David C. Randall

Genetic Manipulation of Intraspinal Plasticity after Spinal Cord Injury Alters the Severity of Autonomic Dysreflexia

Severe spinal cord injuries above mid-thoracic levels can lead to a potentially life-threatening hypertensive condition termed autonomic dysreflexia, which is often triggered by painful distension of pelvic viscera (bladder or bowel) and consequent sensory fiber activation, including nociceptive C-fibers. Interruption of tonically active medullo-spinal pathways after injury causes disinhibition of thoracolumbar sympathetic preganglionic neurons, and intraspinal sprouting of nerve growth factor (NGF)-responsive primary afferent fibers is thought to contribute to their hyperactivity. We investigated spinal levels that are critical for eliciting autonomic dysreflexia using a model of noxious colorectal distension (CRD) after complete spinal transection at the fourth thoracic segment in rats. Post-traumatic sprouting of calcitonin gene-related peptide (CGRP)-immunoreactive primary afferent fibers was selectively altered at specific spinal levels caudal to the injury with bilateral microinjections of adenovirus encoding the growth-promoting NGF or growth-inhibitory semaphorin 3A (Sema3a) compared with control green fluorescent protein (GFP). Two weeks later, cardio-physiological responses to CRD were assessed among treatment groups before histological analysis of afferent fiber density at the injection sites. Dysreflexic hypertension was significantly higher with NGF overexpression in lumbosacral segments compared with GFP, whereas similar overexpression of Sema3a significantly reduced noxious CRD-evoked hypertension. Quantitative analysis of CGRP immunostaining in the spinal dorsal horns showed a significant correlation between the extent of fiber sprouting into the spinal segments injected and the severity of autonomic dysreflexia. These results demonstrate that site-directed genetic manipulation of axon guidance molecules after complete spinal cord injury can alter endogenous circuitry to modulate plasticity-induced autonomic pathophysiology.

Hypertension and Disrupted Blood Pressure Circadian Rhythm in Type 2 Diabetic db/db Mice

Human Type 2 diabetes is associated with increased incidence of hypertension and disrupted blood pressure (BP) circadian rhythm. Db/db mice have been used extensively as a model of Type 2 diabetes, but their BP is not well characterized. In this study, we used radiotelemetry to define BP and the circadian rhythm in db/db mice. We found that the systolic, diastolic, and mean arterial pressures were each significantly increased by 11, 8, and 9 mmHg in db/db mice compared with controls. In contrast, no difference was observed in pulse pressure or heart rate. Interestingly, both the length of time db/db mice were active (locomotor) and the intensity of locomotor activity were significantly decreased in db/db mice. In contrast to controls, the 12-h light period average BP in db/db mice did not dip significantly from the 12-h dark period. A partial Fourier analysis of the continuous 72-h BP data revealed that the power and the amplitude of the 24-h period length rhythm were significantly decreased in db/db mice compared with the controls. The acrophase was centered at 0141 in control mice, but became scattered from 1805 to 0236 in db/db mice. In addition to BP, the circadian rhythms of heart rate and locomotor activity were also disrupted in db/db mice. The mean arterial pressure during the light period correlates with plasma glucose, insulin, and body weight. Moreover, the oscillations of the clock genes DBP and Bmal1 but not Per1 were significantly dampened in db/db mouse aorta compared with controls. In summary, our data show that db/db mice are hypertensive with a disrupted BP, heart rate, and locomotor circadian rhythm. Such changes are associated with dampened oscillations of clock genes DBP and Bmal1 in vasculature.

First-order differential-delay equation for the baroreflex predicts the 0.4-Hz blood pressure rhythm in rats

