One of the most important features of hippocampal long-term potentiation (LTP) is that it is elicited by naturally occurring patterns of neuronal activity. Long term depression of synaptic transmission in hippocampus is induced by low-frequency stimulation protocols that completely different from those required for LTP induction. Depotentiation is another form of synaptic plasticity that has been hypothesized as a candidate for LTP counterpart in learning processes. Depotentiation can be induced by theta-frequency stimulation patterns similar to those required for the LTP induction. We showed that short stimulating sequences applied at theta frequency to Schaffer collaterals can produce complex population spikes in the CA1 fi eld of rat hippocampal slices if they were preincubated in modifi ed artifi cial cerebrospinal fl uid. Such thetastimulation induces LTP if the complex spikes emerge. In these conditions, associative LTP is also produced if weak and strong pathways are stimulated synchronously. We found that associative LTP induced by synchronous theta stimulation is reversed after application of asynchronous theta stimulation. Thus, opposite changes of the effi cacy of synaptic transmission can be induced by similar stimulation protocols. Our results confi rm the hypothesis that in some conditions depotentiation may underlie learning rules in the CA1 fi eld of hippocampus.
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