Properties of Excitatory Synaptic Events in Neurons of Primary Somatosensory Cortex of Neonatal Rats

We have characterized the development of synaptic responses from neurons of rat parietal cortex. Whole-cell recording was used in slice preparations in vitro. Neurons were stained with biocytin to allow their identification, and the sample included pyramidal neurons and Cajal-Retzius cells. Dye-coupling of 3-12 cells was frequently observed from the day of birth (P0) to P3. On average, when recorded with Cs(+)-filled electrodes, the input resistances of neonatal cells were large (mean = 1.1 G omega) and resting membrane potentials were relatively depolarized (mean = -45 mV) when compared to mature neocortical neurons. The application of an NMDA receptor antagonist usually hyperpolarized cells by 5-10 mV and increased their input resistance (mean increase = 83%), suggesting that immature neurons are tonically activated by excitatory amino acids (EAA) in our preparation. Excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) could be obtained from animals as young as P0 by brief stimulation of the subplate. Synaptic responses at these early ages had long durations, often lasting over hundreds of milliseconds, they reversed polarity around 0 mV, and they were blocked by tetrodotoxin and EAA antagonists. Pharmacology and current-voltage relationships demonstrated the presence of both NMDA receptor- and non-NMDA receptor-dependent components in most EPSPs. Unlike synaptic responses of mature neurons, neonatal synaptic responses were composed largely of NMDA receptor-dependent components. We did not observe inhibitory synaptic inputs before P6. In some neurons, single shocks to the subplate region initiated spontaneous EPSPs that lasted > 1 min. This study clearly demonstrates functional synapses in the neocortex of rats on the day of birth. Large NMDA receptor-mediated EPSPs with long duration could have a major influence on the development of cortical circuits in the neonate.