The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1625-1628
Copyright ©2002 by the American Physiological Society

RAPID COMMUNICATION

Activity-Dependent Plasticity of Calcium Clearance From Crayfish Motor Axons

Department of Biological Sciences, State University of New York, Albany, New York 12222

Fengler, Brian T. and Gregory A. Lnenicka. Activity-Dependent Plasticity of Calcium Clearance From Crayfish Motor Axons. J. Neurophysiol. 87: 1625-1628, 2002. Previous studies of a crayfish explant culture demonstrated that regenerating motor axons with high impulse activity develop more rapid clearance of cytoplasmic free Ca²⁺ than those with low impulse activity. We examined whether Ca²⁺ clearance in mature axons also showed activity-dependent plasticity. We studied the phasic and tonic axons of the motor bundle innervating the crayfish closer muscle that display large differences in impulse activity. To compare their Ca²⁺ regulation, we applied the Ca²⁺ ionophore Br-23187 (1 µM) and measured the increase in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) with fura-2. After 55 min of ionophore application, the increase in [Ca²⁺]_i in the phasic axons (1,326 ± 192 nM) was significantly greater than in the tonic axons (359 ± 148 nM). This resulted from stronger Ca²⁺ clearance in the tonic axon rather than less Ca²⁺ influx because blocking Ca²⁺ clearance by Na/Ca exchange and mitochondria eliminated these differences in [Ca²⁺]_i. Next we determined whether Ca²⁺ clearance from the phasic axon could be strengthened by a prolonged increase in impulse activity. The phasic axon was stimulated in vivo at 5 Hz for 1 h/day for 5 days, and 1-3 days after stimulation, Ca²⁺ clearance was again examined. After 55 min of Br-23187 (1 µM) exposure, the increase in [Ca²⁺]_i in the stimulated phasic axon was only 232 ± 123 nM, which was much less than in the control phasic axons and similar to that in the tonic axons. Thus Ca²⁺-clearance mechanisms adapt to changes in impulse activity both in growing and mature axons.