Evidence that reversed glutamate uptake contributes significantly to glutamate release following experimental injury to the rat spinal cord

Released excitatory amino acids contribute significantly to secondary damage following spinal cord injury. Reversal of normal transport due to cell membrane depolarization may contribute to this release. We tested this by administering dihydrokainic acid (DHK), a non-transported glutamate uptake blocker, into the rat spinal cord by microdialysis in association with contusion spinal cord injury. Glutamate release in response to injury was reduced by 34% (P<0.05) when 3 mM DHK was administered within the microdialysis fiber, suggesting that reversed transport is an important contributor to glutamate release upon spinal cord injury.

Adenosine release upon spinal cord injury

The hypothesis that release of adenosine following spinal cord injury (SCI) may provide neuroprotective feedback is explored. Consistent with this hypothesis, substantial release of adenosine, estimated to reach 100 microM in the extracellular space, was detected by microdialysis sampling immediately following contusion SCI. There is also considerable release of excitatory amino acids following SCI. The latter was not affected by administration of the general adenosine receptor antagonist theophylline and the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine, implying that the adenosine released following SCI does not significantly influence the release of neurotoxic amino acids. Administration of the concentration of glutamate released upon SCI into the spinal cord caused only about 1% as much release of adenosine as did injury, evidence that elevated excitatory amino acids do not elicit an appreciable fraction of the release of adenosine that follows SCI. Results obtained suggest that release of endogenous adenosine is not neuroprotective by blocking release of excitatory amino acids following SCI.

Changes in amino acid concentrations over time and space around an impact injury and their diffusion through the rat spinal cord

Release of amino acids, particularly the neurotoxin glutamate, in and around the site of an experimental spinal cord injury was characterized over time by microdialysis. Increases in amino acid concentrations caused by injury decline steeply and then slowly over distance from the impact area, becoming undetectable beyond about 5 mm from the injury epicenter. Diffusion profiles determined in the cord by administering amino acids through one microdialysis fiber and sampling them in a parallel fiber declined steeply with distance. Distant increases coincided temporally with those in the injury epicenter. We conclude that elevated amino acids more than about 1 mm into the periimpact zone are predominantly released in that region rather than diffusing into it from the trauma epicenter. In the outer areas of lesion development, glutamate does not appear to reach concentrations ordinarily toxic, and elevated concentrations do not persist nearly as long as the therapeutic window of NBQX in any part of the lesion. Therefore, the mechanisms whereby excitatory amino acid antagonists reduce the dimensions of injury lesions are unclear. However, sensitization of neurons following impact injury could be important in amino acid neurotoxicity.

Considerations in the determination by microdialysis of resting extracellular amino acid concentrations and release upon spinal cord injury

The following issues are further addressed: (1) Is there considerable leakage of amino acids from the circulation into the space around microdialysis probes, or are amino acid concentrations naturally much higher in the interstitial space than is generally thought? (2) Do observed high interstitial concentrations or depletion of substances in the intracellular space by microdialysis affect release measurements upon spinal cord injury? Amino acid concentrations around microdialysis fibres in the spinal cord of rats were found to approach those in the circulation and to be much higher than interstitial concentrations previously estimated in the CNS. However, much lower concentrations of amino acids were derived in the hippocampus by analogous experiments. Considerable Evans Blue/albumin leaked from the circulation into the interstitial space in the spinal cord immediately after fibre insertion. However, this movement diminished considerably by 4 h later, demonstrating substantial resealing of the blood-brain barrier, at least to large molecules. There is either substantial damage-induced movement of amino acids from the circulation into the dialysis zone after insertion of a microdialysis probe, or there is much less impediment to movement of amino acids across the blood-brain barrier in the spinal cord than in the brain. At low flow rates through the fibre, adding concentrations of amino acids to the inside of the fibre equal to the concentrations around the fibre to prevent their depletion by removal through the microdialysis fibre did not affect increases in concentrations of amino acids in microdialysates following injury. Thus the high concentrations of amino acids present around microdialysis fibres following their insertion do not seem to disturb measurements of amino acid release upon spinal cord injury.

Microdialysis studies of the role of chemical agents in secondary damage upon spinal cord injury

Assorted microdialysis studies of the roles endogenous chemical agents may play in secondary damage upon spinal cord injury (SCI) are described. Issues addressed include the concentrations reached upon injury, mechanisms of release upon injury, and effects of drugs on injury-elicited increases in glutamate concentrations. An important question in identifying an agent of secondary damage upon central nervous system (CNS) trauma is not simply whether the substance is released upon injury, but whether it reaches harmful levels. To resolve this requires establishing the concentration attained and then determining whether administering that level damages neurons. To make microdialysis measurements of amino acids in the CNS more quantitative, we characterized the effects of insertion of a microdialysis fiber on leakage of glutamate from the circulation and explored the effects of depletion by microdialysis on release caused by SCI. Very high glutamate concentrations were found around the fiber for several hours after fiber insertion and 2 days later, and there was substantial leakage of alpha-aminoisobutyric acid from the circulation into the dialysis zone for several hours after fiber insertion. Glutamate concentrations reached upon SCI under nondepleting conditions were similar to those estimated earlier under depleting conditions. Mg2+ release was detectable when microdialysis probes were perfused with Mg2+-free fluid, but not when the concentrations in the perfusing fluid approximated those in the interstitial space. It is concluded (1) that insertion of microdialysis probes into CNS tissue can cause long-lasting leakage of amino acids from the circulation into the space around the fiber, (2) that this leakage can obscure concentration changes that otherwise occur, and (3) that depletion of substances in the fluid around the fiber may cause increases in concentration to be observed that do not normally happen. We also describe demonstrations that administration of methylprednisolone and dihydrokainic acid diminish increases in glutamate concentrations caused by SCI, showing that microdialysis can be used to explore effects of drugs on actions of damaging substances.

A sampling artifact in the microdialysis study of changes in extracellular [Mg2+] upon spinal cord injury

Whether depletion of a substance from the extracellular fluid by removal through the dialysis probe can affect the measured efflux of that substance into the extracellular fluid upon injury is examined. When Mg(2+)-free artificial cerebrospinal fluid was used to perfuse the microdialysis fiber, a significant increase in collected Mg2+ was observed immediately following injury. However, when [Mg2+] inside the fiber was equal to the basal concentration in the extracellular fluid, no increase in [Mg2+] was detected. The difference between the two sets of results is attributed to efflux down an increased intracellular/extracellular [Mg2+] gradient that occurs when the basal extracellular concentration is not balanced within the fiber, but not otherwise. This is a previously unreported problem with microdialysis.