We have described a 0.4-Hz rhythm in renal sympathetic nerve activity (SNA) that is tightly coupled to 0.4-Hz oscillations in blood pressure in the unanesthetized rat. In previous work, the relationship between SNA and fluctuations in mean arterial blood pressure (MAP) was described by a set of two first-order differential equations. We have now modified our earlier model to test the feasibility that the 0.4-Hz rhythm can be explained by the baroreflex without requiring a neural oscillator. In this baroreflex model, a linear feedback term replaces the sympathetic drive to the cardiovascular system. The time delay in the feedback loop is set equal to the time delay on the efferent side, approximately 0.5 s (as determined in the initial model), plus a time delay of 0.2 s on the afferent side for a total time delay of approximately 0.7 s. A stability analysis of this new model yields feedback resonant frequencies close to 0.4 Hz. Because of the time delay in the feedback loop, the proportional gain may not exceed a value on the order of 10 to maintain stability. The addition of a derivative feedback term increases the systems stability for a positive range of derivative gains. We conclude that the known physiological time delay for the sympathetic portion of the baroreflex can account for the observed 0.4-Hz rhythm in rat MAP and that the sensitivity of the baroreceptors to the rate of change in blood pressure, as well as average blood pressure, would enhance the natural stability of the baroreflex.We have described a 0.4-Hz rhythm in renal sympathetic nerve activity (SNA) that is tightly coupled to 0.4-Hz oscillations in blood pressure in the unanesthetized rat. In previous work, the relationship between SNA and fluctuations in mean arterial blood pressure (MAP) was described by a set of two first-order differential equations. We have now modified our earlier model to test the feasibility that the 0.4-Hz rhythm can be explained by the baroreflex without requiring a neural oscillator. In this baroreflex model, a linear feedback term replaces the sympathetic drive to the cardiovascular system. The time delay in the feedback loop is set equal to the time delay on the efferent side, ∼0.5 s (as determined in the initial model), plus a time delay of 0.2 s on the afferent side for a total time delay of ∼0.7 s. A stability analysis of this new model yields feedback resonant frequencies close to 0.4 Hz. Because of the time delay in the feedback loop, the proportional gain may not exceed a value on the order of 10 to maintain stability. The addition of a derivative feedback term increases the systems stability for a positive range of derivative gains. We conclude that the known physiological time delay for the sympathetic portion of the baroreflex can account for the observed 0.4-Hz rhythm in rat MAP and that the sensitivity of the baroreceptors to the rate of change in blood pressure, as well as average blood pressure, would enhance the natural stability of the baroreflex.

Selective vagal postganglionic innervation of the sinoatrial and atrioventricular nodes in the non-human primate

The distribution of parasympathetic postganglionic nerves to the atrioventricular (AVN) and sinoatrial nodal (SAN) regions was investigated in the non-human primate heart. Eight male monkeys (Macaca fascicularis) weighing 5.5-7.0 kg. were anesthetized (alpha-chloralose, 50 mg/kg and urethane, 500 mg/kg) and instrumented to measure arterial pressure, electrocardiogram, atrial and ventricular electrograms. The cervical vagi were electrically stimulated (20 Hz, 4 V, 2 ms) before and after selective denervation (D) of the AVN and/or SAN. Vagal stimulation was repeated during atrial pacing to assess parasympathetic modulation of AVN conduction. Ablation of parasympathetic pathways to the AVN, accomplished by the disruption of the epicardial fat and surface muscle layer at the junction of the inferior vena cava and inferior left atrium eliminated (P less than 0.01) the dromotropic effects of vagal stimulation without affecting the heart rate response (right vagus, before D, paced: atrial rate 218.0 +/- 6.3, ventricular rate 67.1 +/- 23.7; after D: atrial rate 210.3 +/- 6.4, ventricular rate 210.3 +/- 6.4 beats/min, means +/- S.D.). In sharp contrast, surgical dissection of the fat pad overlying the right pulmonary vein-superior vena cava junction significantly (P greater than 0.01) attenuated negative chronotropic effects of vagal stimulation (left vagus, before D the R-R interval increased by 832.7 +/- 146.4 ms, 209.5% increase; after D 37.4 +/- 18.0 ms, 8.8% increase). These data demonstrate discrete vagal efferent pathways innervate both the SAN and AVN regions of the non-human primate heart.

Sympathetic nervous activity and arterial pressure responses during rest and acute behavioral stress in SHR versus WKY rats

The object of this experiment is to compare changes in renal sympathetic nerve activity (SNA), mean arterial blood pressure (MAP) and heart rate (HR) during rest and behavioral stress in 12-14 week old spontaneously hypertensive rats (SHR; N = 12) and normotensive Wistar-Kyoto (WKY; N = 12) controls. Animals were behaviorally trained by following a 15 s auditory conditional stimulus (CS+) with a 1/2 s tail shock. Resting MAP was higher (p < 0.001) in SHR (154 +/- 3 mmHg, mean +/- SEM) compared to WKY (116 +/- 3 mmHg); conversely, there was no difference in the average resting HR. The pattern of the SNA and MAP changes during the CS+ was similar across groups, but the amplitude was larger in the SHR. The CS+ stress stimulus evoked an initial transient MAP increase averaging 14 +/- 2 mmHg in the SHR compared to 4 +/- 1 mmHg in the WKY. This pressor response was preceded by a sudden burst of SNA averaging 177 +/- 22% over baseline in SHR versus 105 +/- 13% for the WKY. HR decreased in SHR only during the second component of the CS+ trial despite the large increase in SNA. We conclude that (1) SHR have higher reactivity than WKY to stress in SNA and MAP; (2) both SHR and WKY have greater SNA and MAP responses to CS+ than CS-(i.e., the discriminative paradigm was effective); (3) control of sympathetic and parasympathetic nervous activity during sustained stress differs remarkably in hypertensive and normotensive subjects; and (4) SHR blood pressure effector mechanisms may have a higher responsiveness to sympathetic nervous activity as compared to WKY.

