Spinal sympathetic interneurons: Their identification and roles after spinal cord injury

Mapping the Iceberg of Autonomic Recovery: Mechanistic Underpinnings of Neuromodulation following Spinal Cord Injury

Spinal cord injury leads to disruption in autonomic control resulting in cardiovascular, bowel, and lower urinary tract dysfunctions, all of which significantly reduce health-related quality of life. Although spinal cord stimulation shows promise for promoting autonomic recovery, the underlying mechanisms are unclear. Based on current preclinical and clinical evidence, this narrative review provides the most plausible mechanisms underlying the effects of spinal cord stimulation for autonomic recovery, including activation of the somatoautonomic reflex and induction of neuroplastic changes in the spinal cord. Areas where evidence is limited are highlighted in an effort to guide the scientific community to further explore these mechanisms and advance the clinical translation of spinal cord stimulation for autonomic recovery.

Chronic Pain-Associated Cardiovascular Disease: The Role of Sympathetic Nerve Activity

Chronic pain affects many people world-wide, and this number is continuously increasing. There is a clear link between chronic pain and the development of cardiovascular disease through activation of the sympathetic nervous system. The purpose of this review is to provide evidence from the literature that highlights the direct relationship between sympathetic nervous system dysfunction and chronic pain. We hypothesize that maladaptive changes within a common neural network regulating the sympathetic nervous system and pain perception contribute to sympathetic overactivation and cardiovascular disease in the setting of chronic pain. We review clinical evidence and highlight the basic neurocircuitry linking the sympathetic and nociceptive networks and the overlap between the neural networks controlling the two.

Autonomic Dysreflexia following Spinal Cord Injury

Autonomic dysreflexia (AD) is a potentially life-threatening condition of the autonomic nervous system following spinal cord injury at or above T6. One of the most common symptoms is a sudden increase in blood pressure induced by afferent sensory stimulation owing to unmodulated reflex sympathetic hyperactivity. Such episodes of high blood pressure might be associated with a high risk of cerebral or retinal hemorrhage, seizures, heart failure, or pulmonary edema. In-depth knowledge is, therefore, crucial for the proper management of the AD, especially for spine surgeons, who encounter these patients quite often in their clinical practice. Systematical review of the literature dealing with strategies to prevent and manage this challenging condition was done by two independent reviewers. Studies that failed to assess primary (prevention, treatment strategies and management) and secondary outcomes (clinical symptomatology, presentation) were excluded. A bibliographical search revealed 85 eligible studies that provide a variety of preventive and treatment measures for the subjects affected by AD. As these measures are predominantly based on noncontrolled trials, long-term prospectively controlled multicenter studies are warranted to validate these preventive and therapeutic proposals.

Spinal Reflex Control of Arterial Blood Pressure: The Role of TRP Channels and Their Endogenous Eicosanoid Modulators

The spinal cord is an important integrative center for blood pressure control. Spinal sensory fibers send projections to sympathetic preganglionic neurons of the thoracic spinal cord and drive sympathetically-mediated increases in blood pressure. While these reflexes responses occur in able-bodied individuals, they are exaggerated following interruption of descending control - such as occurs following spinal cord injury. Similar reflex control of blood pressure may exist in disease states, other than spinal cord injury, where there is altered input to sympathetic preganglionic neurons. This review primarily focuses on mechanisms wherein visceral afferent information traveling via spinal nerves influences sympathetic nerve activity and blood pressure. There is an abundance of evidence for the widespread presence of this spinal reflex arch originating from virtually every visceral organ and thus having a substantial role in blood pressure control. Additionally, this review highlights specific endogenous eicosanoid species, which modulate the activity of afferent fibers involved in this reflex, through their interactions with transient receptor potential (TRP) cation channels.

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.

Disruption of sensation-dependent bladder emptying due to bladder overdistension in a complete spinal cord injury: A case report

BACKGROUND

Autonomic dysreflexia (AD) can be a life-threatening condition in patients with spinal cord injury. It is important to prevent bladder overdistension in these patients as it may trigger AD. Sensation-dependent bladder emptying (SDBE), as a method of bladder management, improves the quality of life and allows physiologic voiding. In this study, we report disruption of the SDBE habit after bladder overdistension leading to AD with chest pain.

