Morphological changes in hippocampal dentate gyrus synapses following spatial learning in rats are transient

The hippocampus is believed to play a crucial role in the formation of memory for spatial tasks. In the present study quantitative electron microscopy was used to investigate morphological changes in the hippocampal dentate gyrus of 3-month-old male rats at 3, 9 and 24 h after training to find a hidden platform in a Morris water maze. Average escape latency (time taken to reach the platform) in all trained groups decreased progressively with increased training but data from a probe trial (quadrant analysis test) at the end of training indicated that only animals in the 9- and 24-h groups, not the 3-h group, displayed significant retention of platform location. Unbiased stereological methods were used to estimate synapse and neuronal density at each time point after training. The majority of synapses had unperforated postsynaptic densities, were localized on small dendritic spines and were classed as axo-spinous. In comparison to age-matched untrained rats, significant but transient increases were observed in axo-spinous synapse density and synapse-to-neuron ratio 9 h after the start of training, but not at earlier (3 h) or later (24 h) times. These changes at 9 h post-training were accompanied by transient decreases in both mean synaptic height and area of postsynaptic density. No such changes were observed in an exercise-matched control group of rats, indicating that the transient synaptic changes in the dentate gyrus are most likely to be specifically related to processes involved in memory formation for the spatial learning task.