A precise and sensitive method for measurement of spinal cord blood flow

Inter-relationships between spinal cord blood flow, neuronal death and neurological function in rabbit spinal cord ischemia

Following rabbit spinal cord ischemia we measured histology, neurological outcome and spinal cord blood flow to investigate relationships between these quantities over a range of ischemia durations. Rabbits underwent reversible spinal cord ischemia induced by temporary aortic artery occlusion for zero to 65 min. In one cohort, surviving spinal cord cells were counted after 4 days of reperfusion using computer image analysis (n=23). Neurological outcome (paraplegic or non-paraplegic) was assessed in another group (n=381) after 18 h of reperfusion. A third group (n=8) had spinal cord blood flow measured by radioactive tracer immediately after ischemia. Histological analysis showed fewer motor neurons and astrocytes surviving as ischemia duration increased. Following 13 min (95% confidence interval (CI), 6 to 27) of ischemia 50% of low lumbar cord motor neurons remained alive; 50% of rabbits were paraplegic following 28.4 (26.7 to 30.0) min of ischemia; 28% (17% to 48%) of low lumbar motor neurons survived at this duration; 5% of rabbits were permanently paraplegic following 13.6 (10.6 to 16.1) min of ischemia, although only 52% (30% to 87%) of lower spinal cord motor neurons survived. Low lumbar cord blood flow was reduced to 2% of flow in non-ischemic thoracic cord after 25 and 60 min of ischemia (0.52 and 0.59 ml/100 g/min, respectively). These data support a fundamental hypothesis on which experimental ischemia is based: that substantial neuronal death follows directly from reversible ischemia and leads to permanent neurological deficit.

Evidence that nitric oxide- and opioid-containing interneurons innervate vessels in the dorsal horn of the spinal cord of rats

In the dorsal horn of the spinal cord, activation of small fibre nociceptive afferents leads to the release of nitric oxide and enkephalins by interneurons. In this work we encountered unexpected relationships among local spinal cord dorsal horn blood flow, specific forms of afferent input, nitric oxide and intrinsic opioids. Selective rises in rat lumbar dorsal cord blood flow using laser Doppler flowmetry and microelectrode hydrogen clearance polarography were generated by ipsilateral, 'nociceptive' low (3 Hz) frequency stimulation of sciatic afferents. Inhibitors of nitric oxide synthase (NOS) prevented rises in flow during stimulation without influencing baseline flow. Ipsilateral hindpaw intradermal injection of capsaicin, a nociceptive activator, also generated large rises in flow sensitive to NOS inhibition. During NOS blockade or morphine administration there were unexpected acute declines in the dorsal cord blood flow strictly confined to low frequency stimulation epochs. This acute vasoconstrictive effect was prevented by administration of an opioid receptor antagonist. Using immunohistochemistry, terminals apparently innervating dorsal spinal cord blood vessels were labelled with antibodies against neuronal NOS and met-enkephalin. We conclude that local nitric oxide and opioids, probably from interneurons, have competitive actions on dorsal horn microvessels once interneurons are activated during a nociceptive barrage. Collateral innervation of blood vessels may explain this property.

Induction of neuronal and inducible nitric oxide synthase in the motoneurons of spinal cord following transient abdominal aorta occlusion in rats

BACKGROUND

Motoneurons in the spinal cord are especially vulnerable to ischemic injury and selectively destroyed after transient ischemia. Nitric oxide (NO) has been implicated in both neurodegneration and neuroprotection to ischemic insult. To evaluate the role of NO in pathophysiology to spinal cord ischemia, the expression of neuronal and inducible nitric oxide synthase (n-NOS and i-NOS) and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) in the motoneurons of the lumbosacral spinal cord was examined in a rat model with transient abdominal aorta (TAA) occlusion.

