Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+]i rise in fetal rat motoneurons in situ.
Ca(2+) 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 [Ca(2+)](i) by up to 100 nm. The GABA- and glycine-evoked [Ca(2+)](i) transients were suppressed by bicuculline and strychnine, respectively. Their magnitude decreased by approximately 50% between embryonic days 15.5 and 19.5. The [Ca(2+)](i) increases were abolished by Ca(2+)-free superfusate and attenuated by approximately 65% by nifedipine, showing that the responses were mediated by voltage-activated Ca(2+) channels. The [Ca(2+)](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 [Ca(2+)](i) increases by >75%. Subsequent removal of Cl(-) reconstituted the responses and increased, upon repeated agonist application, the peak [Ca(2+)](i) rises to values above control. Removal of HCO(3)(-) from the Cl(-)-free (bumetanide-containing) superfusate reversibly abolished both the agonist-induced [Ca(2+)](i) rises and depolarizations that were reestablished by formate anions. In Cl(-)-containing superfusate, removal of HCO(3)(-) decreased both the peak and duration of the agonist-evoked membrane depolarization and [Ca(2+)](i) response. Our findings show that HCO(3)(-) efflux has a major contribution to depolarizations mediated by GABA(A) and glycine receptor-coupled anion channels in prenatal neurons. We hypothesize that the HCO(3)(-)-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.
Kulik
A
A
Physiologisches Institut, Universität Göttingen, D-37073 Göttingen, Germany.
Nishimaru
H
H
Ballanyi
K
K
D000367:Age Factors; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D018755:GABA Agonists / Q000494:pharmacology; D058785:GABA-A Receptor Agonists; D058786:GABA-B Receptor Agonists; D005998:Glycine / Q000378:metabolism* / Q000494:pharmacology; D066298:In Vitro Techniques; D008161:Lumbosacral Region; D009046:Motor Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D018009:Receptors, Glycine / Q000037:antagonists & inhibitors; D013116:Spinal Cord / Q000166:cytology / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
D001639:Bicarbonates; D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D002712:Chlorides; D018755:GABA Agonists; D058785:GABA-A Receptor Agonists; D058786:GABA-B Receptor Agonists; D018009:Receptors, Glycine; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D002118:Calcium; D005998:Glycine