Effects of middle cerebral artery occlusion on baroreceptor reflex control of heart rate in the rat

Baroreflex Curve Fitting Using a WYSIWYG Boltzmann Sigmoidal Equation

Arterial baroreflex assessment using vasoactive substances enables investigators to collect data pairs over a wide range of blood pressures and reflex reactions. These data pairs relate intervals between heartbeats or sympathetic neural activity to blood pressure values. In an X-Y plot the data points scatter around a sigmoidal curve. After fitting the parameters of a sigmoidal function to the data, the graph’s characteristics represent a rather comprehensive quantitative reflex description. Variants of the 4-parameter Boltzmann sigmoidal equation are widely used for curve fitting. Unfortunately, their ‘slope parameters’ do not correspond to the graph’s actual slope which complicates the analysis and bears the risk of misreporting. We propose a modified Boltzmann sigmoidal function with preserved goodness of fit whose parameters are one-to-one equivalent to the sigmoidal curve’s characteristics.

Dexamethasone attenuates the modulatory effect of the insular cortex on the baroreflex in anesthetized rat

The arterial baroreflex (BR) is an important neural mechanism for the stabilization of arterial pressure (AP). It is known that the insular cortex (IC) and other parts of the central autonomic network (CAN) are able to modulate the BR arc, altering baroreflex sensitivity (BRS). In addition, the sensitivity of the BR changes under the influence of hormones, in particular glucocorticoids (GC). It has been suggested that GC may influence BRS by altering the ability of the IC to modulate the BR. This hypothesis has been tested in experiments on rats anesthetized with urethane. It was found that microelectrostimulation of the visceral area in the left IC causes a short-term drop in AP, which is accompanied by bradycardia, and impairs BRS. The synthetic GC dexamethasone (DEX) did not significantly affect the magnitude of depressor responses but increased BRS and impaired the effect of IC stimulation on the BR. The results obtained confirm the hypothesis put forward and suggest that GC can attenuate the inhibitory effects of the IC on the BR arc, thereby enhancing the sensitivity of the BR.

Rostral cuneiform nucleus and the defence reaction: Direct and indirect midbrain-medullary 5-HT mechanisms in baroreflex inhibition

BACKGROUND AND PURPOSE

Activation of the defence reaction inhibits the baroreflex response via the intermediate rostro-ventromedial medulla (B3 raphé) and nucleus tractus solitarius (NTS). Our aim was to determine whether and how baroreflex inhibition, induced by the disinhibition of the rostral cuneiform nucleus (part of the defence pathway), involves 5-HT neurons in B3 and 5-HT₃ receptors in the NTS.

EXPERIMENTAL APPROACH

We performed immunohistochemistry and anatomical experiments to determine whether raphé 5-HT cells expressing Fos were directly targeted by the rostral cuneiform nucleus. The effect of blocking raphé 5-HT neurotransmission and NTS 5-HT₃ receptors on cuneiform-induced inhibition of the baroreflex cardiac response were also analysed.

KEY RESULTS

Bicuculline, microinjected into the rostral cuneiform nucleus, induced an increase of double-labelled Fos-5-HT-IR cells in both the lateral paragigantocellular nucleus (LPGi) and raphé magnus. The anterograde tracer Phaseolus vulgaris leucoaggutinin injected into the rostral cuneiform nucleus revealed a dense projection to the LPGi but not raphé magnus. Cuneiform-induced baroreflex inhibition was prevented by B3 injection of 8-OH-DPAT, a selective 5-HT_1A receptor agonist. Cuneiform disinhibition also failed to inhibit the baroreflex bradycardia after NTS microinjection of the 5-HT₃ receptor antagonist granisetron and in 5-HT₃ receptor knockout mice.

CONCLUSION AND IMPLICATIONS

The rostral cuneiform nucleus participates in the defence inhibition of the baroreflex bradycardia via direct activation of the LPGi and via a projection to the raphé magnus to activate NTS 5-HT₃ receptors and inhibit second-order baroreflex neurons. These data bring new insights in primary and secondary mechanisms involved in vital baroreflex prevention during stress.

