A specific microdialysis probe combined with high performance liquid chromatography analysis was used to compare the basal levels of extracellular amino acids (AAs) in the dorsal horn (DH) between two groups of patients undergoing spinal surgery for the treatment of peripheral neuropathic pain (n = 5) or disabling spasticity (n = 5). A stabilized concentration was reached in the dialysates 45 min after probe implantation for excitatory AAs (glutamate and aspartate) and inhibitory AAs (GABA and glycine). A significant increase in the ratios aspartate/GABA and aspartate/glycine was found in the group of patients suffering from neuropathic pain. This study shows the feasibility of a microdialysis investigation in the DH of patients during a neurosurgical operation and supports in humans the hypothesis of an imbalance between excitatory AAs and inhibitory AAs within the DH in neuropathic pain states, as suggested by previous animal studies.
Microdialysis sampling combined with capillary electrophoresis is emerging as a new approach in drug studies. It allows the continuous monitoring, in vivo or in vitro, of changes in free endogenous compounds as well as in drug substances, following the administration of pharmacological agents. The low volume requirement of capillary electrophoresis for injection allows the collection of dialysates during short sampling times, leading to a precise temporal description of drug-induced biochemical changes or pharmacokinetics. Various protocols can be used for analyzing endogenous compounds and drug substances in microdialysis samples. Capillary electrophoresis with laser-induced fluorescence detection often affords the high sensitivity level which is needed in most studies. Furthermore, the direct on-line coupling of microdialysis, derivatization of samples, and electrophoretic analysis now brings a separation-based biosensor, allowing a real-time description of chemical events with a high molecular specificity. Microdialysis sampling combined with capillary electrophoresis has recently been used to assess pharmacodynamic and pharmacokinetic characteristics of various drugs in animal studies; it may also represent a new approach in clinical pharmacology in the near future.
Abstract Capillary zone electrophoresis with laser‐induced fluorescence detection has been shown to give rapid separations with high resolution and sensitivity. These advantages are documented for catecholamines analysis in the present work, which shows that separation of subnanomolar concentrations of dopamine and noradrenaline (detection limits of 8.6 × 10 −11 M, corresponding to a detected amount of 143 zeptomoles of each catecholamine) can be performed in 82 s with an efficiency of several million theoretical plates.
✓ The aim of this study was to develop, for the first time in the human spinal dorsal horn (DH), an in vivo method for the study of amino acids (AAs). A microdialysis technique was used to sample AAs in the extracellular fluid of the DH apex in eight patients in whom surgery in the dorsal root entry zone (DREZ) was performed. Before making microsurgical lesions, specific concentric-type microdialysis probes were implanted over a 60-minute period in the DREZ and directed to the DH apex (10 implantations). The AA concentrations in the dialysates were determined using high-performance liquid chromatography with fluorescence detection. The concentrations of excitatory AAs (glutamate and aspartate) and inhibitory AAs (γ-aminobutyric acid and glycine) decreased and were stabilized by 45 minutes after probe implantation, whereas the levels of nonneurotransmitter AAs (alanine and threonine) were not stabilized at 60 minutes. The ability of the probe to track the changes of extracellular AAs was demonstrated. Neither intra- nor postoperative microdialysis-related complications were observed (with a follow up of 18 months). The present study demonstrates that microdialysis can be performed safely in the human DH during DREZ lesioning. Despite technical and analytical limitations related to the intraoperative conditions, this technique offers new possibilities for clinical research on neurotransmitters involved in some relevant pathological states, especially in chronic pain and spasticity.
