Developmental switch in the short-term modification of unitary EPSPs evoked in layer 2/3 and layer 5 pyramidal neurons of rat neocortex

Amplitudes of EPSPs evoked by repetitive presynaptic action potentials can either decrease (synaptic depression) or increase (synaptic facilitation). To determine whether facilitation and depression in the connections between neocortical pyramidal cells varied with the identity of the pre- or the po stsynaptic cell and whether they changed during postnatal development, whol e-cell voltage recordings were made simultaneously from two or three pyrami dal cells in layers 2/3 and 5 of the rat sensorimotor cortex, Unitary EPSPs were evoked when pre- and postsynaptic neurons were in the same and in dif ferent layers. In young [postnatal day 14 (P14)] cortex, EPSPs evoked in al l connected neurons depressed. The degree of depression was layer specific and was determined by the identity of the presynaptic cell. EPSPs evoked by stimulation of presynaptic layer 5 neurons depressed significantly more th an did those evoked by stimulation of layer 2/3 neurons. In mature cortex ( P28), however, the EPSPs evoked in these connected neurons facilitated to a comparable degree regardless of the layer in which pre- and postsynaptic n eurons were located. The results suggest that in young cortex the degree of synaptic depression in connected pyramidal cells is determined primarily b y whether the presynaptic cell was in layer 2/3 or 5 and that maturation of the cortex involves a developmental switch from depression to facilitation between P14 and P28 that eliminates the layer-specific differences. A func tional consequence of this switch is that in mature cortex the spread of ex citation between neocortical pyramidal neurons is enhanced when action pote ntials occur in bursts.