Morphometric analysis of experimental spinal cord injury in the cat: the relation of injury intensity to survival of myelinated axons.

The pattern of axonal destruction and demyelination that occurs in experimental contusion injury of cat thoracic spinal cord was studied by line sampling of axons in 1 micron thick plastic sections with the light microscope. Injuries were produced by a weight-drop apparatus, with the vertebral body (T9) below the impact stabilized by supports under the transverse processes. The effects of two combinations of weight and height were examined: 10 or 13 g dropped 20 cm onto an impact area of 5 mm diameter. Animals were kept for 3-5 months after injury, then fixed by perfusion for histological analysis. The number of surviving myelinated axons was found to vary both with the weight used and with the size of the spinal cord. A measure of impact intensity was derived from the calculated momentum of the weight at impact divided by the cross sectional area of the cord (interpolated from dimensions measured rostral and caudal of the lesion following fixation). At impact intensities greater than 0.02 kg-m/s/cm2 there was practically no survival of axons at the center of the injury site, combined with almost complete breakdown of the pial margin. Between 0.08 and 0.2 kg-m/s/cm2 the number of surviving axons varied between 100,000 and 2,000, approximating a negative exponential function (r = -0.88). The number of axons surviving in the outer 100 microns of the cord varied practically linearly (r = -0.82) between near normal and less than 1% of normal over the same range of injury intensity. The number of surviving axons decreased with depth from the pia, also approximating a negative exponential function, with a 10-fold decrease in density over approximately 500 microns. The average slope of this relation with depth remained similar over the range of injury intensity examined, though the slope appeared inversely related to variation in axonal survival for different individuals at a given intensity. It is argued that the loss of axons is probably determined primarily by mechanical stretch at the time of impact. Its centrifugal pattern may be explained by longitudinal displacement of the central contents of the cord, reflecting the viscoelastic "boundary layer" properties of parenchymal flow within the meningeal tube. This is illustrated with reference to the behavior of a gelatin model under compression. The preferential loss of large caliber axons and the characteristic shift to abnormally thin myelin sheaths (resulting from post-traumatic demyelination) both varied in extent independently of injury intensity and overall axonal survival.(ABSTRACT TRUNCATED AT 400 WORDS)

Spinal cord sodium, potassium, calcium, and water concentration changes in rats after graded contusion injury.

Spinal cord Na, K, Ca, and H2O changes were measured 6 h after graded contusion injuries in 40 Sprague-Dawley rats. A 10 g weight was dropped 1.25 cm (n = 6), 2.5 cm (n = 7), 5.0 cm (n = 6), or 7.5 cm (n = 7) onto the thoracic spinal cord of 26 rats. An additional 10 rats served as laminectomy controls and 4 rats were unoperated controls. At 6 h after surgery or injury, the spinal cords were rapidly cut into 4 mm segments, weighed to obtain tissue wet weights (W), dried for 14-16 h at 97 degrees C in a vacuum oven (30 mmHg), and reweighed for tissue dry weights (D). Water concentrations ([H2O]d) were estimated from (W-D)/D in units of ml/g D. Ionic concentrations ([Na]d, [K]d, and [Ca]d) of the tissue samples were measured by atomic absorption spectroscopy with units of mumol/g D. Ionic shifts (delta [Na]d, delta [K]d, delta [Ca]d) were calculated by subtracting laminectomy control values from those measured in injured cords. Laminectomy alone significantly increased [Na]d and [H2O]d compared to unoperated controls. Mean +/- standard deviations of [H2O]d, [Na]d, [K]d, and [Ca]d were, respectively, 1.95 +/- 0.07, 182.6 +/- 5.9, 277.2 +/- 11.8, and 12.1 +/- 1.4 in unoperated controls; 2.12 +/- 0.08, 238.6 +/- 9.2, 277.8 +/- 9.2, and 11.7 +/- 1.1 in laminectomy controls. At the impact site, [K]d fell by 14-37% and [H2O]d rose by 14-24%, [Na]d by 13-64%, and [Ca]d by 65-137% of laminectomy control values. delta [Na]d, delta [K]d, and delta [Ca]d correlated linearly with impact velocities; [Ca]d increased by 1.0% per cm/sec (r = 0.995, p less than 0.005), [Na]d increased 0.67% per cm/sec (r = 0.950, p less than 0.01), and [K]d decreased 0.34% per cm/sec (r = 0.964, p less than 0.01). Neither delta [H2O] nor delta [Na]d + delta [K]d consistently predicted impact velocity. [Na]d + [K]d correlated with [H2O]d with a slope of 177.4 mumol/ml (r = 0.697, p less than 0.005). Since Na and K constitute greater than 95% of tissue inorganic ions, the slope approximates net ionic shift per ml of water entry or the ionic osmolarity of edema fluid. These results indicate that increasing contusions produce graded ionic shifts and that edema does not predict contusion severity. These data support our hypothesis that net ionic shifts cause edema in injured spinal cords.

