At present, the type of dialysis probe most commonly used has a concentric structure (Fig. 1). This kind of probe has been used in a wealth of experimental research
Microdialysis. Fig. 1. Schematic representation of a concentric microdialysis probe made up of the following components (see text for details): 27 gauge stainless steel tubing (1), 25 gauge stainless steel tubing (2), epoxy resin (3 and 9), dental cement (4), hot-melt adhesive (5), polyethylene tubings (6 and 11), fused silica capillary tubings (7 and 10), and dialysis membrane (8). Small arrows indicate the interchange process through dialysis membrane. Larger arrows indicate the direction of perfusion fluid.
because it is well suited for reaching deep structures and/ or small nuclei of the brain. A detailed description of the materials and suppliers can be found in Adell and Artigas (1998). Briefly, the body of the probe is made up of 20-mm long 25 gauge stainless steel tubing. The inflow and outflow tubes threaded through the 25 gauge tubing consist of fused silica capillary tubing of 0.11 mm OD, 0.04 mm ID. The upper exposed end of fused silica tubings are inserted into a 7-mm piece of 27 gauge (0.41 mm OD, 0.20 mm ID) stainless steel tubing. The junction of the 27 and 25 gauge stainless steel tubings is sealed with epoxy glue and covered with dental cement to harden the assembly. Sampling of monoaminergic neurotransmit-ters usually requires a dialysis membrane consisting of a regenerated cellulose hollow fiber (0.17 mm OD, 0.15 mm ID), with a molecular weight cutoff of 6,000 Da, which is placed over the protruding lower portion of the inlet fused silica tubing and glued with epoxy resin to the inside surface of the 25 gauge stainless steel tubing. The tip of the hollow fiber is sealed also with epoxy glue. The length of the dialysis membrane exposed to the tissue varies according to the brain area to be examined. Finally, the 27 gauge protective steel tubes are friction-fitted with 20-mm lengths of polyethylene tubing of 0.61 mm OD, 0.28 mm ID to facilitate the connection of the probe to the perfusion pump and outflow line. These polyethylene and steel tubes are secured together with hot-melt glue.
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