MATERIALS AND METHODS

Male Sprague-Dawley rats were divided into sham-operated (n = 12) and TAA occlusion (n = 24) groups. TAA occlusion was induced by placement of a microvascular clamp around the abdominal aorta for 20 min. Three sham-operated and six TAA occlusion animals were sacrificed at each time interval at 4, 24, and 48 h and 7 days after operation. Tissue sections obtained from the lumbosacral spinal cord were processed for n-NOS, i-NOS, NADPH-d, and hematoxylin-eosin (HE) staining. Histological changes of motoneurons in ventral horn were assessed by HE staining.

RESULTS

In sham-operated control animals, n-NOS-, i-NOS-, and NADPH-d-positive neurons were barely detectable in the ventral horn of the spinal cord. At 4 h after TTA occlusion, n-NOS and NADPH-d expression became evident in the motoneurons and was markedly enhanced at 24 and 48 h. i-NOS expression was also induced in the ventral horn motoneurons of the lumbosacral spinal cord at the same time points. Enzymatic expression in the motoneurons was diminished 7 days after operation. Hyperchromatic neurons indicative of cell death were observed in HE-stained specimens 7 days following TAA occlusion.

CONCLUSIONS

The rapid induction of n-NOS, i-NOS, and NADPH-d in the motoneurons of ventral horn suggests that NO may be involved in the selective and delayed neuronal death in the spinal cord to the ischemic insult.

The effects of caffeic acid phenethyl ester (CAPE) on spinal cord ischemia/reperfusion injury in rabbits

OBJECTIVE

Oxygen-derived free radicals have been implicated in the pathogenesis of spinal cord neuronal injury after both trauma and ischemia-reperfusion. Caffeic acid phenethyl ester (CAPE), an active component of propolis extract, exhibits antioxidant properties. This experimental study was designed to determine the effect of CAPE on ischemia-reperfusion of spinal cord in rabbits.

METHODS

Forty-one New Zealand white rabbits were used in the study. The animals undergone aortic occlusion were divided into three groups each consisting of 11 rabbits: methylprednisolone (MP), CAPE, and control. CAPE 10 micromol/kg, methyl prednisolone (MP) 30 mg/kg or similar dose saline were injected intraperitoneally before surgical intervention. Animals were subjected to 21 min of cross-clamp time. At the end of occlusion time, the clamps were removed and restoration of the blood flow was verified visually. Animals in sham group (n = 8) underwent a surgical procedure similar to the other groups but the aorta was not occluded. Neurological status was scored by assessment of hindlimb motor function deficit.

RESULTS

The scores in CAPE group was different from control groups at 48 h (3.91+/-0.5 vs. 2.91+/-0.7; P = 0.0013). Spinal cord specimens were obtained to determine the tissue levels of malondialdehyde, superoxide dismutase, catalase, and histological changes. Malondialdehyde levels in control group were increased significantly when compared to sham group (124.22+/-24.36 and 41.92+/-10.08 nmol/g wet tissue, P = 0.0003). MDA levels in the CAPE group were lower than MP group and differences between the two groups were statistically significant (56.77+/-15.265 and 107.74+/-19.31 nmol/g wet tissue, P = 0.0001). We did not observe additional tissue injury in CAPE group when compared to control group. SOD and CAT activities were not concordant in all the groups.

CONCLUSIONS

These results suggest that CAPE may be an available agent to protect the spinal cord from ischemia-reperfusion injury.

