Postnatal changes in the overall postsynaptic currents evoked in CA1 pyramidal neurons of the rat hippocampus

Evoked fast postsynaptic currents (fPSCs) during the postnatal development of rats (postnatal day 6-70, P6-P70) were systematically examined in hippocampal CA1 pyramidal neurons using whole-cell recordings with biocytin-filled electrodes. Focal stimulation of the stratum radiatum in the CA1 region elicited fPSCs in 80% of the neurons P6-7, 90% of P9-10, and 100% of > or =P11. In neurons P6-7, the fPSCs were exclusively inward and had multiple (on average 5.6) peaks. The fPSCs increased in amplitude with the growth of dendritic arborization, but decreased in the number of peaks. A distinct outward fPSC following the inward fPSC emerged in neurons > or =P11 and was abolished by bicuculline (50 microM). Bicuculline increased the amplitude and duration of the initial inward fPSC (fEPSC) in all age groups and characteristically recruited the polysynaptic second component of fEPSCs in neurons P11-P21. No spontaneous periodic inward current was detected in any age group after blocking GABAA receptors. The coapplication of DL-2-amino-5-phosphonopentanoic acid (AP5, 100 microM) with bicuculline did not eliminate the polysynaptic second component, but the second component was only elicited in slices in which the CA3 region was kept intact. Moreover, the bicuculline- and AP5-resistant second component was due to the burst activity of CA3 pyramidal neurons, which were excited through excitatory recurrents of the Schaffer collaterals. Plausible physiological functions of the generation of the second component in vivo were discussed.