The Journal of Neuroscience, July 1, 1998, 18(13):4966-4972
Activity-Dependent Development of Calcium Regulation in Growing
Motor Axons
Gregory A.
Lnenicka,
Kathleen F.
Arcaro, and
John M.
Calabro
Neurobiology Research Center, Department of Biological Sciences,
University at Albany, State University of New York, Albany, New York
12222
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.
Key words:
calcium regulation; activity-dependent; fura-2; cell
culture; growth cones; crayfish
Copyright © 1998 Society for Neuroscience 0270-6474/98/18134966-07$05.00/0