Spinal cord gray matter layers rich in NADPH diaphorase-positive neurons are refractory to ischemia-reperfusion-induced injury: a histochemical and silver impregnation study in rabbit

Silver impregnation analysis of neuronal damage and concurrent histochemical characterization of NADPH diaphorase-positive neuronal pools in the rabbit lumbosacral segments was performed during and after transient spinal cord ischemia. Strongly enhanced staining of NADPH diaphorase-positive neurons and their processes appeared in the superficial dorsal horn (laminae I-III), the pericentral region (lamina X) of lower lumbar segments, the lateral collateral pathway, and mainly in neurons of the sacral parasympathetic nucleus in the S2 segment at the end of 40 min of abdominal aorta ligation or 1 day after reperfusion. Despite the development of extensive neuronal degeneration in the central gray matter (laminae IV-VII) between 1 and 4 days after ischemia, a number of nonnecrotizing neurons localized in the areas corresponding with the distribution of NADPH diaphorase-positive neurons was detected, suggesting a selective resistance of these classes of neurons against transient ischemic insult. While the precise mechanism of the observed resistance is not known, it is postulated that region-specific synthesis of nitric oxide and its vasodilatatory effect during the period of incomplete spinal ischemia may account for the observed selective resistance of these spinal cord neurons to transient ischemia.

Epidural perfusion cooling protection against protracted spinal cord ischemia in rabbits

The protective effect of a modified epidural cooling technique was assessed in a rabbit spinal cord ischemia model. The epidural space around the lumbar segments with induced ischemia was continually perfused with cold (5 degrees C) isotonic saline via two communicating spinal canal openings. This procedure allowed the spinal cord to be kept deeply hypothermic (< 15 degrees C within central gray matter) during the ischemic period. The animals were subjected to either normothermic ischemia (Group A) or hypothermic ischemia (Group B). Each group contained three subgroups of animals undergoing 20, 40, or 60 minutes of aortic ligation. Their neurological outcomes were evaluated up to 48 hours postischemia, and the intergroup differences were compared. Two days postischemia, all of the animals were sacrificed by transcardial perfusion-fixation and their lumbar segments were processed for histopathological examination. In addition, in animals with 60-minute ischemia, spinal somatosensory evoked potentials were recorded during surgical intervention and again after 48 hours. In the normothermic animals, a high incidence of paraplegia was detected: in 40% after 20 minutes of ischemia, in 75% after 40 minutes, and in 100% after 60 minutes. In contrast, all of the hypothermic animals exhibited full neurological recovery even after 60 minutes of ischemia. Both electrophysiological and histological observations clearly correlated with the neurological findings. The results suggest that deep spinal cord hypothermia produced by epidural perfusion cooling provides effective protection against protracted spinal cord ischemia in rabbits.

Blood flow and vascular permeability during motor dysfunction in a rabbit model of spinal cord ischemia

BACKGROUND AND PURPOSE

Delayed deterioration of neurological function after central nervous system ischemia is a well-documented clinical problem. The purpose of our study was to elucidate the role of spinal cord blood flow and spinal cord-blood barrier integrity in the evolution of delayed neurological deterioration after transient spinal cord ischemia in rabbits.

METHODS

Anesthetized rabbits were subjected to lumbar spinal cord ischemia (25 minutes) and variable periods of reperfusion (30 minutes to 48 hours after ischemia). Regional spinal cord blood flow was monitored by carbon-14-labeled iodoantipyrine autoradiography; vascular permeability was assessed by quantitative microhistofluorescence of Evans blue-albumin in frozen sections of spinal cord. Hindlimb motor function was assessed by standard scoring system and tissue edema by wet/dry weight method.

RESULTS

Hindlimb motor function indicated complete paralysis during ischemia and partial gradual recovery upon reperfusion (up to 8 hours), followed by progressive deterioration to severe deficits over 48 hours. Severe vascular permeability disruption was noticed early (30 minutes) after reperfusion, but almost complete recovery reestablished at 8 hours was followed by a secondary progressive increase in vascular permeability. Blood flow was reduced by 20-30% (p less than 0.01) 4 hours after ischemia in the gray matter, but hyperemia (200-300%, p less than 0.01) was observed 12-24 hours after ischemia. Spinal cord water content increased by 5.7% (p less than 0.05) 24 hours after ischemia.

