MehringElectrode2001

Non-linear electrical properties of sharp microelec for experiments using intracellular current injection

C. Mehring, F. Kümmell, V. Rodriguez, M. Nawrot, A. Aertsen, D. Heck

Neurobiologie und Biophysik, Institut füur Biologie III
Albert-Ludwigs-Universität, Freiburg
www.brainworks.uni-freiburg.de

We measured the dependence of electrical resistance of sharp electrodes (20-200 MOhm) and patch pipettes (2-6 M$\Omega$) on electrode tip current for current amplitudes typically used in physiological experiments. To investigate the dynamic properties, we determined the voltage response of the electrode-bath interface to current steps with different amplitudes and offsets. Sharp electrodes filled with high-molar (1-3 M) pipette solution inserted in a physiological "extracellular" (ACSF) or "intracellular" (current clamp patch) solution showed a highly non-linear current/resistance relationship. This property was almost independent of the electrode solution. The patch pipette resistance, in contrast, was virtually constant.
     The step response of sharp microelectrodes exhibited -- next to a fast exponential component -- an additive slow component. The fast component ($\tau\ll$ 1ms) reflects the charging of the electrode capacitance. The slow component showed complex dynamics, depending on sign, amplitude and offset of the current step. Thus, the classical model of the electrode as a parallel RC-circuit with constant resistance and capacitance may not be adequate. Instead, the resistance of sharp electrodes changes with current in a time-dependent manner, reaching a stable state only after several seconds. Consequently, the resistance and the capacitance of the electrodes cannot be compensated for dynamic currents, and voltage responses to dynamic current injection measured with sharp electrodes may be significantly affected by the current- and time-dependent electrode resistance.


Supported by the Boehringer-Ingelheim Fonds and the Deutsche Forschungsgemeinschaft (DFG/SFB-505).