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Nitric oxide in the insular cortex modulates baroreflex responses in a cGMP-independent pathway

Insular cortex is a brain structure involved in the modulation of autonomic activity and cardiovascular function. The nitric oxide/cyclic guanosine-3',5'-monophosphate pathway is a prominent signaling mechanism in the central nervous system, controlling behavioral and physiological responses. Nevertheless, despite evidence regarding the presence of nitric oxide-synthesizing neurons in the insular cortex, its role in the control of autonomic and cardiovascular function has never been reported. Thus, the present study aimed to investigate the involvement of nitric oxide/cyclic guanosine-3',5'-monophosphate pathway mediated by neuronal nitric oxide synthase (nNOS) activation within the insular cortex in the modulation of baroreflex responses in unanesthetized rats. For this, we evaluated the effect of bilateral microinjection of either the nitric oxide scavenger carboxy-PTIO, the selective neuronal nitric oxide synthase inhibitor N^ω-Propyl-l-arginine or the soluble guanylate cyclase inhibitor ODQ into the insular cortex on the bradycardia evoked by blood pressure increases in response to intravenous infusion of phenylephrine, and the tachycardia caused by blood pressure decreases evoked by intravenous infusion of sodium nitroprusside. Bilateral microinjection of either NPLA or carboxy-PTIO into the insular cortex increased the reflex bradycardic response, whereas the reflex tachycardia was decreased by these treatments. Bilateral microinjection of the soluble guanylate cyclase inhibitor into the insular cortex did not affect any parameter of baroreflex function evaluated. Overall, our findings provide evidence that insular cortex nitrergic signaling, acting via neuronal nitric oxide synthase, plays a prominent role in control of baroreflex function. However, control of reflex responses seems to be independent of soluble guanylate cyclase activation.

Linear and nonlinear parameters of heart rate variability in ischemic stroke patients

INTRODUCTION

Cardiovascular system presents cortical modulation. Post-stroke outcome can be highly influenced by autonomic nervous system disruption. Heart rate variability (HRV) analysis is a simple non-invasive method to assess sympatho-vagal balance.

OBJECTIVES

The purpose of this study was to investigate cardiac autonomic activity in ischemic stroke patients and to asses HRV nonlinear parameters beside linear ones.

METHODS

We analyzed HRV parameters in 15 right and 15 left middle cerebral artery ischemic stroke patients, in rest condition and during challenge (standing and deep breathing). Data were compared with 15 age- and sex-matched healthy controls.

RESULTS

There was an asymmetric response after autonomic stimulation tests depending on the cortical lateralization in ischemic stroke patients. In resting state, left hemisphere stroke patients presented enhanced parasympathetic control of the heart rate (higher values for RMSSD, pNN50 and HF in normalized units). Right hemisphere ischemic stroke patients displayed a reduced cardiac parasympathetic modulation during deep breathing test. Beside time and frequency domain, using short-term ECG monitoring, cardiac parasympathetic modulation can also be assessed by nonlinear parameter SD1, that presented strong positive correlation with time and frequency domain parameters RMSSD, pNN50, HFnu, while DFA α1 index presented negative correlation with the same indices and positive correlation with the LFnu and LF/HF ratio, indicating a positive association with the sympatho-vagal balance.

CONCLUSIONS

Cardiac monitoring in clinical routine using HRV analysis in order to identify autonomic imbalance may highlight cardiac dysfunctions, thus helping preventing potential cardiovascular complications, especially in right hemisphere ischemic stroke patients with sympathetic hyperactivation.

