even if the facility is equipped with a powerful air-conditioning supply. Subsequently, external weather changes interact with the environment within the facility, influencing the production of embryos. Furthermore, noise and/or vibrations as generated by construction works, negatively affect the embryo yield.

Global disturbances in autonomic function yield cardiovascular instability and hypertension in the chromogranin a null mouse

We reported previously that chromogranin A (Chga) knockout (KO) mice are hypertensive and hyperadrenergic. Here we sought to determine the basis of such alterations by probing physiological, biochemical, and pharmacological responses to perturbations of the autonomic nervous system. In the conscious state, KO mice had substantially elevated basal high blood pressure (BP) and heart rate (HR); immobilization stress caused increments in systolic BP and HR in both wild-type (WT) and KO mice, with higher maxima but blunted increments in the KO state. Catestatin (CST; CHGA(352-372)) selectively diminished stress-induced increments in BP and HR in KO mice, implicating CST as an antihypertensive peptide, even in stressful conditions. Heightened plasma catecholamines in KO mice returned to WT level after CST. Stress caused further increments in catecholamines in WT mice but no change in KO mice. KO mice displayed diminished baroreflex sensitivity in response to either phenylephrine or sodium nitroprusside, accounting for exaggerated pressor and depressor responses to these compounds; baroreceptor function was normalized by CST. To probe the relative roles of endogenous/basal sympathetic vs. parasympathetic tone in control of BP and HR, we used the muscarinic-cholinergic antagonist atropine or the beta-adrenergic antagonist propranolol; HR and BP responses to each antagonist were exaggerated in KO animals. We conclude that ablation of Chga expression results in global disturbances in autonomic function, both sympathetic and parasympathetic, that can be abrogated (or rescued), at least in part, by replacement of CST. The results point to mechanisms whereby CHGA and its CST fragment act to control cardiovascular homeostasis.

Effect of intrauterine fetal programming on response to postnatal shaker stress in endothelial nitric oxide knockout mouse model

OBJECTIVE

To determine whether the intrauterine environment affects the postnatal vascular response to stress in a model of fetal programming induced by endothelial nitric oxide synthase deficiency.

STUDY DESIGN

Homozygous nitric oxide synthase knockout and wild-type controls were crossbred to obtain maternally and paternally derived heterozygous offspring. At 14 weeks of age, in vivo blood pressure measurements by telemetry, and in vitro carotid arteries vascular reactivity studies were performed in male offspring after subjecting them to shaker stress.

RESULTS

Maternally derived heterozygous offspring, compared with paternally derived heterozygous offspring, had significantly higher systolic blood pressure, mean arterial blood pressure, and pulse pressure before, as well as after introduction of the shaker stress. The difference in the latter between maternally and paternally derived heterozygous offspring was accentuated after stress. Maternally derived heterozygous offspring also had significantly higher contractile responses to phenylephrine when compared with paternally derived heterozygous offspring, and this was abolished after incubation with L-NAME.

CONCLUSION

The adverse uterine environment affects the postnatal vascular response to stress.

Kinetics and persistence of cardiovascular and locomotor effects of immobilization stress and influence of ACTH treatment

Stress triggers crucial responses, including elevated blood pressure and heart rate (HR), to handle the emergency and restore homeostasis. However, continuation of these effects following cessation of the stress is implicated with many stress-related disorders. Here, we examine the kinetics and persistence of cardiovascular and locomotor responses to single and repeated immobilization stress (IMO), with and without prior treatment with adrenocorticotropic hormone (ACTH). Radiotelemetry probes were implanted into male Sprague-Dawley rats to continually monitor mean arterial pressure (MAP), HR and locomotor activity. Rats were subjected to IMO for 2 h daily (10 a.m. to noon, 6 consecutive days). The first IMO induced the greatest change in MAP (about 30 mm Hg) and HR (about 200 bpm). Following each IMO, MAP and HR were elevated during the remaining light phase and in the subsequent dark phase, HR was lower than prior to IMO. We further examined whether elevation of ACTH to a level similar to IMO will elicit similar effects, and if it will alter subsequent responses to IMO. Injection of ACTH (13 IU/kg, s.c.) triggered a short-lived rise in MAP, and decreased HR. After six daily injections of ACTH and recovery time (8 days), rats were immobilized as above. The cardiovascular responses were similar during the IMO, but the ACTH-pretreated group displayed differences following cessation of the IMO. In addition, IMO led to a large reduction of locomotor activity during the dark (normally active) phase to levels similar to the light phase. Following the IMOs, locomotor activity recovered more slowly in the ACTH-pretreated group. The study revealed that IMO-triggered cardiovascular and locomotor responses are evident after termination of the stress. In addition, prior exposure to ACTH delayed recovery in cardiovascular and locomotor functions following cessation of stress.

