Autonomic nerve activity and cardiovascular changes during discrete seizures in rats.

Activity in both divisions of the autonomic nervous system (ANS) can increase during seizures and result in tachy- or bradyarrhythmias. We sought to determine the patterns of ANS activity that led to heart rate (HR) changes and whether the character of ANS and HR changes can impact the seizures themselves. Simultaneous recordings of vagus nerve and cervical sympathetic ganglionic or nerve activity, EEG, ECG, and blood pressure were acquired from 16 urethane-anesthetized rats that received systemic kainic acid to induce seizures. After initial continuous seizure activity, discrete seizures were observed in 11/16 rats. Individual seizures were classified based on HR changes as tachycardic (n = 3), bradycardic (n = 17), or one of two more severe categories in which (a) the seizure appeared to be terminated by severe bradyarrhythmia (n = 5) or (b) the animal died (n = 6). Interestingly, even simple bradycardic seizures had episodes of dramatically increased respiratory effort, which we interpret as evidence of airway occlusion based on muscle artifacts in the recordings with transient blood pressure decreases. In the severe outcomes, ANS activity increased during seizures until it caused a drastic HR reduction (>50%), in which case seizure and ANS activity decreased dramatically. Sympathetic activity during this late vulnerable period was important for survival. We conclude that individual seizures produce (a) stereotypical changes in autonomic activity and HR, (b) persistence of sympathetic tone helps to protect against death, and (c) bradycardic seizures may indicate increased risk for seizure-associated obstructive apnea.

High Frequency Oscillations in Rat Hippocampal Slices: Origin, Frequency Characteristics, and Spread.

Field potential oscillations reflect repetitive firing and synaptic activity by ensembles of neurons in certain areas of the brain. They can be distinguished as slow (e.g., alpha, delta, and theta), fast (e.g., beta and gamma), and high frequency oscillations (HFOs). Neuronal oscillations are involved in a variety of physiological and pathophysiological states such as cognition, consciousness, and seizures. The laminar structure of rat hippocampus affords a way to study these oscillations in hippocampal slices. Rat ventral hippocampal brain slices were cut and maintained in a recording chamber that permitted 64 simultaneous extracellular recordings in the presence of artificial CSF. Brief single stimulus pulses were applied with bipolar electrodes to the CA3 or CA1 regions of hippocampus. Single pulses triggered epileptiform population events that included HFOs in the 150-250 Hz range in the presence of GABAA receptor blockade or kainic acid. HFOs also occurred spontaneously in the presence of kainic acid. The oscillations had the largest amplitude in the CA3c cell layer, regardless of the drug, and were synchronized throughout the cell layer. AMPA receptor blockade stopped these HFOs, whereas NMDA receptor blockade did not. Gap junction activation did not restore HFOs in the presence of AMPA receptor blockade. Our findings suggest that a population of excitatory neurons in CA3c may be a primary focus of seizure-like activity in Ammon's Horn. We suggest that the interconnection of CA3c is different from the rest of CA3.

Assessment of arterial stiffness from pedal artery Korotkoff sound recordings: feasibility and potential utility.

Brachial artery (BA) Korotkoff sound (KS) timing reflects arterial stiffness. We recorded pedal artery (PA) KS in 68 healthy subjects using an electronic stethoscope and electrocardiography. Intervals between QRS complex of the electrocardiogram and KS waveform peaks (termed the QKD interval) were measured for 60 seconds, averaged, and QKD velocity (v) calculated. Carotid-BA and carotid-PA pulse wave velocities (PWVs) were measured by applanation tonometry. Analyzable KS recordings were obtained from BA and PA in 100% and 92% subjects. PA QKDv decreased less than BA QKDv with progressive cuff inflation. At diastolic blood pressure + 20 mm Hg (maximal yield), BA QKDv was independently associated with weight and pulse pressure, whereas PA QKDv was related to weight and age. PA QKDv correlated with its corresponding PWV stronger than BA QKDv. In conclusion, PA KS is optimally recorded at diastolic blood pressure + 20 mm Hg; PA QKDv is correlated with age and better correlates with PWV than does BA QKDv. This technique may provide a simple arterial stiffness measure.

Vagal control of cardiac electrical activity and wall motion during ventricular fibrillation in large animals.

