Modulation by extracellular pH of low- and high-voltage-activated calcium currents of rat thalamic relay neurons

The effects of changes in the extracellular pH (pH(o)) on low-voltage- (LVA ) and high-voltage( HVA) activated calcium currents of acutely isolated rel ay neurons of the ventrobasal thalamic complex (VB) were examined using the whole cell patch-clamp technique. Modest extracellular alkalinization (pH 7.3 to 7.7) reversibly enlarged LVA calcium currents by 18.6 +/- 3.2% (mean +/- SE, n = 6), whereas extracellular acidification (pH 7.3 to 6.9) decrea sed the current by 24.8 +/- 3.1% (n = 9). Normalized current amplitudes (I/ I-7.3) fitted as a function of pHo revealed an apparent pK(a) of 6.9. Both, half-maximal activation voltage and steady-state inactivation were signifi cantly shifted to more negative voltages by 2-4 mV on extracellular alkalin ization and to more positive voltages by 2-3 mV on extracellular acidificat ion, respectively. Recovery from inactivation of LVA calcium currents was n ot significantly affected by changes in pHo. In contrast, HVA calcium curre nts were less sensitive to changes in pHo. Although extracellular alkaliniz ation increased maximal HVA current by 6.0 +/- 2.0% (n = 7) and extracellul ar acidification decreased it by 11.9 +/- 0.02% (n = 11), both activation a nd steady-state inactivation were only marginally affected by the moderate changes in pHo used in the present study. The results show that calcium cur rents of thalamic relay neurons exhibit different pHo sensitivity. Therefor e activity-related extracellular pH transients might selectively modulate c ertain aspects of the electrogenic behavior of thalamic relay neurons.