CASE SUMMARY

A 47-year-old male with a diagnosis of C4 American Spinal Cord Injury Association impairment scale A had been emptying his bladder using the clean intermittent catheterization method with an itchy sensation in the nose as a sensory indication for a full bladder for 23 years, and the usual urine volume was about 300-400 mL. At the time of this study, the patient had delayed catheterization for approximately five hours. He developed severe abdominal pain and headache and had to visit the emergency room for bladder overdistension (800 mL) and a high systolic blood pressure (205 mmHg). After control of AD, a hypersensitive bladder was observed despite using anticholinergic agents. The sensation indicating bladder fullness changed from nose itching to pain in the abdomen and precordial area. Moreover, the volume of the painful bladder filling sensation became highly variable and was noted when the bladder urine volume exceeded only 100 mL. The patient refused intermittent clean catheterization. Finally, a cystostomy was performed, which relieved the symptoms.

CONCLUSION

Patients using physiologic feedback, such as SDBE, for bladder management are recommended to avoid bladder overdistension.

The gabapentinoid drugs and their abuse potential

The gabapentinoid drugs, gabapentin and pregabalin, are first-line treatments for neuropathic pain. The epidemics of chronic pain and opioid misuse have given rise to the widespread use of non-opioid drugs such as the gabapentinoids for treatment. Unfortunately, the widespread use of gabapentinoid drugs has resulted in reports of misuse and abuse. Here we summarize the clinical reports of gabapentinoid abuse in different patient populations to help inform clinical practice of chronic pain management.

Morphological maladaptations in sympathetic preganglionic neurons following an experimental high-thoracic spinal cord injury

Spinal cord injury (SCI) disrupts the supraspinal vasomotor pathways to sympathetic preganglionic neurons (SPNs) leading to impaired blood pressure (BP) control that often results in episodes of autonomic dysreflexia and orthostatic hypotension. The physiological cardiovascular consequences of SCI are largely attributed to the plastic changes in spinal SPNs induced by their partial deafferentation. While multiple studies have investigated the morphological changes in SPNs following SCI with contrasting reports. Here we investigated the morphological changes in SPNs rostral and caudal to a high thoracic (T3) SCI at 1-, 4- and 8-weeks post injury. SPNs were identified using Nicotinamide adenine dinucleotide hydrogen phosphate-diaphorase (NADPH- diaphorase) staining and were quantified for soma size and various dendritic measurements. We show that rostral to the lesion, soma size was increased at 1 week along with increased dendritic arbor. The total dendritic length was also increased at chronic stage (8 weeks post SCI). Caudal to the lesion, the soma size or dendritic lengths did not change with SCI. However, dendritic branching was enhanced within a week post SCI and remained elevated throughout the chronic stages. These findings demonstrate that SPNs undergo significant structural changes form sub-acute to chronic stages post-SCI that likely determines their functional consequences. These changes are discussed in context of physiological cardiovascular outcomes post-SCI.

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.

Intraspinal Plasticity Associated With the Development of Autonomic Dysreflexia After Complete Spinal Cord Injury

Traumatic spinal cord injury (SCI) leads to disruption of sensory, motor and autonomic function, and triggers structural, physiological and biochemical changes that cause reorganization of existing circuits that affect functional recovery. Propriospinal neurons (PN) appear to be very plastic within the inhibitory microenvironment of the injured spinal cord by forming compensatory circuits that aid in relaying information across the lesion site and, thus, are being investigated for their potential to promote locomotor recovery after experimental SCI. Yet the role of PN plasticity in autonomic dysfunction is not well characterized, notably, the disruption of supraspinal modulatory signals to spinal sympathetic neurons after SCI at the sixth thoracic spinal segment or above resulting in autonomic dysreflexia (AD). This condition is characterized by unmodulated sympathetic reflexes triggering sporadic hypertension associated with baroreflex mediated bradycardia in response to noxious yet unperceived stimuli below the injury to reduce blood pressure. AD is frequently triggered by pelvic visceral distension (bowel and bladder), and there are documented structural relationships between injury-induced sprouting of pelvic visceral afferent C-fibers. Their excitation of lumbosacral PN, in turn, sprout and relay noxious visceral sensory stimuli to rostral disinhibited thoracic sympathetic preganglionic neurons (SPN) that manifest hypertension. Herein, we review evidence for maladaptive plasticity of PN in neural circuits mediating heightened sympathetic reflexes after complete high thoracic SCI that manifest cardiovascular dysfunction, as well as contemporary research methodologies being employed to unveil the precise contribution of PN plasticity to the pathophysiology underlying AD development.

