Spinal cord injury enhances arterial expression and reactivity of α1-adrenergic receptors-mechanistic investigation into autonomic dysreflexia

Comorbidity of cardiorespiratory and locomotor dysfunction following cervical spinal cord injury in the rat

Cervical spinal cord injury interrupts supraspinal pathways innervating thoracic sympathetic preganglionic neurons and results in cardiovascular dysfunction. Both respiratory and locomotor functions were also impaired due to damages of motoneuron pools controlling respiratory and forelimb muscles, respectively. However, no study has investigated autonomic and somatic motor functions in the same animal model. The present study aimed to establish a cervical spinal cord injury model to evaluate cardiorespiratory response and locomotor activity in unanesthetized rats. Cardiovascular response and respiratory behavior following laminectomy or cervical spinal contusion were measured using noninvasive blood pressure analyzer and plethysmography systems, respectively. Locomotor activity was evaluated by an open-field test and a locomotor rating scale. The results demonstrated that mean arterial blood pressure and heart rate were significantly reduced in contused rats compared with uninjured rats at the acute injured stage. Tidal volume was also significantly reduced during the acute and subchronic stages. Moreover, locomotor function was severely impaired, evidenced by decreasing moving ability and locomotor rating scores from the acute to chronic injured stages. Retrograde neurotracer results revealed that cervical spinal cord injury caused a reduction in number of phrenic and triceps motoneurons. Immunofluorescence staining revealed a significant attenuation of serotonergic, noradrenergic, glutamatergic, and GABAergic fibers innervating the thoracic sympathetic preganglionic neurons in chronically contused rats. These results revealed the pathological mechanism underlying the comorbidity of cardiorespiratory and locomotor dysfunction following cervical spinal cord injury. We proposed that this animal model can be used to evaluate the therapeutic efficacy of potential strategies to improve different physiological functions.NEW & NOTEWORTHY The present study establishes a preclinical rodent model to comprehensively investigate physiological functions under unanesthetized condition following cervical spinal cord contusion. The results demonstrated that cervical spinal cord contusion is associated with impairments in cardiovascular, respiratory, and locomotor function. Respiratory and forelimb motoneurons and neurochemical innervations of sympathetic preganglionic neurons were damaged following injury. This animal model can be used to evaluate the therapeutic efficacy of potential strategies to improve different physiological functions.

Consequences of spinal cord injury on the sympathetic nervous system

Spinal cord injury (SCI) damages multiple structures at the lesion site, including ascending, descending, and propriospinal axons; interrupting the conduction of information up and down the spinal cord. Additionally, axons associated with the autonomic nervous system that control involuntary physiological functions course through the spinal cord. Moreover, sympathetic, and parasympathetic preganglionic neurons reside in the spinal cord. Thus, depending on the level of an SCI, autonomic function can be greatly impacted by the trauma resulting in dysfunction of various organs. For example, SCI can lead to dysregulation of a variety of organs, such as the pineal gland, the heart and vasculature, lungs, spleen, kidneys, and bladder. Indeed, it is becoming more apparent that many disorders that negatively affect quality-of-life for SCI individuals have a basis in dysregulation of the sympathetic nervous system. Here, we will review how SCI impacts the sympathetic nervous system and how that negatively impacts target organs that receive sympathetic innervation. A deeper understanding of this may offer potential therapeutic insight into how to improve health and quality-of-life for those living with SCI.

