Neocortical Synaptic Proliferation Following Forebrain-Dependent Trace Associative Learning

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Many behavioral studies have suggested that learning induces neocortical synaptic modifications. However, neocortical synaptic modifications following forebrain-dependent trace associative learning has not been closely examined. Acquisition of whisker-trace-eyeblink (WTEB) conditioning, a forebrain-dependent trace associative task, has been reported to modulate the expression of cytochrome oxidase, a marker for metabolic activity, in the conditioned barrels, suggesting that trace associative conditioning induces neocortical synaptic plasticity. However, neocortical synaptic plasticity has never been directly examined following this trace associative task. To assess neocortical synaptic modifications, the present study examined synapsin I expression following WTEB conditioning. Synapsin I is part of a phosphoprotein family involved in neuronal regulation of neurotransmitter release that also exhibits an expression pattern closely correlating to synapse number. Findings from this study demonstrated that synapsin I expression is elevated in primary somatosensory neocortex in trace-paired-conditioned mice compared with unpaired-conditioned (stimulation-control) mice and naïve mice, suggesting that WTEB conditioning induces synaptic proliferation. Additional findings from the present study examining cytochrome oxidase expression replicated previous findings demonstrating that WTEB conditioning induces a learning-specific expansion of the cytochrome oxidase staining expression for conditioned barrels. Together, these results suggest that synaptic proliferation is contributing to the learning-induced metabolic augmentation previously observed in conditioned barrels following WTEB conditioning. Furthermore, these results suggest that trace associative learning facilitates neocortical synaptic modification.

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