Sex Differences in Insular Cortex Gyri Responses to the Valsalva Maneuver

Sex differences in autonomic regulation may underlie cardiovascular disease variations between females and males. One key autonomic brain region is the insular cortex, which typically consists of five main gyri in each hemisphere, and shows a topographical organization of autonomic function across those gyri. The present study aims to identify possible sex differences in organization of autonomic function in the insula. We studied brain functional magnetic resonance imaging (fMRI) responses to a series of four 18-s Valsalva maneuvers in 22 healthy females (age ± SD: 50.0 ± 7.9 years) and 36 healthy males (45.3 ± 9.2 years). Comparisons of heart rate (HR) and fMRI signals were performed with repeated measures ANOVA (threshold P < 0.05 for all findings). All subjects achieved the target 30 mmHg expiratory pressure for all challenges. Typical HR responses were elicited by the maneuver, including HR increases from ~4 s into the strain period (Phase II) and rapid declines to below baseline 5–10 s, following strain release (Phase IV). Small, but significant, sex differences in HR percent change occurred during the sympathetic-dominant Phase II (female < male) and parasympathetic-dominant Phase IV (female > male, i.e., greater undershoot in males). The insular cortices showed similar patterns in all gyri, with greater signal decreases in males than females. Both sexes exhibited an anterior–posterior topographical organization of insular responses during Phase II, with anterior gyri showing higher responses than more posterior gyri. The exception was the right anterior-most gyrus in females, which had lower responses than the four other right gyri. Responses were lateralized, with right-sided dominance during Phase II in both sexes, except the right anterior-most gyrus in females, which showed lower responses than the left. The findings confirm the anterior and right-sided sympathetic dominance of the insula. Although sex differences were prominent in response magnitude, organization differences between males and females were limited to the right anterior-most gyrus, which showed a lower fMRI response in females vs. males (and vs. other gyri in females). The sex differences suggest a possible differing baseline state of brain physiology or tonic functional activity between females and males, especially in the right anterior-most gyrus.

Cardiovascular and single-unit responses to L-glutamate injection into the posterior insular cortex in rat

The insular cortex in rat is a longitudinal strip that runs along the rostral half of the rhinal fissure. The previous studies showed connections between the posterior insular cortex (PIC) and some major cardiovascular centers. Based on the stimulation site, electrical or chemical stimulation of the PIC induced an increase or a decrease in blood pressure (BP) and heart rate (HR). There is no report of simultaneous cardiovascular and single-unit recording microinjection of Glut in the PIC. In this study, L-glutamate was microinjected into the PIC of urethane anesthetized rats and arterial pressure, HR and single-unit responses were recorded simultaneously. Also the response of the neurons to baroreceptor activation was explored. Glut produced five types of long oscillatory, pressor, depressor, bradycardic and tachycardic cardiovascular responses, with no association between pressure and HR responses. We also observed five single-unit responses, consisting of short excitatory, long oscillatory, excitatory, inhibitory and mixed responses. There was an association between oscillation in BP and in single-unit response. There were some differences between the two sides especially for single-unit responses. In conclusion, there were five types of cardiovascular and five types of single-unit responses, to Glut microinjection into PIC, from which three types were correlated. The left side of the PIC is involved more in the cardiovascular functions. These data along with the fact that most recorded neurons responded to baroreceptor activation, might imply the presence of feedback systems in the PIC, producing irregularity in BP and HR.

Both α1- and α2-adrenoceptors in the insular cortex are involved in the cardiovascular responses to acute restraint stress in rats

The insular cortex (IC) is a limbic structure involved in cardiovascular responses observed during aversive threats. However, the specific neurotransmitter mediating IC control of cardiovascular adjustments to stress is yet unknown. Therefore, in the present study we investigated the role of local IC adrenoceptors in the cardiovascular responses elicited by acute restraint stress in rats. Bilateral microinjection of different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α1-adrenoceptor antagonist WB4101 into the IC reduced both the arterial pressure and heart rate increases elicited by restraint stress. However, local IC treatment with different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α2-adrenoceptor antagonist RX821002 reduced restraint-evoked tachycardia without affecting the pressor response. The present findings are the first direct evidence showing the involvement of IC adrenoceptors in cardiovascular adjustments observed during aversive threats. Our findings indicate that IC noradrenergic neurotransmission acting through activation of both α1- and α2-adrenoceptors has a facilitatory influence on pressor response to acute restraint stress. Moreover, IC α1-adrenoceptors also play a facilitatory role on restraint-evoked tachycardiac response.