Effects of C2 hemisection on respiratory and cardiovascular functions in rats

High cervical spinal cord injuries induce permanent neuromotor and autonomic deficits. These injuries impact both central respiratory and cardiovascular functions through modulation of the sympathetic nervous system. So far, cardiovascular studies have focused on models of complete contusion or transection at the lower cervical and thoracic levels and diaphragm activity evaluations using invasive methods. The present study aimed to evaluate the impact of C2 hemisection on different parameters representing vital functions (i.e., respiratory function, cardiovascular, and renal filtration parameters) at the moment of injury and 7 days post-injury in rats. No ventilatory parameters evaluated by plethysmography were impacted during quiet breathing after 7 days post-injury, whereas permanent diaphragm hemiplegia was observed by ultrasound and confirmed by diaphragmatic electromyography in anesthetized rats. Interestingly, the mean arterial pressure was reduced immediately after C2 hemisection, with complete compensation at 7 days post-injury. Renal filtration was unaffected at 7 days post-injury; however, remnant systolic dysfunction characterized by a reduced left ventricular ejection fraction persisted at 7 days post-injury. Taken together, these results demonstrated that following C2 hemisection, diaphragm activity and systolic function are impacted up to 7 days post-injury, whereas the respiratory and cardiovascular systems display vast adaptation to maintain ventilatory parameters and blood pressure homeostasis, with the latter likely sustained by the remaining descending sympathetic inputs spared by the initial injury. A better broad characterization of the physiopathology of high cervical spinal cord injuries covering a longer time period post-injury could be beneficial for understanding evaluations of putative therapeutics to further increase cardiorespiratory recovery.

Toward rebalancing blood pressure instability after spinal cord injury with spinal cord electrical stimulation: A mini review and critique of the evolving literature

High-level spinal cord injury commonly leads to blood pressure instability. This manifests clinically as orthostatic hypotension (OH), where blood pressure can drop to the point of loss of consciousness, and autonomic dysreflexia (AD), where systolic blood pressure can climb to over 300 mmHg in response to an unperceived noxious stimulus. These blood pressure fluctuations can occur multiple times a day, contributing to increased vessel shear stress and heightened risk of cardiovascular disease. The pathophysiology of both of these conditions is rooted in impairments in regulation of spinal cord sympathetic preganglionic neurons, which control blood pressure by mediating vascular resistance and catecholamine release. Recently, spinal cord electrical stimulation has provided evidence that it may modulate these blood pressure imbalances. Early proposed mechanisms suggest activation of spinal cord dorsal horn neurons that ultimately act upon the sympathetic preganglionic neuronal pathways. For OH, spinal cord stimulation likely induces local activation of these neurons to generate baseline sympathetic tone and accompanying vasoconstriction. The mechanisms for spinal stimulation regulating AD are less clear, though some suggest it activates inhibitory circuits to dampen the overactive sympathetic response. While questions remain, spinal cord electrical stimulation is an intriguing new modality that may restore blood pressure regulation following spinal cord injury.

The ability of baroreflex activation to improve blood pressure and resistance vessel function in spontaneously hypertensive rats is dependent on stimulation parameters

Baroreflex activation by electric stimulation of the carotid sinus (CS) effectively lowers blood pressure. However, the degree to which differences between stimulation protocols impinge on cardiovascular outcomes has not been defined. To address this, we examined the effects of short- and long-duration (SD and LD) CS stimulation on hemodynamic and vascular function in spontaneously hypertensive rats (SHRs). We fit animals with miniature electrical stimulators coupled to electrodes positioned around the left CS nerve that delivered intermittent 5/25 s ON/OFF (SD) or 20/20 s ON/OFF (LD) square pulses (1 ms, 3 V, 30 Hz) continuously applied for 48 h in conscious animals. A sham-operated control group was also studied. We measured mean arterial pressure (MAP), systolic blood pressure variability (SBPV), heart rate (HR), and heart rate variability (HRV) for 60 min before stimulation, 24 h into the protocol, and 60 min after stimulation had stopped. SD stimulation reversibly lowered MAP and HR during stimulation. LD stimulation evoked a decrease in MAP that was sustained even after stimulation was stopped. Neither SD nor LD had any effect on SBPV or HRV when recorded after stimulation, indicating no adaptation in autonomic activity. Both the contractile response to phenylephrine and the relaxation response to acetylcholine were increased in mesenteric resistance vessels isolated from LD-stimulated rats only. In conclusion, the ability of baroreflex activation to modulate hemodynamics and induce lasting vascular adaptation is critically dependent on the electrical parameters and duration of CS stimulation.

