Respiratory sinus arrhythmia in freely moving and anesthetized rats

Quantification of respiratory sinus arrhythmia in control dogs and in dogs with various severity classes of myxomatous mitral valve disease

INTRODUCTION

Loss of respiratory sinus arrhythmia (RSA) is a negative prognostic factor in dogs with myxomatous mitral valve disease (MMVD). The aim of this study was to calculate the percentage (%) of RSA in healthy dogs and dogs in various MMVD classes.

ANIMALS

Control and MMVD dogs were prospectively included in the study.

MATERIALS AND METHODS

Respiratory sinus arrhythmia was calculated from a dual channel electrocardiography and breathing curve recording using the peak-to-trough method, in percent of the average heart rate.

RESULTS

One hundred and forty-nine dogs were studied, including 24 control and 125 MMVD dogs of different severity classes. An overall %RSA decrease was documented with increasing disease severity up to the Ca class along with a relative %RSA increase in the Cc class. The %RSA magnitude differed between B2 and Ca (P<0.001), and between Ca and Cc (P = 0.001) groups, respectively. The %RSA showed a medium negative correlation with the La:Ao ratio (r2 = -0.568, P<0.001) and with the E-wave velocity (r2 = -0.561, P<0.001).

DISCUSSIONS

A decrease in %RSA was shown with increased disease severity up to acute congestive heart failure (CHF). Dogs receiving cardiac therapy leading to stabilized CHF might restore their ability to exhibit RSA, often revealing a higher %RSA compared to those in acute CHF.

STUDY LIMITATIONS

Low number of respiratory cycles for analysis. Therapy effect not evaluated.

CONCLUSIONS

The findings of this study can serve as the basis for future risk stratification and carry the potential of proving an additional clinical marker for diagnostic and therapeutic decisions making when managing MMVD dogs.

Respiratory sinus arrhythmia in spontaneously breathing, unanesthetized newborn and adult Wistar rats

We examined respiratory sinus arrhythmia (RSA) and possible interaction with respiratory frequency (fR) and heart rate (HR) in spontaneously breathing, unanesthetized newborn Wistar rats (2- to 5-day-old; n = 54) and the adult rats (8-week-old; n = 34). Instantaneous heart rate (inst-HR) was calculated as the reciprocal of the inter-beat-interval. For each breath, RSA was determined as the difference between the maximum and minimum inst-HR value. The absolute RSA or RSA% (RSA per HR) were calculated as the average RSA of 10 consecutive breaths. RSA (or RSA%) in the newborn rats was significantly lower than that in the adult rats. Correlation coefficient between RSA (or RSA%) and 1/fR or HR/fR, but not HR, was significant in newborn rats, whereas only that between RSA (or RSA%) and HR was significant in adult rats. The power spectrum density of heartbeat fluctuation was detectable in both age groups. The present findings suggest that RSA exists and could be influenced by fR, rather than HR, in newborn rats.

"Brain-breath" interactions: respiration-timing-dependent impact on functional brain networks and beyond

Breathing is a natural daily action that one cannot do without, and it sensitively and intensely changes under various situations. What if this essential act of breathing can impact our overall well-being? Recent studies have demonstrated that breathing oscillations couple with higher brain functions, i.e., perception, motor actions, and cognition. Moreover, the timing of breathing, a phase transition from exhalation to inhalation, modulates specific cortical activity and accuracy in cognitive tasks. To determine possible respiratory roles in attentional and memory processes and functional neural networks, we discussed how breathing interacts with the brain that are measured by electrophysiology and functional neuroimaging: (i) respiration-dependent modulation of mental health and cognition; (ii) respiratory rhythm generation and respiratory pontomedullary networks in the brainstem; (iii) respiration-dependent effects on specific brainstem regions and functional neural networks (e.g., glutamatergic PreBötzinger complex neurons, GABAergic parafacial neurons, adrenergic C1 neurons, parabrachial nucleus, locus coeruleus, temporoparietal junction, default-mode network, ventral attention network, and cingulo-opercular salience network); and (iv) a potential application of breathing manipulation in mental health care. These outlines and considerations of "brain-breath" interactions lead to a better understanding of the interoceptive and cognitive mechanisms that underlie brain-body interactions in health conditions and in stress-related and neuropsychiatric disorders.

Gelatin-Based Metamaterial Hydrogel Films with High Conformality for Ultra-Soft Tissue Monitoring

Implantable hydrogel-based bioelectronics (IHB) can precisely monitor human health and diagnose diseases. However, achieving biodegradability, biocompatibility, and high conformality with soft tissues poses significant challenges for IHB. Gelatin is the most suitable candidate for IHB since it is a collagen hydrolysate and a substantial part of the extracellular matrix found naturally in most tissues. This study used 3D printing ultrafine fiber networks with metamaterial design to embed into ultra-low elastic modulus hydrogel to create a novel gelatin-based conductive film (GCF) with mechanical programmability. The regulation of GCF nearly covers soft tissue mechanics, an elastic modulus from 20 to 420 kPa, and a Poisson's ratio from - 0.25 to 0.52. The negative Poisson's ratio promotes conformality with soft tissues to improve the efficiency of biological interfaces. The GCF can monitor heartbeat signals and respiratory rate by determining cardiac deformation due to its high conformability. Notably, the gelatin characteristics of the biodegradable GCF enable the sensor to monitor and support tissue restoration. The GCF metamaterial design offers a unique idea for bioelectronics to develop implantable sensors that integrate monitoring and tissue repair and a customized method for endowing implanted sensors to be highly conformal with soft tissues.

