Activity-dependent development of calcium regulation in growing motor axons

In cultured nerve cord explants from the crayfish (Procambarus clarkii), the normal impulse activity levels of growing motor axons determine their response to Ca²⁺ influx. During depolarization or Ca²⁺ ionophore application, normally active tonic motor axons continue to grow, whereas inactive phasic motor axons retract and often degenerate. To determine the role of Ca²⁺ regulation in this difference, we measured the intracellular free Ca²⁺ concentration ([Ca²⁺](i)) with fura-2. Growth cones from tonic axons normally had a higher [Ca²⁺](i) than those from phasic axons. When depolarized with 60 mM K ⁺, growth cones and neurites from phasic axons had a [Ca²⁺](i) three to four times higher than did those from tonic axons. This difference in Ca²⁺ regulation includes greater Ca²⁺-handling capacity for growing tonic axons; the increase in [Ca²⁺](i) produced by the Ca²⁺ ionophore 4-bromo-A23187 (0.25 μM) iS four to five times greater in phasic than in tonic axons, and the decline in [Ca²⁺](i) at the end of a depolarizing pulse is three to four times faster in tonic axons than phasic ones. Blocking impulses in growing tonic axons for 2-3 d with tetrodotoxin reduces their capacity to regulate [Ca²⁺](i). Thus, growing tonic and phasic axons have differences in Ca²⁺ regulation that develop as a result of their different activity levels. These activity-dependent differences in Ca²⁺ regulation influence axon growth and degeneration and probably influence other neuronal processes that are mediated by changes in [Ca²⁺](i).