Adaptations of peripheral vasoconstrictor pathways after spinal cord injury

Longitudinal Assessment of Autonomic Function during the Acute Phase of Spinal Cord Injury: Use of Low-Frequency Blood Pressure Variability as a Quantitative Measure of Autonomic Function

High-level spinal cord injury (SCI) can disrupt cardiovascular autonomic function. However, the evolution of cardiovascular autonomic function in the acute phase following injury is unknown. We evaluated the timing, severity, progression, and implications of cardiovascular autonomic injury following acute SCI. We tested 63 individuals with acute traumatic SCI (aged 48 ± 2 years) at five time-points: <2 weeks, and 1, 3, 6-12, and >12 months post-injury. Supine beat-to-beat systolic arterial pressure (SAP) and R-R interval (RRI) were recorded and low-frequency variability (LF SAP and LF RRI) determined. Cross-spectral analyses were used to determine baroreflex function (low frequency) and cardiorespiratory interactions (high frequency). Known electrocardiographic (ECG) markers for arrhythmia and self-reported symptoms of cardiovascular dysfunction were determined. Comparisons were made with historical data from individuals with chronic SCI and able-bodied controls. Most individuals had high-level (74%) motor/sensory incomplete (63%) lesions. All participants had decreased LF SAP at <2 weeks (2.22 ± 0.65 mm Hg²). Autonomic injury was defined as high-level SCI with LF SAP <2 mm Hg². Two distinct groups emerged by 1 month: autonomically complete SCI with sustained low LF SAP (0.76 ± 0.17 mm Hg²) and autonomically incomplete SCI with increased LF SAP (5.46 ± 1.0 mm Hg², p < 0.05). Autonomically complete injuries did not recover over time. Cardiovascular symptoms were prevalent and worsened with time, especially in those with autonomically complete lesions, and chronic SCI. Baroreflex function and cardiorespiratory interactions were impaired after SCI. Risk of arrhythmia increased immediately after SCI, and remained elevated throughout the acute phase. Acute SCI is associated with severe cardiovascular dysfunction. LF SAP provides a simple, non-invasive, translatable, quantitative assessment of autonomic function, and is most informative 1 month after injury.

Spinal cord injury alters purinergic neurotransmission to mesenteric arteries in rats

Complications associated with spinal cord injury (SCI) result from unregulated reflexes below the lesion level. Understanding neurotransmission distal to the SCI could improve quality of life by mitigating complications. The long-term impact of SCI on neurovascular transmission is poorly understood, but reduced sympathetic activity below the site of SCI enhances arterial neurotransmission (1). We studied sympathetic neurovascular transmission using a rat model of long-term paraplegia (T2-3) and tetraplegia (C6-7). Sixteen weeks after SCI, T2-3 and C6-7 rats had lower blood pressure (BP) than sham rats (103  ±  2 and 97  ±  4 vs. 117  ±  6 mmHg, P < 0.05). T2-3 rats had tachycardia (410  ±  6 beats/min), and C6-7 rats had bradycardia (299  ±  10 beats/min) compared with intact rats (321  ±  4 beats/min, P < 0.05). Purinergic excitatory junction potentials (EJPs) were measured in mesenteric arteries (MA) using microlectrodes, and norepinephrine (NE) release was measured using amperometry. NE release was similar in all groups, while EJP frequency-response curves from T2-3 and C6-7 rats were left-shifted vs. sham rats. EJPs in T2-3 and C6-7 rats showed facilitation followed by run-down during stimulation trains (10 Hz, 50 stimuli). MA reactivity to exogenous NE and ATP was similar in all rats. In T2-3 and C6-7 rats, NE content was increased in left cardiac ventricles compared with intact rats, but was not changed in MA, kidney, or spleen. Our data indicate that peripheral purinergic, but not adrenergic, neurotransmission increases following SCI via enhanced ATP release from periarterial nerves. Sympathetic BP support is reduced after SCI, but improving neurotransmitter release might maintain cardiovascular stability in individuals living with SCI.NEW & NOTEWORTHY This study revealed increased purinergic, but not noradrenergic, neurotransmission to mesenteric arteries in rats with spinal cord injury (SCI). An increased releasable pool of ATP in periarterial sympathetic nerves may contribute to autonomic dysreflexia following SCI, suggesting that purinergic neurotransmission may be a therapeutic target for maintaining stable blood pressure in individuals living with SCI. The selective increase in ATP release suggests that ATP and norepinephrine may be stored in separate synaptic vesicles in periarterial sympathetic varicosities.

