Iontophoretic application of acetylcholine: Advantages of high resistance micropipettes in connection with an electronic current pump.

Desensitization of ACh-receptors occurs when ACh-micropipettes with resistances smaller than 100 MΩ are used, even if an applied 'braking' current is set to an optimum. High sensitivity with almost no desensitization can be obtained with pipettes of higher resistances, but the current passing capability is normally reduced and a high time constant makes ACh-pulses of 1 msec duration a problem. These drawbacks can be reduced by using a potent electronic constant current source which simultaneously neutralizes the pipette's capacity. The resistance range from 120 to 200 MΩ seems to be optimal for microapplication of ACh, especially in exploring regions of low sensitivity. Rather uniform pipettes in this range can be obtained by applying a cooling air jet during the pull of the pipettes. [ABSTRACT FROM AUTHOR]