Posttraumatic syrinx formation: experimental study.

An experimental study was performed to examine posttraumatic spinal cord cavitation in an animal model by evaluating immediate and delayed computed tomographic (CT) scans obtained after administration of intrathecal contrast material. Four cats underwent midthoracic laminectomy and spinal cord contusion using a standard 400 g-cm model. All animals were studied by CT with intrathecal contrast enhancement before and 4-5 days, 3-4 weeks, and 7-13 weeks after experimental cord contusion. Either metrizamide or iopamidol was used as the contrast agent. Two of the four cats had CT and pathologic evidence of cord cavitation at the site of injury. Another animal had uptake of contrast material into the spinal cord without pathologic evidence of cyst formation, which was believed to represent malacic change. The fourth animal had a normal-appearing cord by both CT and pathologic criteria. Animals that received metrizamide after cord contusion had generalized myoclonic seizures. This did not occur when iopamidol was administered.

Experimental spinal cord injury: treatment with naloxone.

We studied the effect of the opiate antagonist naloxone on the recovery of cats injured with a 400-g-cm impact injury to T-9. The animals were evaluated by recording somatosensory evoked potentials and performing weekly neurological examinations. Several dose schedules were followed. Six of eight cats that received an intravenous or intraperitoneal bolus of naloxone (10 mg/kg) 45 minutes after injury regained the ability to walk. Recovery occurred in only one of five animals that were treated with an infusion of naloxone, 10 mg/kg/hour, and in none of five animals given 1 mg/kg as a bolus. Because these results are not related to any observed change in blood pressure, we believe that naloxone may be achieving its effect through the preservation of spinal cord blood flow, as well as other mechanisms that have yet to be defined.

Effect of aminophylline and isoproterenol on spinal cord blood flow after impact injury.

A study of the effects of spinal cord injury upon spinal cord blood flow was carried out in cats. A 400 mg-cm impact produced an overall reduction in spinal cord blood flow of 24% in the white matter and 30% in the gray matter, as determined by 14C-antipyrine autoradiography. At the level of the injury, white-matter flow was 8.1 ml/100 gm/min, a reduction of 49%, and in the gray matter, 12.5 ml/100 gm/min, a reduction of 76%. Treatment with aminophylline and isoproterenol improved the overall blood flow in the spinal cord. At the level of the injury, white-matter flow after this treatment was no longer significantly different from control values. The gray-matter flow remained decreased to 26.2 ml/100 gm/min, a reduction of only 47%. It is proposed that aminophylline and isoproterenol may increase cyclic adenosine monophosphate (AMP) and prevent platelet aggregation along the endothelial surfaces of the microcirculation, and may thereby help to maintain improved perfusion of the injured spinal cord.

Vestibulospinal monitoring in experimental spinal trauma.

Vestibulospinal tract function was monitored in experimental contusion of the spinal cord in cats, and compared with somatosensory cortical evoked potentials. Both white and gray matter portions of the vestibular and somatosensory pathways were evaluated in cord injuries at T-7 and L-4. Severe contusions of 20 gm-20 cm force impact resulted in a rapid (less than 1 second) abolition of thoracic white matter conductivity, but a somewhat slower (4 to 5 minutes) loss of lumbar gray matter responses. A paradoxical transient recovery of white matter conductivity occurred 1 to 2 hours after injury, despite eventual progression to central hemorrhagic necrosis at the contusion site. In contrast, mild contusions (20 gm-10 cm force impact) produced only a temporary loss of neuronal activity: white matter for 1 to 2 hours, and gray matter for 30 to 40 minutes. In general, vestibular and somatosensory potentials showed similar sensitivity to contusion, although the former tended to recover earlier. We conclude that contusion injury causes two types of neuronal dysfunction in spinal cord: 1) a low-threshold concussion-related loss of activity lasting 30 to 120 minutes; and 2) a higher threshold necrotic process, requiring 1 to 2 hours to develop, which apparently spreads from gray to white matter.

The role of the sympathetic nervous system in pressor responses induced by spinal injury.

