Parasympathetic nerves influence cerebral blood flow during hypertension in rat

Effect of sphenopalatine ganglion block on intracranial pressure and cerebral venous outflow oxygenation during craniotomy for supratentorial brain tumours

BACKGROUND

Intraoperative intracranial pressure (ICP) control continues to be a challenge for anaesthetists during craniotomies. Although many standard brain-dehydrating protocols are available, they may be ineffective in certain surgical situations and may result in harm either to the systemic or cerebral circulation. Sphenopalatine ganglion block (SPGB) can reverse the vasodilatory effects of anaesthesia during craniotomy.

METHODS

This prospective randomised study was carried from June 2020 to February 2021. Fifty-two patients were randomly allocated into two groups, the block group (B) and the non-block control group (Non). Twenty-six patients were enrolled in the (B) group and received a bilateral transnasal SPG block with 2% lidocaine using a hallow culture swab prior to anaesthesia induction. Intraoperative monitoring was performed using standard American Society of Anesthesiologists (ASA) monitors in addition to invasive monitoring using intra-arterial cannulas and jugular venous bulb catheters. Subdural ICP monitors were also employed. The arterio-jugular oxygen difference in mmol/l (AjvDO2) was then calculated. Mean flow velocity cm/s (MFV) and pulsatility index (PI) were monitored in both groups using Transcranial Doppler. Haemodynamic data were recorded every 30 min from induction of anaesthesia until the closure of the dura.

RESULTS

There was a significant difference in ICP prior to the dural opening between the block group (B), mean ± sd 7.58 ± 1.47, and the non-block group (Non), mean ± sd (11.69 ± 1.72), p-value < 0.001. There was no significant difference in MFV between (B) group, mean ± sd 72.65 ± 2.28 and (Non) group, mean ± sd 71.19 ± 3.09 before intubation (baseline values). While there was a significant difference after intubation between block group, mean ± sd 72.12 ± 1.77 and non - block group, mean ± sd 74.62 ± 5.07, p-value = 0.02. There was an insignificant difference between (B) and (Non) groups before intubation regarding PI values, while PI was significantly higher in (B) group than the (Non) group after intubation where mean ± sd was 1.17 ± 0.05 versus 0.96 ± 0.09, respectively, p-value = 0.001. There was no significant difference regarding cerebral oxygenation between the groups.

CONCLUSIONS

SPGB can control factors that increase CBF during anaesthesia by the block of parasympathetic vasodilatory fibres to the arterial system in the anterior cerebral circulation, while neither hindering cerebral venous drainage nor impairing cerebral oxygenation, as it gives no supply to cerebral veins and does not affect basal CBF. Additionally, it does not affect systemic circulation.

Wavelet decomposition analysis is a clinically relevant strategy to evaluate cerebrovascular buffering of blood pressure after spinal cord injury

The capacity of the cerebrovasculature to buffer changes in blood pressure (BP) is crucial to prevent stroke, the incidence of which is three- to fourfold elevated after spinal cord injury (SCI). Disruption of descending sympathetic pathways within the spinal cord due to cervical SCI may result in impaired cerebrovascular buffering. Only linear analyses of cerebrovascular buffering of BP, such as transfer function, have been used in SCI research. This approach does not account for inherent nonlinearity and nonstationarity components of cerebrovascular regulation, often depends on perturbations of BP to increase the statistical power, and does not account for the influence of arterial CO₂ tension. Here, we used a nonlinear and nonstationary analysis approach termed wavelet decomposition analysis (WDA), which recently identified novel sympathetic influences on cerebrovascular buffering of BP occurring in the ultra-low-frequency range (ULF; 0.02-0.03Hz). WDA does not require BP perturbations and can account for influences of CO₂ tension. Supine resting beat-by-beat BP (Finometer), middle cerebral artery blood velocity (transcranial Doppler), and end-tidal CO₂ tension were recorded in cervical SCI ( n = 14) and uninjured ( n = 16) individuals. WDA revealed that cerebral blood flow more closely follows changes in BP in the ULF range ( P = 0.0021, Cohen's d = 0.89), which may be interpreted as an impairment in cerebrovascular buffering of BP. This persisted after accounting for CO₂. Transfer function metrics were not different in the ULF range, but phase was reduced at 0.07-0.2 Hz ( P = 0.03, Cohen's d = 0.31). Sympathetically mediated cerebrovascular buffering of BP is impaired after SCI, and WDA is a powerful strategy for evaluating cerebrovascular buffering in clinical populations.

Parasympathetic innervation of vertebrobasilar arteries: is this a potential clinical target?

