Behavioral tagging: Synaptic event or cellular alteration?

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Recently a number of studies were published addressing the translation of the synaptic tagging and capture hypothesis (STCH), developed and tested in hippocampal slices, into learning and long-term memory mechanisms in intact animals. Based on behavioral observations, these studies show similar results as in the original STCH experiments. A weak memory, induced by weak training in one hippocampus dependent behavioral task is converted into a long-term memory, by timely related induction of a long-term memory, induced by strong training in a second hippocampus dependent task. The induction of long-term memory should result in the synthesis of plasticity related proteins (PRP), which can be captured by the synapses involved in the weak memory trace and thus be converted into a long-term memory. Here it is outlined, that these results can alternatively be explained by non-synaptic mechanisms like changes in neuronal intrinsic excitability (IE). The plasticity of short and long-term IE show some common features with synaptic plasticity. Thus, behavioral tagging experiments, purely based on behavioral observations, do not necessarily support that synaptic tagging and capture mechanisms are involved in memory formations, unless it has been shown that the weak and strong training protocols result in synaptic early and late potentiation, or the STCH could be extended to short and long-term forms of intrinsic excitability.

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