Fine line between LTP and LTD...

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Bruder Jakob
Bruder Jakob's picture
Fine line between LTP and LTD...

Hello,
I am trying to make LTP recordings from hippocampal slices (400micron) in ACSF (in the dentate gyrus, stimulating the perforant path and recording the middle third of the molecular layer). However, I do not get systematic changes in my field EPSP responses following the High frequency stimulation (HFS): at times I get no response, and many times I get LTD.
I know that many things may interfere with or enhance potentiation of fEPSPs namely: the type of perfusion  chamber (I am using submerged), magnesium and calcium concentrations used, the stimulating protocol...  I have adjusted my protocols to mimic certain studies where LTP in DG  in ACSF has been observed (Saxe 2006, Farioli-Vecchiol 2008, Shimazu 2006, Van Praag 1999, Vasuta 2007....).
Based on the Wolf Singer lab, the difference between LTP and LTD with a protocol that normally induces LTP (HFS) is the post-synaptic Ca2+ concentration elevation. So I am wondering how to raise the post-synaptic Ca2+ concentration, again, with field recordings!? I am already using 2mM extra-cellular Ca2+ and 1,3mM MgS04 in the ACSF...
Someone told me to raise the stimulation intensity by 50% during the HFS only. This worked only once and in presence of Bicuculline, but I must also get LTP in ACSF! Are there some other little LTP tricks like this one to get LTP systematically instead of LTD or no change??? At the moment my stimulation protocol is: 4 trains of 0,5 sec at 100Hz; 30 sec train interval; 0,1 ms pulse duration.
Thank you all!
Desperate for answers...