Spinal cord injury consistently evokes a transient 3- to 4-minute rise is systemic pressure, followed by prolonged hypotension. Because the role of the sympathetic nervous system in these blood pressure changes is not clear, the pressure responses were studied using systematic ablation of the peripheral sympathetic nervous system. In total, 24 cats were subjected to bilateral thoracic sympathectomy, adrenalectomy, splanchnicectomy, combinations of the preceding, sham operation, or no treatment. Either 3 or 24 hours after the ablations, the blood pressure responses were evoked by 400 gm-cm contusions of the thoracic cord. Although neither thoracic sympathectomy nor adrenalectomy alone abolished the hypertensive phase, the combination of the two procedures did. This suggests that both the thoracic sympathetic ganglia and the adrenal glands participate in the pressor response. Thoracic sympathectomy affected primarily the early part, whereas adrenalectomy diminished the later part of the hypertensive response. This correlates with the function of the former being neurally and the latter being humorally mediated. None of the sympathetic lesions consistently affected the hypotensive phase. Spinal contusion injury produces widespread sympathetic activation, mediating the hypertensive changes.

Effect of naloxone on posttraumatic ischemia in experimental spinal contusion.

The effect of naloxone on blood flow and somatosensory evoked potentials was studied in cats subjected to 400 gm-cm contusion injuries of the thoracic spinal cord. Eight cats were treated with 10 mg/kg naloxone 45 to 60 minutes after injury, 11 cats were given 10 ml of saline instead of naloxone, and six cats were neither injured nor treated. Hydrogen clearance was used to measure blood flow in the lateral white columns at the contusion site. Naloxone, given intravenously, significantly inproved the blood flow rates in the lateral column white matter. At 2 hours after injury, the mean blood flow in the saline-treated cats fell to 50% (p greater than 0.01) of preinjury flow rates, whereas it increased 6% (p greater than 0.50) in naloxone-treated cats, and 12% (p greater than 0.50) in uninjured cats. At the 3rd hour after injury, the respective flows fell 47% (p less than 0.01), and 6% (p greater than 0.50), and increased 15% (p greater than 0.50) of the preinjury flow rates. The naloxone-treated cats had striking preservation of sensory function and somatosensory evoked potentials at 24 hours after injury. At 24 hours, responses had returned in all the naloxone-treated cats and in only 11% of the saline-treated cats. The probability of this combination of events occurring by chance is 0.0030. The authors conclude that naloxone may be useful for the treatment of spinal cord injury. The mechanism of the effect is unknown.

21-Aminosteroid reduces ion shifts and edema in the rat middle cerebral artery occlusion model of regional ischemia.

U74006F is a member of a new family of steroid drugs called 21-aminosteroids, which are potent inhibitors of lipid peroxidation with little or no glucocorticoid or mineralocorticoid activity. We investigated the effects of U74006F on the early ionic edema produced by middle cerebral artery occlusion in rats. Intravenous doses of 3 mg/kg U74006F were given 10 minutes and 3 hours after occlusion. Tissue concentrations of Na+, K+, and water at and around the infarct site were measured by atomic absorption spectroscopy and by wet-dry weight measurements 24 hours after occlusion. Compared with vehicle treatment, U74006F treatment reduced brain water entry, Na+ accumulation, K+ loss, and net ion shift by 25-50% in most brain areas sampled in the frontal and parietal cortex. However, reductions of ionic edema were most prominent and reached significance (p less than 0.005, unpaired two-tailed t test) mostly in the frontoparietal and parietal cortex areas adjacent to the infarct site. Our findings suggest that a steroid drug without glucocorticoid or mineralocorticoid activity can reduce edema in cerebral ischemia but that the effects are largely limited to tissues in which collateral blood flow may be present.

The vestibulospinal free fall response: a test of descending function in spinal-injured cats.

A major problem in spinal cord injury research is quantification of motor function in animals. Most investigators in the field currently use neurologic scoring systems, relying on subjective observations of complex behaviours and assigning scores based on arbitrary criteria. These scoring scales are prone to observer bias and are nonspecific. We describe here a simple, reproducible, noninvasive, and objective test of a limited aspect of spinal motor function in cats, based on a well-known involuntary response of animals to sudden free fall. Free fall responses, or FFRs, have been studied in many species, including man, and are thought to be carried in ventral and lateral column pathways, i.e., vestibulospinal, reticulospinal, and rubrospinal tracts. We recorded the FFRs from hind and forelimb muscles of 100 cats before and after thoracic spinal cord injury. Hindlimb FFRs were shown to have three quantifiable components: a fast synchronous activation (E1) followed by a short silent period during which spinal segmental reflexes are inhibited (I1) and a late desynchronized excitatory burst (E2). Thoracic spinal injury produced hindlimb FFR losses ranging from greatly reduced amplitude to complete absence of response. Residual FFRs correlated with the extent of ventral column preservation and locomotory ability. Individual FFR components can be preserved. For example, some injured cats exhibited only 11 responses. Our work suggests that FFRs are a reliable and sensitive test of motor recovery in spinal cord injury.

