Activity-dependent reduction in voltage-dependent calcium current in a crayfish motoneuron

The effect of increased impulse activity upon voltage-dependent Ca²⁺ currents was studied in the cell body of a crayfish phasic motoneuron using two-electrode voltage-clamp technique. Increased electrical activity in this relatively inactive motoneuron produces a short-term and long-term reduction in the voltage-dependent Ca²⁺ current. Both forms of activity-dependent reduction in Ca²⁺ current are Ca²⁺ dependent. The short-term reduction in Ca²⁺ current appears to involve the Ca²⁺-dependent inactivation of Ca²⁺ channels, previously described in a variety of neurons. The long-term reduction in Ca²⁺ current is produced by prolonged Ca²⁺ influx and persists for days: in vivo stimulation of the phasic motor axon at 5 Hz for 1 hr results in a 30% reduction in Ca²⁺ current density, which persists for at least 3 d. Both the short-term and long-term reductions in Ca²⁺ current appear to result from changes in a single type of high-voltage-activated (HVA) Ca²⁺ channel. Inhibition of protein synthesis attenuates the long- term reduction in Ca²⁺ current and has no effect upon the short-term Ca²⁺ current reduction. During the long-term reduction in Ca²⁺ current, it appears that Ca²⁺ channels located distant to the site of Ca²⁺ influx are affected. The relationship of these results to a previously described Ca²⁺- dependent reduction in transmitter release is discussed.