Microdialysis sampling was originally developed to study the chemistry of cerebrospinal fluid. The concept of continuous sampling by brain microdialysis was originally described at Yale University in 1972 (“Dialytrode for Long Term Intracerebral Perfusion in Awake Monkeys”, J. Delgado, F. DeFeurdi, R. Roth, D. Ryugo and B. Mitruka, Arch Int. Pharmacodyn. (1972) 198: 9-21.) Neuroscientists have employed brain microdialysis as a monitoring tool for more than two decades. The use of a brain microdialysis probe is therefore the best developed of all possible applications.
Several different probe styles have been used. The original transcerebral probes consisted of a single length of dialysis tubing which stretched from one side of the head to the other. This approach integrated the chemical activity of all brain tissue in contact with the membrane but was not useful for differentiating the activity of individual brain regions. In loop probes a length of dialysis tubing was bent in half to form a smaller probe which could be inserted within specific brain tissues. These tended to be easy for researchers to construct, but still created more tissue damage than desirable. Pin probes, which depend on a small cylinder of dialysis membrane mounted on a thin cannula, were favored as the design creating less tissue damage. Pin probes evolved from the push-pull cannula technique originally developed by Gaddum for sampling of cerebrospinal fluid. The earliest published reference to a pin probe was at Emory University, in 1982 in a thesis written by R.D. Johnson working under Prof. J.B. Justice. Pin probes can be placed into the target tissue with the greatest accuracy. They can be removed and reinserted into guides mounted on the skull. BR probes manufactured by BAS use the pin design.
BR probes offer a small profile (340 micrometer OD) to minimize tissue damage and use a membrane with excellent recovery characteristics and a long shelf life (Polyacrylonitrile, MWCO = 30,000 daltons).
The BR Microdialysis Probe is shown with its accessories. Starting at the left is a BR probe in the shipping vial. The probe can return to the vial for in vitro calibrations or short-term storage once wetted. The next view shows the probe removed from the vial. Notice the slot in the vial which secures the locking key on the probe head. The optional intracerebral guide is shown next with the stylet removed. The stylet remains in place during stereotaxic insertion of the guide. It is removed just prior to insertion of the probe. The intracerebral guide is always placed just above the tissue to be studied. The probe is designed so that the membrane portion will extend beyond the end of the guide cannula. The target tissue will be not be pierced until the probe is inserted in the guide for the first time. On the far right, a probe has been fully inserted into the guide. In this photograph, a rubber O-ring on the guide cannula is the feature which locks the probe. The rubber O-ring offers a “snap” fit and release. For studies in active animals, another type of guide cannula with a steel “Omega-ring” is used because of the strength of steel. The steel ring rotates to around the guide to lock or release the probe. Both types of guides permit the probe to be removed after the sampling experiment is completed and reinserted at another time. The combination of an O-ring guide and probe is completely non-metallic and suitable for NMR imaging. The Omega-ring guide is not used for NMR because of the steel in the Omega-ring.
BAS also offers a pin style brain microdialysis probe (model IBR) with an additional cannula which can be used for infusions or injections of fluids directly to the tissue being dialyzed. Although many drugs can be administered to the tissue via addition to the dialysis perfusion fluid, this approach requires extra effort to determine the actual dosage of drug delivered. Compounds with a molecular weight above the MW cutoff of the membrane are too large to be dialyzed into the tissue. The infusion cannula in the IBR probe is an open cylinder with a volume of 0.2 microliters. Fluid injected or infused into this cylinder will exit at the tip of the dialysis membrane and contact the tissue directly. An IBR probe is therefore suitable for injections of high molecular weight compounds such as dyes, proteins, DNA, RNA, carbohydrates, etc.
The IBR Microdialysis Probe permits the delivery of fluid directly to a tissue being dialyzed. This is accomplished by including a third cannula inside a brain microdialysis probe which exits from the tip of the probe. Fluid injected into the brain tissue through this cannula is likely to travel upwards, along the path created by insertion of the probe, and wash the tissue in contact with the dialysis membrane.
IBR probes fit into the same intracerebral guides used for BR brain microdialysis probes. The color-coded injection cannula (red) uses the same type of flanged tubing connector used to connect FEP or PEEK tubing to the probe inlet (yellow) and outlet (green).
Applications of BR Brain Microdialysis Probes
• Monitor changes in levels of neurotransmitters or low molecular weight (< 5000 da) chemicals in the brain of a conscious animal.
• Non-metallic probe and intracerebral guide (O-ring) facilitates NMR imaging of implant site.
• Standard BR probes are suitable for rodents such as rats, hamsters and guinea pigs.
• Custom BR probes may be constructed for larger animals with longer cannula and membranes.
• Use the IBR infusion/dialysis probe to deliver drug directly to brain tissue while simultaneously monitoring tissue response to drug.
• Probes may be sterilized by the user using ethylene oxide (ETO) gas or plasma peroxide.
• Use shipping vial for in vitro calibration and short-term storage.
BAS Raturn software recorded animal activity during an experiment in which amphetamine was delivered directly to the striatum via an IBR probe, and biogenic amines were simultaneously monitored in dialysates from the same probe. The illustrations show the original Raturn data file, and graphs comparing left and right sensor data in 10 minute segments. In the Raturn, the left sensor signifies clockwise rotation and the right sensor signifies counter clockwise rotation.