Functional neuroanatomy of autonomic regulation

Considerable effort has been put into animal studies establishing the sites in the brain that are responsible for control of the autonomic nervous system. These studies relied on an electrophysiological or neurochemical response to the activation of peripheral autonomic receptors or chemical or electrical stimulation of central sites. A large number of excellent reviews summarize the results of these studies. More recently, functional imaging has been used to not only confirm the electrophysiological and anatomical studies in animals, but has allowed a more complete understanding of how the brain responds as a whole for effecting autonomic control. The earliest studies to examine forebrain control during functional imaging utilized tests that involved active participation of the subjects and included maximal inspiration, Valsalva manoeuvre, isometric handgrip and cold compress application. There were a few issues that arose from these studies. First, they involved areas of the brain that included active decision making, they were more prone to inducing movement artefact, and some of these tests could activate noxious regions in the brain in addition to autonomic sites. In fact, this dual modality activation represented a more severe complication for investigators determining nociceptive sites in the brain, since virtually all of their stimuli had concomitant autonomic responses. More recent investigations attempted to resolve these issues with more selective passive and active stimuli. In spite of the very different approaches taken to visceral activation in functional imaging studies, a consistent picture of the key areas involved in autonomic control has emerged.

Sympathetic control of short-term heart rate variability and its pharmacological modulation

The static relationship between heart rate (HR) and the activity of either vagal or sympathetic nerves is roughly linear within the physiological range of HR variations. The dynamic control of HR by autonomic nerves is characterized by a fixed time delay between the onset of changes in nerve activity and the onset of changes in HR. This delay is much longer for sympathetically than for vagally mediated changes in HR. In addition, the kinetics of the HR responses shows the properties of a low-pass filter with short (vagal) and long (sympathetic) time constants. These differences might be secondary to differences in nervous conduction times, width of synaptic cleft, kinetics of receptor activation and post-receptor events. Because of the accentuated low-pass filter characteristics of the HR response to sympathetic modulation, sympathetic influences are almost restricted to the very-low-frequency component of HR variability, but the chronotropic effects of vagal stimulation usually predominate over those of sympathetic stimulation in this frequency band. Oscillations in cardiac sympathetic nerve activity are not involved in respiratory sinus arrhythmia (high-frequency component) and make a minor contribution to HR oscillations of approximately 10-s period (low-frequency component of approximately 0.1 Hz), at least in the supine position. In the latter case, HR oscillations are derived mainly from a baroreflex, vagally mediated response to blood pressure Mayer waves. Beta-blockers and centrally acting sympathoinhibitory drugs share the ability to improve the baroreflex control of HR, possibly through vagal facilitation, which might be beneficial in several cardiovascular diseases.

Brain perfusion assessed by 99mTc-ECD SPECT imaging in pediatric patients with neurally mediated reflex syncope

BACKGROUND

The involvement of cardiogenic and neurogenic mechanisms in neurally mediated reflex syncope is well documented. In our previous studies in patients with neurally mediated reflex syncope, we have found evidence for differential regulation of the noradrenergic receptors in tilt-positive and tilt-negative patients. The present work concentrates on the observations of differences in regional brain perfusion using brain SPECT via injecting the patient at the completion of the tilt test.

METHODS AND RESULTS

The following study was designed to assess the reduction and regional differences in cerebral blood flow by means of SPECT using technetium-99m labeled V-oxo-1,2-N1ethylenedylbisl-cysteine diethylester (ECD) in patients with an injection during tilt testing. Twenty patients with NMS were included in the study with a mean age of 12.2 years (age range; 8-16 years). HUT was positive in 10 patients and negative in 10 patients. When tilt (+) and tilt (-) were evaluated together, regional cortical/cerebellum ratios were ranging from 0.85 to 1.25 in different cortical areas with highest variability of perfusion index in left frontoparietal cortex. The lowest perfusion index values were observed in the left anterior frontal region followed by the left prefrontal-frontoparietal-anterior, parietal-orbito frontal, and anterior temporal regions where perfusion is predominantly supplied via the anterior and middle cerebral arteries, while these differences did not reach statistical significance in a single dominant region compared to the other regions examined using ANOVA (P > 0.05) with this sample size. Decreases in [99mTc]ECD uptake were more widespread regionally on the left hemisphere than were decreases in right side of the brain. However when tilt- and tilt+ groups were compared, perfusion was significantly lower in the right periinsular posterior parietal and temporal regions (P < 0.05) in tilt + group.

CONCLUSION

These tilt induced regional differences in brain perfusion suggest the distinct roles of middle cerebral artery dominant territory-related vasodepressor compensation mechanisms in neurally mediated reflex syncope phenomena where cerebral lateralization of cardiac control and insular ischemia may play an important role.