Telemetric long-term assessment of autonomic function in experimental heart failure

Despite medical advances in the treatment of heart failure (HF), mortality remains high. It has been shown that alterations of the autonomic-nervous-system (ANS) are associated with HF progression and increased mortality. Preclinical models are required to evaluate the effectiveness of novel treatments modulating the autonomic imbalance. However, there are neither standard models nor diagnostic methods established to measure sympathetic and parasympathetic outflow continuously. Digital technologies might be a reliable tool for continuous assessment of autonomic function within experimental HF models. Telemetry devices and pacemakers were implanted in beagle dogs (n = 6). HF was induced by ventricular pacing. Cardiac hemodynamics, plasma catecholamines and parameter describing the ANS ((heart rate variability (HRV), deceleration capacity (DC), and baroreflex sensitivity (BRS)) were continuously measured at baseline, during HF conditions and during recovery phase. The pacing regime led to the expected depression in cardiac hemodynamics. Telemetric assessment of the ANS function showed a significant decrease in Total power, DC, and Heart rate recovery, whereas BRS was not significantly affected. In contrast, plasma catecholamines, revealing sympathetic activity, showed only a significant increase in the recovery phase. A precise diagnostic of the ANS in the context of HF is becoming increasingly important in experimental models. Up to now, these models have shown many limitations. Here we present the continuous assessment of the autonomic function in the progression of HF. We could demonstrate the advantage of highly resolved ANS measurement by HR and BP derived parameters due to early detection of an autonomic imbalance in the progression of HF.

Exogenous glutamine ameliorates diabetic nephropathy in a rat model of type 2 diabetes mellitus through its antioxidant and anti-inflammatory activities

This study aimed to evaluate the effects of glutamine (Gln) on diabetic nephropathy and other complications in a rat model of type 2 diabetes mellitus. Streptozotocin/nicotinamide induced diabetic rats were enrolled as an animal model of type 2 diabetes mellitus. Animals were divided into control, diabetic, and Gln (1000 mg/l in drinking water, eight weeks) treated diabetic groups. Gln alleviated renal inflammatory and oxidative stress biomarkers (tumour necrosis factor-alpha, interleukin 6, glutathione peroxidase, total superoxide dismutase, and glutathione), decreased serum uric acid and creatinine, and restored renal histopathological changes (glomerular volume, sclerosis, and leukocyte infiltration). Additionally, Gln ameliorated other complications, including systemic oxidative stress (serum malondialdehyde and nitric oxide, serum and liver glutathione, glutathione peroxidase, and total superoxide dismutase, and liver catalase), insulin resistance, hyperglycaemia, and hyperlipidaemia. Collectively, Gln attenuates diabetic nephropathy and other complications in type 2 diabetes mellitus in rats through its antioxidant and anti-inflammatory activities.

Rostral ventrolateral medulla, retropontine region and autonomic regulations

The rostral half of the ventrolateral medulla (RVLM) and adjacent ventrolateral retropontine region (henceforth RVLMRP) have been divided into various sectors by neuroscientists interested in breathing or autonomic regulations. The RVLMRP regulates respiration, glycemia, vigilance and inflammation, in addition to blood pressure. It contains interoceptors that respond to acidification, hypoxia and intracranial pressure and its rostral end contains the retrotrapezoid nucleus (RTN) which is the main central respiratory chemoreceptor. Acid detection by the RTN is an intrinsic property of the principal neurons that is enhanced by paracrine influences from surrounding astrocytes and CO₂-dependent vascular constriction. RTN mediates the hypercapnic ventilatory response via complex projections to the respiratory pattern generator (CPG). The RVLM contributes to autonomic response patterns via differential recruitment of several subtypes of adrenergic (C1) and non-adrenergic neurons that directly innervate sympathetic and parasympathetic preganglionic neurons. The RVLM also innervates many brainstem and hypothalamic nuclei that contribute, albeit less directly, to autonomic responses. All lower brainstem noradrenergic clusters including the locus coeruleus are among these targets. Sympathetic tone to the circulatory system is regulated by subsets of presympathetic RVLM neurons whose activity is continuously restrained by the baroreceptors and modulated by the respiratory CPG. The inhibitory input from baroreceptors and the excitatory input from the respiratory CPG originate from neurons located in or close to the rhythm generating region of the respiratory CPG (preBötzinger complex).

D-Limonene Alleviates Acute Kidney Injury Following Gentamicin Administration in Rats: Role of NF-κB Pathway, Mitochondrial Apoptosis, Oxidative Stress, and PCNA

Clinical application of gentamicin (GM) is well known to be associated with the development of acute kidney injury (AKI). This study was the first to investigate the possible protective effects of D-limonene (D-lim) on AKI following GM administration in rats. 32 rats arranged in four groups (n = 8): (1) the control group received saline intraperitoneally (0.5 ml/day) and orally (0.5 ml/day), (2) the D-lim group received D-lim (100 mg/kg) orally and saline (0.5 ml/day) intraperitoneally, (3) the GM group received GM (100 mg/kg/day) intraperitoneally and saline (0.5 ml/day) orally, and (4) the treated group received intraperitoneal GM (100 mg/kg) and oral D-lim (100 mg/kg). All treatments were performed daily for 12 consecutive days. Results revealed that D-lim ameliorated GM-induced AKI, oxidative stress, mitochondrial apoptosis, and inflammation. D-lim showed nephroprotective effects as reflected by the decrease in serum urea and creatinine and improvement of renal histopathological changes. D-lim alleviated GM-induced oxidative stress by increasing the activities of renal catalase, serum and renal glutathione peroxidase, and renal superoxide dismutase and decreasing renal malondialdehyde and serum nitric oxide levels. Intriguingly, D-lim suppressed mitochondrial apoptosis by considerably downregulating Bax and caspase-3 (Casp-3) mRNA and protein expressions and markedly enhancing Bcl2 mRNA and protein expressions. Furthermore, D-lim significantly decreases GM-induced inflammatory response through downregulation of NF-κB, IL-6, and TNF-α mRNA and/or protein expressions and decrease in renal myeloperoxidase activity. Finally, D-lim remarkably downregulated PCNA protein expression in the treated group compared with the GM group. In brief, this study showed that D-lim alleviated AKI following GM administration in rats, partially through its antioxidant, anti-inflammatory, and antiapoptotic activities as well as downregulation of PCNA expression.

