Accessing the brain’s interstitial space for sampling and monitoring of small-molecular-weight constituents is only effective with the proper fluid-delivery tools in place. Microdialysis techniques depend on programmable syringe-based infusion devices capable of steady and exact microflow volume rates for controlled support. At the method’s core is the semipermeable membrane-containing probe, through which, via constant perfusion, molecules from the extracellular space are collected.1 Membranes differ in their pore size and material, but flow through the membrane is based on the diffusion of molecules along their concentration gradient (i.e., Fick’s law), where those greater than 20,000 Da are generally limited.1,2
Microdialysis Monitoring: A Procedural Close-Up
In vivo microdialysis is perhaps most advantageous in its ability to sample extracellular fluid from the brain of a live animal, as opposed to analyzing post-mortem tissue. Moreover, the principle of “reverse or retro dialysis” allows for low-molecular-weight drugs to be delivered to predefined regions of the brain.2 In this way, microdialysis is an end-to-end approach for researchers interested in sample analysis pre- and post-drug administration, while making direct observations of the animal’s behavior.
Once the semipermeable membrane-containing probe is surgically implanted, a perfusion pump delivers the perfusate at a precise and slow rate of 0.3 µl/min to 3.0 µl/min.1,2 A lower perfusion rate is pivotal for preventing increased pressure in the probe, analyte depletion, and thereby an overall decreased analyte concentration. In addition to flow rate, sample recovery may be affected by factors including the composition of the perfusate and/or membrane, the size of the membrane’s active area, temperature, and the nature of the analyte’s biochemical characteristics.1 Enhanced sensitivity of equipment and proper experimental planning can do away with most, if not all of these factors. Choosing a flow rate that will yield an appropriate amount of the analyte governs your success and is dependent on the perfusion pump design; it should distribute volume at a smooth, stable, and reproducible rate.
Operation Microdialysis: An Applications Overview
The applications of microdialysis in neuroscience research are widespread, ranging from addiction and neurodegeneration, to psychiatry and behavior. In the 50 years since its inception, fine-tuning efforts and successful deployment for neurochemical quantification has made microdialysis the universal approach for directly obtaining and measuring levels of neurotransmitters, neuropeptides, and hormones.3-5 Today, the method has been applied to virtually all tissues for the monitoring of endogenous or exogenous extracellular free molecules.
In the context of the brain, most research is preclinical and has used animal models ranging from mice to dogs, to sheep, pigs, and nonhuman primates. However, the organ’s inherent physical (e.g., highly restricting blood brain barrier) and functional (e.g., highly effective efflux transporters) limitations has made accessing the parenchyma classically difficult. Microdialysis has been an invaluable tool for both the collection and isolation of informative molecules and for the pharmacological profiling of brain-targeting drugs, (e.g., antipsychotics and antidepressants) and their effect on neurochemical release in various sections of the brain.
1. V.I. Chefer, et al., “Overview of brain microdialysis,” Curr Protoc Neurosci CHAPTER (2009): Unit 7.1.
2. A.S. Darvesh, “In vivo brain microdialysis: Advances in neuropsychopharmacology and drug discovery,” Expert Opin Drug Discov 6:109-127, 2011.
3. L. Bito L, et al., “The concentration of free amino acids and other electrolytes in cerebrospinal fluid: in vivo dialysis of brain and blood plasma of the dog,” J Neurochem 13:1057-1067, 1966.
4. J.M.R Delgado, et al., “Dialytrode for long term intracerebral perfusion in awake monkeys. Arch Int Pharmacodyn 198:9-21, 1972.
5. U. Ungerstedt and C. Pycock, “Functional correlates of dopamine neurotransmission,” Bull Schweitz Akad Med Wiss 1278:1-13, 1974.
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