Differential depression at excitatory and inhibitory synapses in visual cortex

The function of cortical circuits depends critically on the balance between excitation and inhibition. This balance reflects not only the relative num bers of excitatory and inhibitory synapses but also their relative strength s. Recent studies of excitatory synapses in visual and somatosensory cortic es have emphasized that synaptic strength is not a fixed quantity but is a dynamic variable that reflects recent presynaptic activity. Here, we compar e the dynamics of synaptic transmission at excitatory and inhibitory synaps es onto visual cortical pyramidal neurons. We find that inhibitory synapses show less overall depression than excitatory synapses and that the kinetic s of recovery from depression also differ between the two classes of synaps e. When excitatory and inhibitory synapses are stimulated concurrently, thi s differential depression produces a time- and frequency-dependent shift in the reversal potential of the composite postsynaptic current. These result s indicate that the balance between excitation and inhibition can change dy namically as a function of activity.