Acute Cardiovascular Consequences of Anterior Descending Coronary Artery Occlusion in Unanesthetized Monkey

Summary The acute response of the unanesthetized monkey to occlusion of the anterior descending coronary artery consists of a pressor response, tachycardia and increase in d(LVP)/dt. This response resembles that reported for comparable situations in man, and may be due to elevated cardiac sympathetic tone. Anatomical and physiological characteristics of the non-human primate suggest that these animals provide attractive models for experimental study of anterior MI.

Roles of cardiac output and peripheral resistance in mediating blood pressure response to stress in rats

The change in arterial blood pressure (BP) in response to presentation of an acute behavioral stress (i.e., classical conditioning) in rat includes an initial rapid rise (C1) followed by a delayed, but more sustained, pressor response (C2). The purpose of this experiment is to determine the patterns of change in cardiac output (CO) and total peripheral vascular resistance (TPR) that are associated with the behaviorally induced pressor response. A blood flow probe was implanted around the ascending aorta, and a catheter was implanted in a femoral artery in 10 male Sprague-Dawley rats. The rats were trained by a 15-s tone (CS+) followed by a 0.5-s tail shock; another tone (CS-), never followed by shock, served as a behavioral control. BP responded to the stressful stimulus (CS+) by a rapid C1 increase (8 ± 1 mmHg; mean ± SE) followed by the delayed C2 response (2 ± 0.3 mmHg); the unconditioned response to shock was a 9 ± 2 mmHg increase in BP. The C1 BP increase produced a significant increase in TPR (10 ± 1 dyn ⋅ s/cm5); CO was not significantly changed. TPR decreased during C2 (-4 ± 2 dyn ⋅ s/cm5), whereas CO was significantly increased (2 ± 1 ml/min). These data contribute to our understanding of how the autonomic nervous system organizes the cardiovascular response to a suddenly perceived behavioral stress.The change in arterial blood pressure (BP) in response to presentation of an acute behavioral stress (i.e., classical conditioning) in rat includes an initial rapid rise (C1) followed by a delayed, but more sustained, pressor response (C2). The purpose of this experiment is to determine the patterns of change in cardiac output (CO) and total peripheral vascular resistance (TPR) that are associated with the behaviorally induced pressor response. A blood flow probe was implanted around the ascending aorta, and a catheter was implanted in a femoral artery in 10 male Sprague-Dawley rats. The rats were trained by a 15-s tone (CS+) followed by a 0.5-s tail shock; another tone (CS-), never followed by shock, served as a behavioral control. BP responded to the stressful stimulus (CS+) by a rapid C1 increase (8 +/- 1 mmHg; mean +/- SE) followed by the delayed C2 response (2 +/- 0.3 mmHg); the unconditioned response to shock was a 9 +/- 2 mmHg increase in BP. The C1 BP increase produced a significant increase in TPR (10 +/- 1 dyn.s/cm5); CO was not significantly changed. TPR decreased during C2 (-4 +/- 2 dyn.s/cm5), whereas CO was significantly increased (2 +/- 1 ml/min). These data contribute to our understanding of how the autonomic nervous system organizes the cardiovascular response to a suddenly perceived behavioral stress.

Ablation of posterior atrial ganglionated plexus potentiates sympathetic tachycardia to behavioral stress

