The mechanisms of synaptic transmission in the rat hippocampus at birth are assumed to be fundamentally different from those found in the adult. It has been reported that in the CA3-CA1 pyramidal cells a conversion of "silent" glutamatergic synapses to conductive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) synapses starts gradually after P2. Further, GABA via its depolarizing action seems to give rise to grossly synchronous yet slow calcium oscillations. Therefore, GABA is generally thought to have a purely excitatory rather than an inhibitory role during the first postnatal week. In the present study field potential recordings and gramicidin perforated and whole cell clamp techniques as well as K(+)-selective microelectrodes were used to examine the relative contributions of AMPA and GABA(A) receptors to network activity of CA3-CA1 pyramidal cells in the newborn rat hippocampus. As early as postnatal day (P0-P2), highly coherent spontaneous firing of CA3 pyramidal cells was seen in vitro. Negative-going extracellular spikes confined to periodic bursts (interval 16 +/- 3 s) consisting of 2.9 +/- 0.1 spikes were observed in stratum pyramidale. The spikes were accompanied by AMPA-R-mediated postsynaptic currents (PSCs) in simultaneously recorded pyramidal neurons (7.6 +/- 3.0 unitary currents per burst). In CA1 pyramidal cells synchronous discharging of CA3 circuitry produced a barrage of AMPA currents at >20 Hz frequencies, thus demonstrating a transfer of the fast CA3 network activity to CA1 area. Despite its depolarizing action, GABA(A)-R-mediated transmission appeared to exert inhibition in the CA3 pyramidal cell population. The GABA(A)-R antagonist bicuculline hypersynchronized the output of glutamatergic CA3 circuitry and increased the network-driven excitatory input to the pyramidal neurons, whereas the GABA(A)-R agonist muscimol (100 nM) did the opposite. However, the occurrence of unitary GABA(A)-R currents was increased after muscimol application from 0.66 +/- 0.16 s(-1) to 1.43 +/- 0.29 s(-1). It was concluded that AMPA synapses are critical in the generation of spontaneous high-frequency bursts in CA3 as well as in CA3-CA1 transmission as early as P0-P2 in rat hippocampus. Concurrently, although GABA(A)-R-mediated depolarization may excite hippocampal interneurons, in CA3 pyramidal neurons it can restrain excitatory inputs and limit the size of the activated neuronal population.