Quantitative evaluation of ontogenetic change in heart rate and its autonomic regulation in newborn mice with the use of a noninvasive piezoelectric sensor

A reliable basal heart rate (HR) measurement in freely moving newborn mice was accomplished for the first time by using a novel noninvasive piezoelectric transducer (PZT) sensor. The basal HR was approximately 320 beats/min at postnatal day (P)0 and increased with age to approximately 690 beats/min at P14. Contribution of autonomic control to HR was then assessed. Sympathetic blockade with metoprolol significantly reduced basal HR at both P6 (-236 +/- 23 beats/min; mean +/- SE) and P12 (-105 +/- 8 beats/min), but atropine was without effect, indicating the predominant tonic adrenergic stimulation and absence of vagal control for basal HR in newborn mice. In contrast to stable basal HR during 5-min recording, HR measured by ECG (ECG-HR) was markedly decreased because of the restraint stress of attaching ECG electrodes, with accompanying freezing behavior. ECG-HR lowered and further decreased gradually during 5 min (slow cardiodeceleration) at P0-P3 and rapidly decreased and gradually recovered within 5 min (transient bradycardia) at P9-P14. The response was not uniform in P4-P8 mice: they showed either of these two patterns or sustained bradycardia (9-29%), and the number of mice that showed transient bradycardia increased with age (30-100%) during the period. Studies with autonomic blockade suggest that the slow cardiodeceleration and transient bradycardia are mediated mainly by withdrawal of adrenergic stimulation and phasic vagal activation, respectively, and the autonomic control of HR response to restraint stress is likely to change from the withdrawal of adrenergic stimulation to the phasic vagal activation at different stages during P4-P8 in individual mice. The PZT sensor may offer excellent opportunities to monitor basal HR of small animals noninvasively.

Heart rate variability in mice: a theoretical and practical guide

The mouse is the animal model principally used to study biological processes in mammals. The mutation, overexpression or knockout of one or several genes can provide insight into human disease. In cardiovascular research, evaluation of autonomic nervous function is an essential tool for a better understanding of the pathophysiological conditions in which cardiomyopathy arises and develops. Analysis of heart rate variability is the least invasive method to evaluate the sympathovagal balance on the sino-atrial level. The need to perform this technique on freely moving mice emerged in the 1990s, but despite previous studies it has been difficult to set up and standardize a common protocol. The multitudes of techniques used, plus subtle differences in methodology, impede the comparison and clear interpretation of results. This article aims to make a survey of heart rate variability analysis and to establish a standardized protocol for the assessment of the autonomic neural regulation of heart rate in mice.

Deficiency in angiotensin AT1a receptors prevents diabetes-induced hypertension

The renin-angiotensin system has been implicated in the etiology of the cardiovascular complications of diabetes. Our studies extend these findings to show a specific role for angiotensin AT1a receptors in mediating diabetes-induced hypertension. Male angiotensin AT1a knockout (AT1aKO) and wild-type (AT1aWT) mice with arterial telemetric catheters were injected with streptozotocin (STZ; 150 mg/kg ip). The STZ dose was selected on the basis of a dose-response experiment in C57/BL mice. Blood glucose, water intake, body weight, blood pressure (BP), and heart rate (HR) were measured over a 2-wk period. Estimates of BP and HR variance (BPV and HRV) and their low- and high-frequency domains were also determined. STZ induced similar levels of hyperglycemia and polydypsia in the groups. Mean arterial pressure (MAP) was increased from 100 ± 6 to 124 ± 6 mmHg in diabetic AT1aWT. MAP was unchanged in AT1aKO (80 ± 4 vs. 85 ± 5 mmHg, basal vs. STZ). Treatment with an ACE inhibitor, captopril, produced a greater reduction in MAP (−18%) in diabetic AT1aWT than in AT1aKO (−3.4%). BPV was lower in AT1aKO (19 ± 0.5 vs. 9 ± 2 mmHg2, AT1aWT vs. AT1aKO). Diabetes reduced BPV but only in AT1aWT (19 ± 0.5 vs. 8 ± 1 mmHg2, basal vs. STZ). There were no changes in HR in either group. In AT1aKO, STZ increased HRV and its high-frequency domain with no changes seen in AT1aWT. Results document that ANG AT1a receptors are critical in diabetes-induced hypertension and in cardiac autonomic responses.