Basilar artery occlusion in rats.

The basilar artery is one of the three major sources of blood supply to the circle of Willis. To investigate the effects of basilar artery occlusion, we surgically exposed and coagulated the basilar artery in 25 rats. Basilar artery occlusion at any single point between the foramen magnum and the circle of Willis in 11 rats did not produce histologically detectable infarcts in the brain at 12-24 hours. Two-point occlusions of the basilar artery in 12 rats produced variable infarcts between the occlusion sites but no ischemic lesions elsewhere. After either single- or double-point occlusions, the proximal basilar artery refilled within 2-3 minutes. When the basilar artery was occluded above and below the origins of the anterior inferior cerebellar arteries, the artery segments between the occlusion points initially collapsed but refilled within 2-3 minutes in two rats. Basilar artery occlusions invariably suppressed cortical somatosensory evoked potentials by greater than 50%. Regardless of whether a brain stem infarct developed, somatosensory evoked potential amplitudes recovered to greater than baseline levels by 4 hours in seven of 17 rats and returned to baseline levels by 24 hours in every rat tested. We conclude that the occluded basilar artery receives extensive retrograde collateral blood flow and that somatosensory evoked potentials are exquisitely sensitive to basilar artery occlusion but are insensitive to whether brain stem infarcts develop.

Ascorbic acid: a putative biochemical marker of irreversible neurologic functional loss following spinal cord injury.

The development of permanent paraplegia in spinal injured cats is accompanied by a large progressive decline in total ascorbic acid (AA) and a transient increase in oxidized (AAox) ascorbate. Since AA is involved in a variety of processes required for normal central nervous system (CNS) performance we suggested that such large ascorbate loss may contribute to derangements in spinal cord function following injury. We now demonstrate that methylprednisolone (15 mg/kg) and naloxone (10 mg/kg), two treatments that preserve neurologic function in this model, rapidly block deteriorating ascorbate status. Naloxone at 1 mg/kg, a treatment providing no therapeutic benefit, has no protective effect on ascorbate. The results strongly support the hypothesis that loss of ascorbate homeostasis reflects irreversible loss of neurologic function following spinal cord injury.

Tissue Na, K, and Ca changes in regional cerebral ischemia: their measurement and interpretation.

A simple and reliable method of quantifying tissue damage is described. This method, based on atomic absorption spectroscopic determinations of Na, K, and Ca concentrations in small brain samples, was applied to the rat middle cerebral artery occlusion model (MCAo). At the infarct site by 24 hours, Na concentration more than doubled, Ca concentration increased by greater than 70%, and K concentration fell nearly 80%; these changes are consistent with a greater than 80% disruption of cells. A remarkable acceleration of ionic shifts occurred between 4 and 6 hours after MCAo. At 4 hours, only 20-30% of the ionic shifts found at 24 hours had occurred; by 6 hours, 80-100% of the ionic shifts found at 24 hours had taken place. Since the measurements reflect ionic movement into and out of the tissue, they are likely to represent irreversible tissue damage. Although blood brain barrier breakdown may have contributed to an increased rate of ionic shifts, large ionic gradients must have been present between the extracellular space and the vascular compartment at 4-6 hours to drive the ionic shifts. Our results suggest an upper time limit of 4 hours for treatments of acute ischemic tissue damage in the rat MCAo model. The methods and analytical approach described may be useful for determining the time window for therapeutic intervention in acute CNS injuries, as well as for evaluating treatment effects.

Further studies on free-radical pathology in the major central nervous system disorders: effect of very high doses of methylprednisolone on the functional outcome, morphology, and chemistry of experimental spinal cord impact injury.

The hypothesis that pathologic free-radical reactions are initiated and catalyzed in the major central nervous system (CNS) disorders has been further supported by the current acute spinal cord injury work that has demonstrated the appearance of specific, cholesterol free-radical oxidation products. The significance of these products is suggested by the fact that: (i) they increase with time after injury; (ii) their production is curtailed with a steroidal antioxidant; (iii) high antioxidant doses of the steroidal antioxidant which curtail the development of free-radical product prevent tissue degeneration and permit functional restoration. The role of pathologic free-radical reactions is also inferred from the loss of ascorbic acid, a principal CNS antioxidant, and of extractable cholesterol. These losses are also prevented by the steroidal antioxidant. This model system is among others in the CNS which offer distinctive opportunities to study, in vivo, the onset and progression of membrane damaging free-radical reactions within well-defined parameters of time, extent of tissue injury, correlation with changes in membrane enzymes, and correlation with readily measurable in vivo functions.