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Developmental Changes in Spontaneous GABAA‐mediated Synaptic Events in Rat Hippocampal CA3 Neurons

Ongoing spontaneous postsynaptic potentials (SPSPs) were intracellularly recorded at 34‐36°C from hippocampal CA3 neurons in slices obtained from postnatal days (P) 0 – 6 and 7 – 31. SPSPs occurred randomly, and their frequency distribution was fitted by a single exponential function. They were little affected by kynurenic acid, but were reversibly blocked by bicuculline, implying that they were mediated by GABAA receptors. The mean amplitude was 4.53 ± 0.89 mV in control conditions and 4.07 ± 0.79 mV in kynurenic acid. In kynurenic acid (with CsCl‐filled microelectrodes), SPSPs reversed polarity at 2.4 ± 2 mV. When tetrodotoxin (1 μM) was added to kynurenic acid solution, GABAA‐mediated miniature postsynaptic potentials (MPSPs) were recorded. Under these conditions large events disappeared. The mean amplitude of MPSPs was 2.51 ± 0.43 mV. The mean frequency decreased from 2.96 ± 1.04 Hz in kynurenic acid to 0.4 ± 0.15 Hz in kynurenic acid plus tetrodotoxin. In contrast with P0 – P6, at P7 – P31 SPSPs were significantly affected by kynurenic acid. The mean amplitude of SPSPs shifted from 4.71 ± 0.82 mV in control conditions to 3.79 ± 0.76 mV in kynurenic acid. At this developmental stage, the reversal potential of GABAA‐mediated SPSPs shifted towards more negative values (‐23.7 ± 1.3 mV). Addition of tetrodotoxin to kynurenic acid solution abolished larger events and revealed GABAergic MPSPs. The mean amplitude of MPSPs was 2.72 ± 0.5 mV, a value very close to that observed at P0 – P6. Synaptic currents were recorded at 22 – 24°C from voltage‐clamped CA3 pyramidal neurons (at P6) using the tight‐seal whole‐cell recording technique. Cells were dialysed with CsCl solution and held at ‐70 mV. Spontaneous GABAA‐mediated miniature postsynaptic currents (MPSCs) were recorded in the presence of kynurenic acid and tetrodotoxin. The decay time of MPSCs was fitted with a single exponential and was 29 ± 3 ms. No correlation was found between the peak amplitude of individual events and their rise or decay time constant. The mean amplitude distribution of MPSCs was 12 ± 4.3 pA. In outside‐out patches from acutely dissociated CA3 hippocampal neurons, GABA (50 μM) activated single‐channel events of 24 and 35 pS conductance. Therefore one quantal current represents the simultaneous opening of five to seven GABAA receptor channels on the postsynaptic cell. These data show also that in the immediate postnatal period spontaneous glutamatergic potentials are poorly developed and start appearing towards the end of the first postnatal week concomitant with the shift of GABA from the depolarizing to the hyperpolarizing direction.