PROPERTIES OF THE HYPERPOLARIZATION-ACTIVATED CATION CURRENT I-H IN RAT MIDBRAIN DOPAMINERGIC-NEURONS

Intracellular electrophysiological recordings in current- and voltage-clamp mode were obtained from dopaminergic neurons of the rat mesencephalon in an in vitro slice preparation. In current-clamp mode, a time-dependent anomalous rectification (TDR) of the membrane was observed in response to hyperpolarizing current pulses. In single-electrode voltage-clamp mode, a slowly developing inward current (I-h) underlying the TDR was studied by hyperpolarizing voltage commands from a holding potential of -50 to -60 mV, I-h started to be activated at -69 mV, was fully activated at -129 to -141 mV, with half-maximal activation at -87 mV, and showed no inactivation with time. The time course of development of lh followed a single exponential, and its time constant was voltage-dependent. At -81 mV, I-h activated with a time constant of 379 +/- 47.6 ms, whereas at -129 mV I-h activated with a time constant of 65 +/- 2.2 ms. Its estimated reversal potential was -35 +/- 4 mV. Raising the extracellular concentration of K+ from 2.5 to 6.5 and to 12.5 mM increased the amplitude of I-h while reducing the extracellular concentration of Na+ from 153.2 to 27.2 mM caused a reduction in amplitude of I-h. Bath application of caesium (1-5 mM) reversibly reduced or blocked the TDR/I-h. Perfusion of tetrodotoxin (0.5-1 mu M), tetraethylammonium (10-20 mM) or barium (0.3-2 mM) did not significantly affect I-h. I-h was also present in cells impaled with CsCl-filled electrodes. When I-h was substantially reduced by extracellular caesium (1 mM) the firing rate of the dopaminergic cells, which consisted of a spontaneous pacemaker discharge of action potentials, was not clearly changed. In addition, the holding current in voltage-clamp experiments at -50 to -60 mV was not affected by 1 mM caesium. We conclude that although the I-h current is a typical feature of the dopaminergic neurons, it is neither a significant factor underlying the spontaneous pacemaker activity nor does it contribute substantially to the setting of the normal resting potential level of the membrane. On the other hand, since I-h starts at voltages lower than or equal to -69 mV (below firing threshold), it may play a modulatory role in the cell's excitability by limiting the amplitude and duration of any prolonged hyperpolarizing events in the dopaminergic cells.