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

The effects of external Cs + on magnocellular neurosecretory cells were studied during intracellular recordings from 93 supraoptic nucleus neurones in superfused explants of rat hypothalamus. Bath application of 3–5 m m Cs + provoked reversible membrane depolarisation 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 positive potentials. Voltage-clamp analysis showed that external Cs + blocked the hyperpolarisation-activated inward current, I H . Under current clamp, application of ZD 7288, a selective blocker of I H , caused an increase in membrane resistance at voltages ≤−65 mV. Voltage-current analysis further revealed that blockade of I H caused hyperpolarisation when the initial voltage was < −60 mV but had no effect at more positive values. Current- and voltage-clamp analysis of the effects of Cs + in the presence of ZD 7288, or ZD 7288 and tetraethyl ammonium (TEA), revealed an increase 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 voltage independent and reversed at −100 mV. The reversal potential shifted by +22.7 mV when external [K + ] was increased from 3 to 9 m m . We conclude that, in addition to blocking I H , external Cs + blocks a leakage K + current that contributes significantly to the resting potential of rat magnocellular neurosecretory cells.