Effects of progressive hypoxia on long tract neural conduction in the spinal cord

Microvascular system of the lumbar dorsal root ganglia in rats. Part II: neurogenic control of intraganglionic blood flow

OBJECT

The dorsal root ganglion (DRG) should not be overlooked when considering the mechanism of low-back pain and sciatica, so it is important to understand the morphological features of the vascular system supplying the DRG. However, the neurogenic control of intraganglionic blood flow has received little attention in the past. The authors used an immunohistochemical technique to investigate the presence and distribution of autonomic and sensory nerves in blood vessels of the DRG.

METHODS

Ten Wistar rats were used. To investigate the mechanism of vasomotion on the lumbar DRG, the authors used immunohistochemical methods. Sections were incubated overnight with antisera to tyrosine hydroxylase (TH), aromatic L-amino-acid decarboxylase (AADC), 5-hydroxytryptamine, substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), somatostatin (SOM), neuropeptide Y (NPY), leucine-enkephalin, and cholineacetyl transferase (Ch-E). The avidin-biotin complex method was used as the immunohistochemical procedure, and the sections were observed under a light microscope.

RESULTS

In the immunohistochemical study, TH-, AADC-, SP-, CGRP-, VIP-, SOM-, NPY-, and Ch-E-positive fibers were seen within the walls of blood vessels in the DRG. This study revealed the existence of a comprehensive perivascular adrenergic, cholinergic, and peptidergic innervation of intraganglionic blood vessels, with a possible role in neurogenic regulation (autoregulation) of intraganglionic circulation.

CONCLUSIONS

The presence of perivascular nerve plexuses around intraganglionic microvessels suggests that autonomic nerves play an important role in intraganglionic circulation.

Vasomotion of intraradicular microvessels in rat

STUDY DESIGN

This study is to investigate the changes of vasomotion of intraradicular microvessels in vivo.

OBJECTIVE

We have observed microvascular corrosion casts of the lumbar nerve root by scanning electron microscopy and used an immunohistochemical technique to investigate the presence and distribution of autonomic and sensory nerve in blood vessels of the nerve root.

SUMMARY OF BACKGROUND DATA

It is generally considered that the genesis of radiculopathy associated with the degenerative conditions of the spine may result from both mechanical compression and circulatory disturbance. However, the neurogenic control of intraradicular blood flow has received little attention in the past.

METHODS

For three-dimensional observation of intraradicular vessels, we used scanning electron microscopic examination of microvascular corrosion casts in ten Wister rats. To investigate the mechanism of vasomotion of the nerve root, we used immunohistochemical methods. The sections were incubated overnight with antisera to tyrosine hydroxylase, choline acetyl transferase, substance P, calcitonin-gene-related peptide, vasoactive intestinal peptide, somatostatin, neuropeptide Y, leucine-enkephalin, cholecystokinin octapeptide, brain-nitric oxide synthase, and endothelium-nitric oxide synthase. Abidin-biotin complex method was used as the immunohistochemical procedure and the sections were observed under the light microscope.

RESULTS

The general view of whole vascular casts of the lumbar spinal cord and nerve roots showed a high density of vessels. Bifurcation or anastomoses of capillaries approximately took place at right angles in a T-shaped pattern and capillaries showed a lot of ring-like compressions. This ring-like compression on the cast may represent a vascular sphincter in the microvessels. This study also reveals the existence of perivascular adrenergic, cholinergic, peptidergic, and nitroxydergic innervation with a possible role in neurogenic regulation of nerve root circulation.

CONCLUSION

Perivascular nerve plexuses around intraradicular microvessels suggest that the autonomic nerves play an important role in intraradicular circulation.

Spinal Cord Blood Flow Change by Intravenous Midazolam During Isoflurane Anesthesia

We investigated the effects of IV midazolam on spinal cord blood flow in 32 cats anesthetized with isoflurane. Cats underwent laminectomy, and the lumbar spinal cord was exposed. A platinum electrode was inserted stereotaxically into the spinal cord to a depth of 1 mm-2 mm lateral to midline at L2. Arterial blood pressure, heart rate, and spinal cord blood flow (using the hydrogen clearance method) were measured before and at 5, 15, 30, 60, 90, and 120 min after an IV bolus of midazolam (0, 1, 2, or 4 mg/kg in saline 5 mL; n = 8 cats per dose). Arterial blood pressure was not affected by 0 or 1 mg/kg of midazolam but was decreased for 30 min by 2 or 4 mg/kg of midazolam. Heart rate did not change. Spinal cord blood flow was increased for 90 min by midazolam 1 mg/kg and for 15 min by midazolam 2 mg/kg but was not changed by midazolam 4 mg/kg. In conclusion, 1 mg/kg of midazolam increased feline spinal cord blood flow without changing arterial blood pressure. In contrast, a larger dose of midazolam (4 mg/kg) did not change spinal cord blood flow but substantially decreased arterial blood pressure during isoflurane anesthesia.

