The rapid development of structural plasticity through short water maze training: A DTI study

Diffusion MRI is sensitive to the microstructure of tissue and allows the study of structural plasticity over short time scales of only hours. The initial temporal and spatial progression of this process, however, has yet to be elucidated. With the aim of examining early temporal progression of structural plasticity, we subjected rats to short training periods on a task in the Morris water maze (MWM), a paradigm previously shown to induce rapid changes in diffusion tensor imaging (DTI) indices. Two groups of rats were each divided into subgroups that consecutively completed 1, 2 or 3 sets of short trials (up to 60 s) in the MWM. Each set comprised 4 trials (1 from each quadrant of the WMW), with a specific time interval between sets. To assess the effect of the duration of the task on the evolving changes in DTI indices, we allowed a rest of 45 min between sets in one group of rats and a 2-h rest in the other. All rats were scanned with a DTI protocol before and 45 min after their last trial. We found that a few minutes of training in a new task sufficed to generate changes in diffusion indices. The earliest changes in DTI (measured after one set of trials) progressed with further training (measured after two sets), but within a few more minutes (after three sets) they reached a plateau. Lengthening the duration of the overall task by prolonging the time interval between sessions did not alter this pattern of change, suggesting that at least within this short time scale such changes are task-dependent, but not time dependent. Our results demonstrate the progression of structural neuroplasticity at different stages of exposure to a novel experience, and show that DTI can be used to trace, in vivo, the localization of structural plasticity induced by training.