Functional Synaptic Projections onto Subplate Neurons in Neonatal Rat Somatosensory Cortex

Subplate neurons (SPn) play an important role in the formation of thalamocortical connections during early development and show glutamatergic and GABAergic spontaneous synaptic activity. We characterized these synaptic inputs by performing whole-cell recordings from SPn in somatosensory cortical slices of postnatal day 0-3 rats. At -70 mV, electrical stimulation of the thalamocortical afferents elicited in 68% of the SPn a monosynaptic CNQX-sensitive postsynaptic current (PSC). These fast PSCs were mediated by AMPA receptors, because they were prolonged by cyclothiazide and blocked by GYKI 52466. On membrane depolarization, thalamocortical stimulation elicited in 50% of the cells an additional slow monosynaptic component mediated by NMDA receptors. Stimulation of the cortical plate evoked in 72% of SPn a monosynaptic AMPA receptor-mediated PSC with an additional NMDA component at depolarized membrane potentials and in 40% of the investigated cells polysynaptic responses, depending on GABA(A) and NMDA receptors. Stimulation of the subplate elicited in 67% of SPn a monosynaptic dual-component PSC mediated by AMPA and NMDA receptors activated at -70 mV and in 12% of SPn a monosynaptic single-component PSC mediated by AMPA receptors with an additional NMDA component activated at depolarized membrane potentials. A monosynaptic GABAergic response could be observed in 68% of SPn after stimulation of the subplate. In gramicidin-perforated patch recordings, bath application of GABA caused membrane depolarization to -40 mV and elicited action potentials. These results demonstrate that SPn receive distinct functional synaptic inputs arising from the thalamus, cortical plate, and subplate, indicating that SPn are capable of integrating and processing information from cortical and subcortical regions.