Medial prefrontal cortex modulation of the baroreflex parasympathetic component in the rat

The ventral portion of the medial prefrontal cortex (vMPFC) that comprises the prelimbic and infralimbic cortex is involved in arterial blood pressure and heart rate control. In the present study, we attempted to verify the effect of an acute and reversible blockade of vMPFC activity by local bilateral microinjections of either lidocaine (a local anesthetic) or CoCl2 (a nonselective synapse blocker) on the baroreflex response of unanesthetized rats. Bilateral microinjection of lidocaine into the vMPFC did not affect the tachycardiac response to mean arterial pressure (MAP) decreases caused by i.v. infusion of sodium nitroprusside or the baroreflex gain in unanesthetized rats. However, lidocaine caused a reversible shift of the reflex threshold pressure toward higher (MAP) increases in response to i.v. infusion of phenylephrine, thus indicating an action on the parasympathetic component of the baroreflex. The effects of the blockade of local synapses in the vMPFC by CoCl2 were similar to those observed after the acute ablation of that area caused by lidocaine. Bilateral microinjection of CoCl2 into the vMPFC also caused a shift of the reflex threshold pressure bradycardiac responses to MAP increases toward higher MAP values, without affecting the baroreflex gain. In conclusion, our data indicate that the vMPFC is involved in baroreflex control, and more specifically in the modulation of the parasympathetic baroreflex component. The temporary ablation of this area by local microinjections of lidocaine caused a shift of the reflex threshold pressure toward higher MAP values, which is compatible with the idea that the vMPFC has a modulatory action on the baroreflex. The observation that CoCl2 and lidocaine microinjections had similar effects on the baroreflex also suggests that this modulation involves local synaptic neurotransmission within the vMPFC.

Insular cortex lesions alter baroreceptor sensitivity in the urethane-anesthetized rat

Cardiovascular representation has been demonstrated within the insular cortex and lateralization has been previously inferred. In this study, baroreceptor gain was investigated in response to the systemic injection of the pressor agent phenylephrine (PE) and the depressor agent sodium nitroprusside (SNP) in 57 urethane-anesthetized, male Sprague-Dawley rats before and after single lesion placement. Lesions mainly confined to the anterior insula (left or right) or the adjacent cortex were without significant effect on baroreceptor gain. Left posterior insular lesions, however, significantly increased baroreceptor gain (p<0.0001) whereas right posterior insular lesions had no effect on baroreceptor gain although heart rate and blood pressure were both significantly increased after lesion placement (p<0.05). These data suggest that: (1) the posterior insula (and not surrounding cortex or anterior insula) is primarily involved in cardiovascular control; (2) the left insular cortex may be chiefly concerned with parasympathetic cardiac regulation. Conversely, the right posterior insular cortex may regulate both cardiac and vasomotor sympathetic tone, as has been suggested in other species.

Cardiovascular changes during focal cerebral ischemia in rats

BACKGROUND AND PURPOSE

Recent studies have suggested that cerebral infarction influences autonomic activity and may contribute to sudden death. The goal of this study was to examine effects of focal cerebral infarction on mean arterial pressure and heart rate.

METHODS

Halothane-anesthetized rats were assigned to two groups: stroke (n = 10), in which the middle cerebral artery or an adjacent vessel was embolized with a silicone cylinder, and sham (n = 8), in which rats were sham embolized (saline). Arterial pressure and heart rate were measured for 90 minutes and again 24 hours after vascular occlusion. A change in electroencephalographic amplitude of -45% after embolization was used to determine if a significant degree of infarction was present.

RESULTS

Vascular occlusion produced a significant increase in mean arterial pressure at 10, 60, and 90 minutes (p < 0.05). Changes in heart rate were significantly greater (p < 0.05) than in sham-treated rats at 10 and 30 minutes after embolization. In contrast, mean arterial pressure and heart rate measured 24 hours after embolization were similar in both groups. Anatomic analysis of the infarcted areas demonstrated that either insular cortex or amygdala was affected in all embolized rats.

CONCLUSIONS

This study indicates that cerebral infarction produces a transient elevation of mean arterial pressure and heart rate. However, within 24 hours both parameters returned to preinfarcted levels. Our findings are consistent with clinical reports that indicate that mean arterial pressure and heart rate of stroke patients are similar to those of other groups when they are admitted to the hospital, although other cardiovascular parameters are greatly altered.