Developmental synaptic changes increase the range of integrative capabilities of an identified excitatory neocortical connection

Excitatory synaptic transmission between pyramidal cells and fast-spiking ( FS) interneurons of layer V of the motor cortex was investigated in acute s lices by using paired recordings at 30 degrees C combined with morphologica l analysis. The presynaptic and postsynaptic properties at these identified central synapses were compared between 3- and 5-week-old rats. At these tw o postnatal developmental stages, unitary EPSCs were mediated by the activa tion of AMPA receptors with fast kinetics at a holding potential of -72 mV. The amplitude distribution analysis of the EPSCs indicates that, at both s tages, pyramidal-FS connections consisted of multiple functional release si tes. The apparent quantal size obtained by decreasing the external calcium ([Ca2+](e)) varied from 11 to 29 pA near resting membrane potential, In you ng rats, pairs of presynaptic action potentials elicited unitary synaptic r esponses that displayed paired-pulse depression at all tested frequencies. In older animals, inputs from different pyramidal cells onto the same FS in terneuron had different paired-pulse response characteristics and, at most of these connections, a switch from depression to facilitation occurred whe n decreasing the rate of presynaptic stimulation. The balance between facil itation and depression endows pyramidal-FS connections from 5-week-old anim als with wide integrative capabilities and confers unique functional proper ties to each synapse.