Stability of oFPR and optogenetic induction of LTD. Field potentials were elicited in layer II/III by applying laser flashes in layer V (1 ms at 0.04 mW). A. Normalized fEPSP amplitudes evoked by pulses of blue laser at 0.033 Hz were stable throughout the 60-min recording period (n=3). B–D. LTD induction by LFS: The graphs show normalized fEPSP peak amplitudes recorded before and after conditioning stimulation (900 pulses) at 1 Hz (B, n=5), 2 Hz (C, n=4), or 4 Hz (D, n=4). Only optogenetic stimulation at 1 Hz resulted in long-term depression of fEPSP amplitude (B), suggesting that the optogenetic LTD induction was frequency-dependent. Insert in B: representative average field potential traces before (black trace) and after (gray trace) LFS in a slice.