Distinct roles of angiotensin receptors in autonomic dysreflexia following high-level spinal cord injury in mice

Autonomic dysreflexia (AD), a syndrome caused by loss of supraspinal control over sympathetic activity and amplified vascular reflex upon sensory stimuli below injury level, is a major health problem in high-level spinal cord injury (SCI). After supraspinal sympathetic control of the vasculature below the lesion is lost, the renin-angiotensin system (RAS) is thought to be involved in AD by regulating blood pressure and vascular reactivity. In this study, we aimed to assess the role of different RAS receptors during AD following SCI. Therefore, we induced AD by colorectal distention (CRD) in wild-type mice and mice deficient in the RAS components angiotensin (Ang) II type 1a receptor (AT1a) (Agtr1a^-/-) and Ang-(1-7) receptor Mas (Mas^-/-) four weeks after complete transection of spinal cord at thoracic level 4 (T4). Systemic blood pressure measurements and wire myography technique were performed to assess hemodynamics and the reactivity of peripheral arteries, respectively. CRD increased mean arterial blood pressure (MAP) and decreased heart rate (HR) in all three animal groups. However, we found less increases in MAP in Mas^-/- mice compared to control mice after CRD, whereas AT1a deficiency did not affect the hemodynamic response. We found that the reactivity of wild-type and Mas^-/- mesenteric arteries, which are innervated from ganglia distal but close to thoracic level T4, was diminished in response to Ang II in AD after T4-SCI, but this difference was not observed in the absence of AT1a receptors. CRD did not influence the reactivity of femoral arteries which are innervated from ganglia more distal to thoracic level T4, in response to Ang II in AD. In conclusion, we identified a specific role of the Ang-(1-7) receptor Mas in regulating the systemic blood pressure increase in AD in T4-SCI mice. Furthermore, AT1a signaling is not involved in this hemodynamic response, but underlies increased vascular reactivity in mesenteric arteries in response to Ang II, where it may contribute to adaptive changes in regional blood flow.

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.

Autonomic dysreflexia: a cardiovascular disorder following spinal cord injury

Autonomic dysreflexia (AD) is a serious cardiovascular disorder in patients with spinal cord injury (SCI). The primary underlying cause of AD is loss of supraspinal control over sympathetic preganglionic neurons (SPNs) caudal to the injury, which renders the SPNs hyper-responsive to stimulation. Central maladaptive plasticity, including C-fiber sprouting and propriospinal fiber proliferation exaggerates noxious afferent transmission to the SPNs, causing them to release massive sympathetic discharges that result in severe hypertensive episodes. In parallel, upregulated peripheral vascular sensitivity following SCI exacerbates the hypertensive response by augmenting gastric and pelvic vasoconstriction. Currently, the majority of clinically employed treatments for AD involve anti-hypertensive medications and Botox injections to the bladder. Although these approaches mitigate the severity of AD, they only yield transient effects and target the effector organs, rather than addressing the primary issue of central sympathetic dysregulation. As such, strategies that aim to restore supraspinal reinnervation of SPNs to improve cardiovascular sympathetic regulation are likely more effective for AD. Recent pre-clinical investigations show that cell transplantation therapy is efficacious in reestablishing spinal sympathetic connections and improving hemodynamic performance, which holds promise as a potential therapeutic approach.