Evidence of axon connectivity across a spinal cord transection in rats treated with epidural stimulation and motor training combined with olfactory ensheathing cell transplantation

Olfactory ensheathing cells (OECs) are unique glia that support axon outgrowth in the olfactory system, and when used as cellular therapy after spinal cord injury, improve recovery and axon regeneration. Here we assessed the effects of combining OEC transplantation with another promising therapy, epidural electrical stimulation during a rehabilitative motor task. Sprague-Dawley rats received a mid-thoracic transection and transplantation of OECs or fibroblasts (FBs) followed by lumbar stimulation while climbing an inclined grid. We injected pseudorabies virus (PRV) into hindlimb muscles 7 months post-injury to assess connectivity across the transection. Analyses showed that the number of serotonergic (5-HT) axons that crossed the rostral scar border and the area of neurofilament-positive axons in the injury site were both greater in OEC- than FB-treated rats. We detected PRV-labeled cells rostral to the transection and remarkable evidence of 5-HT and PRV axons crossing the injury site in 1 OEC- and 1 FB-treated rat. The axons that crossed suggested either axon regeneration (OEC) or small areas of probable tissue sparing (FB). Most PRV-labeled thoracic neurons were detected in laminae VII or X, and ~25% expressed Chx10, a marker for V2a interneurons. These findings suggest potential regeneration or sparing of circuits that connect thoracic interneurons to lumbar somatic motor neurons. Despite evidence of axonal connectivity, no behavioral changes were detected in this small-scale study. Together these data suggest that when supplemented with epidural stimulation and climbing, OEC transplantation can increase axonal growth across the injury site and may promote recovery of propriospinal circuitry.

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.

How sympathetic are your spinal cord circuits?

NEW FINDINGS

What is the topic of this review? This review focuses on the role of gap junctions and interneurones in sympathetic control at the spinal cord level. What advances does it highlight? The review considers the importance of these local spinal circuits in contributing to rhythmic autonomic activity and enabling appropriate responses to homeostatic perturbations. Sympathetic control of end organs relies on the activity of sympathetic preganglionic neurones (SPNs) within the spinal cord. These SPNs exhibit heterogeneity with respect to function, neurochemistry, location, descending inputs and patterns of activity. Part of this heterogeneity is bestowed by local spinal circuitry. Our understanding of the role of these local circuits, including the significance of connections between the SPNs themselves through specialized gap junctions, is patchy. This report focuses on interneurones and gap junctions within these circuits. Gap junctions play a role in sympathetic control; they are located on SPNs in the intermediolateral cell column. Mefloquine, a chemical that blocks these gap junctions, reduces local rhythmic activity in the spinal cord slice and disrupts autonomic control in the working heart-brainstem preparation. The role that these gap junctions may play in health and disease in adult animals remains to be elucidated fully. Presympathetic interneurones are located in laminae V, VII and X and the intermediolateral cell column; those in lamina X are GABAergic and directly inhibit SPNs. The GABAergic inputs onto SPNs exert their effects through activation of synaptic and extrasynaptic receptors, which stabilize the membrane at negative potentials. The GABAergic interneurones contribute to rhythmic patterns of activity that can be generated in the spinal cord, because bicuculline reduces network oscillatory activity. These studies indicate that local spinal cord circuitry is critical in enabling appropriate levels and patterning of activity in sympathetic outflow. We need to understand how these circuits may be harnessed in the situation of spinal cord injury.

Symptomatic arrhythmias due to syringomyelia-induced severe autonomic dysfunction

Syringomyelia is characterized by cavity formation in the spinal cord, most often at C2-Th9 level. Clinical manifestation reflects extent and localization of the spinal cord injury.

