Redox changes in perfusates following intracerebral penetration of microdialysis probes

A review of flux considerations for in vivo neurochemical measurements

The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

Intracarotid delivery of drugs: the potential and the pitfalls

The major efforts to selectively deliver drugs to the brain in the past decade have relied on smart molecular techniques to penetrate the blood-brain barrier, whereas intraarterial drug delivery has drawn relatively little attention. Meanwhile, rapid progress has been made in the field of endovascular surgery. Modern endovascular procedures can permit highly targeted drug delivery by the intracarotid route. Intracarotid drug delivery can be the primary route of drug delivery or it could be used to facilitate the delivery of smart neuropharmaceuticals. There have been few attempts to systematically understand the kinetics of intracarotid drugs. Anecdotal data suggest that intracarotid drug delivery is effective in the treatment of cerebral vasospasm, thromboembolic strokes, and neoplasms. Neuroanesthesiologists are frequently involved in the care of such high-risk patients. Therefore, it is necessary to understand the applications of intracarotid drug delivery and the unusual kinetics of intracarotid drugs.

Coupling microdialysis with flow-injection chemiluminescence detection for a protein–drug interaction study

The interaction of metronidazole (MTZ) and human serum albumin (HSA) was studied using the coupling system of on-line microdialysis sampling with flow-injection chemiluminescence detection (FI-MD-CL). The interested drug and HSA were mixed in different molar ratios in 0.067 mol L(-1) phosphate buffer (pH 7.4) and incubated at 37 degrees C in a water-bath. Then the microdialysis probe was put into the MTZ-HSA mixed solution and sampled at a perfusion rate of 5 microL min(-1). The microdialysates was determined using flow-injection chemiluminescence. In vitro recovery (R) of MTZ under experimental conditions was approximately 25.2% with a R.S.D. of about 3.2%. The values estimated for the binding constant (K) and the number of the binding sites (n) were found to be 1.50 x 10(3)L mol(-1) and 1.89, respectively. The values of nK obtained using Scatchard analysis and Klotz plot were found to be quite similar. The method provided a reliable and simple technique for the study of drug-protein interaction.

Stem cells for neurodegenerative disorders: where can we go from here?

The use of stem cells in cell replacement therapy for neurodegenerative diseases has received a great deal of scientific and public interest in recent years. This is due to the remarkable pace at which paradigm-changing discoveries have been made regarding the neurogenic potential of embryonic, fetal, and adult cells. Over the last decade, clinical fetal tissue transplants have demonstrated that dopaminergic neurons can survive long term and provide functional clinical benefits for patients with Parkinson's disease. Pluripotent embryonic stem cells and multipotent neural stem cells may provide renewable sources that could replace these primary fetal grafts. Considerable advancement has been made in generating cultures with high numbers of neurons in general and of dopaminergic neurons using a varied array of techniques. However, much of this encouraging progress still remains to be tested on long-term expanded human cultures. Further problems include the low survival rate of these cells following transplantation and the tumorigenic tendencies of embryo-derived cells. However, pre-differentiation or genetic modification of stem cell cultures prior to transplantation may help lead to the generation of high numbers of cells of the desired phenotype following grafting. Boosting particular factors or substrates in the culture media may also protect grafted neurons from oxidative and metabolic stress, and provide epigenetic trophic support. Possible endogenous sources of cells for brain repair include the transdifferentiation of various types of adult cells into neurons. Despite the excitement generated by examples of this phenomenon, further work is needed in order to identify the precise instructive cues that generate neural cells from many other tissue types, and whether or not the new cells are functionally normal. Furthermore, issues such as cell homogeneity and fusion need to be addressed further before the true potential of transdifferentiation can be known. Endogenous stem cells also reside in the neurogenic zones of the adult brain (ventricle lining and hippocampus). Further elucidation of the mechanisms that stimulate cell division and migration are required in order to learn how to amplify the small amount of new cells generated by the adult brain and to direct these cells to areas of injury or degeneration. Finally, a more fundamental understanding of brain injury and disease is required in order to circumvent local brain environmental restrictions on endogenous cell differentiation and survival.

Biochemical approaches to studying neurotoxicity

This overview provides an introduction to biochemical analysis of toxicant effects on the nervous system. It includes a brief discussion of the salient features of the nervous system and a review of the various approaches used to detect and identify neurotoxicants and their modes of action.

Kainic acid causes redox changes in cerebral cortex extracellular fluid: NMDA receptor activity increases ascorbic acid whereas seizure activity increases uric acid

Kainic acid (KA) causes seizures and extensive brain damage in rats. To study the effects of KA on the redox state in cerebral cortex extracellular fluid (ECF), ascorbic and uric acid concentrations were measured in intracerebral microdialysis samples before and after systemic KA administration (ip). During seizures, concentrations of ascorbic and uric acid increased 500 and 100%, respectively. When midazolam was given with KA to prevent seizures, ascorbic acid still increased 400%, but uric acid increased only transiently. When the NMDA receptor antagonist aminophosphonovaleric acid (APV) was included in the microdialysis perfusion media, ascorbic acid levels decreased during baseline perfusion in a concentration-dependent manner. APV then suppressed the KA-induced increase in ascorbic acid levels, without blocking seizure activity. In summary, increased uric acid levels in brain ECF activity after KA administration are related to the induced seizure, but ascorbic acid levels are associated with NMDA receptor activity.

Cerebral penetration injury leads to H2O2 generation in microdialysis samples

Delayed tissue damage is proposed to be caused by reactive oxygen species. We investigated the effects of microdialysis probe penetration into rat piriform cortex on hydrogen peroxide (H2O2) in brain extracellular fluid (ECF). H2O2 decreased immediately after probe insertion into the brain, but increased over 300% in samples within minutes after collection. We assessed H2O2 changes in vitro in microdialysis perfusion media containing various ascorbic acid concentrations and confirmed ascorbic acid is a source of H2O2. We conclude that decreased H2O2 concentrations in perfusion media as it passes through the brain reflect an extracellular antioxidant effect, whereas the increase in H2O2 with time after sample collection indicates that H2O2 generating substances are present in ECF. Thus, the potential for producing reactive oxygen species in brain ECF exists following penetration injury, especially if transition metals are released into the neuronal microenvironment.