Acute Ramelteon Treatment Maintains the Cardiac Rhythms of Rats during Non-REM Sleep

Sleep curtailment negatively affects cardiac activities and thus should be ameliorated by pharmacological methods. One of the therapeutic targets is melatonin receptors, which tune circadian rhythms. Ramelteon, a melatonin MT1/MT2 receptor agonist, has recently been developed to modulate sleep-wake rhythms. To date, the sleep-promoting effect of ramelteon has been widely delineated, but whether ramelteon treatment physiologically influences cardiac function is not well understood. To address this question, we recorded electrocardiograms, electromyograms, and electrocorticograms in the frontal cortex and the olfactory bulb of unrestrained rats treated with either ramelteon or vehicle. We detected vigilance states based on physiological measurements and analyzed cardiac and muscular activities. We found that during non-rapid eye movement (non-REM) sleep, heartrate variability was maintained by ramelteon treatment. Analysis of the electromyograms confirmed that neither microarousal during non-REM sleep nor the occupancy of phasic periods during REM sleep was altered by ramelteon. Our results indicate that ramelteon has a remedial effect on cardiac activity by keeping the heartrate variability and may reduce cardiac dysfunction during sleep.

PreBötzinger complex neurons drive respiratory modulation of blood pressure and heart rate

Heart rate and blood pressure oscillate in phase with respiratory activity. A component of these oscillations is generated centrally, with respiratory neurons entraining the activity of pre-sympathetic and parasympathetic cardiovascular neurons. Using a combination of optogenetic inhibition and excitation in vivo and in situ in rats, as well as neuronal tracing, we demonstrate that preBötzinger Complex (preBötC) neurons, which form the kernel for inspiratory rhythm generation, directly modulate cardiovascular activity. Specifically, inhibitory preBötC neurons modulate cardiac parasympathetic neuron activity whilst excitatory preBötC neurons modulate sympathetic vasomotor neuron activity, generating heart rate and blood pressure oscillations in phase with respiration. Our data reveal yet more functions entrained to the activity of the preBötC, with a role in generating cardiorespiratory oscillations. The findings have implications for cardiovascular pathologies, such as hypertension and heart failure, where respiratory entrainment of heart rate is diminished and respiratory entrainment of blood pressure exaggerated.

Baroreflex functionality in the eye of diffusion tensor imaging

By applying diffusion tensor imaging (DTI) as a physiological tool to evaluate changes in functional connectivity between key brainstem nuclei in the baroreflex neural circuits of mice and rats, recent work has revealed several hitherto unidentified phenomena regarding baroreflex functionality. (1) The presence of robust functional connectivity between nucleus tractus solitarii (NTS) and nucleus ambiguus (NA) or rostral ventrolateral medulla (RVLM) offers a holistic view on the moment-to-moment modus operandi of the cardiac vagal baroreflex or baroreflex-mediated sympathetic vasomotor tone. (2) Under pathophysiological conditions (e.g. neurogenic hypertension), the disruption of functional connectivity between key nuclei in the baroreflex circuits is reversible. However, fatality ensues on progression from pathophysiological to pathological conditions (e.g. hepatic encephalopathy) when the functional connectivity between NTS and NA or RVLM is irreversibly severed. (3) The absence of functional connectivity between the NTS and caudal ventrolateral medulla (CVLM) necessitates partial rewiring of the classical neural circuit that includes CVLM as an inhibitory intermediate between the NTS and RVLM. (4) Sustained functional connectivity between the NTS and NA is responsible for the vital period between brain death and the inevitable cardiac death. (5) Reduced functional connectivity between the NTS and RVLM or NA points to inherent anomalous baroreflex functionality in floxed and Cre-Lox mice. (6) Disrupted NTS-NA functional connectivity in Flk-1 (VEGFR2) deficient mice offers an explanation for the hypertensive side-effect of anti-vascular endothelial growth factor therapy (anti-VEGF) therapy. These newly identified baroreflex functionalities revealed by DTI bear clinical and therapeutic implications.

Cardiorespiratory interactions in humans and animals: rhythms for life

The cardiorespiratory system exhibits oscillations from a range of sources. One of the most studied oscillations is heart rate variability, which is thought to be beneficial and can serve as an index of a healthy cardiovascular system. Heart rate variability is dampened in many diseases including depression, autoimmune diseases, hypertension, and heart failure. Thus, understanding the interactions that lead to heart rate variability, and its physiological role, could help with prevention, diagnosis, and treatment of cardiovascular diseases. In this review, we consider three types of cardiorespiratory interactions: respiratory sinus arrhythmia (variability in heart rate at the frequency of breathing), cardioventilatory coupling (synchronization between the heart beat and the onset of inspiration), and respiratory stroke volume synchronization (the constant phase difference between the right and the left stroke volumes over one respiratory cycle). While the exact physiological role of these oscillations continues to be debated, the redundancies in the mechanisms responsible for its generation and its strong evolutionary conservation point to the importance of cardiorespiratory interactions. The putative mechanisms driving cardiorespiratory oscillations as well as the physiological significance of these oscillations will be reviewed. We suggest that cardiorespiratory interactions have the capacity to both dampen the variability in systemic blood flow as well as improve the efficiency of work done by the heart while maintaining physiological levels of arterial CO2. Given that reduction in variability is a prognostic indicator of disease, we argue that restoration of this variability via pharmaceutical or device-based approaches may be beneficial in prolonging life.

