Brain microdialysis sampling

Parsons LH, Kerr TM, Weiss F. 1998. Simple microbore high-performance liquid chromatographic method for the determination of dopamine and cocaine from a single in vivo brain microdialysis sample. J Chromatogr B 709 35-45. 

Most electrode materials that are employed in LCEC can also be used for CEEC. The most commonly employed working electrode is a carbon fiber. Carbon fibers come in many different sizes and can also be etched to smaller diameters. Common applications of CEEC with carbon fiber electrodes are the detection of catecholamines in single neuronal cells and amino acids in brain microdialysis samples following derivatization with NDA/CN. 

In a separate study, amino acids were detected in a brain microdialysis sample by CEEC without derivatization. A copper electrode was employed as the working electrode and the CE separation was accomplished using a zwitterionic buffer at a high pH. Under these conditions, Zhou and Lunte were able to detect several amino acids in microdialysis samples [30]. 

Malone, M. Zou, H. Lunte, S.M. Smyth, M.R. Determination of Tryptophan and Kynurenine in Brain Microdialysis Samples by Capillary Electrophoresis with Electrochemical Detection. J. Chromatogr. A 1995 700, 73—80. 

To date, few detection methods are available for analysis of purine molecules in brain microdialysis samples. Most methods infuse purine molecules (retro-dialysis) directly to a brain region of interest then examine alterations in baseline levels of other neurotransmitters, such as DA [150]. When ATP or adenosine is detected in the brain using microdialysis, the dialysate samples are separated and detected by LC connected to either UV absorbance or fluorescence [72—76]. Striatal baseline levels of adenosine have been reported between 40 and 210 nM in the rat, while striatal ATP levels in the mouse are 60—170 nM using chemiluminescence [165]. What is most striking about extracellular adenosine and ATP measurements are the large basal ranges reported, which may be due to differences in LC methods, detector sensitivity, or the derivatiza-tion step required for fluorescence detection. 

Uutela P, Karhu L, Piepponen P, Kaenmaki M, Ketola RA, Kostiainen R. Discovery of dopamine glucuronide in rat and mouse brain microdialysis samples using liquid chromatography tandem mass spectrometry. Anal Chem 2009 81 427—34. 

Zhang MY, Hughes ZA, Kerns EH, Lin Q, Beyer CE. Development of a liquid chromatography/ tandem mass spectrometry method for the quantitation of acetylcholine and related neurotransmitters in brain microdialysis samples. J Pharm Biomed Anal 2007 44 586-93. 

Kaul S, Faiman MD, Lunte CE. Determination of GABA, glutamate and carbama-thione in brain microdialysis samples by capillary electrophoresis with fluorescence detection. Electrophoresis 2011 32 284—91. 

Lanckmans K, Clinckers R, Van Eeckhaut A, Sane S, Smolders I, Michotte Y (2006) Use of microbore LC-MS/MS for the quantification of oxcarbazepine and its active metabolite in rat brain microdialysis samples. J Chromatogr B 831 205-212... 

Figure 27.17 Separation and determination of amino acids by reversed phase gradient LCEC of isoindole derivatives. The sample was obtained from an awake monkey using a microdialysis sampling probe to collect amino acids from the extracellular fluid of the brain.

Microdialysis sampling has been applied to numerous tissues, especially the brain since the brain is sensitive to alterations in volume and ionic composition. Ultrafiltration has been primarily used for peripheral tissue sampling from subcutaneous tissue since the removal of fluid from the brain is believed to cause alterations in brain chemistry.2 For basic research use, microdialysis sampling devices are typically called microdialysis probes. For clinical studies, the device is called a microdialysis sampling catheter since in clinical medicine a catheter is defined as a small tube that can be implanted. 

Microdialysis sampling was first developed to allow direct collection of chemicals involved with neurotransmission within the mammalian brain. Developing this... 

A consensus value from this carefully designed study from McNay and Sherwin is that 1.25 mM glucose be included in microdialysis sampling perfusion fluids used for brain dialysis. This consensus matches previously reported values obtained using NMR techniques.95... 

