Co-localization of substance P and dopamine beta-hydroxylase with growth-associated protein-43 is lost caudal to a spinal cord transection

Disordered cardiovascular control after spinal cord injury

Damage to the spinal cord disrupts autonomic pathways, perturbing cardiovascular homeostasis. Cardiovascular dysfunction increases with higher levels of injury and greater severity. Disordered blood pressure control after spinal cord injury (SCI) has significant ramifications as cord-injured people have an increased risk of developing heart disease and stroke; cardiovascular dysfunction is currently a leading cause of death among those with SCI. Despite the clinical significance of abnormal cardiovascular control following SCI, this problem has been generally neglected by both the clinical and research community. Both autonomic dysreflexia and orthostatic hypotension are known to prevent and delay rehabilitation, and significantly impair the overall quality of life after SCI. Starting with neurogenic shock immediately after a higher SCI, ensuing cardiovascular dysfunctions include orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. Disordered temperature regulation accompanies these autonomic dysfunctions. This chapter reviews the human and animal studies that have furthered our understanding of the pathophysiology and mechanisms of orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. The cardiovascular dysfunction that occurs during sexual function and exercise is elaborated. New awareness of cardiovascular dysfunction after SCI has led to progress toward inclusion of this important autonomic problem in the overall assessment of the neurological condition of cord-injured people.

Autonomic function following cervical spinal cord injury

Spinal cord injury (SCI) is commonly associated with devastating paralysis. However, this condition also results in a variety of autonomic dysfunctions, primarily: cardiovascular, broncho-pulmonary, urinary, gastrointestinal, sexual, and thermoregulatory. SCI and the resultant unstable autonomic control are responsible for increased mortality from cardiovascular and respiratory disease among individuals with SCI. Injury level and severity directly correlate to the severity of autonomic dysfunctions following SCI. Following high cervical SCI, parasympathetic (vagal) control will remain intact, while the spinal sympathetic circuits will lose their tonic supraspinal autonomic control. On the other hand, in individuals with injury below the 5th thoracic segment, both the sympathetic and parasympathetic control of the heart and broncho-pulmonary tree are intact. As a result of injury level, individuals with quadriplegia versus those with paraplegia will have very different cardiovascular and respiratory responses. Furthermore, similar relationships can exist between the level of SCI and function of other organs that are under autonomic control (bladder, bowel, sweat glands, etc.). It is also important to appreciate that high cervical injuries result in significant respiratory dysfunctions due to the involvement of the diaphragm and a larger portion of the accessory respiratory muscles. Early recognition and timely management of autonomic dysfunctions in individuals with SCI are crucial for the long term health outcomes in this population.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

Autonomic dysreflexia after spinal cord injury: central mechanisms and strategies for prevention

Spinal reflexes dominate cardiovascular control after spinal cord injury (SCI). These reflexes are no longer restrained by descending control and they can be impacted by degenerative and plastic changes within the injured cord. Autonomic dysreflexia is a condition of episodic hypertension that stems from spinal reflexes initiated by sensory input entering the spinal cord caudal to the site of injury. This hypertension greatly detracts from the quality of life for people with cord injury and can be life-threatening. Changes in the spinal cord contribute substantially to the development of this condition. Rodent models are ideal for investigating these changes. Within the spinal cord, injury-induced plasticity leads to nerve growth factor (NGF)-dependent enlargement of the central arbor of a sub-population of sensory neurons. This enlarged arbor can provide increased afferent input to the spinal reflex, intensifying autonomic dysreflexia. Treatments such as antibodies against NGF can limit this afferent sprouting, and diminish the magnitude of dysreflexia. To assess treatments, a compression model of SCI that leads to progressive secondary damage, and also to some white matter sparing, is very useful. The types of spinal reflexes that likely mediate autonomic dysreflexia are highly susceptible to inhibitory influences of bulbospinal pathways traversing the white matter. Compression models of cord injury reveal that treatments that spare white matter axons also markedly reduce autonomic dysreflexia. One such treatment is an antibody to the integrin CD11d expressed by inflammatory leukocytes that enter the cord acutely after injury and cause significant secondary damage. This antibody blocks integrin-mediated leukocyte entry, resulting in greatly reduced white-matter damage and decreased autonomic dysreflexia after cord injury. Understanding the mechanisms for autonomic dysreflexia will provide us with strategies for treatments that, if given early after cord injury, can prevent this serious disorder from developing.

Dorsal column lesion reduces mechanical allodynia in the induction, but not the maintenance, phase in spinal hemisected rats

The dorsal column-medial lemniscal (DC-ML) system is known to be a route of ascending input signals for mechanical allodynia following peripheral nerve injury. We examined whether the pain signals after spinal hemisection were transmitted via the DC-ML system in the induction and maintenance phases of the neuropathic pain. Under enflurane anesthesia, rats were subjected to spinal hemisection at T13 level and bilateral DC lesion was made at T8 level 1 day or 3 weeks after the hemisection. The DC lesion 1 day after the hemisection significantly reduced the mechanical, but not cold, allodynia, whereas the DC lesion 3 weeks after the hemisection did not change both mechanical and cold allodynia. These results suggest that the signals for mechanical allodynia following spinal hemisection should be transmitted via the DC-ML system in the induction, but not maintenance, phase.