CASES

20-year old woman was admitted for recurrent rest-related presyncopes with sudden manifestation. Paroxysms of sinus bradycardia with SA and AV blocks were repeatedly documented during symptoms. There was normal echocardiographic finding, (para) infectious etiology was not proved. Character of the ECG findings raised suspicion on neurogenic cause. Autonomic nervous system testing demonstrated abnormalities reflecting predominant sympathetic dysfunction. Suspicion on incipient myelopathy was subsequently confirmed by MRI, which discovered syringomyelia at Th5 level as the only pathology. A 52-year old man with hypotrophic quadruparesis resulting from perinatal brain injury was sent for 2-years lasting symptoms (sudden palpitation, sweating, muscle tightness, shaking) with progressive worsening. Symptoms occurred in association with sudden increase of sinus rhythm rate and blood pressure that were provoked by minimal physical activity. Presence of significant autonomic dysregulation with baroreflex hyperreactivity in orthostatic test and symptomatic postural orthostatic tachycardia with verticalization-associated hypertension were proved. MRI revealed syringomyelia at C7 and Th7 level affecting sympathetic centers at these levels. Sympathetic fibers dysfunction at C-Th spinal level may cause significant autonomic dysfunction with arrhythmic manifestation.

Local and systemic pathogenesis and consequences of regimen-induced inflammatory responses in patients with head and neck cancer receiving chemoradiation

Treatment-related toxicities are common among patients with head and neck cancer, leading to poor clinical outcomes, reduced quality of life, and increased use of healthcare resources. Over the last decade, much has been learned about the pathogenesis of cancer regimen-related toxicities. Historically, toxicities were separated into those associated with tissue injury and those with behavioural or systemic changes. However, it is now clear that tissue-specific damage such as mucositis, dermatitis, or fibrosis is no longer the sole consequence of direct clonogenic cell death, and a relationship between toxicities that results in their presentation as symptom clusters has been documented and attributed to a common underlying pathobiology. In addition, the finding that patients commonly develop toxicities representing tissue injury outside radiation fields and side effects such as fatigue or cognitive dysfunction suggests the generation of systemic as well as local mediators. As a consequence, it might be appropriate to consider toxicity syndromes, rather than the traditional approach, in which each side effect was considered as an autonomous entity. In this paper, we propose a biologically based explanation which forms the basis for the diverse constellation of toxicities seen in response to current regimens used to treat cancers of the head and neck.

Partial restoration of cardiovascular function by embryonic neural stem cell grafts after complete spinal cord transection

High-level spinal cord injury can lead to cardiovascular dysfunction, including disordered hemodynamics at rest and autonomic dysreflexia during noxious stimulation. To restore supraspinal control of sympathetic preganglionic neurons (SPNs), we grafted embryonic brainstem-derived neural stem cells (BS-NSCs) or spinal cord-derived neural stem cells (SC-NSCs) expressing green fluorescent protein into the T4 complete transection site of adult rats. Animals with injury alone served as controls. Implanting of BS-NSCs but not SC-NSCs resulted in recovery of basal cardiovascular parameters, whereas both cell grafts alleviated autonomic dysreflexia. Subsequent spinal cord retransection above the graft abolished the recovery of basal hemodynamics and reflexic response. BS-NSC graft-derived catecholaminergic and serotonergic neurons showed remarkable long-distance axon growth and topographical innervation of caudal SPNs. Anterograde tracing indicated growth of medullar axons into stem cell grafts and formation of synapses. Thus, grafted embryonic brainstem-derived neurons can act as functional relays to restore supraspinal regulation of denervated SPNs, thereby contributing to cardiovascular functional improvement.

Lower motor neuron findings after upper motor neuron injury: insights from postoperative supplementary motor area syndrome

Hypertonia and hyperreflexia are classically described responses to upper motor neuron injury. However, acute hypotonia and areflexia with motor deficit are hallmark findings after many central nervous system insults such as acute stroke and spinal shock. Historic theories to explain these contradictory findings have implicated a number of potential mechanisms mostly relying on the loss of descending corticospinal input as the underlying etiology. Unfortunately, these simple descriptions consistently fail to adequately explain the pathophysiology and connectivity leading to acute hyporeflexia and delayed hyperreflexia that result from such insult. This article highlights the common observation of acute hyporeflexia after central nervous system insults and explores the underlying anatomy and physiology. Further, evidence for the underlying connectivity is presented and implicates the dominant role of supraspinal inhibitory influence originating in the supplementary motor area descending through the corticospinal tracts. Unlike traditional explanations, this theory more adequately explains the findings of postoperative supplementary motor area syndrome in which hyporeflexia motor deficit is observed acutely in the face of intact primary motor cortex connections to the spinal cord. Further, the proposed connectivity can be generalized to help explain other insults including stroke, atonic seizures, and spinal shock.

