Aim: 

The CNS is capable of transforming synaptic activities in order to register and to store learned information. Moreover, synaptic processes underlying learning should be observed at live -namely, at the very moment at which they are taking place. Synaptic plasticity can be evoked either by high frequency stimulation (HFS; Bliss and Lømo, J Physiol (Lond), 232: 331, 1973) or during the acquisition of new motor and cognitive abilities (Gruart et al., J Neurosci 26: 1077, 2006).

Methods and Results: 

Animals were implanted with chronic electrodes in selected hippocampal sites following procedures described elsewhere Gruart et al., J Neurosci 26: 1077, 2006). Both LTP and synaptic activity in learning evoke lasting changes in synaptic weights, and exhibit common properties such as input specificity, and associability. However, some questions regarding differences between HFS-induced LTP and activity-dependent synaptic plasticity remain unanswered. When LTP was evoked prior to conditioning, animals were unable to acquire conditioned eyeblinks if the training started 8 days after LTP disappearance, and no change was detected in fEPSP evoked at the CA3-CA1 synapse. In contrast, LTP-induced animals learned as did controls when the conditioning test was presented 20 days after LTP had decayed to baseline, and presented a normal increase in fEPSP slopes across conditioning. When evoked during the first two conditioning sessions, LTP prevented both eyeblink conditioning and fEPSP increase. LTP did not disrupt the normal performance of a recall test of a previously acquired eyeblink conditioning. Paired-pulse stimulation showed that, in opposition to synaptic activation during actual learning, LTP courses with a presynaptic disfacilitation. Studies carried out in different knock-in (TrkB), transgenic (APP, PS-1, TrkC), and other types of genetically manipulated mice were also studied.

Conclusion: 

Results offer new insights for the understanding of differential presynaptic changes taking place during LTP and during associative learning.