Tail arteries from chronically spinalized rats have potentiated responses to nerve stimulation in vitro

Patients with severe spinal cord lesions that damage descending autonomic pathways generally have low resting arterial pressure but bladder or colon distension or unheeded injuries may elicit a life-threatening hypertensive episode. Such episodes (known as autonomic dysreflexia) are thought to result from the loss of descending baroreflex inhibition and/or plasticity within the spinal cord. However, it is not clear whether changes in the periphery contribute to the exaggerated reflex vasoconstriction. The effects of spinal transection at T7-8 on nerve- and agonist-evoked contractions of the rat tail artery were investigated in vitro. Isometric contractions of arterial segments were recorded and responses of arteries from spinalized animals ('spinalized arteries') and age-matched and sham-operated controls were compared. Two and eight weeks after transection, nerve stimulation at 0.1-10 Hz produced contractions of greater force and duration in spinalized arteries. At both stages, the alpha-adrenoceptor antagonists prazosin (10 nm) and idazoxan (0.1 microm) produced less blockade of nerve-evoked contraction in spinalized arteries. Two weeks after transection, spinalized arteries were supersensitive to the alpha(1)-adrenoceptor agonist phenylephrine, and the alpha(2)-adrenoceptor agonist, clonidine, but 8 weeks after transection, spinalized arteries were supersensitive only to clonidine. Contractions of spinalized arteries elicited by 60 mm K(+) were larger and decayed more slowly at both stages. These findings demonstrate that spinal transection markedly increases nerve-evoked contractions and this can, in part, be accounted for by increased reactivity of the vascular smooth muscle to vasoconstrictor agents. This hyper-reactivity may contribute to the genesis of autonomic dysreflexia in patients.

The expression of Fos-labeled spinal neurons in response to colorectal distension is enhanced after chronic spinal cord transection in the rat

The present study used Fos-like immunoreactivity to examine neuronal activation in response to colorectal distension in rats at 1 day or 30 days following spinal cord transection or sham transection. Fifty-five Wistar rats were anesthetized and an incision was made to expose the T(5) spinal segment. The dura was reflected away in all rats and a complete transection at the rostral end of the T(5) segment was given to the lesioned group. At 1 day (acute) or 30 days (chronic) post-surgery, conscious rats were subjected to a 2 h period of intermittent colorectal distension. Rats were perfused and spinal segments L(5)-S(2) were removed and processed for Fos-like immunoreactivity. Spinal cord transection alone had no effect on Fos-labeling in either acute or chronic rats. In acute rats, colorectal distension produced significant increases in Fos-labeling in the superficial and deep dorsal horn regions. In chronic rats, colorectal distension produced a three-fold increase in Fos-labeled neurons that was manifest throughout all laminar regions. These results indicate that the number of neurons expressing Fos in response to colorectal distension is much greater after a chronic spinal cord transection than after an acute transection. Since Fos is an indicator of neuronal activation, the results show that many more neurons become active in response to colorectal distension following a chronic spinal injury. This suggests that a functional reorganization of spinal circuits occurs following chronic spinal cord transection. This may ultimately result in altered visceral and somatic functions associated with spinal cord injury in humans.

Changes in synaptic inputs to sympathetic preganglionic neurons after spinal cord injury

Spinal cord injury (SCI) leads to plastic changes in organization that impact significantly on central nervous control of arterial pressure. SCI causes hypotension and autonomic dysreflexia, an episodic hypertension induced by spinal reflexes. Sympathetic preganglionic neurons (SPNs) respond to SCI by retracting and then regrowing their dendrites within 2 weeks of injury. We examined changes in synaptic input to SPNs during this time by comparing the density and amino acid content of synaptic input to choline acetyltransferase (ChAT)-immunoreactive SPNs in the eighth thoracic spinal cord segment (T8) in unoperated rats and in rats at 3 days or at 14 days after spinal cord transection at T4. Postembedding immunogold labeling demonstrated immunoreactivity for glutamate or gamma-aminobutyric acid (GABA) within presynaptic profiles. We counted the number of presynaptic inputs to measured lengths of SPN somatic and dendritic membrane and identified the amino acid in each input. We also assessed gross changes in the morphology of SPNs using retrograde labeling with cholera toxin B and light microscopy to determine the structural changes that were present at the time of evaluation of synaptic density and amino acid content. At 3 days after SCI, we found that retrogradely labeled SPNs had shrunken somata and greatly shortened dendrites. Synaptic density (inputs per 10-microm membrane) decreased on ChAT-immunoreactive somata by 34% but increased on dendrites by 66%. Almost half of the inputs to SPNs lacked amino acids. By 14 days, the density of synaptic inputs to dendrites and somata decreased by 50% and 70%, respectively, concurrent with dendrite regrowth. The proportion of glutamatergic inputs to SPNs in spinal cord-transected rats ( approximately 40%) was less than that in unoperated rats, whereas the GABAergic proportion (60-68%) increased. In summary, SPNs participate in vasomotor control after SCI despite profound denervation. An altered balance of excitatory and inhibitory inputs may explain injury-induced hypotension.

Functional recovery of paraplegic rats and motor axon regeneration in their spinal cords by olfactory ensheathing glia

Axonal regeneration in the lesioned mammalian central nervous system is abortive, and this causes permanent disabilities in individuals with spinal cord injuries. In adult rats, olfactory ensheathing glia (OEG) transplants successfully led to functional and structural recovery after complete spinal cord transection. From 3 to 7 months post surgery, all OEG-transplanted animals recovered locomotor functions and sensorimotor reflexes. They presented voluntary hindlimb movements, they supported their body weight, and their hindlimbs responded to light skin contact and proprioceptive stimuli. In addition, relevant motor axons (corticospinal, raphespinal, and coeruleospinal) regenerated for long distances within caudal cord stumps. Therefore, OEG transplantation provides a useful repair strategy in adult mammals with traumatic spinal cord injuries. Our results with these cells could lead to new therapies for the treatment of spinal cord lesions in humans.