This review aims to summarise the contemporary evidence for the presence and function of the parasympathetic innervation of the cerebral circulation with emphasis on the vertebral and basilar arteries (the posterior cerebral circulation). We consider whether the parasympathetic innervation of blood vessels could be used as a means to increase cerebral blood flow. This may have clinical implications for pathologies associated with cerebral hypoperfusion such as stroke, dementia and hypertension. Relative to the anterior cerebral circulation little is known of the origins and neurochemical phenotypes of the parasympathetic innervation of the vertebrobasilar arteries. These vessels normally provide blood flow to the brainstem and cerebellum but can, via the Circle of Willis upon stenosis of the internal carotid arteries, supply blood to the anterior cerebral circulation too. We review the multiple types of parasympathetic fibres and their distinct transmitter mechanisms and how these vary with age, disease and species. We highlight the importance of parasympathetic fibres for mediating the vasodilatory response to sympathetic activation. Current trials are investigating the possibility of electrically stimulating the postganglionic parasympathetic ganglia to improve cerebal blood flow to reduce the penumbra following stroke. We conclude that although there are substantial gaps in our understanding of the origins of parasympathetic innervation of the vertebrobasilar arteries, activation of this system under some conditions might bring therapeutic benefits.

Nitric oxide and cerebrovascular regulation

The cerebrovascular regulation involves highly complex mechanisms to assure that the brain is perfused at all times. These mechanisms depend on all components of the neurovascular units: neurons, glia, and vascular cells. All these cell types can produce nitric oxide (NO), a powerful vasodilator through different NO synthases. Many studies underlined the key role of NO in the maintenance of resting cerebral blood flow (CBF) as well as in the mechanisms that control cerebrovascular tone: autoregulation and neurovascular coupling. However, although the role of NO in the control of CBF has been largely investigated, the complexity of the NO system and the lack of specific NO synthase inhibitors led to still unresolved questions such as the origin of NO and the pathways by which it controls the vascular tone. In this chapter, the role of NO in the regulation of CBF is critically reviewed and discussed in the context of the neurovascular unit and the general principles of cerebrovascular regulation.

MRI study of cerebral, retinal and choroidal blood flow responses to acute hypertension

Blood flow (BF) in many tissues is stable during significant fluctuations in systemic arterial blood pressure or perfusion pressure under normal conditions. The regulatory mechanisms responsible for this non-passive BF behavior include both local and neural control mechanisms. This study evaluated cerebral BF (CBF), retinal BF (RBF) and choroidal BF (ChBF) responses to acute blood pressure increases in rats using magnetic resonance imaging (MRI). A transient increase in blood pressure inside the MRI scanner was achieved by mechanically inflating a balloon catheter to occlude the descending aorta near the diaphragm. We verified the rat model of mechanical occlusion and MRI approach by first measuring blood-flow regulatory responses to changing BP in the brain under normoxia and hypercapnia where the phenomenon is well documented. Retinal and choroidal blood-flow responses to transient increased arterial pressure were then investigated. In response to an acute increase in blood pressure, RBF exhibited autoregulatory behavior and ChBF exhibited baroregulation similar to that seen in the cerebral circulation. This approach may prove useful to investigate retinal and choroidal vascular dysregulation in rat models of retinal diseases with suspected vascular etiology.

Nitroxidergic innervation of human cerebral arteries

A dense network of nerves containing neuronal nitric oxide synthase is present in cerebral vessels from experimental animals. The nerves may regulate cerebrovascular tone, protect the brain from stroke, and contribute to cluster headaches in humans; but studies in humans have shown only modest nitroxidergic innervation of cerebral vessels. We tested the hypothesis that nerve fibers containing neuronal nitric oxide synthase richly innervate human cerebral arteries. We used immunohistochemical techniques at post mortem and found dense neuronal nitric oxide synthase nerve staining in human cerebral vessel walls consistent with participation of nitroxidergic fibers in human physiological and pathophysiological processes.

Altered heart rhythm dynamics in very low birth weight infants with impending intraventricular hemorrhage

OBJECTIVE

Intraventricular hemorrhage remains an important problem among very low birth weight infants and may result in long-term neurodevelopmental disabilities. Neonatologists have been unable to accurately predict impending intraventricular hemorrhage. Because alterations in the autonomic nervous system's control of heart rhythm have been associated with intraventricular hemorrhage after its development, we sought to determine if early subtle alterations of heart rhythm could be predictive of impending intraventricular hemorrhage in very low birth weight infants.

METHODS

This case-control study included 10 newborn very low birth weight infants with intraventricular hemorrhage (5 grade IV, 4 grade III, and 1 grade II) and 14 control infants without intraventricular hemorrhage. Heart rhythm data from the first day of life before the development of intraventricular hemorrhage were evaluated. Detrended fluctuation analysis, a nonlinear fractal heart rate variability method, was used to assess the fractal dynamics of the heart rhythm. Fractal scaling exponents were calculated by using this analysis.

