Mechanisms of norepinephrine accumulation within sites of spinal cord injury

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Spinal cord contusion models

Most human spinal cord injuries involve contusions of the spinal cord. Many investigators have long used weight-drop contusion animal models to study the pathophysiology and genetic responses of spinal cord injury. All spinal cord injury therapies tested to date in clinical trial were validated in such models. In recent years, the trend has been towards use of rats for spinal cord injury studies. The MASCIS Impactor is a well-standardized rat spinal cord contusion model that produces very consistent graded spinal cord damage that linearly predicts 24-h lesion volumes, 6-week white matter sparing, and locomotor recovery in rats. All aspects of the model, including anesthesia for male and female rats, age rather than body weight criteria, and arterial blood gases were empirically selected to enhance the consistency of injury.

Catecholamine-induced hypertension in lumbosacral paraplegia: five case reports

Hypertension in the patient with SCI is relatively rare and generally restricted to patients with high-level injuries where autonomic dysreflexia can occur. Resting blood pressure in individuals with SCI has been described as lower than that in the normal population. This report describes five previously normotensive teenagers with subsequent paraplegia as a result of gunshot wounds who presented with hypertension secondary to idiopathic elevation of plasma or urinary catecholamine levels. A clonidine suppression test was used as a neuroprobe to inhibit centrally mediated sympathetic outflow, excluding the probability of an extra-axial autonomous catecholamine-secreting tumor as the possible source of hypertension. Positive suppression was achieved in four patients (41%, 37.2%, 4.8%, and 37.2% decreases). One patient had values corresponding to orthostatic changes (an increase of 63%) because of poor compliance with the test. This patient was lost to follow-up; in the remaining four, hypertension resolved at 12, 8, 9, and 6 weeks postinjury. The increased circulating catecholamine level appears to be promoted by a centrally mediated response to the SCI. Elevated blood pressure probably results from an upgraded receptor regulation or an increased receptor sensitivity on the affected cells in the absence of restraining spinal reflexes. The pathophysiology of such hypertension seems to be secondary to autonomic dysfunction and, although it may be transient, it should be treated promptly and reevaluated periodically until stabilization is achieved.

Central nervous system bioaminergic responses to mechanical trauma. An experimental study

Changes in biogenic amines in the brain and spinal cord following penetrating injury were studied in male Wistar rats using high-performance liquid chromatography with electrochemical detection. Rapid increase in hemispheric concentration of these substances was noted beginning shortly after trauma. This trend continued until they were about three to four times control levels by about 24 to 48 hours postinjury. In the spinal cord, however, there was an initial sharp reduction in regional concentrations 2 hours postinjury followed by a slow rise thereafter. By 48 hours postinjury, levels of norepinephrine, dopamine, and serotonin of the cords of injured animals were still less than those of nontraumatized controls. This variation in the central nervous system bioaminergic response with the level of injury raises questions as to its precise role in neurological damage following mechanical insult.

Serotonergic response to spinal distraction trauma in experimental scoliosis

The effects of distraction injury to the spinal cord on serotonin (5HT) content and metabolism in a rat model of scoliosis were studied. Previous studies in this laboratory (Salzman et al., 1987a) have identified the 5HT response as a major component of the posttraumatic progression of spinal injury after impact trauma in the rabbit. The present study was designed to determine the universality of this response by examining a different model of injury in a different species. The results demonstrate that distraction trauma in the rat, like impact injury in the rabbit, is associated with a rapid and robust increase in the local spinal cord content and metabolism of 5HT and a long-term depletion of 5HT below the site of injury. The roles of the blood platelet and the raphe-spinal tract in the acute response and the disruption of axoplasmic transport during the chronic phase of injury are discussed.

Effect of transection on the blood-spinal cord barrier of the rat after isolation from descending sources

The disruption of descending pathways and subsequent release of vasoactive neurotransmitters may contribute to the abnormal vascular permeability observed after spinal cord injury. Therefore, the relationship between disruption of long descending fiber tracts in the rat spinal cord and the development of blood-spinal cord barrier breakdown to the protein horseradish peroxidase (HRP) was evaluated. This was accomplished by first transecting the cord in order to deplete transmitter stores in the distal (caudal) segments. One month after this isolation procedure, a second transection was made several segments distal to the first transection. The axial distribution of barrier permeability to HRP was evaluated at both the light and electron microscopic levels in this 'isolated cord' preparation. Camera lucida drawings, delineating the distribution of tracer leakage in the spinal cord, were used to quantify the extent of protein extravasation. Vascular leakage of the tracer was identified as early as 1 h postinjury but was restricted to segments adjacent to the second transection. By 1 day after injury, protein extravasation was more marked, as compared to the earlier time points, and axial spread of barrier breakdown occurred along more distal vascular sites. Abnormal permeability to HRP was confirmed at the ultrastructural level where the protein was present within vesicles in the endothelium and the surrounding smooth muscle layer and basal lamina. The tracer was also identified in the cytoplasmic compartment of neurons and glia and within the adjacent extracellular space.