The role of the posterior atrial ganglionated plexus (PAGP) in heart rate (HR) control was tested in unanesthetized dogs ( n = 8). Resting HR was unchanged before (85 ± 20 beats/min, mean ± SD) versus after (87 ± 18 beats/min) surgical ablation of these intrinsic cardiac ganglia (PAGPX). However, the peak tachycardia to a 30-s stressful stimulus was significantly increased ( P < 0.05) from +53 ± 22 beats/min before the denervation to +77 ± 13 beats/min after PAGPX. Conversely, the peak HR increase during the stress after β-adrenergic blockade was the same before (36 ± 24 beats/min) versus after (38 ± 14 beats/min) PAGPX. Moreover, the HR response to a neutral behavioral stimulus, which is mediated primarily by withdrawal of parasympathetic inhibition of the sinoatrial (SA) node, was unaltered by PAGPX. Thus the augmented tachycardia subsequent to PAGPX was attributable primarily to increased sympathetic action at the SA node. These findings indicate that a major role of PAGP parasympathetic neurons is to inhibit sympathoexcitatory effects on HR, probably either via interactions between neurons comprising the intrinsic plexus(es) or perhaps via presynaptic inhibition of sympathetic neurotransmitter release. This organization would allow parasympathetic ganglia within the PAGP to selectively modify sympathetic input to the SA node independent of direct vagal inhibition of pacemaker activity.The role of the posterior atrial ganglionated plexus (PAGP) in heart rate (HR) control was tested in unanesthetized dogs (n = 8). Resting HR was unchanged before (85 +/- 20 beats/min, mean +/- SD) versus after (87 +/- 18 beats/min) surgical ablation of these intrinsic cardiac ganglia (PAGPX). However, the peak tachycardia to a 30-s stressful stimulus was significantly increased (P < 0.05) from +53 +/- 22 beats/min before the denervation to +77 +/- 13 beats/min after PAGPX. Conversely, the peak HR increase during the stress after beta-adrenergic blockade was the same before (36 +/- 24 beats/min) versus after (38 +/- 14 beats/min) PAGPX. Moreover, the HR response to a neutral behavioral stimulus, which is mediated primarily by withdrawal of parasympathetic inhibition of the sinoatrial (SA) node, was unaltered by PAGPX. Thus the augmented tachycardia subsequent to PAGPX was attributable primarily to increased sympathetic action at the SA node. These findings indicate that a major role of PAGP parasympathetic neurons is to inhibit sympathoexcitatory effects on HR, probably either via interactions between neurons comprising the intrinsic plexus(es) or perhaps via presynaptic inhibition of sympathetic neurotransmitter release. This organization would allow parasympathetic ganglia within the PAGP to selectively modify sympathetic input to the SA node independent of direct vagal inhibition of pacemaker activity.

Responses of Individual Cardiac Chambers to Stimulation of the Cervical Vagosympathetic Trunk in Atropinized Dogs

Employing multiple chamber pressure recording techniques, the presence of fibers within the right and left cervical vagosympathetic trunks which innervate both right and left ventricles has been demonstrated. Electrical excitation of the cervical vagosympathetic in the atropinized dog elicited a distinct increase in both rate of rise and in maximal systolic intraventricular pressures, with or without cardiac acceleration. It persisted during electrical pacing of the right ventricle. In many animals, the a-wave of the atrial pressure traces also showed augmentation, but this was not consistent or essential for the ventricular response. Although pulmonary vasoconstriction was not ruled out as a concurrent event, its contribution to right ventricular augmentation was not essential as demonstrated in bilateral, isovolumetric ventricular preparations. The evidence indicates the presence of adrenergic fibers in the cervical vagosympathetic trunk which are distributed to all four cardiac chambers. Their effects on the heart are abolished by the β-blocking agent, propranolol, and are unaffected by eserine administration.

Stability of the heart rate power spectrum over time in the conscious dog.

The purpose of this experiment was to test the stability of the heart rate (HR) power spectrum over time in conscious dogs. HR was recorded for 1 h for each of six animals on 2 days. A Fast Fourier transform was used to derive the HR power spectrum for the 12 contiguous 5‐min epochs comprising the 1‐h recordings. Changes in frequency and amplitude of the various spectral peaks were quantitatively examined. We confirm the presence of two major concentrations of power centered around 0.02 (low frequency peak) and 0.32 Hz (high frequency peak). However, we observed variations in these spectral peaks, especially their amplitudes, both within each hour and from day 1 to day 2. The amplitudes of these two spectral peaks tended to vary reciprocally. HR power spectra based on 5 min of recorded data were also derived from an additional eight animals in both the lying and standing positions; the power spectra from these short recordings were sufficiently sensitive to detect redistributions in power due to changes in posture in all eight dogs. We conclude that: 1) data should be recorded for relatively long periods (e.g., 1 h) to characterize the HR power spectrum; 2) some variability in frequency and amplitude will persist across spectra even when based on longer data bases; 3) care should be taken to ensure that the subjects behavioral state is stable within the recording period; 4) shorter (e.g., 5 min) data bases are not suitable except for detecting relatively robust changes in the HR power spectrum.—Brown, D. R.; Randall, D. C.; Knapp, C. F.; Lee, K. C.; Yingling, J. D. Stability of the heart rate power spectrum over time in the conscious dog. FASEB J. 3: 1644‐1650; 1989.