Spinal cord injury increases the reactivity of rat tail artery to angiotensin II

Studies in individuals with spinal cord injury (SCI) suggest the vasculature is hyperreactive to angiotensin II (Ang II). In the present study, the effects of SCI on the reactivity of the rat tail and mesenteric arteries to Ang II have been investigated. In addition, the effects of SCI on the facilitatory action of Ang II on nerve-evoked contractions of these vessels were determined. Isometric contractions of artery segments from T11 (tail artery) or T4 (mesenteric arteries) spinal cord-transected rats and sham-operated rats were compared 6–7 weeks postoperatively. In both tail and mesenteric arteries, SCI increased nerve-evoked contractions. In tail arteries, SCI also greatly increased Ang II-evoked contractions and the facilitatory effect of Ang II on nerve-evoked contractions. By contrast, SCI did not detectably change the responses of mesenteric arteries to Ang II. These findings provide the first direct evidence that SCI increases the reactivity of arterial vessels to Ang II. In addition, in tail artery, the findings indicate that Ang II may contribute to modifying their responses following SCI.

Abdominal Muscle Training Can Enhance Cough After Spinal Cord Injury

Background. Respiratory complications in people with high-level spinal cord injury (SCI) are a major cause of morbidity and mortality, particularly because of a reduced ability to cough as a result of abdominal muscle paralysis. Objective. We investigated the effect of cough training combined with functional electrical stimulation (FES) over the abdominal muscles for 6 weeks to observe whether training could improve cough strength. Methods. Fifteen SCI subjects (C4-T5) trained for 6 weeks, 5 days per week (5 sets of 10 coughs per day) in a randomized crossover design study. Subjects coughed voluntarily at the same time as a train of electrical stimulation was delivered over the abdominal muscles via posterolaterally positioned electrodes (50 Hz, 3 seconds). Measurements were made of esophageal (Pes) and gastric (Pga) expiratory pressures and the peak expiratory flow (PEFcough) produced at the 3 time points of before, during, and after the training. Results. During voluntary coughs, FES cough stimulation improved Pga, Pes, and PEFcough acutely, 20-fold, 4-fold, and 50%, respectively. Six weeks of cough training significantly increased Pga (37.1 ± 2.0 to 46.5 ± 2.9 cm H2O), Pes (35.4 ± 2.7 to 48.1 ± 2.9 cm H2O), and PEFcough (3.1 ± 0.1 to 3.6 ± 0.1 L/s). Cough training also improved pressures and flow during voluntary unstimulated coughs. Conclusions. FES of abdominal muscles acutely increases mechanical output in coughing in high-level SCI subjects. Six weeks of cough training further increases gastric and esophageal cough pressures and expiratory cough flow during stimulated cough maneuvers.

Plasticity of TRPV1-Expressing Sensory Neurons Mediating Autonomic Dysreflexia Following Spinal Cord Injury

Spinal cord injury (SCI) triggers profound changes in visceral and somatic targets of sensory neurons below the level of injury. Despite this, little is known about the influence of injury to the spinal cord on sensory ganglia. One of the defining characteristics of sensory neurons is the size of their cell body: for example, nociceptors are smaller in size than mechanoreceptors or proprioceptors. In these experiments, we first used a comprehensive immunohistochemical approach to characterize the size distribution of sensory neurons after high- and low-thoracic SCI. Male Wistar rats (300 g) received a spinal cord transection (T3 or T10) or sham-injury. At 30 days post-injury, dorsal root ganglia (DRGs) and spinal cords were harvested and analyzed immunohistochemically. In a wide survey of primary afferents, only those expressing the capsaicin receptor (TRPV1) exhibited somal hypertrophy after T3 SCI. Hypertrophy only occurred caudal to SCI and was pronounced in ganglia far distal to SCI (i.e., in L4-S1 DRGs). Injury-induced hypertrophy was accompanied by a small expansion of central territory in the lumbar spinal dorsal horn and by evidence of TRPV1 upregulation. Importantly, hypertrophy of TRPV1-positive neurons was modest after T10 SCI. Given the specific effects of T3 SCI on TRPV1-positive afferents, we hypothesized that these afferents contribute to autonomic dysreflexia (AD). Rats with T3 SCI received vehicle or capsaicin via intrathecal injection at 2 or 28 days post-SCI; at 30 days, AD was assessed by recording intra-arterial blood pressure during colo-rectal distension (CRD). In both groups of capsaicin-treated animals, the severity of AD was dramatically reduced. While AD is multi-factorial in origin, TRPV1-positive afferents are clearly involved in AD elicited by CRD. These findings implicate TRPV1-positive afferents in the initiation of AD and suggest that TRPV1 may be a therapeutic target for amelioration or prevention of AD after high SCI.

