Ionic basis of the caesium-induced depolarisation in rat supraoptic nucleus neurones

The effects of external Cs+ on magnocellular neurosecretory cells were stud ied during intracellular recordings from 93 supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Bath application of 3-5 mm Cs+ provoked reversible membrane depolarisati on and increased firing rate in all of the neurones tested. Voltage-current analysis revealed an increase in membrane resistance between - 120 and -55 mV. The increase in resistance was greater below -85 mV than at more posit ive potentials. 3. Voltage-clamp analysis showed that external Cs' blocked the hyperpolaris ation-activated inward current, I-H. Under current clamp, application of ZD 7288, a selective blocker of I-H, caused an increase in membrane resistanc e at voltages less than or equal to -65 mV. Voltage-current analysis furthe r revealed that blockade of I-H caused hyperpolarisation when the initial v oltage was < -60 mV but had no effect at more positive values. 4. Current- and voltage-clamp analysis of the effects of Cs+ in the presenc e of ZD 7288, or ZD 7288 and tetraethyl ammonium (TEA), revealed an increas e in membrane resistance throughout the range of voltages tested (-120 to - 45 mV). The current blocked by Cs+ in the absence of I-H was essentially vo ltage independent and reversed at -100 mV. The reversal potential shifted b y +22.7 mV when external [K+] was increased from 3 to 9 mm. We conclude tha t, in addition to blocking I-H, external Cs+ blocks a leakage K+ current th at contributes significantly to the resting potential of rat magnocellular neurosecretory cells.