4.4. In Vivo Microdialysis

Mice were anesthetized with chloral hydrate (400 mg/kg, i.p.) and placed in a stereotaxic frame. Dialysis probes (2 mm membrane length, 0.24 mm outer diameter, Cuprophane, 6-kDa cutoff, CMA-11, CMA/Microdialysis, Solna, Sweden) with CMA-11 guide cannulae were implanted into the right striatum. The stereotaxic coordinates for implantation of microdialysis probes were (in mm): AP 0.0, DV 4.4, ML 2.5 relative to bregma. The placement of the probe was verified by histological examination after the experiments. Following surgery, animals were returned to their home cages with free access to food and water. Twenty-four hours after surgery, the dialysis probe was connected to a syringe pump and perfused at flow rate 1 μL/min with artificial cerebrospinal fluid (CSF; composition in mM: Na⁺ 150; K⁺ 3.0; Ca²⁺ 1.4; PO₄³⁻ 1.0; Cl⁻ 155; ESA, Bedford, MA, USA) with 0.25 mM ascorbate, pH 7.3. To measure the ”true” extracellular concentration of dopamine, a quantitative “low perfusion rate” microdialysis was performed [13,17]. CSF was perfused at a flow rate of 0.1 μL/min for 6–7 h and collected in a tube containing 2 μL of 0.4 M HClO₄ every 90 min. Perfusate samples were assayed for dopamine using HPLC-ECD. Monoamines and metabolites were separated on a microbore reverse-phase column (C-18, 5 Wm, 1U150 mm, Unijet, BAS, West Lafayette, IN, USA) with a mobile phase consisting of 0.03 M citrate-phosphate buffer with 2.1 mM octyl sodium sulfate, 0.1 mM EDTA, 10 mM NaCl, and 17% methanol (pH 3.6) at a flow rate of 90 μL/min and detected by a 3-mm glass carbon electrode (Unijet, BAS, West Lafayette, IN, USA) set at +0.8 V. The volume of injection was 5 μL [38].