Quantitative analysis of dysautonomia in patients with autonomic dysreflexia

Autonomic dysreflexia (AD) is a life-threatening condition for individuals with cervical or high-thoracic spinal cord injury (SCI). The profile of autonomic dysfunction in AD using validated clinical autonomic tests has not been described so far, although it could be useful to identify SCI patients at greater risk of developing AD non-invasively. With this objective, 37 SCI patients (27% female) were recruited, and hemodynamic and cardiac parameters were continuously monitored to determine the presence of AD, defined as an increase of systolic blood pressure of 20 mmHg or higher after bladder filling with saline. Then, standard autonomic function testing was performed, including Deep Breathing, Valsalva Manoeuvre and Tilt Table Test. Finally, baroreflex sensitivity (BRS), and spectral analysis of heart rate and blood pressure variability were measured at rest. Catecholamines and vasopressin levels were also measured at supine and upright positions. The severity of SCI was assessed through clinical and radiological examinations. AD was observed in 73.3% of SCI patients, being 63.6% of them asymptomatic during the dysreflexive episode. AD patients displayed a drop in sympathetic outflow, as determined by decreased noradrenalin plasma levels, reduced sympathovagal balance and increased BRS. In line with decreased sympathetic activity, the incidence of neurogenic orthostatic hypotension was higher in AD patients. Our results provide novel evidence regarding the autonomic dysfunction in SCI patients with AD compared to non-AD patients, posing non-invasively measured autonomic parameters as a powerful clinical tool to predict AD in SCI patients.

Transcriptome Analysis Reveals the Effects of Troxerutin and Cerebroprotein Hydrolysate Injection on Injured Spinal Cords in Rats

Spinal cord injury (SCI) is a serious condition that results in disability and has a high morbidity rate; its treatment is very difficult. Although troxerutin and cerebroprotein hydrolysate (TCH) injections have been extensively used in clinics in China for the treatment of traumatic brain injury (TBI) and cerebral stroke, the potential efficacy of TCH injection in the treatment of SCI has never been revealed. In this study, the effects of administering TCH injections on neurological recovery in post-SCI rats were first tested with regard to the behavior and histology; subsequently, the specific expression profile of mRNAs and long noncoding RNAs (LncRNAs) in their spinal cords were conducted using RNA sequencing (RNA-seq). The LncRNA-mRNA networks were also elucidated. After SCI, we found that TCH injection with the right dose is effective for the recovery of locomotion function and repairing of the damaged tissue in the spinal cord; TCH injection is also discovered to have a role in the regulation of 443 differentially expressed genes (DEGs) and 27 differentially expressed LncRNAs (DELs) that are identified to have multiple functions, including locomotion, blood vessel morphogenesis, thiamine metabolism, Hippo signaling pathway, and axon guidance, by applying the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and Gene Ontology (GO) analysis. In addition, it is revealed that, after SCI, the highly expressed LncRNA AABR07071383.1 in the post-SCI cis/trans-regulates the expression of mRNA Acpp mRNA that encodes a key enzyme involved in the metabolic process of thiamine in the abirritation of the dorsal root ganglion (DRG), which implies that TCH injection may be more effective when administered with benfotiamine (a common treatment drug).

Anxiety masquerading as autonomic dysreflexia

Context: Autonomic dysreflexia (AD) is characterized by a sudden onset of hypertension in those with spinal cord injuries (SCI) at T6 or above. Prompt recognition and treatment of AD is important. Herein are two individuals with SCI and acute hypertension due to anxiety that may easily have been confused for AD. Findings: Case 1: A 77-year-old male with C4 AIS D SCI and a history of anxiety and AD presented for urodynamics to evaluate the effectiveness of onabotulinumtoxinA injections into his urethral sphincter. After lying down on the urodynamics table, his systolic blood pressure (SBP) was noted to rise, from 138 to 170 mmHg over six minutes without any instrumentation or position change. Upon announcing urodynamics would be postponed and preparing for AD management, his SBP returned to below baseline within two minutes. Case 2: A 57-year-old male with T12 AIS A SCI and a history of anxiety presented for routine urodynamics. His baseline SBP was 140 mmHg. During bladder filling, incidental SBP was elevated to 170 mmHg. Filling was stopped and he was told the study was complete. One minute later, prior to draining his bladder, SBP decreased to 150 mmHg. Conclusions: An individual's psychological state should be considered in addition to traditional management of AD when an individual with SCI presents with hypertension. Moreover, Case 2 may possibly explain reports of individuals presenting with AD with injuries below T6.