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.

Delayed detection of motor pathway dysfunction after selective reduction of thoracic spinal cord blood flow in pigs

OBJECTIVE

Clinical monitoring of myogenic motor evoked potentials to transcranial stimulation provides rapid evaluation of motor-pathway function during surgical procedures in which spinal cord ischemia can occur. However, a severe reduction of spinal cord blood flow that remains confined to the thoracic spinal cord might render ischemic only the descending axons of the corticospinal pathway. In this situation lower-limb motor evoked potentials could respond relatively late compared with a similar spinal cord blood flow reduction of the lumbar spinal cord that renders predominantly motoneurons ischemic.

METHODS

Selective thoracic and lumbar spinal cord ischemia was induced by sequential clamping of segmental arteries during continuous assessment of laser-Doppler spinal cord blood flow at the thoracic and lumbar spinal cord. Myogenic motor evoked potentials were recorded from the upper and lower limbs. The time to loss of motor evoked potentials was compared (n = 11) during reduction of laser-Doppler spinal cord blood flow below 25% of baseline (ischemic segment), and flow was maintained at greater than 75% of baseline in the nonischemic segment, both during thoracic and lumbar spinal cord ischemia.

RESULTS

Average laser-Doppler spinal cord blood flow in the ischemic segment was similar during thoracic (26% +/- 15% [+/- SD]) and lumbar (26% +/- 16%) ischemia, whereas normal flow was maintained in the nonischemic segment. The time to motor evoked potentials loss was considerably longer after thoracic spinal cord ischemia (15 +/- 11 minutes) than after lumbar spinal cord ischemia (3 +/- 2 minutes, P <.005).

CONCLUSION

In this experimental model of selective spinal cord ischemia, a severe reduction of lumbar spinal cord blood flow results in rapid loss of myogenic motor evoked potentials, whereas a similar blood flow reduction in the thoracic spinal cord results in relatively slow loss of motor evoked potentials. The effectiveness of motor evoked potentials to rapidly assess spinal cord integrity might be limited when spinal cord ischemia is confined to the thoracic segments.

Neuronal damage following intraspinal injection of a nitric oxide synthase inhibitor in the rat

Intraspinal microinjection of the nonspecific nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) was used to determine if inhibition of NOS results in morphological changes in the rat spinal cord. Following spinal injections of 100-750 mM L-NAME (pH 7.0), 1.0-500 mM L-NAME (pH 2.5-5.4), or L-NAME + L-arginine, quantitative analysis of morphological changes revealed a positive dose-response relationship between L-NAME and neuronal loss. This effect was blocked by L-arginine and was inversely related to spinal levels of NOS enzyme activity. Results of this study have shown the importance of basal NOS activity in maintaining the structural integrity of spinal neurons. It is proposed that the effects of L-NAME on nitric oxide (NO) production leads to decreased blood flow, secondary to vasoconstriction, and a hypoxic-ischemic reaction in spinal tissue. The results suggest that a potential contributing factor to neuronal damage in pathological conditions such as spinal cord injury may be the decreased production of nitric oxide.

Effect of lidocaine after experimental cerebral ischemia induced by air embolism

To investigate possible approaches to the treatment of neural damage induced by air embolism and other forms of acute cerebral ischemia, somatosensory evoked potentials (SEP's) were measured after cerebral air embolism in the anesthetized cat. Air was introduced into the carotid artery in increments of 0.08 ml until the SEP amplitude was reduced to approximately 10% or less of baseline values. Either a saline or lidocaine infusion was begun 5 minutes after inducing cerebral ischemia. In the saline-treated group, SEP amplitude was reduced to 6.7% +/- 1.6% (mean +/- standard error of the mean) of baseline, with a return to 32.6% +/- 4.7% of baseline over a 2-hour period. In the lidocaine-treated group, SEP amplitude was reduced to 5.9% +/- 1.5%, with a return to 77.3% +/- 6.2% over a 2-hour period. The results suggest that lidocaine administration facilitates the return of neural function after acute cerebral ischemia induced by air embolism.