Vagal inputs control pacemaking and conduction systems in the heart. Anatomical evidence suggests a direct ventricular action, but functional evidence that separates direct and indirect (via the conduction system) vagal actions is less well established. We studied vagus nerve stimulation (VNS) during sinus rhythm and ventricular fibrillation (VF) in pigs and sheep to determine: 1) the range of unilateral and bilateral actions (inotropic and chronotropic) and 2) whether VNS alters left ventricular motion and/or electrical activity during VF, a model of abnormal electrical conduction of the left ventricle that excludes sinus and atrioventricular nodal function. Adult pigs (N=8) and sheep (N=10) were anesthetized with urethane and mechanically ventilated. VNS was performed in animals at 1, 2, 5, 10, 20, 50, and 100Hz for 20s. VF was induced with direct current to the ventricles or occlusion of the left anterior descending coronary artery. In 4 pigs and 3 sheep, left ventricular wall motion was assessed from endocardial excursion in epicardial echocardiography. In sheep and pigs, the best frequency among those tested for VNS during sinus rhythm to produce sustained electrical and mechanical ventricular standstill was 50Hz for unilateral or bilateral stimulation. When applied during VF, bilateral VNS increased the variability of the dominant VF frequency, indicating a direct impact on the excitability of ventricular myocytes, and decreased endocardial excursion by more than 50% during VF. We conclude that the vagus nerve directly modulates left ventricular function independently from its effects on the conduction system.

Quantitative video laryngoscopy to monitor recovery from recurrent laryngeal nerve injury in the rat.

Recovery from unilateral vocal-fold paralysis is lengthy, unpredictable, and often incomplete, highlighting the need for better treatments of the injured recurrent laryngeal nerve. To be able to monitor recovery of vocal-fold motion in studies with rats, we developed a procedure for quantitative video laryngoscopy. An asymmetry index was defined as a continuous and robust measure of unequal vocal-fold motion and calculated from spectral-density plots of vocal-fold displacements. In a cohort of 8 animals, unilateral vocal-fold paralysis was observed within seconds after clamping of the right recurrent laryngeal nerve and was accompanied by a markedly negative asymmetry index. Over the next month, the asymmetry index gradually returned to zero, concomitant with a visible recovery of vocal-fold motion. Our results suggest that quantitative video laryngoscopy is a sensitive and discriminating method for monitoring recovery from recurrent laryngeal nerve injury and set the stage for testing novel surgical and pharmacological treatments of unilateral vocal-fold paralysis.

Relation of autonomic and cardiac abnormalities to ventricular fibrillation in a rat model of epilepsy.

Cardiac autonomic, conduction, and structural changes may occur in epilepsy and may contribute to sudden unexpected death in epilepsy (SUDEP), e.g. by increasing the risk for ventricular fibrillation (VF). In a model of chronic seizures in rats, we sought to study (1) cardiac and autonomic derangements that accompany the epileptic state, (2) whether chronically seizing rats experienced more significant cardiac effects after severe acute seizures, and (3) the susceptibility of chronically seizing rats to VF arising from autonomic and hypoxemic changes, which commonly occur during seizures. Sprague-Dawely rats were injected with saline or kainic acid to induce chronic seizures. At 2-3 months or 7-11 months after injection, these rats were studied with both 12-lead electrocardiography (to assess heart rate variability and QT dispersion) and echocardiography under ketamine/xylazine or urethane anesthesia. Hearts were subsequently excised, weighed, and examined histologically. Epileptic rats exhibited decreased vagal tone, increased QT dispersion, and eccentric cardiac hypertrophy without significant cardiac fibrosis, especially at 7-11 months post-injection. Of these three findings, vagal tone was inversely correlated with heart weights. Epileptic rats exhibited diminished systolic function compared to controls after severe acute seizures. However, animals with long-standing chronic seizures were less susceptible to autonomic/hypoxemia-driven VF, and their susceptibility inversely correlated with mean left ventricular wall thickness on histology. On the basis of this model, we conclude that cardiac changes accompany epilepsy and these can lead to significant seizure-associated cardiac performance decreases, but these cardiac changes actually lower the probability of VF.

Determination of heart rate variability with an electronic stethoscope.

INTRODUCTION: Heart rate variability (HRV) is widely used to characterize cardiac autonomic function by measuring beat-to-beat alterations in heart rate. Decreased HRV has been found predictive of worse cardiovascular (CV) outcomes. HRV is determined from time intervals between QRS complexes recorded by electrocardiography (ECG) for several minutes to 24 h. Although cardiac auscultation with a stethoscope is performed routinely on patients, the human ear cannot detect heart sound time intervals. The electronic stethoscope digitally processes heart sounds, from which cardiac time intervals can be obtained. METHODS: Accordingly, the objective of this study was to determine the feasibility of obtaining HRV from electronically recorded heart sounds. We prospectively studied 50 subjects with and without CV risk factors/disease and simultaneously recorded single lead ECG and heart sounds for 2 min. RESULTS: Time and frequency measures of HRV were calculated from R-R and S1-S1 intervals and were compared using intra-class correlation coefficients (ICC). CONCLUSION: The majority of the indices were strongly correlated (ICC 0.73-1.0), while the remaining indices were moderately correlated (ICC 0.56-0.63). In conclusion, we found HRV measures determined from S1-S1 are in agreement with those determined by single lead ECG, and we demonstrate and discuss differences in the measures in detail. In addition to characterizing cardiac murmurs and time intervals, the electronic stethoscope holds promise as a convenient low-cost tool to determine HRV in the hospital and outpatient settings as a practical extension of the physical examination.

