Ga. Lnenicka et al., ACTIVITY-DEPENDENT DEVELOPMENT OF CALCIUM REGULATION IN GROWING MOTORAXONS, The Journal of neuroscience, 18(13), 1998, pp. 4966-4972
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+ ionophor
e application, normally active tonic motor axons continue to grow, whe
reas inactive phasic motor axons retract and often degenerate. To dete
rmine 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+], than those from p
hasic axons. When depolarized with 60 mM K+, growth cones and neurites
from phasic axons had a [Ca2+], three to four times higher than did t
hose from tonic axons. This difference in Ca2+ regulation includes gre
ater Ca2+-handling capacity for growing tonic axons; the increase in [
Ca2+], produced by the Ca2+ ionophore 4-bromo-A23187 (0.25 mu M) is fo
ur to five times greater in phasic than in tonic axons, and the declin
e in [Ca2+](i) at the end of a depolarizing pulse is three to four tim
es faster in tonic axons than phasic ones. Blocking impulses in growin
g tonic axons for 2-3 d with tetrodotoxin reduces their capacity to re
gulate [Ca2+](i). Thus, growing tonic and phasic axons have difference
s in Ca2+ regulation that develop as a result of their different activ
ity levels. These activity-dependent differences in Ca2+ regulation in
fluence axon growth and degeneration and probably influence other neur
onal processes that are mediated by changes in [Ca2+](i).