Somatosympathetic Vasoconstrictor Reflexes in Human Spinal Cord Injury: Responses to Innocuous and Noxious Sensory Stimulation below Lesion

It is known that the sudden increases in blood pressure associated with autonomic dysreflexia in people with spinal cord injury (SCI) are due to a spinally mediated reflex activation of sympathetic vasoconstrictor neurons supplying skeletal muscle and the gut. Apart from visceral inputs, such as those originating from a distended bladder, there is a prevailing opinion that autonomic dysreflexia can be triggered by noxious stimulation below the lesion. However, do noxious inputs really cause an increase in blood pressure in SCI? Using microelectrodes inserted into a peripheral nerve to record sympathetic nerve activity we had previously shown that selective stimulation of small-diameter afferents in muscle or skin, induced by bolus injection of hypertonic saline into the tibialis anterior muscle or the overlying skin, evokes a sustained increase in muscle sympathetic nerve activity and blood pressure and a transient increase in skin sympathetic nerve activity and decrease in skin blood flow in able-bodied subjects. We postulated that these sympathetic responses would be exaggerated in SCI, with a purely noxious stimulus causing long-lasting increases in blood pressure and long-lasting decreases in skin blood flow. Surprisingly, though, we found that intramuscular or subcutaneous injection of hypertonic saline into the leg caused negligible changes in these parameters. Conversely, weak electrical stimulation over the abdominal wall, which in able-bodied subjects is not painful and activates large-diameter cutaneous afferents, caused a marked increase in blood pressure in SCI but not in able-bodied subjects. This suggests that it is activation of large-diameter somatic afferents, not small-diameter afferents, that triggers increases in sympathetic outflow in SCI. Whether the responses to activation of large-diameter afferents reflect plastic changes in the spinal cord in SCI is unknown.

Vasomotor disturbances in complex regional pain syndrome--a review

Complex regional pain syndromes (CRPS) are characterized by vascular disturbances primary affecting the microcirculation in the distal part of the involved extremity. In the acute stage inhibited sympathetic vasoconstriction and exaggerated neurogenic inflammation driven by central and peripheral mechanisms, respectively, seem to be the major pathophysiological mechanisms inducing vasodilation. During the chronic course of the disease as well as early in some patients vasoconstriction dominates the clinical picture induced by changes in the microcirculation itself such as endothelial dysfunction or vascular hyperreactivity, whereas sympathetic vasoconstrictor activity returns and neurogenic inflammation is less severe. It can be suggested that the interaction between different mechanisms underlying vasomotor disturbances as well as the severity of each single mechanism in the individual patient have a great impact on the variety of the overall clinical picture in CRPS. Irrespective of the underlying pathophysiology, measurements of skin temperature differences between the affected and the contralateral extremity can serve as a diagnostic tool in CRPS, in particular when sensitivity and specificity is increased by considering dynamic alterations in skin temperature asymmetries.

Recurrent autonomic dysreflexia exacerbates vascular dysfunction after spinal cord injury

BACKGROUND CONTEXT

Individuals with high spinal cord injury (SCI) are prone to significant fluctuation in blood pressure with episodes of very high and low blood pressure during autonomic dysreflexia (AD) and orthostatic hypotension, respectively. We do not know how such blood pressure lability affects the vasculature.

