DIFFERING MECHANISMS OF EXPRESSION FOR SHORT-TERM AND LONG-TERM POTENTIATION

Long-term potentiation (LTP) is a use-dependent form of synaptic plast icity that is of great interest as a cellular mechanism that may contr ibute to memory storage. It is the sustained phase of population excit atory postsynaptic potential induced by high-frequency stimulation (HF S). HFS can also induce short-term potentiation (STP), a decremental p otentiation lasting similar to 15 min. It has been unclear whether STP is simply a reversible form of LTP elicited by subthreshold stimuli o r whether it is an independently expressed form of synaptic plasticity . We have attempted to clarify the relationship between LTP and STP in the extracellular recording technique in area CA1 of the adult rat hi ppocampal slice preparation to test four predictions of the hypothesis that LTP and STP are expressed via the same mechanism. First, occludi ng LTP expression should block STP expression. Saturating LTP under si x different conditions, however, did not occlude STP expression. Secon d, occluding STP expression should occlude LTP expression. The partial or full occlusion of STP by two maneuvers (increasing the stimulus in tensity used for HFS or applying 3-isobutyl-1-methylxanthine), however , did not occlude LTP expression. Third, LTP increases and decreases p aired pulse facilitation (PPF), and STP should have the same effect. S TP did not change PFF, however. The first three results, then, suggest that STP and LTP are expressed via different mechanisms. Fourth, STP should be maximal near the LTP induction threshold, and then decrease above it. Surprisingly, STP was maximal at or very close to the LTP in duction threshold, but it did not decrease above this threshold. This relationship suggests the possibility that STP and LTP share an induct ion step(s). What is the function of the independently expressed STP? We find that LTP can be induced by two HFSs, each of which is subthres hold for LTP, if the second is given during STP from the first. This s uggests that STP can temporarily lower the LTP induction threshold. Th ree lines of evidence, then, suggest that STP and LTP may be expressed via different mechanisms; however, the proximity of STP saturation to LTP induction suggests that they may share an induction step(s). STP may also have the very important function of temporarily lowering the LTP induction threshold. Fin ally, these data suggestion caution in in terpreting LTP data obtained < 20-30 min after HFS, because they may b e contaminated by STP, which appears to have different underlying mech anisms.