Monoaminergic responses to spinal trauma. Participation of serotonin in posttraumatic progression of neural damage

The monoamines norepinephrine (NE), dopamine (DA), and serotonin (5-HT) and their major metabolites were measured in the spinal cord of rabbits following laminectomy or impact injury to the thoracic cord. Samples were taken 30 minutes, 60 minutes, 4 hours, and 6 weeks after injury. Utilization ratios (metabolite/transmitter) were calculated from the data. Turnover rates for NE and DA were also calculated at 30 minutes using the alpha-methylparatyrosine method. Trauma resulted in rapid and sustained elevations in 5-HT concentration at and around the injury site. The catecholamines were depleted slightly at the injury site. Levels of 5-hydroxyindole-3-acetic acid were elevated at 30 minutes but fell to baseline by 4 hours, resulting in a decrease in the 5-HT utilization ratio. The utilization and turnover of NE was increased at the injury site, while DA function was not affected. The large short-term increase in 5-HT levels may have been due to extravasation of platelet 5-HT stores into spinal tissue, rather than due to changes in neuronal 5-HT metabolism. At 6 weeks after injury, each monoamine and metabolite appeared to accumulate in spinal cord tissue proximal to the insult. Distal to the injury, depleted amine stores displayed augmented utilization. The data are discussed in terms of a serotonergic hypothesis of the progression of neural damage after trauma, with the interaction of 5-HT with raphe-spinal nerve terminals as a principal event.

Dopamine partially mediates the cardiovascular effects of naloxone after spinal injury

Following spinal injury, the opiate antagonist naloxone selectively elevates plasma dopamine levels, with the dopamine changes significantly correlated with improved cardiovascular function. Moreover, the cardiovascular effects of naloxone are significantly attenuated by pretreatment with the dopamine antagonist domperidone. From these data, it is concluded that the cardiovascular effects of naloxone after spinal injury are in part dopamine mediated.

The effect of norepinephrine on the spinal cord circulation and its possible implications in the pathogenesis of acute spinal trauma

The effect of intra-arterially administered norepinephrine (NE) upon spinal cord blood flow (SCBF), before and after disruption of the blood-cord barrier was studied in dogs. Barrier disruption was accomplished with an intra-arterial bolus injection of 2.5 M urea. Multiple ligations of branches of the posterior aorta and cannula placements ensured that the urea was directed to the lumbar and sacral segments of the cord. The SCBF was measured by the hydrogen clearance method. Intra-arterial urea by itself had no significant effect on SCBF. The intra-arterial infusion of NE (12 microgram/min and 30 microgram/min) was without overall effect on SCBF. However, if the blood-cord barrier had been previously disrupted with hypertonic urea, both concentrations of NE resulted in large reductions in SCBF. No such reductions in SCBF were seen with blood-cord barrier disruption and NE if the animals had been pre-treated with the alpha-blocker, phenoxybenzamine (1.5 mg/kg). Some aspects of the possible involvement of NE in the pathophysiology of acute spinal injury are discussed.

Microscopic analysis of early histopathological spinal cord alterations following trauma in normal and cathecholamine-depleted cats

The progressive histopathological sequence over the first 3 hr after a 400 g-cm blunt injury to the spinal cord of catecholamine (CA)-intact and CA-depleted cats is described. Norepinephrine levels were measured in all animals. The experiments were designed to determine the role of CA in progressive hemorrhagic necrosis of the spinal cord by removing CA from one group of animals prior to trauma. A second group of CA-intact animals was subjected to identical experimentally-induced trauma. Upon analysis of the histopathological changes, it appears that the sequential nature of the development of hemorrhagic necrosis is both qualitatively and quantitatively similar in both experimental groups. The general conclusion is drawn that catecholamines in greater than normal amounts are present at the lesion site after trauma, but these catecholamines are probably not involved in the process of central hemorrhagic necrosis.

Hemorrhagic white matter infarction in three critically ill patients

Extensive areas of hemorrhagic softening developed in the cerebral white matter in three critically ill patients. The anatomic peculiarities of the vasculature of this region, as well as the possible roles of edema and vasospasm, are weighed as factors in the pathogenesis of these unusual lesions.