CONCLUSIONS

This study demonstrates that delayed neurological and motor deterioration after spinal cord ischemia is associated with severe progressive breakdown of spinal cord-blood barrier integrity that develops late (hours) after the injury. Our data suggest that no ischemic insult in early or late reperfusion is associated with delayed motor deterioration.

The blood-brain barrier permeability in graded postischemic spinal cord reoxygenation in rabbits

Postischemic blood-brain barrier permeability changes were studied using a rabbit spinal cord ischemia model followed by normoxic recirculation (group I) or graded postischemic reoxygenation (group II). No signs of Evans blue leakage were found in lumbar segments 3 h after normoxic blood recirculation. After 6 h, the fluorescence was apparent in the perivascular space and in the pericytes, followed by a massive penetration of the tracer into the neuropil and perikarya at 12 h survival; 18 h after normoxic reperfusion, the fluorescence was localized in the cytoplasm of the middle-sized and large neurons. Graded postischemic reoxygenation of lumbar segments applied during the same survival periods had a highly protective effect on vascular membrane permeability. The structural components of the vascular wall as well as neuropil and perikarya remained after its application entirely tracer free.

Blood flow and electrolytes in spinal cord ischemia

The contribution of reoxygenation-reperfusion injury to ischemic brain damage has been clearly demonstrated but not in the spinal cord. To evaluate this phenomenon in spinal cord ischemia, we measured spinal cord blood flow (SCBF) by [14C]iodoantipyrine and electrolytes in rabbits after 10 or 40 min ischemia followed by 30 min or 4 days recirculation. Ischemia for 10 or 40 min reduced blood flow in the lower lumbar segments L5-L7 (30 ml/100 g/min) to 5 and 10% of control. After 30 min of recirculation moderate hyperemia (25-40% above control) was observed in segments L5-L7 which was not related to the degree of functional impairment. Na+, water, and Ca2+ increased and K+ decreased after 40 min ischemia, but were unchanged after 10 min ischemia. Recirculation for 30 min after 40 min of ischemia resulted in a progressive rise in Ca2+ which correlated with irreversible spinal cord injury.

Pharmacologic studies of the neuroprotective actions of a glutamate antagonist in ischemia

Ischemia may increase glutamate release, which can lead to neuronal damage. The therapeutic value of drugs that antagonize glutamate's effects are being investigated in CNS ischemia. This study examined the efficacy of a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten- 5,10-imine hydrogen maleate], in reducing ischemic injury. We explored the limits of this therapy and different properties of MK-801 that might be involved in its neuroprotective actions. Two focal CNS ischemia models were used, a multiple cerebral embolic model (MCEM) and a rabbit spinal cord ischemia model (RSCIM). When animals were treated 5 minutes after the onset of injury, MK-801 was effective in reducing ischemic damage in both models. However, when treatment was delayed 10 minutes after the ischemic insult in the MCEM, no neuroprotection was observed even when the MK-801 dose was increased eightfold. We also did not find a beneficial effect of MK-801 pretreatment with a dose that was one tenth of the effective dose in the RSCIM. Studies using the (-) MK-801 isomer showed that MK-801 neuroprotection exhibited stereoselectivity. The contribution of anticonvulsant activity and sedation to MK-801's neuroprotective actions was examined indirectly using phenytoin and midazolam, respectively. Neither drug was effective in reducing ischemic injury in the MCEM. This suggests that MK-801's neuroprotective efficacy in ischemia is mediated through its NMDA receptor antagonist activity independent of its anticonvulsant or sedative properties. These results support the hypothesis that excessive NMDA receptor excitation may be involved in ischemic neuronal damage.

