Inactivation of calcium currents in granule cells cultured from mouse cerebellum.

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Abstract
1. Cells dissociated from mouse cerebellum were grown in vitro. Ca2+ channel currents were recorded from granule cells with the patch‐clamp technique under conditions which suppressed currents through Na+ and K+ channels and minimized run‐down of current through Ca2+ channels. 2. A strong depolarizing voltage step from a hyperpolarized holding potential produced inward Ca2+ channel current that decayed exponentially to a non‐zero level. Inward current decayed to approximately 40% of its peak amplitude (range 20‐90%). 3. The inward current increased in amplitude when Ca2+ was replaced with Ba2+ or after raising the concentration of extracellular Ba2+, but the rate of decay of current was unaffected. 4. The current‐voltage (I‐V) relation showed that peak or sustained current increased with voltage pulses more positive than approximately ‐30 mV, reached a maximum amplitude near +20 mV and became progressively smaller with larger depolarizations. 5. The tail currents produced after rapidly repolarizing the membrane potential to ‐70 mV from a positive test pulse decayed along a single exponential time course with a time constant of approximately 0.5 ms. The amplitude of tail current measured at a fixed repolarization potential increased as the pre‐pulse was made more positive and reached a maximum with pre‐pulses more positive than +40 mV. A plot of normalized amplitude of the tail current as a function of the pre‐pulse potential was fitted with a Boltzmann relation with V1/2 = approximately + 8 mV and steepness k = 14 mV. 6. Shifting the holding potential to more positive potentials reduced the amplitude of the Ca2+ channel current elicited by the fixed voltage step and abolished the decay of the inward current. The peak current was normalized to the maximum peak current elicited from a very negative holding potential and plotted as a function of holding potential. The points were fitted with a Boltzmann relation for inactivation with V1/2 = approximately ‐57 mV and steepness k = 14 mV. 7. The onset of inactivation was studied in two‐pulse experiments in which the duration of conditioning pre‐pulse was varied. Increasing the duration of a pre‐pulse to a fixed potential reduced the peak inward current evoked by the second test pulse. Plotting normalized current as a function of pre‐pulse duration showed that inactivation developed along a double exponential time course. Both fast and slow time constants decreased as the pre‐pulse potential was made more positive.(ABSTRACT TRUNCATED AT 400 WORDS)