Autonomic boundary conditions for ventricular fibrillation and their implications for a novel defibrillation technique.

The sympathetic and parasympathetic divisions of the autonomic nervous system modulate cardiac rhythm and the probability of arrhythmia occurrence. Both increased sympathetic drive and hypoxia increase the likelihood for ventricular fibrillation (VF). Vagus nerve stimulation (VNS) can protect from fatal arrhythmias via cholinergic and nitrergic action. We sought to determine boundary conditions for VF and defibrillation by autonomic manipulations accompanied or not by hypoxic changes in urethane-anesthetized rats. VF was induced with (1) vagotomy, (2) systemic high-dose (>15 mg/kg) isoproterenol, and (3) hypoxemia. When VNS (50 Hz) produced cardiac standstill, it converted every VF episode (59/59). A nitric oxide synthase inhibitor did not reduce VNS efficacy (13/14 episodes converted), but addition of atropine reduced VNS efficacy (11/27 episodes converted). VF can be induced by autonomic derangements only under constrained conditions, including sympathetic over-activation, reduced parasympathetic input, and hypoxemia. VNS can provide an alternative method to defibrillate via its cholinergic action.

Neurogenic pathway mediated remote preconditioning protects the brain from transient focal ischemic injury.

Attenuation of ischemic injury can be achieved by priming the brain with a sub-lethal ischemic insult, a phenomenon known as ischemic preconditioning (IP). We sought to determine if subjecting a distant organ, such as the lower limb, to a similar priming ischemic insult would result in protection of the brain from a subsequent severe ischemic injury, as induced by middle cerebral artery occlusion (MCAo) and if this protection is mediated via a neurogenic pathway. Adult Wistar male rats were subjected to either remote preconditioning (RPC) or sham surgery and then subsequently underwent 2 h MCAo 24, 48 or 72 h after the RPC/sham RPC stimulus. Of the animals undergoing RPC, only those that sustained MCAo 24 h later showed significantly smaller cerebral infarct volumes (150.34±30.91 mm(3)) and better clinical neurological outcomes (1.15±0.69) as compared to the sham RPC group (infarct volume 250.25±26.98mm(3); neurological score 1.80±0.87) (P<0.05). RPC animals sustaining MCAo at 48 and 72 h later did not show significant differences in cerebral infarct volumes or clinical neurological outcomes as compared to the sham RPC group. Furthermore, attenuation of the neuroprotective effect by the ganglion blocker hexamethonium suggested a neurogenically mediated pathway responsible for this phenomenon. Remote sub-lethal ischemic injury to both lower limbs results in cerebral protection from subsequent ischemia within 24 h of initiation of the RPC stimulus and this protection in part may be mediated via a neurogenic pathway.

Role of SREBP-1 in the development of parasympathetic dysfunction in the hearts of type 1 diabetic Akita mice.

RATIONALE: Diabetic autonomic neuropathy (DAN), a major complication of diabetes mellitus, is characterized, in part, by impaired cardiac parasympathetic responsiveness. Parasympathetic stimulation of the heart involves activation of an acetylcholine-gated K+ current, I(KAch), via a (GIRK1)2/(GIRK4)2 K+ channel. Sterol regulatory element binding protein-1 (SREBP-1) is a lipid-sensitive transcription factor. OBJECTIVE: We describe a unique SREBP-1-dependent mechanism for insulin regulation of cardiac parasympathetic response in a mouse model for DAN. METHODS AND RESULTS: Using implantable EKG transmitters, we demonstrated that compared with wild-type, Ins2(Akita) type I diabetic mice demonstrated a decrease in the negative chronotropic response to carbamylcholine characterized by a 2.4-fold decrease in the duration of bradycardia, a 52+/-8% decrease in atrial expression of GIRK1 (P<0.01), and a 31.3+/-2.1% decrease in SREBP-1 (P<0.05). Whole-cell patch-clamp studies of atrial myocytes from Akita mice exhibited a markedly decreased carbamylcholine stimulation of I(KAch) with a peak value of -181+/-31 pA/pF compared with -451+/-62 pA/pF (P<0.01) in cells from wild-type mice. Western blot analysis of extracts of Akita mice demonstrated that insulin treatment increased the expression of GIRK1, SREBP-1, and I(KAch) activity in atrial myocytes from these mice to levels in wild-type mice. Insulin treatment of cultured atrial myocytes stimulated GIRK1 expression 2.68+/-0.12-fold (P<0.01), which was reversed by overexpression of dominant negative SREBP-1. Finally, adenoviral expression of SREBP-1 in Akita atrial myocytes reversed the impaired I(KAch) to levels in cells from wild-type mice. CONCLUSIONS: These results support a unique molecular mechanism for insulin regulation of GIRK1 expression and parasympathetic response via SREBP-1, which might play a role in the pathogenesis of DAN in response to insulin deficiency in the diabetic heart.