Modulation of the hyperpolarization-activated cation current of rat thalamic relay neurones by intracellular pH

1. Properties of the hyperpolarization-activated cation current (I-h) were investigated in thalamocortical neurones of an in vitro slice preparation o f the rat ventrobasal thalamic complex (VB) before and during changes of pi pette pH (pH(p)), intracellular pH (pH(i)) and bath pH (pH(b)) using the wh ole-cell patch-clamp technique and fluorescence ratio imaging of the pH ind icator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. Recording of I-h with predefined pH(p) revealed significant shifts in th e voltage dependence of I-h activation (V-1/2) of 4-5 mV to more positive v alues for a pH(p) of 7.5 and 2-3 mV to more negative values for a pH(p) of 6.7 as compared to control values (pH(p) = 7.1). 3. Application of the weak acid lactate (20 mM), which produced a slow mono phasic intracellular acidification, induced a reversible negative shift of V-1/2 of up to 3 mV. Application of 20 mM TMA, which caused a distinct intr acellular alkalinization, shifted V-1/2 to 4-5 mV more positive values. 4. In slices bathed in Hepes-buffered saline, no significant pH(o) dependen ce of I-h was observed. Changing pH(o) by altering the extracellular [HCO3- ] in the presence of constant pCO(2) also revealed no significant pH(o) dep endence of I-h. 5. Rhythmic stimulation of thalamocortical neurones with repetitive depolar izing pulse trains caused an intracellular acidification, which reversibly decreased the amplitude and time course of activation of I-h. 6. The results of the present study indicate that shifts in pH(i) result in a significant modulation of the gating properties of I-h channels in TC ne urones. Through this mechanism activity-dependent shifts in pH(i) may contr ibute to the up- and downregulation of I-h.