Ultrastructural analysis reveals avoidance conditioning to induce a transient increase in hippocampal dentate spine density in the 6 hour post-training period of consolidation

Abstract

Concepts underlying memory consolidation invoke change in synapse structure and function. Such concepts relate to change in connectivity pattern enabled by increased synapse number, change in synaptic configuration resulting from overproduction and selective pruning, or structural change in synapse transmission zones. This study undertook the unbiased estimation of learning associated change in dendritic spine number on granule cells in the hippocampal dentate gyrus. Rats were trained to acquire a passive avoidance response after which spine number in the mid-molecular layer of the dorsal dentate gyrus were estimated at increasing post-training times. This showed there to be an increase in spine density with time after training which was initiated at 3 h, and maximal at 6 h. The increase at this latter time was not detected in passive control animals. At 72 h post-training spine density was seen to return to basal levels. These results are consistent with the various models for synapse connectivity change in memory formation whether they relate to altered number or connectivity pattern.