Instantaneous inward rectification in the Mauthner cell: a postsynaptic booster for excitatory inputs.

Single and double electrode voltage clamp techniques have been used to analyse the leakage conductance of the goldfish Mauthner cell. The results indicate that the input conductance of this neuron is maximal at the resting potential, having an average value of 4.68 microS. Approximately 50% of this conductance is voltage dependent and could be inactivated by large hyperpolarizing command pulses (30-50 mV magnitude) of 20-35 ms duration. The magnitude and apparent time constant of the inactivation are both functions of membrane potential, such that the relaxation of the inward current increased and occurred more rapidly for greater hyperpolarizations. The presence of instantaneous inward tail currents when the imposed voltage steps were terminated and membrane potential was returned to the resting level indicated that the conductance mechanism inactivated during hyperpolarization had generated a small outward current at the resting level. Thus, we propose that a major fraction of the Mauthner cell's input conductance is a voltage-dependent K+ conductance. Hyperpolarizing inactivation is a feature of an inward K+ rectifier in other cell types, and when the recording microelectrode was located in the Mauthner cell lateral dendrite, it was possible to demonstrate characteristics consistent with rectification, namely a low conductance for outward depolarizing currents and a high conductance for inward currents. The inward rectifier of the Mauthner cell is different from that of other neurons in that it is already maximally activated at the resting potential and therefore is a major determinant of that parameter. In addition, its activation and inactivation kinetics are quite fast.(ABSTRACT TRUNCATED AT 250 WORDS)