Early neurohistopathological changes of canine lumbosacral spinal cord segments in ischemia-reperfusion-induced paraplegia

Mapping the canine lumbosacral spinal cord neurons damaged by ischemia-reperfusion after high thoracic aorta ligation was performed using the Nauta degenerating method. Highly Nauta-positive perikarya of the long ascending projection systems in the 4th to 6th dorsal layer, interneurons in the 7th layer and motoneurons in the 8th and 9th layers in L3-S3 segments subjected to 30 min of ischemia and 30 min of reperfusion were localized and their laminar distribution was specified. Spastic paraplegia fully developed 2 days postoperatively after 30 min of aortic ligation is neurohistopathologically characterized by occurrence of enlarged Nauta-positive boutons with prevailing localization in the 4th to 8th layer of the gray matter.

Effect of partial ischemia on phospholipids and postischemic lipid peroxidation in rabbit spinal cord

Rabbit spinal cord, subjected to severe partial ischemia induced by abdominal aorta ligation tightly below the renal arteries, was analyzed for phospholipid composition and levels of lipid peroxidation products after 10, 20, and 40 min of the insult. Under conditions when spinal cord blood flow was decreased below 5% of control, concentrations of inositol and ethanolamine phospholipids were decreased by 30% and 10%, respectively. Phosphatidic acid concentration was also altered during ischemia. No accumulation of thiobarbituric acid reactive substances (TBA-RS), conjugated dienes and fluorescent lipid soluble material was found throughout the ischemic period. Pattern of TBA-RS, conjugated diene, and fluorophore formation during postischemic in vitro incubation without and with a peroxidation couple (Fe2+, ascorbic acid) showed increased susceptibility to postischemic lipid peroxidation in tissues after 20 and 40 min of ischemia.

Improvement of energy state and basic modifications of neuropathological damage in rabbits as a result of graded postischemic spinal cord reoxygenation

The role of graded postischemic reoxygenation applied at the end of 20 min of spinal cord ischemia was studied with respect to the intraspinal pO2 tension, energy state, and histopathological sequelae. Graded postischemic reoxygenation can induce a positive shift in the intraspinal pO2 tension, but normal postischemic reoxygenation with normotensive pO2 blood tension inevitably causes the postischemic intraspinal pO2 overshoot. Graded postischemic reoxygenation significantly improves the energy state expressed by higher adenosine triphosphate (ATP), phosphocreatine (PCr) and glucose levels. Using the Nauta impregnating degenerating method, clear histopathological differences were found in the L3-S3 segments after 20 min of ischemia. Apparently divergent damage was observed when normal reoxygenation or graded postischemic reoxygenation was used. Diametrically different histopathological outcomes were obtained with normal reoxygenation and graded postischemic reoxygenation 2 and 4 days postoperatively.

Effects of calcium channel blockers on neurologic outcome after focal ischemia in rabbits

To determine the efficacy of calcium channel blockers in preserving neurologic function after central nervous system ischemia, we studied three such agents in two animal models. We treated rabbits after inducing ischemia in the brain with intra-arterial microspheres and in the spinal cord using a removable aortic occluding device. We found no benefit, in terms of neurologic functional outcome, from lidoflazine, nimodipine, or nicardipine. All three agents elevated regional blood flow in the spinal cord. We conclude that calcium antagonists are not likely to prove beneficial if used alone in the treatment of focal central nervous system ischemia.

Studies of the influence of biogenic amines on central nervous system ischemia

Several serotonin inhibitors have been shown to reduce neurologic deficits in experimental CNS ischemia. Using biochemical and histological methods we tested the effects of the serotonin inhibitors cyproheptadine and brom-LSD in a highly reproducible rabbit spinal cord ischemia model. Detailed mapping of regional spinal cord blood flow was used to guide sampling for the biochemical studies. We found that it is possible to study biochemical and morphological aspects of spinal cord ischemia in great detail using a combination of quite precise techniques. However, at this level of resolution there were no substantial changes in biogenic amine concentrations in severely ischemic or marginally perfused tissue after the durations of ischemia that cause the onset of irreversible tissue damage. Treatment with doses of serotonin inhibitors that produce preservation of neurological function did not cause significant alterations of tissue concentrations of biogenic amines or tissue morphology in treated versus untreated animals.