Role of peripheral nerve fibres in acute and chronic inflammation in arthritis

The peripheral nervous system takes an active part in inflammatory processes by regulating effector cell function and reallocation of energy to the immune system. During acute inflammation, rapid neuronal reorganization and change of activity takes place. The hallmarks of this process are an increase in systemic sympathetic activity, a decrease in systemic parasympathetic activity and loss of sympathetic nerve fibres from sites of inflammation concomitant with increased innervation with sensory nerve fibres and increased sensory nerve fibre activity. On a systemic level, the increase in sympathetic activity (and decrease in parasympathetic activity) is necessary to provide enough energy to nourish the activated immune system. In locally inflamed tissue, the decrease in sympathetic nerve fibre density results in reduced anti-inflammatory signalling and, together with neuropeptides released from sensory nerve fibres, promotes local inflammation. In acute inflammation, this 'inflammatory configuration' of the peripheral nervous system favours the rapid clearance of antigenic threats. However, in chronic autoimmune inflammation, these changes of the peripheral nervous system lead to an unfavourable situation with ongoing energy reallocation and continuous local destruction. As an example of a chronic inflammatory condition, we discuss evidence for neuroimmune regulation in autoimmune arthritis with a focus on the sympathetic nervous system.

Electrophysiological and morphological characterization of propriospinal interneurons in the thoracic spinal cord

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤ 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

Neuropeptide coding of sympathetic preganglionic neurons; focus on adrenally projecting populations

Chemical coding of sympathetic preganglionic neurons (SPN) suggests that the chemical content of subpopulations of SPN can define their function. Since neuropeptides, once synthesized are transported to the axon terminal, most demonstrated chemical coding has been identified using immunoreactive terminals at the target organ. Here, we use a different approach to identify and quantify the subpopulations of SPN that contain the mRNA for pituitary adenylate cyclase activating polypeptide (PACAP) or enkephalin. Using double-labeled immunohistochemistry combined with in situ hybridization (ISH) we firstly identified the distribution of these mRNAs in the spinal cord and determined quantitatively, in Sprague-Dawley rats, that many SPN at the T4-T10 spinal level contain preproPACAP (PPP+, 80 ± 3%, n=3), whereas a very small percentage contain preproenkephalin (PPE+, 4 ± 2%, n=4). A similar neurochemical distribution was found at C8-T3 spinal level. These data suggest that PACAP potentially regulates a large number of functions dictated by SPN whereas enkephalins are involved in few functions. We extended the study to explore those SPN that control adrenal chromaffin cells. We found 97 ± 5% of adrenally projecting SPN (AP-SPN) to be PPP+ (n=4) with only 47 ± 3% that were PPE+ (n=5). These data indicate that adrenally projecting PACAPergic SPN regulate both adrenal adrenaline (Ad) and noradrenaline (NAd) release whereas the enkephalinergic SPN subpopulation must control a (sub) population of chromaffin cells - most likely those that release Ad. The sensory innervation of the adrenal gland was also determined. Of the few adrenally projecting dorsal root ganglia (AP-DRG) observed, 74 ± 12% were PPP+ (n=3), whereas 1 ± 1% were PPE+ (n=3). Therefore, if sensory neurons release peptides to the adrenal medulla, PACAP is most likely involved. Together, these data provide a neurochemical basis for differential control of sympathetic outflow particularly that to the adrenal medulla.

Serum leptin levels following acute experimental spinal cord injury

BACKGROUND/OBJECTIVE

Spinal cord injury influences many hormones that are known to be involved in the modulation of neurotrophic, neurogenic, and neuroprotective events. Recent studies showed that leptin could be neuroprotective, enhancing neuronal survival in vitro and in vivo. The objective of this study was to evaluate the pattern of the serum leptin levels in rats during acute traumatic SCI.

METHODS

Forty male Sprague-Dawley rats were divided randomly into 4 groups. In the control group, neither laminectomy nor SCI was performed; only laminectomy was performed without SCI in the sham group. In the cervical and thoracic spinal trauma groups, laminectomies were performed following the same trauma procedure. Blood samples were drawn 2, 6, 12, and 24 hours after the procedures and assayed immediately.

RESULTS

In the first 2 hours, levels of leptin were similar in control and sham-operated groups and higher in neurotrauma groups (P < 0.05). At the sixth hour, leptin levels increased in the sham-operated group, decreased in the neurotrauma groups (P < 0.05), and did not change in the control group (P > 0.05). At the 12th hour, the levels of leptin increased in all groups (P > 0.05). At the 24th hour, they decreased in the control, sham-operated, and cervical groups (P < 0.05); levels did not change in the thoracic group (P > 0.05). The decrease was higher in the control group than in the other groups (P < 0.05).