Effectiveness of arginine vasopressin for the management of refractory hemorrhagic shock in a patient with autonomic dysreflexia caused by spinal cord injury

Background

Arginine vasopressin has been used for the management of refractory vasodilatory shock. However, it is still unclear whether arginine vasopressin is useful for hypotension in patients with spinal cord injury.

Case description

A 78-year-old man with autonomic dysreflexia and paralysis below the level corresponding to Th2 due to spinal cord injury previously underwent cholecystectomy. During the surgery, accidental hemorrhage led him to refractory hemorrhagic shock unresponsive to fluid resuscitation and catecholamine. Lasting hypotension was improved with arginine vasopressin.

Conclusion

We described a rare case report on the use of arginine vasopressin for management of refractory hemorrhagic shock in a patient with autonomic dysreflexia.

Exploring detailed characteristics of autonomic dysreflexia

OBJECTIVES

Identify clinical and pathophysiologic insights into autonomic dysreflexia (AD) in patients with spinal cord injury (SCI).

STUDY DESIGN

Analysis of prospectively gathered AD dataset.

SETTING

Inpatient Veterans Affairs SCI unit.

PARTICIPANTS

78 male patients with SCI who experienced AD.

METHODS

Statistical methods were utilized to identify the frequency of relative bradycardia vs. tachycardia during AD, the effectiveness of supplemental opioids in managing suspected nociceptive pain mediated AD, the effect of chronicity of SCI on response to pharmacological management of AD, and the response to nitroglycerin ointment in suspected bladder related AD.

RESULTS

445 episodes of AD were analyzed. The frequency of relative bradycardia and tachycardia with AD were 0.3% and 68.0% respectively. The addition of opioids to an antihypertensive medication protocol did not significantly decrease AD episode duration or magnitude of systolic blood pressure (SBP) change. A strongly matched positive linear correlation was identified between the duration of pharmacologically treated AD episodes and chronicity of SCI (R2=0.83). Bladder related AD episodes treated with nitroglycerin ointment had a faster onset of action (10.8 minutes vs. 15.9 minutes), faster time to reach a safe target blood pressure (16.5 minutes vs 20.9 minutes), and greater decrease in SBP (84.3mmHg vs. 68.6mmHg) than non-bladder related episodes (P=0.19, 0.23, and 0.02 respectively).

CONCLUSIONS

AD may commonly occur with relative tachycardia. While further investigation is needed on the effects of chronicity of SCI and pharmacologic management, this study raises multiple directions for future research to understand clinical signs and treatment variables of AD following SCI.

Recent update on basic mechanisms of spinal cord injury

Spinal cord injury (SCI) is a life-shattering neurological condition that affects between 250,000 and 500,000 individuals each year with an estimated two to three million people worldwide living with an SCI-related disability. The incidence in the USA and Canada is more than that in other countries with motor vehicle accidents being the most common cause, while violence being most common in the developing nations. Its incidence is two- to fivefold higher in males, with a peak in younger adults. Apart from the economic burden associated with medical care costs, SCI predominantly affects a younger adult population. Therefore, the psychological impact of adaptation of an average healthy individual as a paraplegic or quadriplegic with bladder, bowel, or sexual dysfunction in their early life can be devastating. People with SCI are two to five times more likely to die prematurely, with worse survival rates in low- and middle-income countries. This devastating disorder has a complex and multifaceted mechanism. Recently, a lot of research has been published on the restoration of locomotor activity and the therapeutic strategies. Therefore, it is imperative for the treating physicians to understand the complex underlying pathophysiological mechanisms of SCI.

Exendin-4 improves cardiovascular function and survival in flow-induced pulmonary hypertension

OBJECTIVES

Systemic left-to-right shunting causes pulmonary arteriopathy, leading to progressive cardiopulmonary failure and a poor prognosis. In this study, we examined the extraglycemic effect of a synthetic glucagon-like peptide, exendin-4, on pulmonary arteriopathy regression and cardiopulmonary function in nondiabetic rats.

