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The Journal of Neuroscience, November 1, 2000, 20(21):7905-7913
Role of Bicarbonate and Chloride in GABA- and Glycine-Induced
Depolarization and [Ca2+]i Rise in Fetal Rat
Motoneurons In Situ
Anna
Kulik,
Hiroshi
Nishimaru, and
Klaus
Ballanyi
Physiologisches Institut, Universität Göttingen,
D-37073 Göttingen, Germany
Ca2+ imaging and (perforated) patch recording
were used to analyze the mechanism of GABA- and glycine-induced
depolarizations in lumbar motoneurons of spinal cord slices from fetal
rats. In fura-2 ester-loaded cells, the agonist-induced depolarizations increased [Ca2+]i by up to 100 nM. The GABA- and glycine-evoked
[Ca2+]i transients were suppressed by
bicuculline and strychnine, respectively. Their magnitude decreased by
~50% between embryonic days 15.5 and 19.5. The
[Ca2+]i increases were abolished by
Ca2+-free superfusate and attenuated by ~65% by
nifedipine, showing that the responses were mediated by
voltage-activated Ca2+ channels. The
[Ca2+]i rises were potentiated by
>300% immediately after removal of Cl from the
superfusate but recovered to values of 50-200% of control during
repeated agonist administration in Cl-free saline.
Bumetanide gradually suppressed the
[Ca2+]i increases by >75%.
Subsequent removal of Cl reconstituted the
responses and increased, upon repeated agonist application, the peak
[Ca2+]i rises to values above control.
Removal of HCO3 from the
Cl-free (bumetanide-containing) superfusate
reversibly abolished both the agonist-induced
[Ca2+]i rises and depolarizations that
were reestablished by formate anions. In
Cl-containing superfusate, removal of
HCO3 decreased both the peak and
duration of the agonist-evoked membrane depolarization and
[Ca2+]i response. Our findings show
that HCO3 efflux has a major
contribution to depolarizations mediated by GABAA and
glycine receptor-coupled anion channels in prenatal neurons. We
hypothesize that the HCO3-dependent
depolarizing component, which is likely to produce an intracellular
acidosis, might play an important role during the early postnatal
period when the Cl-dependent component gradually
shifts to hyperpolarization.
Key words:
bicarbonate; calcium; chloride pump; development; imaging; motor neurons; neuronal maturation
Copyright © 2000 Society for Neuroscience 0270-6474/00/20217905-09$05.00/0
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