Direct projections from hypothalamic orexin neurons to brainstem cardiac vagal neurons

Orexin neurons are known to augment the sympathetic control of cardiovascular function, however the role of orexin neurons in parasympathetic cardiac regulation remains unclear. To test the hypothesis that orexin neurons contribute to parasympathetic control we selectively expressed channelrhodopsin-2 (ChR2) in orexin neurons in orexin-Cre transgenic rats and examined postsynaptic currents in cardiac vagal neurons (CVNs) in the dorsal motor nucleus of the vagus (DMV). Simultaneous photostimulation and recording in ChR2-expressing orexin neurons in the lateral hypothalamus resulted in reliable action potential firing as well as large whole-cell currents suggesting a strong expression of ChR2 and reliable optogenetic excitation. Photostimulation of ChR2-expressing fibers in the DMV elicited short-latency (ranging from 3.2ms to 8.5ms) postsynaptic currents in 16 out of 44 CVNs tested. These responses were heterogeneous and included excitatory glutamatergic (63%) and inhibitory GABAergic (37%) postsynaptic currents. The results from this study suggest different sub-population of orexin neurons may exert diverse influences on brainstem CVNs and therefore may play distinct functional roles in parasympathetic control of the heart.

Parasympathetic Vagal Control of Cardiac Function

This brief review focuses on four new topics, with novel and clinically significant consequences, concerning the powerful influence of parasympathetic activity on cardiac function. In this short summary, we will highlight very recent and important work, published in the last 3-4 years, that (1) challenges the paradigm that parasympathetic activity to the heart is involved in the control of heart rate but plays little role in other cardiac functions, (2) characterizes important long-range synaptic pathways to parasympathetic cardiac vagal neurons that are involved in "higher" brain functions (such as arousal and emotional challenges), (3) asks whether implantable chronic vagal nerve stimulation is a promising clinical tool for treating cardiovascular diseases, and (4) describes newly identified neuropeptides and other modulators that can influence the generation and maintenance of parasympathetic activity to the heart.

Utility of a Novel Biofeedback Device for Within-Breath Modulation of Heart Rate in Rats: A Quantitative Comparison of Vagus Nerve vs. Right Atrial Pacing

In an emerging bioelectronics era, there is a clinical need for physiological devices incorporating biofeedback that permits natural and demand-dependent control in real time. Here, we describe a novel device termed a central pattern generator (CPG) that uses cutting edge analog circuitry producing temporally controlled, electrical stimulus outputs based on the real time integration of physiological feedback. Motivated by the fact that respiratory sinus arrhythmia (RSA), which is the cyclical changes in heart rate every breath, is an essential component of heart rate variability (HRV) (an indicator of cardiac health), we have explored the versatility and efficiency of the CPG for producing respiratory modulation of heart rate in anesthetized, spontaneously breathing rats. Diaphragmatic electromyographic activity was used as the input to the device and its output connected to either the right cervical vagus nerve or the right atrium for pacing heart rate. We found that the CPG could induce respiratory related heart rate modulation that closely mimicked RSA. Whether connected to the vagus nerve or right atrium, the versatility of the device was demonstrated by permitting: (i) heart rate modulation in any phase of the respiratory cycle, (ii) control of the magnitude of heart rate modulation, and (iii) instant adaptation to changes in respiratory frequency. Vagal nerve pacing was only possible following transection of the nerve limiting its effective use chronically. Pacing via the right atrium permitted better flexibility and control of heart rate above its intrinsic level. This investigation now lays the foundation for future studies using this biofeedback technology permitting closer analysis of both the function and dysfunction of RSA.

Chronic intermittent hypoxia alters neurotransmission from lateral paragigantocellular nucleus to parasympathetic cardiac neurons in the brain stem

Patients with sleep-related disorders, including obstructive sleep apnea (OSA), have an increased risk of cardiovascular diseases. OSA events are more severe in rapid eye movement (REM) sleep. REM sleep further increases the risk of adverse cardiovascular events by diminishing cardioprotective parasympathetic activity. The mechanisms underlying REM sleep-related reduction in parasympathetic activity likely include activation of inhibitory input to cardiac vagal neurons (CVNs) in the brain stem originating from the lateral paragigantocellular nucleus (LPGi), a nucleus that plays a role in REM sleep control. This study tests the hypothesis that chronic intermittent hypoxia and hypercapnia (CIHH), an animal model of OSA, inhibits CVNs because of exaggeration of the GABAergic pathway from the LPGi to CVNs. GABAergic neurotransmission to CVNs evoked by electrical stimulation of the LPGi was examined with whole cell patch-clamp recordings in an in vitro brain slice preparation in rats exposed to CIHH and control rats. GABAergic synaptic events were enhanced after 4-wk CIHH in both male and female rats, to a greater degree in males. Acute hypoxia and hypercapnia (H/H) reversibly diminished the LPGi-evoked GABAergic neurotransmission to CVNs. However, GABAergic synaptic events were enhanced after acute H/H in CIHH male animals. Orexin-A elicited a reversible inhibition of LPGi-evoked GABAergic currents in control animals but evoked no significant changes in CIHH male rats. In conclusion, exaggerated inhibitory neurotransmission from the LPGi to CVNs in CIHH animals would reduce cardioprotective parasympathetic activity and enhance the risk of adverse cardiovascular events.

The nucleus of the solitary tract and the coordination of respiratory and sympathetic activities

It is well known that breathing introduces rhythmical oscillations in the heart rate and arterial pressure levels. Sympathetic oscillations coupled to the respiratory activity have been suggested as an important homeostatic mechanism optimizing tissue perfusion and blood gas uptake/delivery. This respiratory-sympathetic coupling is strengthened in conditions of blood gas challenges (hypoxia and hypercapnia) as a result of the synchronized activation of brainstem respiratory and sympathetic neurons, culminating with the emergence of entrained cardiovascular and respiratory reflex responses. Studies have proposed that the ventrolateral region of the medulla oblongata is a major site of synaptic interaction between respiratory and sympathetic neurons. However, other brainstem regions also play a relevant role in the patterning of respiratory and sympathetic motor outputs. Recent findings suggest that the neurons of the nucleus of the solitary tract (NTS), in the dorsal medulla, are essential for the processing and coordination of respiratory and sympathetic responses to hypoxia. The NTS is the first synaptic station of the cardiorespiratory afferent inputs, including peripheral chemoreceptors, baroreceptors and pulmonary stretch receptors. The synaptic profile of the NTS neurons receiving the excitatory drive from afferent inputs is complex and involves distinct neurotransmitters, including glutamate, ATP and acetylcholine. In the present review we discuss the role of the NTS circuitry in coordinating sympathetic and respiratory reflex responses. We also analyze the neuroplasticity of NTS neurons and their contribution for the development of cardiorespiratory dysfunctions, as observed in neurogenic hypertension, obstructive sleep apnea and metabolic disorders.

