Long-term depression and associativity in rat primary motor cortex following thalamic stimulation

Associativity is an attractive property of LTP in terms of its possible mechanism as a model for memory storage. In this study, we compare the effects of homosynaptic vs. associative stimulation on the induction of LTP and LTD in the neocortex of freely behaving rats. Using a callosal input to the motor cortex as a 'strong' input (one that potentiates reliably following homosynaptic stimulation), we paired activity of this pathway with a 'weak' thalamocortical pathway (one that does not potentiate when stimulated homosynaptically). Surprisingly, homosynaptic HFS caused a lasting depression of the field EPSP in the thalamocortical pathway. Analysis of this effect revealed that it was largely polysynaptic. Associative HFS (HFS applied to both pathways) not only failed to induce an LTP effect in the thalamocortical pathway, it increased the magnitude of the depression. Associative HFS did, however, facilitate LTP induction in the 'strong' callosal pathway. When comparing the effects of homosynaptic and associative LTD induction (HFS on one pathway anticorrelated with LFS on the other), we found that both protocols induced a similar magnitude of depression. These results show that HFS applied to the thalamocortical pathway causes a depression and this depression is enhanced, not reversed, by associative pairing with a strong input.