A review of all the different basal concentrations found for glucose in rat brain is well beyond the scope of this chapter. However, these two brief examples highlight one of the constant concerns with microdialysis sampling in sensitive tissues such as the brain. The main concern is how the measurement event itself is actually affecting the analyte that is being measured. 

De Lange ECM, De Boer AG, Breimer DD (2000) Methodological issues in microdialysis sampling for pharmacokinetic studies. Adv Drug Deliv Rev 45 125-148 Elmquist WF, Sawchuk RJ (1997) Application of microdialysis in pharmacokinetic studies. Pharm Res 14 267-288 Evrard PA, Deridder G, Verbeeck RK (1996) Intravenous microdialysis in the mouse and the rat development and pharmacokinetic application of a new probe. Pharm Res 13 12-17 Jacobson I, Sandberg M, Hamberger A (1985) Mass transfer in brain dialysis devices a new method for the estimation of extracellular amino acids concentration. J Neurosci Methods 15 263-268... 

Boonen K, Baggerman G, Hertog WD, Husson SJ, Overbergh L, Mathieu C, Schoofs L. Neuropeptides of the islets of Langerhans A peptidomics study. Gen. Comp. Endocrinol. 2007 152 231-241. Haskins WE, Watson CJ, Cellar NA, Powell DH, Kennedy RT. Discovery and Neurochemical screening of peptides in brain extracellular fluid by chemical analysis of in vivo microdialysis samples. Anal. Chem. 2004 76 5523-5533. 

In all these cases, the microdialysis sampling technique has great potential. This technique arose historically in the field of neuroscience, having been introduced in 1972 by Delgado et al. [1] for use in brain research as an evolution of the push-pull technique devised by Gaddum back in 1961 [2] and of the longterm dialysis sac implantation explored by Bito since 1966 [3]. 

In brain research, microdialysis sampling employing a miniaturized dialysis unit (probe) containing a dialysis membrane of a few millimeters length has become popular. The probe is implanted into the tissue or organ of the test animal and is infused with an isotonic solution (typically at 0.5-25 L/min). A steady-state osmotic flux across the membrane removes molecules with a mass below the cutoff of the membrane from the extracellular matrix. Microdialysis yields relatively clean samples of volumes in the range 20-100 jU-L. However, the recovery of neuropeptides can be as low as 0.5-15%, leading to a low neuropeptide concentration in the samples [5,6]. 

If the ionic strength and pH of the perfusate do not match those of the extracellular environment the physiological conditions will be perturbed. Results from experiments performed under these conditions could then be a product of factors that are not accounted for experimentally. While this can be less important in a blood vessel than in the brain, where all processes are affected by electrolyte levels, the perfusate composition is very important in the reliability of the results. Analysis of microdialysis samples with mass spectrometric detection offers a special challenge because the high salt concentration of the dialysate could affect the sensitivity of the spectrometer. 

Kennedy s group used microdialysis sampling to monitor extracellular dopamine concentration in the brains of rats. The dialysate, mixed online with 6 mM naphthalene-2,3-dicarboxaldehye and 10 mM potassium cyanide in a reaction capillary, was periodically analyzed by MEKC at 90-s intervals [60]. The MEKC system consisted of a 10 p,m ID, 369 p,m OD, and 16 cm (14.5 cm to the detector) capillary, 30 mM phosphate buffer (pH 7.4) containing 6.5 mM SDS and 2 mM 2-hydoxypropyl-)3-CD, with LIE detection using the 413-nm line of a 14-mW diode-pumped laser, and an electric field of 850 V/cm. The detection limit for dopamine was 2 nM when sampling by microdialysis. The separation capability of the developed method is illustrated in Figure 3.10 where... 

One of the advantages of CE is that it can be employed for the analysis of very small samples. Generally, only a few nanoliters of sample are required for analysis. Hence, CE has been employed for the analysis of amino acids in single cells, small tissue extracts, and microdialysis samples. Figure 5 shows a separation of NDA amino acids from a rat brain di-alysate using CE with electrochemical detection. 

---