Chronic intermittent hypoxia and hypercapnia inhibit the hypothalamic paraventricular nucleus neurotransmission to parasympathetic cardiac neurons in the brain stem

Obstructive sleep apnea is associated with chronic intermittent hypoxia/hypercapnia (CIHH) episodes during sleep that heighten sympathetic and diminish parasympathetic activity to the heart. Although one population of neurons in the paraventricular nucleus of the hypothalamus strongly influences sympathetic tone and has increased activity after CIHH, little is known about the role of this pathway to parasympathetic neurons and how this network is altered in CIHH. We hypothesized that CIHH inhibits the excitatory pathway from the paraventricular nucleus of the hypothalamus to parasympathetic cardiac vagal neurons in the brain stem. To test this hypothesis, channelrhodopsin was selectively expressed, using viral vectors, in neurons in the paraventricular nucleus of the hypothalamus and channelrhodopsin-expressing fibers were photoactivated to evoke postsynaptic currents in cardiac vagal neurons in brain stem slices. Excitatory postsynaptic currents were diminished in animals exposed to CIHH. The paired-pulse and prolonged facilitation of the postsynaptic current amplitudes and frequencies evoked by paired and bursts of photoactivation of channelrhodopsin fibers, respectively, occurred in unexposed rats but were blunted in CIHH animals. In response to an acute challenge of hypoxia/hypercapnia, the amplitude of postsynaptic events was unchanged during, but increased after hypoxia/hypercapnia in unexposed animals. In contrast, postsynaptic currents were inhibited during hypoxia/hypercapnia in rats exposed to CIHH. In conclusion, the excitatory pathway to cardiac vagal neurons is diminished in response to both acute and chronic exposures to hypoxia/hypercapnia. This could elicit a reduced cardioprotective parasympathetic activity and an enhanced risk of adverse cardiovascular events in episodes of apnea and chronic obstructive sleep apnea.

Developmental changes in GABAergic neurotransmission to presympathetic and cardiac parasympathetic neurons in the brainstem

Cardiovascular function is regulated by a dynamic balance composed of sympathetic and parasympathetic activity. Sympathoexcitatory presympathetic neurons (PSNs) in the rostral ventrolateral medulla project directly to cardiac and vasomotor sympathetic preganglionic neurons in the spinal cord. In proximity to the PSNs in the medulla, there are preganglionic cardiac vagal neurons (CVNs) within the nucleus ambiguus, which are critical for parasympathetic control of heart rate. Both CVNs and PSNs receive GABAergic synaptic inputs that change with challenges such as hypoxia and hypercapnia (H/H). Autonomic control of cardiovascular function undergoes significant changes during early postnatal development; however, little is known regarding postnatal maturation of GABAergic neurotransmission to these neurons. In this study, we compared changes in GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs and PSNs under control conditions and during H/H in postnatal day 2-5 (P5), 16-20 (P20), and 27-30 (P30) rats using an in vitro brainstem slice preparation. There was a significant enhancement in GABAergic neurotransmission to both CVNs and PSNs at age P20 compared with P5 and P30, with a more pronounced increase in PSNs. H/H did not significantly alter this enhanced GABAergic neurotransmission to PSNs in P20 animals. However, the frequency of GABAergic IPSCs in PSNs was reduced by H/H in P5 and P30 animals. In CVNs, H/H elicited an inhibition of GABAergic neurotransmission in all ages studied, with the most pronounced inhibition occurring at P20. In conclusion, there are critical development periods at which significant rearrangement occurs in the central regulation of cardiovascular function.

Urethane suppresses hippocampal CA1 neuron excitability via changes in presynaptic glutamate release and in potassium channel activity

Urethane is a widely used anesthetic for animal experiments. Although urethane is thought to minimally interfere with neurophysiological processes and appears to preserve synaptic signal transmission, it has also been reported to produce depressive effects on neuronal excitability. In the present study, we used electrophysiological recordings to investigate the effects of urethane on rat hippocampal CA1 neurons. Whole-cell recordings were employed in a brain slice preparation to record discharges in current-clamp mode and sEPSCs or mEPSCs in voltage-clamp mode. Urethane was found to significantly increase both the interspike interval and the coefficient of variation of the firing. Moreover, it was found that the inter-event intervals of sEPSC/mEPSCs were increased, but the amplitude and the kinetic properties (rise time and decay time) of the sEPSC/mEPSC were not altered by urethane, which implies that potassium leak currents were involved in such effects. The results suggest that urethane significantly suppresses activity of hippocampal CA1 neurons and alters spontaneous pre-synaptic glutamatergic release possibly by activating potassium leak currents.

Central and Peripheral GABA(A) Receptor Regulation of the Heart Rate Depends on the Conscious State of the Animal

Intuitively one might expect that activation of GABAergic inhibitory neurons results in bradycardia. In conscious animals the opposite effect is however observed. GABAergic neurons in nucleus ambiguus hold the ability to control the activity of the parasympathetic vagus nerve that innervates the heart. Upon GABA activation the vagus nerve will be inhibited leaving less parasympathetic impact on the heart. The picture is however blurred in the presence of anaesthesia where both the concentration and type of anaesthetics can result in different effects on the cardiovascular system. This paper reviews cardiovascular outcomes of GABA activation and includes own experiments on anaesthetized animals and isolated hearts. In conclusion, the impact of changes in GABAergic input is very difficult to predict in these settings, emphasizing the need for experiments performed in conscious animals when aiming at determining the cardiovascular effects of compounds acting on GABAergic neurons.