CONCLUSIONS

Activation of endogenous leptin secretion started immediately after the SCI. The level of neurologic lesion (either cervical or thoracic regions) affected the levels of serum leptin differently, but with the exception of the first 12-hour period, this difference did not reach a statistically significant level.

Sympathetic-correlated c-Fos expression in the neonatal rat spinal cord in vitro

An isolated thoracic spinal cord of the neonatal rat in vitro spontaneously generates sympathetic nerve discharge (SND) at ~25 degrees C, but it fails in SND genesis at < or = 10 degrees C. Basal levels of the c-Fos expression in the spinal cords incubated at < or = 10 degrees C and ~25 degrees C were compared to determine the anatomical substrates that might participate in SND genesis. Cells that exhibited c-Fos immunoreactivity were virtually absent in the spinal cords incubated at < or = 10 degrees C. However, in the spinal cords incubated at ~25 degrees C, c-Fos-positive cells were found in the dorsal laminae, the white matter, lamina X, and the intermediolateral cell column (IML). Cell identities were verified by double labeling of c-Fos with neuron-specific nuclear protein (NeuN), glial fibrillary acidic protein (GFAP), or choline acetyltransferase (ChAT). The c-Fos-positive cells distributed in the white matter and lamina X were NeuN-negative or GFAP-positive and were glial cells. Endogenously active neurons showing c-Fos and NeuN double labeling were scattered in the dorsal laminae and concentrated in the IML. Double labeling of c-Fos and ChAT confirmed the presence of active sympathetic preganglionic neurons (SPNs) in the IML. Suppression of SND genesis by tetrodotoxin (TTX) or mecamylamine (MECA, nicotinic receptor blocker) almost abolished c-Fos expression in dorsal laminae, but only mildly affected c-Fos expression in the SPNs. Therefore, c-Fos expression in some SPNs does not require synaptic activation. Our results suggest that spinal SND genesis is initiated from some spontaneously active SPNs, which are capable of TTX- or MECA-resistant c-Fos expression.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

Retrograde adenoviral vector targeting of nociresponsive pontospinal noradrenergic neurons in the rat in vivo

The spinal dorsal horn receives a dense innervation of noradrenaline-containing fibers that originate from pontine neurons in the A5, locus coeruleus (LC), and A7 cell groups. These pontospinal neurons are believed to constitute a component of the endogenous analgesic system. We used an adenoviral vector with a catecholaminergic-selective promoter (AVV-PRS) to retrogradely label the noradrenergic neurons projecting to the lumbar (L4–L5) dorsal horn with enhanced green fluorescent protein (EGFP) or monomeric red fluorescent protein (mRFP). Retrogradely labeled neurons (145 ± 12, n = 14) were found in A5-12%, LC-80% and A7-8% after injection of AVV-PRS-EGFP to the dorsal horn of L4–L5. These neurons were immunopositive for dopamine β-hydroxylase, indicating that they were catecholaminergic. Retrograde labeling was optimal 7 days after injection, persisted for over 4 weeks, and was dependent on viral vector titer. The spinal topography of the noradrenergic projection was examined using EGFP- and mRFP-expressing adenoviral vectors. Pontospinal neurons provide bilateral innervation of the cord and there was little overlap in the distribution of neurons projecting to the cervical and lumbar regions. The axonal arbor of the pontospinal neurons was visualized with GFP immunocytochemistry to show projections to the inferior olive, cerebellum, thalamus, and cortex but not to the hippocampus or caudate putamen. Formalin testing evoked c-fos expression in these pontospinal neurons, suggesting that they were nociresponsive (A5-21%, LC-16%, and A7-26%, n = 8). Thus, we have developed a viral vector-based strategy to selectively, retrogradely target the pontospinal noradrenergic neurons that are likely to be involved in the descending control of nociception.

Leg sympathetic response to noxious skin stimuli is similar in high and low level human spinal cord injury

OBJECTIVE

To determine if sympathetically mediated vasoconstriction in the lower extremities is injury level dependent. Although sympathetic responses have been measured in the limbs of people with high and low level SCI using blood flow measurements, including Doppler ultrasound and venous plethysmography, a direct comparison between injury levels has not been made.

