Properties of Excitatory Synaptic Connections Mediated by the Corpus Callosum in the Developing Rat Neocortex

Despite the major role of excitatory cortico-cortical connections in mediating neocortical activities, little is known about these synapses at the cellular level. Here we have characterized the synaptic properties of long-range excitatory-to-excitatory contacts between visually identified layer V pyramidal neurons of agranular frontal cortex in callosally connected neocortical slices from postnatal day 13 to 21(P13–21) rats. Midline stimulation of the corpus callosum with a minimal stimulation paradigm evoked inward excitatory postsynaptic currents (EPSCs) with an averaged peak amplitude of 56.5 ± 5 pA under conditions of whole cell voltage clamp at −70 mV. EPSCs had fixed latencies from stimulus onset and could follow stimulus trains (1–20 Hz) without changes in kinetic properties. Bath application of 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) abolished these responses completely, indicating that they were mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs). Evoked responses were isolated in picrotoxin to yield purely excitatory PSCs, and a low concentration of NBQX (0.1 μM) was used to partially block AMPARs and prevent epileptiform activity in the tissue. Depolarization of the recorded pyramidal neurons revealed a late, slowly decaying component that reversed at ∼0 mV and was blocked by d-2-amino-5-phosphonovaleric acid. Thus AMPA and N-methyl-d-aspartate receptors (NMDARs) coexist at callosal synapses and are likely to be activated monosynaptically. The peak amplitudes and decay time constants for EPSCs evoked using minimal stimulation (±40 mV) were similar to spontaneously occurring sEPSCs. Typical conductances associated with AMPA and NMDAR-mediated components, deduced from their respective current-voltage (I-V) relationships, were 525 ± 168 and 966 ± 281 pS, respectively. AMPAR-mediated responses showed age-dependent changes in the rectification properties of their I-V relationships. While I-Vs from animals >P15 were linear, those in the younger (<P16) age group were inwardly rectifying. Although Ca2+ permeability in AMPARs can be correlated with inward rectification, outside-out somatic patches from younger animals were characterized by Ca2+-impermeable receptors, suggesting that somatic receptors might be functionally different from those located at synapses. While the biophysical properties of AMPAR components of callosally-evoked EPSCs were similar to those evoked by stimulation of local excitatory connections, the NMDA component displayed input-specific differences. NMDAR-mediated responses for local inputs were activated at more hyperpolarized holding potentials in contrast with those evoked by callosal stimulation. Paired stimuli used to assay presynaptic release properties showed paired-pulse depression (PPD) in animals <P16, which converted to facilitation (PPF) in older animals, suggesting a developmental transition from low probability of transmitter release to high P r at these synapses and/or alterations in the properties of the underlying postsynaptic receptors. Physiologic properties of neocortical e-e connections are thus input specific and subject to developmental changes in their postsynaptic receptors.