PURPOSE

We used a well-characterized animal model of AD to determine whether increasing the frequency of AD during recovery from SCI would exacerbate injury-induced dysfunction in resistance vessels.

STUDY DESIGN/SETTING

Experimental animal study. International Collaboration On Repair Discoveries (ICORD), University of British Columbia, Canada.

METHODS

Complete transection of the T3 spinal cord was performed in male Wistar rats. For 14 days after injury, AD was induced via colorectal distension (CRD; 30 minutes per day) in the experimental group (SCI-CRD). One month after SCI, baseline cardiovascular parameters and severity of CRD-induced AD were assessed in SCI-CRD animals and SCI-only controls. Mesenteric arteries were harvested for in vitro myography to characterize vasoactive responses to phenylephrine (PE) and acetylcholine (ACh).

RESULTS

Mesenteric arteries from SCI-CRD animals exhibited larger maximal responses to PE than arteries from SCI-only controls. Hyperresponsiveness to PE was not a product of endothelial dysfunction because mesenteric arteries from both groups had similar vasodilator responses to ACh. Both SCI-only controls and SCI-CRD animals exhibited CRD-evoked AD 1 month after SCI; however, CRD-induced hypertension was less pronounced in animals that were previously exposed to CRD.

CONCLUSIONS

Injury-induced changes within the vasculature may contribute to the development of AD after SCI. Here, we provide evidence that AD itself has significant and long-lasting effects on vascular function. This finding has implications for the medical management of AD and provides an impetus for maintaining stable blood pressure.

Transdifferentiation of bone marrow stromal cells into cholinergic neuronal phenotype: a potential source for cell therapy in spinal cord injury

BACKGROUND AIMS

Cholinergic neurons are very important cells in spinal cord injuries because of the deficits in motor, autonomic and sensory neurons. In this study, bone marrow stromal cells (BMSC) were evaluated as a source of cholinergic neurons in a rat model of contusive spinal cord injury.

METHODS

BMSC were isolated from adult rats and transdifferentiated into cholinergic neuronal cells. The BMSC were pre-induced with beta-mercaptoethanol (BME), while the induction was done with nerve growth factor (NGF). Neurofilament (NF)-68, -160 and -200 immunostaining was used for evaluating the transdifferentiation of BMSC into a neuronal phenotype. NeuroD expression, a marker for neuroblast differentiation, and Oct-4 expression, a marker for stemness, were evaluated by reverse transcriptase (RT)-polymerase chain reaction (PCR). Choline acetyl transferase (ChAT) immunoreactivity was used for assessing the cholinergic neuronal phenotype. Anti-microtubule-associated protein-2 (MAP-2) and anti-synapsin I antibodies were used as markers for the tendency for synptogenesis. Finally, the induced cells were transplanted into the contused spinal cord and locomotion was evaluated with the Basso-Beattie-Bresnahan (BBB) test.

RESULTS

At the induction stage, there was a decline in the expression of NF-68 associated with a sustained increase in the expression of NF-200, NF-160, ChAT and synapsin I, whereas MAP-2 expression was variable. Transplanted cells were detected 6 weeks after their injection intraspinally and were associated with functional recovery.

CONCLUSIONS

The transdifferentiation of BMSC into a cholinergic phenotype is feasible for replacement therapy in spinal cord injury.

Pressor response to passive walking-like exercise in spinal cord-injured humans

OBJECTIVE

To examine blood pressure responses during passive walking-like exercise in the standing posture (PWE) in spinal cord-injured (SCI) humans.

