Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+](i) rise in fetal rat motoneurons in situ

Ca2+ imaging and (perforated) patch recording were used to analyze the mech anism of GABA- and glycine-induced depolarizations in lumbar motoneurons of spinal cord slices from fetal rats. In fura-2 ester-loaded cells, the agon ist-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 similar to 50% betwe en embryonic days 15.5 and 19.5. The [Ca2+](i) increases were abolished by Ca2+-free superfusate and attenuated by similar to 65% by nifedipine, showi ng that the responses were mediated by voltage-activated Ca2+ channels. The [Ca2+](i) rises were potentiated by >300% immediately after removal of CI- from the superfusate but recovered to values of 50-200% of control during repeated agonist administration in CI--free saline. Bumetanide gradually su ppressed the [Ca2+](i) increases by >75%. Subsequent removal of CI- reconst ituted the responses and increased, upon repeated agonist application, the peak [Ca2+](i) rises to values above control. Removal of HCO3- from the CI- -free (bumetanide-containing) superfusate reversibly abolished both the ago nist-induced [Ca2+](i) rises and depolarizations that were reestablished by formate anions. In CI--containing superfusate, removal of HCO3- decreased both the peak and duration of the agonist-evoked membrane depolarization an d [Ca2+](i) response. Our findings show that HCO3- efflux has a major contr ibution to depolarizations mediated by GABAA and glycine receptor-coupled a nion channels in prenatal neurons. We hypothesize that the HCO3--dependent depolarizing component, which is likely to produce an intracellular acidosi s, might play an important role during the early postnatal period when the CI--dependent component gradually shifts to hyperpolarization.