Methods of assessing vagus nerve activity and reflexes

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

Respiratory modulation of premotor cardiac vagal neurons in the brainstem

The respiratory and cardiovascular systems are highly intertwined, both anatomically and physiologically. Respiratory and cardiovascular neurons are often co-localized in the same brainstem regions, and this is particularly evident in the ventral medulla which contains presympathetic neurons in the rostral ventrolateral medulla, premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus, and the ventral respiratory group, which includes the pre-Botzinger complex. Anatomical studies of respiratory and cardiovascular neurons have demonstrated that many of these neurons have projections and axon collateral processes which extend into their neighboring cardiorespiratory regions providing an anatomical substrate for cardiorespiratory interactions. As other reports in this Special Issue of Respiratory Physiology & Neurobiology focus on interactions between the respiratory network and baroreceptors, neurons in the nucleus tractus solitarius, presympathetic neurons and sympathetic activity, this report will focus on the respiratory modulation of parasympathetic activity and the neurons that generate parasympathetic activity to the heart, cardiac vagal neurons.

Vagal modulation of heart rate variability during atrial fibrillation in pigs

Atrial fibrillation (AF) is the most common sustained cardiac dysrhythmia and is associated with an increased risk for sudden cardiac death. The ventricular rhythm is irregular and displays both non-linear and linear patterns; however, it has not been determined whether vagally derived patterns are manifest within the irregular rhythm. Moreover, indices of increased vagal control are associated with reduced risk of sudden cardiac death. In this study, we sought to determine whether the ventricular rhythm pattern during AF is, in part, modulated by vagal activity. Vagal oscillations were forced at 0.15 Hz by neck suction in 12 pigs with sustained AF with and without glycopyrrolate (0.15 microg/kg, intravenously) vagal blockade. Vagal activity was evaluated using time- and frequency-domain heart rate variability measures. The standard deviation of RR intervals (SDRRI) was significantly increased during vagal activation compared with baseline (P = 0.006). Moreover, SDRRI correlated significantly with spectral power at 0.15 Hz during baseline (r = 0.90, P < .001) and vagal activation (r = 0.86, P < 0.05). Glycopyrrolate blocked the increase in SDRRI (P < 0.001) and blunted spectral power at 0.15 Hz (P < 0.05). These results indicate that: (1) power spectral analysis may be used to assess parasympathetic regulation during AF, and (2) vagal oscillations produce an entrainment of the ventricular rhythm during AF in pigs.

Interactions between heart rate variability and pulmonary gas exchange efficiency in humans

The respiratory component of heart rate variability (respiratory sinus arrhythmia, RSA) has been associated with improved pulmonary gas exchange efficiency in humans via the apparent clustering and scattering of heart beats in time with the inspiratory and expiratory phases of alveolar ventilation, respectively. However, since human RSA causes only marginal redistribution of heart beats to inspiration, we tested the hypothesis that any association between RSA amplitude and pulmonary gas exchange efficiency may be indirect. In 11 patients with fixed-rate cardiac pacemakers and 10 healthy control subjects, we recorded R-R intervals, respiratory flow, end-tidal gas tension and the ventilatory equivalents for carbon dioxide and oxygen during 'fast' (0.25 Hz) and 'slow' paced breathing (0.10 Hz). Mean heart rate, mean arterial blood pressure, mean arterial pressure fluctuations, tidal volume, end-tidal CO(2), and were similar between pacemaker and control groups in both the fast and slow breathing conditions. Although pacemaker patients had no RSA and slow breathing was associated with a 2.5-fold RSA amplitude increase in control subjects (39 +/- 21 versus 97 +/- 45 ms, P < 0.001), comparable (main effect for breathing frequency, F(1,19) = 76.54, P < 0.001) and reductions (main effect for breathing frequency, F(1,19) = 23.90, P < 0.001) were observed for both cohorts during slow breathing. In addition, the degree of (r = 0.36, P = 0.32) and reductions (r = 0.29, P = 0.43) from fast to slow breathing were not correlated to the degree of associated RSA amplitude enhancements in control subjects. These findings suggest that the association between RSA amplitude and pulmonary gas exchange efficiency during variable-frequency paced breathing observed in prior human work is not contingent on RSA being present. Therefore, whether RSA serves an intrinsic physiological function in optimizing pulmonary gas exchange efficiency in humans requires further experimental validation.

Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21-28 and 42-49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42-49-day-old rats was more negative than that of arterioles from 21-28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH(2) or TxA(2) assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.

Heart rate variability in rodents: uses and caveats in toxicological studies

Heart rate variability (HRV) is a measure of cardiac pacing dynamics that has recently garnered a great deal of interest in environmental health studies. While the use of these measures has become popular, much uncertainty remains in the interpretation of results, both in terms of human and animal research. In humans, HRV endpoints, specifically chronic alterations in baseline HRV patterns, have been reasonably well characterized as prognostic indicators of adverse outcomes for a variety of diseases. However, such information is lacking for reversible HRV changes that may be induced by short-term exposures to environmental toxicants. Furthermore, there are minimal substantive data, either acute or chronic, regarding the pathological interpretation or prognostic value of toxicant-induced changes in HRV in rodents. The present report summarizes the physiological and clinical aspects of HRV, the methodological processes for obtaining these endpoints, and previous human and animal studies in the field of environmental health. Furthermore, we include a discussion of important caveats and recommendations for the interpretation of HRV data in animal research.