METHODS

Twelve motor-complete SCI individuals (cervical level 6 to thoracic level 12, ASIA grade: A or B) and twelve able-bodied controls (CON) participated in this study. SCI individuals were divided into a group with injury level at or above thoracic (T) 6 (HSCI, n = 7) and a group with injury level at or below T10 (LSCI, n = 5). Subjects carried out 6-minute quiet standing and then 12-minute PWE at 1 Hz using a gait training apparatus that enables subjects to stand and move their legs passively.

RESULTS

Mean arterial blood pressures (MAPs) at standing in HSCI, LSCI and CON were 69 +/- 5, 83 +/- 4 and 93 +/- 2 mmHg, respectively. MAP changed significantly during PWE only in HSCI and CON, increasing to 88 +/- 4 (P < 0.001) and 98 +/- 1 mmHg (P < 0.01), respectively. The former group showed a larger increase in MAP (P < 0.001).

INTERPRETATION

Spinal sympathetic reflexes can be induced in a region isolated from the brainstem in response to a stimulus originating below the level of the spinal cord injury, and the magnitude of increase in blood pressure is greater in SCI individuals with lesion level at or above T6 due to loss of supraspinal control of the major sympathetic outflow. This central mechanism may be one of the reasons why greater pressor response to PWE was observed in HSCI.

Assessing the integrity of sympathetic pathways in spinal cord injury

STUDY DESIGN

Measurement of cutaneous sympathetic reflexes and hemodynamic responses to brief electrical stimuli applied above (forehead) and below (abdominal wall) a spinal lesion.

OBJECTIVE

To assess the validity of using cutaneous vasoconstriction as a sensitive indicator of increases in sympathetic activity in spinal cord injury.

SETTING

Prince of Wales Medical Research Institute, Australia.

SUBJECTS

Twenty spinal cord injured subjects with injuries ranging from C3-T11 and nine able-bodied controls.

METHOD

Cutaneous electrical stimulation was applied to the forehead and abdominal wall to subjects at unexpected times. Sudomotor and vasomotor responses, as well as continuous arterial pressure, heart rate and respiration were monitored.

RESULTS

Sudomotor (electrodermal) responses to forehead stimulation were scarce in spinal cord injured subjects, whereas cutaneous vasoconstrictor responses (photoelectric pulse plethysmography) provided a sensitive indicator of any remaining central control of sympathetic function below the lesion. Electrical stimulation applied to the abdominal wall evoked vasoconstrictor reflexes below the lesion in the majority of spinal cord injured subjects, whereas only a limited number of electrodermal responses were observed. That these cutaneous vasoconstrictor responses could reflect parallel increases in muscle and splanchnic vasoconstrictor activity was indicated by the increases in blood pressure; patients lacking vasoconstrictor responses rarely showed stimulus-induced blood pressure increases.

CONCLUSION

Our findings show that skin vasomotor responses to somatosensory stimulation provide a more sensitive tool than electrodermal responses for evaluation of sympathetic function below a spinal cord lesion. STATEMENT OF ETHICS: We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during the course of this research, and all experiments were conducted with the understanding and consent of each subject.

Recognition and effective management of autonomic dysreflexia in spinal cord injuries

Autonomic dysreflexia is a potentially life-threatening hypertensive medical emergency that occurs most often in spinal cord-injured individuals with spinal lesions at or above the mid-thoracic spinal cord level. It is a condition that remains poorly recognised outside of spinal cord injury centres, which may result in adverse outcomes including mortality from potentially delayed diagnosis and treatment. Acute autonomic dysreflexia is characterised by severe paroxysmal hypertension associated with throbbing headaches, profuse sweating, nasal stuffiness, flushing of the skin above the level of the lesion, bradycardia, apprehension and anxiety, which is sometimes accompanied by cognitive impairment. The key to effective management is prevention of the condition, by recognition and avoidance of factors that initiate the condition. When it occurs, immediate recognition and reversal of trigger factors along with prompt administration of pharmacological treatment is of paramount importance in order to prevent complications, which include intracranial and retinal haemorrhage, convulsions, cardiac irregularities and death. Promising data from recent animal studies may hold the key to future treatment options.