Abstract
Glass micropipettes of 2-4 μm i.d. are filled with a solution of 8% Alcian blue 8GX (Gurr) in 2 mol/l NaCl by direct injection through an 0.22 μm Millipore filter. The electrode impedances are 5-10 MΩ at 1 Hz and the recording characteristics are indistinguishable from those of micropipettes filled with the conventional 4 mol/l NaCl. The pressure needed to inject the dye can be supplied by any gas cylinder but a medical oxygen cylinder has proved convenient. Connections are made with standard laboratory polythene tubing and nylon taps. The fitting between the tubing and the glass micropipette is made gas tight with a sleeve of the heat-shrinkable tubing used by electronic engineers. A length of fine silver wire is pushed through the polythene into the electrolyte to provide an electrical connection to the preamplifier. A sidearm on the pressure line leads to a clock-face pressure gauge reading 0-60 p.s.i. (0-414 kPa) and another sidearm opens to the atmosphere via a needle valve. To inject Alcian blue the vent to the air is closed and the gas cylinder tap is then opened slowly. As the pressure rises an in vivo impedance monitor automatically ckecks and displays the resistance of the micro-electrode. When dye begins to flow from the tip at 5-50 p.s.i. (35-345 kPa) there is a sharp fall in impedance. To make a small discrete spot the pressure is then rapidly released by opening the vent to the air. The whole injection procedure takes a few seconds. To mark the line of a track an electrode can be withdrawn smoothly through the brain while still injecting dye.