Longitudinal extension of oedema in experimental spinal cord injury--evidence for two types of post-traumatic oedema

As in brain, another, obviously hydrostatic, type of oedema develops in the acutely injured spinal cord of the rabbit in addition to vasogenic oedema. The authors arrived at this finding by means of fluorescent tracer studies histological examinations, and measurements of tissue water, sodium and potassium. Longitudinal spread of vasogenic oedema remained limited to the zone of the epicentre and its surroundings. Hyperhydration, which appears gradually in the remote segments and is associated with increased sodium content, has the characteristics of plasma-ultrafiltrate.

Central necrosis following contusion to the sheep's spinal cord

This paper presents the results of a study on the pathological changes associated with post traumatic central spinal cord necrosis.

A histochemical investigation of catecholamines in spinal cord injury

The catecholamine hypothesis of progressive spinal cord necrosis following mechanical trauma was investigated with the histofluorometric method. Forty-four adult mongrel dog were examined as control, L1 crush-injured, and crush-injured with prior T1 total transection groups. In crush injured dogs, catecholamines were present in a 1 cm length of white matter at the crush site, with the greatest accumulation in the deep lateral and ventral funiculi. Gray matter fluorescence was not enhanced. Prior transection did not abolish the intense accumulation of catecholamines at the site of the cord injury. We propose that the catecholamines accumulating at the cord injured site are not central in origin, but represent an uptake mechanism into white matter as a reflection of cord microperfusion.

Norepinephrine levels in traumatized spinal cord of catecholamine-depleted cats

This study was designed to elucidate the origin of norepinephrine (NE) measured in spinal cord following trauma. In normal cats the NE concentration at the lesion site increased 63% over control 1 h following experimentally produced blunt trauma (400 g-cm). Spinal cords of adrenalectomized cats were also traumatized, but there was no increase in NE levels 1 h post-trauma. Spinal cord NE levels in these animals were not significantly different from normal or adrenalectomized non-injured controls. In the absence of one of the major peripheral sources of catecholamine, the adrenal medulla, our decreased NE levels prompt us to disagree with the hypothesis that NE measured in injured spinal cord is liberated from intrinsic neuronal systems. It is likely that the mechanism of NE accumulation is directly related to increased circulating levels of NE. The authors speculate on a possible reason for the conflicting results obtained by different laboratories in this area of research.

Effect of delayed local cooling on experimental spinal cord injury

The authors report studies indicating that delayed local cooling is effective in minimizing the neurological deficits of experimental spinal cord injury in cats. Cortical evoked responses were useful in predicting the neurological outcome; untreated animals whose evoked response disappeared for 6 hours failed to recover whereas all treated animals in the same group recovered dramatically. Decompression by laminectomy alone proved ineffective. Possible explanations for the therapeutic effects of cooling and the significance of the delay are briefly discussed.

The experimental contusion injury of the spinal cord in sheep

The validity of reproduction of the controlled contusion injury to the spinal cord in the experimental animal is questioned. The dynamic pathology involving the microvasculature within the first two hours is illustrated using light microscopy. After 15-30 minutes swelling of axons and disruption of myelin sheaths become evident in most areas of white matter. After four hours microcysts have formed in the columns of white matter and are evidence of irreversible damage. Swelling of the cord following injury results from congestion, extravasation and intracellular swelling of neurones, rather than from any demonstrable increase in extracellular fluid. Oedema was only demonstrated with perfusion fixation. Isotope and contrast myelography were compared in the identification of the degree and extent of spinal cord swelling. Significant improvement in motor power was found in a group of paraplegic sheep treated with alpha-methyl paratyrosine. There was no significant improvement in the degree of recovery of motor power or sensation in those animals treated with intrathecal methyl prednisolone (Depo-Medrol). The histopathology in the crushed spinal cord tissue of the treated and untreated animals at various intervals of time was compared. Some possible explanations for the different patterns of clinical recovery in the treated animals are discussed.

Use of 6-hydroxydopamine to create lesions in catecholamine neurons in rats

The authors report experiments in rats with 6-hydroxydopamine (6-OHDA), a chemical agent used for creating lesions in catecholamine neurons. Stereotaxic injection of 6-OHDA into the compact zone of the substantia nigra induced degeneration of the nigrostriatal dopamine (DA) system, confirmed by histology and histochemistry of the substantia nigra and by dopamine determinations in the neostriatum. These observations closely mimic those found in Parkinson's disease. In the experimental model a rotatory behavior is induced by DA agonists; this response has been used to test drugs for potential therapeutic use and also in basic studies of the extrapyramidal system. This technique may be useful in developing other experimental models for neurosurgical problems involving catecholamine pathways.

The pathophysiological response to spinal cord injury

✓ In this review of spinal cord injury research, the author has selected contributions which in his opinion best represent modern experimental concepts regarding the mechanism and management of spinal cord injuries. He has placed special emphasis on the controversial issues appropriate to a new, stimulating, and competitive area of research.