RESULTS

Twenty-four infants (mean +/- SD, birth weight: 845 +/- 213g: gestational age: 26.1 +/- 1.9 weeks) participated in the study. The short-term scaling exponent was significantly larger in infants who later developed intraventricular hemorrhage compared with those who did not (0.60 +/- 0.1 vs 0.45 +/- 0.1). A value of 0.52 resulted in 70% sensitivity and positive predictive value and 79% specificity and negative predictive value. The short-term scaling exponent was the only significant predictor of intraventricular hemorrhage.

CONCLUSIONS

Fractal dynamics of the heart rhythm is significantly altered in very low birth weight infants before developing intraventricular hemorrhage and may be predictive of impending intraventricular hemorrhage.

Parasympathetic tonic dilatory influences on cerebral vessels

Parasympathetic nerves from the pterygopalatine ganglia may participate in development of cluster headaches, in vascular responses to hypertension and in modulation of damage due to stroke. Stimulation of the nerves elicits cerebral vasodilatation, but it is not known if the nerves tonically influence cerebrovascular tone. We hypothesized that parasympathetics provide a tonic vasodilator influence and tested that hypothesis by measuring cerebral blood flow in anesthetized rats before and after removal of a pterygopalatine ganglion. Ganglion removal led to reduced cerebral blood flow without changing blood pressure. Thus, parasympathetic nerves provide tonic vasodilatory input to cerebral blood vessels.

Parasympathetic stimulation elicits cerebral vasodilatation in rat

Forebrain arteries receive nitroxidergic input from parasympathetic ganglionic fibers that arise from the pterygopalatine ganglia. Previous studies have shown that ganglionic stimulation in some species led to cerebral vasodilatation while interruption of those fibers interfered with vasodilatation seen during acute hypertension. Because the ganglionic fibers are quite delicate and are easily damaged when the ganglia are approached with published techniques we sought to develop a method that allowed clear exposure of the ganglia and permitted demonstration of cerebral vasodilatation with electrical stimulation of the ganglia in the rat. We had found that an orbital approach during which the eye was retracted for visualization of the ganglion precluded eliciting vasodilatation with ganglionic stimulation. In the current study approaching the ganglion through an incision over the zygomatic arch provided clear exposure of the ganglion and stimulation of the ganglion with that approach led to vasodilatation.

Neuronal nitric oxide mediates cerebral vasodilatation during acute hypertension

Parasympathetic nerves from the pterygopalatine ganglia provide nitroxidergic innervation to forebrain cerebral blood vessels. Disruption of that innervation attenuates cerebral vasodilatation seen during acute hypertension as does systemic administration of a non-selective nitric oxide synthase (NOS) inhibitor. Although such studies suggest that nitric oxide (NO) released from parasympathetic nerves participates in vasodilatation of cerebral vessels during hypertension, that hypothesis has not been tested with selective local inhibition of neuronal NOS (nNOS). We tested that hypothesis through these studies performed in anesthetized rats instrumented for continuous measurement of blood pressure, heart rate and pial arterial diameter through a cranial window. We sought to determine if the nNOS inhibitor propyl-L-arginine delivered directly to the outer surface of a pial artery would (1) attenuate changes in pial arterial diameter during acute hypertension and (2) block nNOS-mediated dilator effects of N-methyl-D-aspartate (NMDA) delivered into the window but (3) not block vasodilatation elicited by acetylcholine (ACh) and mediated by endothelial NOS dilator. Without the nNOS inhibitor arterial diameter abruptly increased 70+/-15% when mean arterial pressure (MAP) reached 183+/-3 mm Hg while with nNOS inhibition diameter increased only 13+/-10% (p<0.05) even when MAP reached 191+/-4 mm Hg (p>0.05). The nNOS inhibitor significantly attenuated vasodilatation induced by NMDA but not ACh delivered into the window. Thus, local nNOS inhibition attenuates breakthrough from autoregulation during hypertension as does complete interruption of the parasympathetic innervation of cerebral vessels. These findings further support the hypothesis that NO released from parasympathetic fibers contributes to cerebral vasodilatation during acute hypertension.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Inhibiting the nucleus tractus solitarii extends cerebrovascular autoregulation during hypertension

Autoregulation maintains cerebral blood flow near basal levels as blood pressure increases, but vasodilation, breakthrough, occurs when hypertension exceeds the autoregulatory range. Loss of breakthrough after transection of baroreceptor nerves suggests that breakthrough is neurally mediated. We hypothesize that central baroreflex interruption will likewise prevent breakthrough. In treated rats, injections of lidocaine into the nucleus tractus solitarii blocked breakthrough and the baroreflex. Therefore, central, like peripheral, baroreflex interruption extends autoregulation during hypertension.