Local application of FFA causes an increase in population spike response to PP stimulation. FFA leaked through the recording pipette while measuring electrophysiological response to PP stimulation. A, A marked increase in population spike is seen with all stimulation intensities, by comparison to a control pipette, used at the same recording location. B, This is not accompanied by a change in population EPSPs. Insetillustrates sample responses recorded with NaCl- and FFA-containing pipettes alternately at the same site. Note the large increase in population spike in the presence of FFA. C, Insertion of a recording pipette at the site that was recorded previously with an NaCl-containing pipette shows a larger population spike with the same EPSP (as in inset in B). Replacement of the recording drug-containing pipette with an NaCl-containing pipette restored the original population spike. This procedure can be repeated several times with considerable stability.