Evidence for proportional synaptic scaling in neocortex of intact animals.

Neocortical neurons in culture can modify their global synaptic strength according to their recent activity level. This process has been termed synaptic scaling. We explored possible effects of synaptic scaling in intact animals by comparing EPSPs of two identified synaptic tracts on single cortical neurons in acute slices. We show that the amplitude and rise-time of unitary thalamocortical and intracortical EPSPs vary among different neurons, but are highly correlated on single cells. Likewise, the AMPA-to-NMDA receptor ratio in EPSPs evoked from the two tracts is correlated in single neurons, although intracortical pathways within layer 5 exhibit a significantly larger NMDA component. These findings demonstrate that, under physiological conditions, cortical neurons actively regulate their own response magnitude, regardless of the input source.