METHODS

Pulmonary hypertension (PH) was induced by monocrotaline (60 mg/kg, subcutaneous) injection followed by the creation of an aortocaval fistula. After 4 weeks, exendin-4 (1 μg/kg/day) was administered intraperitoneally for 3 consecutive weeks, followed by an assessment of cardiopulmonary function, pulmonary artery vasoreactivity, tissue and blood biochemistry, and lung histology.

RESULTS

Exendin-4 significantly reduced right ventricle mass and pulmonary artery pressure, which improved right ventricle function and the survival rate in rats with PH. Tissue and blood interleukin-1β levels decreased, whereas pulmonary artery cyclic adenosine monophosphate levels were restored by exendin-4. Smooth muscle-myosin heavy chain-II and α-smooth muscle actin protein levels increased in the pulmonary arteries of exendin-4-treated rats. Histology showed that exendin-4 decreased the main and intra-acinar pulmonary artery medial thickness.

CONCLUSIONS

Exendin-4 treatment improved pulmonary artery function in flow-induced PH via its direct vasoactive properties, anti-inflammatory effects, and vascular smooth muscle cell phenotypic modulation. Mitigation of pulmonary arteriopathy further potentiated right ventricle performance and reduced overall mortality. These responses were associated with suppressed expression and activity of interleukin-1β and its downstream signaling molecules. Glucagon-like peptide analogs may possess pleiotropic therapeutic potential in flow-induced PH.

Autonomic dysreflexia after spinal cord injury: Systemic pathophysiology and methods of management

Traumatic spinal cord injury (SCI) has widespread physiological effects beyond the disruption of sensory and motor function, notably the loss of normal autonomic and cardiovascular control. Injury at or above the sixth thoracic spinal cord segment segregates critical spinal sympathetic neurons from supraspinal modulation which can result in a syndrome known as autonomic dysreflexia (AD). AD is defined as episodic hypertension and concomitant baroreflex-mediated bradycardia initiated by unmodulated sympathetic reflexes in the decentralized cord. This condition is often triggered by noxious yet unperceived visceral or somatic stimuli below the injury level and if severe enough can require immediate medical attention. Herein, we review the pathophysiological mechanisms germane to the development of AD, including maladaptive plasticity of neural circuits mediating abnormal sympathetic reflexes and hypersensitization of peripheral vasculature that collectively contribute to abnormal hemodynamics after SCI. Further, we discuss the systemic effects of recurrent AD and pharmacological treatments used to manage such episodes. Contemporary research avenues are then presented to better understand the relative contributions of underlying mechanisms and to elucidate the effects of recurring AD on cardiovascular and immune functions for developing more targeted and effective treatments to attenuate the development of this insidious syndrome following high-level SCI.

Changes in sympathetic neurovascular function following spinal cord injury

The effects of spinal cord injury (SCI) on sympathetic neurovascular transmission have generally been ignored. This review describes changes in sympathetic nerve-mediated activation of arterial vessels to which ongoing sympathetic activity has been reduced or silenced following spinal cord transection in rats. In all vessels studied in rats, SCI markedly enhanced their contractile responses to nerve activity. However, the mechanisms that augment neurovascular transmission differ between the rat tail artery and mesenteric artery. In tail artery, the enhancement of neurovascular transmission cannot be attributed to changes in sensitivity of the vascular muscle to α₁- or α₂-adrenoceptor agonists. Instead the contribution of L-type Ca²⁺ channels to activation of the smooth muscle by nerve-released noradrenaline is greatly increased following SCI. By contrast, mesenteric arteries from SCI rats had increased sensitivity to phenylephrine but not to methoxamine. While both phenylephrine and methoxamine are α₁-adrenoceptor agonists, only phenylephrine is a substrate for the neuronal noradrenaline transporter. Therefore the selective increase in sensitivity to phenylephrine suggests that the activity of the neuronal noradrenaline transporter is reduced. While present evidence suggests that sympathetic vasoconstrictor neurons do not contribute to the normal regulation of peripheral resistance below a complete SCI in humans, the available evidence does indicate that these experimental findings in animals are likely to apply after SCI in humans and contribute to autonomic dysreflexia.