Upregulation of the hyperpolarization‐activated cation current in rat thalamic relay neurones by acetazolamide

1 The effect of inhibition of brain carbonic anhydrase (CA) on the hyperpolarization‐activated cation current (Ih) of thalamocortical (TC) neurones of the rat ventrobasal thalamic complex (VB) was investigated in an in vitro slice preparation using the whole‐cell patch‐clamp technique and fluorescence ratio imaging of the pH indicator 2′,7′‐bis(carboxyethyl)‐5(and ‐6)‐carboxyfluorescein (BCECF). 2 Recording of Ih before and after addition of 0.4‐0.8 mM acetazolamide to the bathing fluid revealed a significant shift in the voltage dependence of activation (V½) of 5‐7 mV to more positive potentials. 3 Simultaneous recording of Ih and BCECF fluorescence ratio (F420/F495) revealed an increase in Ih amplitude accompanied by an intracellular alkalinization upon application of acetazolamide. The CA inhibitor ethoxyzolamide (EZA, 50 μm) also led to an intracellular alkalinization and a subsequent 4‐5 mV positive shift of V½ of Ih. 4 Acetazolamide and EZA both profoundly slowed the rapid fall of pHi upon switching from Hepes‐ to CO2/HCO3−‐buffered superfusate, indicating intracellular CA isoforms in TC neurones. 5 In slices bathed in Hepes‐buffered saline, addition of acetazolamide had no effect on the amplitude and time course of activation of Ih, indicating that the action of acetazolamide on Ih was dependent on the presence of HCO3−. 6 Under current‐clamp conditions, the neuronal response to hyperpolarizing current pulses in the presence of acetazolamide was decreased as compared to control. This resulted in a strongly reduced ability of TC neurones to produce rebound Ca2+‐mediated spikes. 7 The present results implied that in TC neurones acetazolamide led to an intracellular alkalinization which causes, due to its pH sensitivity, an increase in Ih.