I assume you have to do record within 15-17 minutes to avoid washout of
the cells internal solution? You could avoid this using perforated
patching...
Also, are you constantly checking series resistance? If this is
changing, then this could easily account for the changes you are
seeing.
Also, are you giving the picrotoxin time to washin before you patch the
cells? It can take a really long time to reach saturation in my hands
(over 4 minutes).
Finally, try a different stimulator, concentric stimulators are great
for local stimulation and other paradigms where you need localized
stimulation, but if you're doing LTP in the hippocampus (I assume
you're in CA1) then a nice big fat twisted pair bi polar electrode is
always more stable, in my experience at least. (are you recruit more
the the shaffer fibres).
