Elsevier

Neuroscience

Volume 80, Issue 1, 25 June 1997, Pages 69-77
Neuroscience

Ultrastructural synaptic correlates of spatial learning in rat hippocampus

https://doi.org/10.1016/S0306-4522(97)00125-5Get rights and content

Abstract

Memory formation is believed to alter neural circuitry at the synaptic level. Although the hippocampus is known to play an important role in spatial learning, no experimental data exist on the synaptic correlates of this process at the ultrastructural level. Here, we have employed quantitative electron microscopy in order to compare the density, size and spatial arrangement of synapses in the dentate gyrus, and in area CA1, of spatially trained (water maze, invisible platform) versus control (visible platform) rats. No training-associated changes of hippocampal volume were found using a stereological estimation (disector) of the volume density of dentate granule, or CA1 pyramidal cells. Nor were changes found in either density, or sizes of synapses (spinous or dendritic), in CA1 or dentate gyrus. However, analysis of synaptic spatial distribution showed a training-associated increase in the frequency of shorter distances (i.e. clustering) between synaptic active zones in CA1, but not dentate, thus indicating alterations in local neural circuitry.

This finding indicates subtle changes in synaptic organisation in area CA1 of the hippocampus following a learning experience, suggesting that spatial memory formation in mammalian hippocampus may involve topographical changes in local circuitry without synapse formation de novo.

Section snippets

Spatial training in the rat

Seventeen male Sprague–Dawley rats (350–400 g) were employed. The water maze training protocol has been described previously.[18]Briefly, in the control, i.e. the non-spatial learning group (eight animals), a rat was released into the pool, at a pseudo-random location, and trained to find a visible escape platform. In the spatial learning group (nine rats), the platform was submerged so the animals once having found the platform, would subsequently have to rely on visible cues outside the maze.

Analysis of behavioural parameters

In the water maze, the decay of the escape latency with further training can be considered an indicator of the learning progress. We quantified the animal's performance in the water maze using the direct relationship between time of session and the escape latency decay, that is, as an exponential regression:t=t0exp(−αT)where: t is the escape latency; T, time of training session; t0, “expected” initial escape latency; and α is the decay constant of the escape latency, which we consider as a

Discussion

The present study has employed a behavioural paradigm in which a comparison is made between a group of animals trained for a spatial memory task, and a group trained in conditions which do not require spatial memory, but are otherwise similar. Thus, animals in the control group were likely to experience motor activity, stress, visual and other sensory input matched with that in the test group, with the exception of the learning component related to the spatial task. However, it cannot be

Conclusion

In our experiments, training of rats on a hippocampal-dependent spatial learning task did not produce significant long-term changes in density or sizes of synapses, in either dentate gyrus, or CA1 areas of hippocampus when morphometric analyses were carried out on animals perfused six days after the last training session. However, more subtle, but significant, changes were found which indicate local re-arrangement of synaptic connections in CA1 associated with learning. These results provide

Acknowledgements

The study was partly funded by BBSRC Grant S02085 to MGS, by the Wellcome Trust and the European Neuroscience Program.

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