Urethane inhibits the GABAergic neurotransmission in the nucleus of the solitary tract of rat brain stem slices

Because urethane is a widely used anesthetic in animal experimentation, in the present study, we evaluated its effects on neurons of the nucleus of the solitary tract (NTS) in brain stem slices from young rats (25-30 days old). Using the whole cell configuration of the patch-clamp technique, spontaneous postsynaptic currents (sPSCs) and evoked excitatory postsynaptic currents (eEPSCs) were recorded. Urethane (20 mM) decreased by approximately 60% the frequency of GABAergic sPSCs (1.0 +/- 0.2 vs. 0.4 +/- 0.1 Hz) but did not change the frequency, amplitude, or half-width of glutamatergic events or TTX-resistant inhibitory sPSCs [miniature inhibitory postsynaptic currents (IPSCs)]. Miniature IPSCs were measured in the presence of urethane plus 1 mM diazepam (1 mM), and no changes were seen in their amplitude. This suggests that the GABA concentration in the NTS synapses is set at saturating level. We also evaluated the effect of urethane on eEPSCs, and no significant change was observed in the amplitude of N-methyl-d-aspartate [NMDA; 44.2 +/- 11.5 vs. 37.6 +/- 10.6 pA (holding potential = 40 mV)] and non-NMDA currents [204.4 +/- 35.5 vs. 196.6 +/- 31.2 pA (holding potential = -70 mV)]. Current-clamp experiments showed that urethane did not alter the action potential characteristics and passive membrane properties. These data suggest that urethane has an inhibitory effect on GABAergic neurons in the NTS but does not change the spontaneous or evoked excitatory responses.

Heart rate variability in conscious neonatal swine: spectral features and responses to short-term intermittent hypoxia

BACKGROUND

Spectral analysis of the cardiac time series has been used as a tool for assessing levels of parasympathetic and sympathetic modulation of the sinoatrial node. In the present investigation we evaluated daily changes in heart rate variability spectra in conscious neonatal piglets that were either neurally intact (n = 5) or had undergone right stellate ganglionectomy (n = 5). The partial stellectomized animals and their intact litter mates were exposed to four days of intermittent hypoxia, each day comprising nine episodes of hypoxia alternating with nine episodes of normoxia. A time control group (n = 7) comprised animals from different litters that were not exposed to intermittent hypoxia. We hypothesized that exposure to intermittent hypoxia would increase sympathetic efferent neuronal modulation of heart rate variability spectra in neurally intact animals and in those with right stellate ganglionectomy, and that his effect would be observed in heart rate variability spectra computed from baseline recordings.

RESULTS

Overall, heart rate variability spectra during baseline conditions were dominated by high frequency activity, a reflection of parasympathetic efferent neuronal innervation and linkage to the ventilatory cycle manifested as respiratory sinus arrhythmia. Exposure to intermittent hypoxia did not alter daily baseline spectral features that would indicate an increase of sympathetic cardiac activity: low frequency (0.05 - 0.15 Hz) activity was unaffected and the ratio of low- to -high frequency activity remained less than unity indicating a predominance of high frequency activity. The resultant spectra were remarkably similar despite differences in cardiac sympathetic efferent neuronal innervation and experimental treatment. When spectra were computed from cardiac time series during representative hypoxic episodes, significant increases in activity across the low frequency region (0.05 - 0.15 Hz) of heart rate variability spectra were noted and were comparable in neurally intact animals and in those with right stellate ganglionectomy.

CONCLUSION

The findings of this investigation provided important information regarding sympathetic efferent neuronal innervation of the heart during the neonatal period. Both neurally intact animals and those with right stellate ganglionectomy had equivalent increases of activity in the low frequency region of heart rate variability spectra during hypoxic stimulation. Such a finding demonstrated the capability of residual cardiac sympathetic neuronal innervation to affect functionally appropriate changes in cardiac chronotropy.

Effect of pentobarbital on spectral characteristics and phase ratios of wave oscillations of cardiac contraction period and time of atrioventricular conduction in cats

Effects of pentobarbital on spectral characteristics and phase ratios of wave oscillations of the cardiac contraction period (RR interval) and time of atrioventricular conduction (AV interval) were studied in experiments of cats. Pentobarbital moderately reduced the mean values of both intervals and significantly reduced their standard deviations and spectral powers in all frequency bands (high-frequency, low-frequency, and very low-frequency). Pentobarbital treatment led to deceleration of breathing, the frequency range of respiratory oscillations of RR and AV intervals shifted in some cases from high to low frequencies; evaluation of spectral power in the intermittent band corresponding to respiration frequency (instead of standard fixed high-frequency band) showed that pentobarbital suppressed the respiratory oscillations in these bands. Pentobarbital induced inversion of phase ratio between respiratory oscillations of RR and AV intervals: oscillations of both intervals before pentobarbital coincided by phase, while after pentobarbital injection they were in antiphase. The mechanisms of the latter phenomenon deserve further investigation.

Impaired cardiac and sympathetic autonomic control in rats differing in acetylcholine receptor sensitivity

Acetylcholine receptors (AChR) are important in premotor and efferent control of autonomic function; however, the extent to which cardiovascular function is affected by genetic variations in AChR sensitivity is unknown. We assessed heart rate variability (HRV) and baroreflex sensitivity (BRS) in rats bred for resistance (FRL) or sensitivity (FSL) to cholinergic agents compared with Sprague-Dawley rats (SD), confirmed by using hypothermic responses evoked by the muscarinic agonist oxotremorine (0.2 mg/kg ip) ( n ≥ 9 rats/group). Arterial pressure, ECG, and splanchnic sympathetic (SNA) and phrenic (PNA) nerve activities were acquired under anesthesia (urethane 1.3 g/kg ip). HRV was assessed in time and frequency domains from short-term R-R interval data, and spontaneous heart rate BRS was obtained by using a sequence method at rest and after administration of atropine methylnitrate (mATR, 2 mg/kg iv). Heart rate and SNA baroreflex gains were assessed by using conventional pharmacological methods. FRL and FSL were normotensive but displayed elevated heart rates, reduced HRV and HF power, and spontaneous BRS compared with SD. mATR had no effect on these parameters in FRL or FSL, indicating reduced cardiovagal tone. FSL exhibited reduced PNA frequency, longer baroreflex latency, and reduced baroreflex gain of heart rate and SNA compared with FRL and SD, indicating in FSL dual impairment of cardiac and circulatory baroreflexes. These findings show that AChR resistance results in reduced cardiac muscarinic receptor function leading to cardiovagal insufficiency. In contrast, AChR sensitivity results in autonomic and respiratory abnormalities arising from alterations in central muscarinic and or other neurotransmitter receptors.