Acute injury to the central nervous system

Central nervous system (CNS) trauma is divided into brain and spinal cord injury. A basic understanding of the pathophysiology of CNS trauma helps the practitioner more accurately evaluate, treat, and prognose cases of CNS trauma. The progressive nature of CNS injuries and the contribution of microvascular ischemic are explored.

Effect of intravenous lidocaine on experimental spinal cord injury

A series of experiments was conducted to study the effect of systemic intravenous administration of lidocaine on neurological recovery after acute experimental spinal cord injury in cats. The spinal cord was injured by the rapid inflation of an epidural balloon at T-6. The physiological integrity of the spinal cord ceased within 2 seconds in all animals, as demonstrated by acute disappearance of the somatosensory evoked response (SER). There was essentially no return of the SER in the five untreated animals when monitored for 4 hours post-injury. All of the pathological specimens from these animals revealed severe central cord hemorrhage. Intravenous lidocaine was begun 15 minutes after the injury in five animals. Three of these animals had significant return of the SER. The pathological specimens from the lidocaine-treated animals revealed either mild or moderate central cord hemorrhage. The results of this experiment suggest that systemic lidocaine administration has a significant beneficial effect in the treatment of acute spinal cord injury.

Protective effect of lidocaine in acute cerebral ischemia induced by air embolism

To investigate possible approaches to the prevention and treatment of neural damage induced by air embolism and other forms of acute cerebral ischemia, a model was used in which cerebral air embolism was produced by infusion of air (0.4 ml) into a vertebral artery of chloralose-anesthetized cats. Neurological function was assessed by measuring cortical somatosensory evoked responses in a group of 10 untreated animals and in a group of eight animals pretreated with intravenous lidocaine (5 mg/kg). In the untreated group, the primary somatosensory amplitude was reduced to 28% +/- 9% (mean +/- standard error) of the value before air embolism, with a return to 60% +/- 8% 1 hour and 73% +/- 12% 2 hours after embolism. In the group pretreated with lidocaine, the primary somatosensory amplitude was reduced to 68% +/- 9% of the value before air embolism, with a return to 92% +/- 3% 1 hour and 97 +/- 2% 2 hours after embolism. Pretreatment with lidocaine also greatly attenuated the acute hypertension and the increase in intracranial pressure following air embolism. These results demonstrate that pretreatment with intravenous lidocaine significantly reduces the neural decrement and increases the recovery of neural function after acute cerebral ischemia induced by air embolism.

Effects of hyperbaric oxygen therapy on long-tract neuronal conduction in the acute phase of spinal cord injury

To study the acute effects of hyperbaric oxygen ventilation (HBO) on long-tract function following spinal cord trauma, the authors employed a technique for monitoring spinal cord evoked potentials (SCEP) as an objective measure of translesion neuronal conduction in cats subjected to transdural impact injuries of the spinal cord. Control animals subjected to injuries of a magnitude of 400 or 500 gm-cm occasionally demonstrated spontaneous return of translesion SCEP within 2 hours of injury when maintained by pentobarbital anesthesia and by ventilation with ambient room air at 1 atmosphere absolute pressure (1 ATA). Animals sustaining corresponding injuries but receiving immediate treatment with HBO at 2 ATA for a period of 3 hours following impact demonstrated variable responses to this treatment modality. Animals sustaining injuries of 400 gm-cm magnitude showed recovery of translesion SCEP in four of five cases, while animals sustaining injuries of 500 gm-cm magnitude responded to HBO treatment by recovery of SCEP no more frequently than did control animals. When the onset of HBO therapy was delayed by 2 hours following impact, there appeared to be no demonstrable protective effect on long-tract neuronal conduction mediated by HBO alone. The observations suggest that HBO treatments can mediate preservation of marginally injured neuronal elements of the spinal cord long tracts during the early phases of traumatic spinal cord injury. These protective effects may be based upon the reversal of focal tissue hypoxia, or by reduction of tissue edema. HBO treatment markedly diminished the protective effects of HBO on long-tract neuronal conduction following traumatic spinal cord injury.