Kinetic analysis of the GABAB-mediated inhibition of the high-threshold Ca2+ current in cultured rat sensory neurones.

1. The action of baclofen on the voltage-gated Ca2+ current (ICa) was studied, using cultured neurones of the newborn rat dorsal root ganglia (DRG). Two major categories of ICa were identified: a small transient current activated positive to -60 mV (low voltage-activated ICa) and a large and slowly inactivating current activated positive to -30 mV (high voltage-activated ICa). 2. Baclofen reversibly blocked the high voltage-activated ICa and slowed the activation phase of the current in a concentration-dependent manner (0.5-50 microM). The half-maximal effective concentration was about 1.5 microM as measured by a peak of ICa. On the contrary, a high concentration of baclofen (100 microM) had no detectable effect on the low voltage-activated ICa. 3. The baclofen-sensitive component of the high voltage-activated ICa was largely inactivated by a depolarized holding potential (Vh) of -40 mV, whereas the baclofen-resistant component was not affected by a change in Vh ranging from -110 to -30 mV. 4. The high voltage-activated ICa had two components of current decay: an inactivating component and a quasi-sustained component, with time constants about 420 and 1220 ms, respectively. The time constant of decay for the inactivating component was not affected by replacement of external Ca2+ with Ba2+, whereas that for the quasi-sustained component was markedly prolonged, suggesting that the decay of this component may be due to Ca(2+)-induced block rather than voltage-dependent inactivation. A high concentration of baclofen (50 microM) selectively blocked the inactivating component. 5. The decay phase of the baclofen-sensitive component of the high voltage-activated ICa was best fitted by a sum of two exponentials, with 29.2 and 481 ms for the fast and slow components, respectively. The time constants of the two components were not affected by an increase in the concentration of baclofen, whereas the amplitudes changed concentration-dependently. 6. The slowed activation of the high voltage-activated ICa by baclofen was partially reversed by a large depolarizing pre-pulse. However, such an acceleration of the current was similarly observed in the control solution. Furthermore, the actual current size increased by the pre-pulse was similar in both the control and baclofen-containing solutions. 7. These results suggest that baclofen selectively blocks the inactivating component of the high voltage-activated ICa which forms a rapid rising phase of this current, thus slowing the activation phase of the total high voltage-activated ICa.