Hypoxia recruits a respiratory-related excitatory pathway to brainstem premotor cardiac vagal neurons in animals exposed to prenatal nicotine

The most ubiquitous form of arrhythmia is respiratory sinus arrhythmia in which the heart beat slows during expiration and heart rate increases during inspiration. Whereas respiratory sinus arrhythmia benefits pulmonary gas exchange respiratory dysfunction presents a major challenge to the cardiorespiratory system. Hypoxia evokes a pronounced bradycardia mediated by increases in parasympathetic cardiac activity. It has been hypothesized that the fatal events in sudden infant death syndrome (SIDS) are exaggerated cardiorespiratory responses to hypoxia. This study tests whether premotor cardiac vagal neurons receive rhythmic respiratory-related excitatory synaptic inputs during normoxia and hypoxia, and if animals exposed to nicotine in the prenatal period have exaggerated responses to hypoxia. Premotor cardiac vagal neurons in the nucleus ambiguus were identified in rats by the presence of a fluorescent tracer in medullary slices that generate rhythmic inspiratory-related motor discharge. Respiratory activity was recorded from the hypoglossal nerve and excitatory synaptic events in cardiac vagal neurons were isolated using patch clamp techniques. Adult female rats were implanted with osmotic minipumps that delivered nicotine at a level approximately equivalent to those that occur in moderate to heavy smokers. During normal eupneic respiration, as well as during hypoxia, premotor cardiac vagal neurons from control animals did not receive any rhythmic respiratory-related excitatory inputs. However in animals exposed to nicotine throughout the prenatal period respiratory bursts during hypoxia dramatically increased the frequency of excitatory synaptic events in cardiac vagal neurons. In summary, in animals exposed to nicotine throughout the prenatal period, but not in unexposed animals, respiratory bursts that occur during hypoxia dramatically increase the frequency of excitatory synaptic events in cardiac vagal neurons. This study establishes a likely neurochemical mechanism for the heart rate responses to hypoxia and a link between prenatal nicotine exposure and exaggerated bradycardia responses during hypoxia that may contribute to sudden infant death syndrome.

Prenatal nicotine exposure recruits an excitatory pathway to brainstem parasympathetic cardioinhibitory neurons during hypoxia/hypercapnia in the rat: implications for sudden infant death syndrome

Maternal cigarette smoking and prenatal nicotine exposure increase the risk for sudden infant death syndrome (SIDS) by 2- to 4-fold, yet despite adverse publicity, nearly one of four pregnant women smoke tobacco. Infants who succumb to SIDS typically experience a severe bradycardia that precedes or is accompanied by centrally mediated life-threatening apneas and gasping. Although the causes of the apnea and bradycardia prevalent in SIDS victims are unknown, it has been hypothesized that these fatal events are exaggerated cardiorespiratory responses to hypoxia or hypercapnia. Changes in heart rate are primarily determined by the activity of cardiac vagal neurons (CVNs) in the brainstem. In this study, we tested whether hypoxia/hypercapnia evokes synaptic pathways to CVNs and whether these cardiorespiratory interactions are altered by prenatal exposure to nicotine. Spontaneous rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 700- to 800-microm medullary sections. CVNs were identified in this preparation by retrograde fluorescent labeling, and excitatory synaptic inputs to CVNs were isolated and studied using patch-clamp electrophysiologic techniques. Hypoxia/hypercapnia did not elicit an increase in excitatory neurotransmission to CVNs in unexposed animals, but in animals that were exposed to nicotine in the prenatal period, hypoxia/hypercapnia recruited an excitatory neurotransmission to CVNs. This study establishes a likely neurochemical mechanism for the exaggerated decrease in heart rate in response to hypoxia/hypercapnia that occurs in SIDS victims.

Paradoxical respiratory sinus arrhythmia in the anesthetized rat

This study examines respiratory sinus arrhythmia (RSA) in the isoflurane-anesthetized rat. In fifteen female Sprague-Dawley rats, we recorded continuous ECG and respiratory airflow before and after bilateral vagotomy. RSA was assessed using power spectral analysis and by plotting the normalised changes in heart period as a function of the time during the respiratory cycle. Contrary to descriptions of RSA in conscious rats, we observed in all rats in the current study a 'reversed' pattern of RSA in which heart rate decelerated during inspiration. Elimination of vagal efferent fibres to the heart by vagotomy did not abolish the presence of reversed RSA suggesting that the pattern of heart period variation is not neural, and may be largely mechanical. Vagotomy altered breathing by increasing respiratory period, tidal volume, and the time to peak inspiratory flow. These changes did not alter the magnitude of RSA but reduced the latency period between inspiratory onset and the onset of respiratory related prolongation of heart period. Periods of positive pressure ventilation were associated with reversal of the inspiratory cardiac-deceleration pattern of RSA to resemble the more widely described pattern of inspiratory cardiac-acceleration. We conclude that RSA is not a suitable measure of vagal tone during anesthesia in the rat and reiterate the caution that needs to be taken when working with anesthetized experimental models of cardiac control.