METHODS

Volunteers with chronic SCI were grouped according to injury level. Above T6: high level (HL, n=7), and T6 and below: low level (LL, n=6). All subjects had complete motor and sensory loss. Leg arterial flows were recorded by venous occlusion plethysmography, and continuous heart rate and mean arterial pressure (MAP) were measured. The conditioning stimulus consisted of transcutaneous stimulation to the arch of the contralateral foot.

RESULTS

HL and LL subjects demonstrated a significant decrease in arterial conductance during stimulation with no significant difference found between groups. As expected, only group HL demonstrated a significant increase in MAP.

CONCLUSIONS

These results support our hypothesis that local (leg) sympathetic responses are similar for both high and low level SCI.

SIGNIFICANCE

While low level SCI does not typically present with autonomic dysreflexia, bouts of increased reflex sympathetic activity could have ramifications for metabolism as well as renal and motor system function.

Urinary bladder irritation alters efficacy of vagal stimulation on rostral medullary neurons in chronic T8 spinalized rats

The presence of pelvic visceral inputs to neurons in the rostral medulla that are responsive to electrical stimulation of the abdominal branches of the vagus nerve (VAG-abd) was investigated in a complete chronic T8 spinal transection rat model. Using extracellular electrophysiological recordings from single medullary reticular formation (MRF) neurons, 371 neurons in 15 rats responsive to pinching the ear (search stimulus) were tested for somato-visceral and viscero-visceral convergent responses to stimulation of the following nerves/territories: VAG-abd, dorsal nerve of the penis, pelvic nerve, distention of urinary bladder and colon, penile stimulation, urethral infusion, and touch/pinch of the entire body surface. In addition to these mechanical and electrical stimuli, a chemical stimulus applied to the bladder was assessed as well. Of the total neurons examined, 205 were tested before and 166 tested beginning 20 min after application of a chemical irritant (2% acetic acid) to the urinary bladder (same rats used pre/post irritation). As with intact controls, many ear-responsive MRF neurons responded to the electrical stimulation of VAG-abd. Although MRF neuron responses failed to be evoked with direct (mechanical and electrical nerve) pelvic visceral stimuli, acute chemical irritation of the urinary bladder produced a significant increase in the number of MRF neurons responsive to stimulation of VAG-abd. The results of this study indicate a central effect that potentially relates to some of the generalized below level pelvic visceral sensations that have been documented in patients with complete spinal cord injury.

Axonal regeneration and development of de novo axons from distal dendrites of adult feline commissural interneurons after a proximal axotomy

Following proximal axotomy, several types of neurons sprout de novo axons from distal dendrites. These processes may represent a means of forming new circuits following spinal cord injury. However, it is not know whether mammalian spinal interneurons, axotomized as a result of a spinal cord injury, develop de novo axons. Our goal was to determine whether spinal commissural interneurons (CINs), axotomized by 3-4-mm midsagittal transection at C3, form de novo axons from distal dendrites. All experiments were performed on adult cats. CINs in C3 were stained with extracellular injections of Neurobiotin at 4-5 weeks post injury. The somata of axotomized CINs were identified by the presence of immunoreactivity for the axonal growth-associated protein-43 (GAP-43). Nearly half of the CINs had de novo axons that emerged from distal dendrites. These axons lacked immunoreactivity for the dendritic protein, microtubule-associated protein2a/b (MAP2a/b); some had GAP-43-immunoreactive terminals; and nearly all had morphological features typical of axons. Dendrites of other CINs did not give rise to de novo axons. These CINs did, however, each have a long axon-like process (L-ALP) that projected directly from the soma or a very proximal dendrite. L-ALPs were devoid of MAP2a/b immunoreactivity. Some of these L-ALPs projected through the lesion and formed bouton-like swellings. These results suggest that proximally axotomized spinal interneurons have the potential to form new connections via de novo axons that emerge from distal dendrites. Others may be capable of regeneration of their original axon.

Autonomic Nervous System Dysfunction After Spinal Cord Injury

The autonomic nervous system (ANS) plays a key role in the regulation of many physiologic processes, mediated by supraspinal control from centers in the central nervous system. The role of autonomic dysfunction in persons with spinal cord injuries is crucial to understand because many aspects of the altered physiology seen in these individuals are directly caused by ANS dysregulation.