Synaptogenesis of mossy fibers induced by spatial water maze overtraining

Synaptic plasticity has been proposed as a mechanism underlying learning an d memory. Synaptic reorganization of hippocampal mossy fibers has been obse rved after experimentally induced epilepsy, and after brief high-frequency activation inducing long-term potentiation. Furthermore, it has been sugges ted that synaptic changes in the hippocampus may occur after spatial learni ng. In this study, by using a zinc-detecting histologic technique (Timm), w e demonstrate a significant increase of mossy fiber terminals in the CA3 st ratum oriens region induced by training rats during 3 days in a spatial Mor ris water maze. In contrast, animals trained for only 1 day and animals tha t were just allowed to swim or were overtrained in a stress-motivated inhib itory avoidance task did not show increments of mossy fiber terminals in th e stratum oriens. Electron microscopy confirmed that synaptic density of mo ssy fiber terminals in the stratum oriens increases significantly in water maze overtrained animals compared with the swimming control animals. Taken together, these results suggest that overtraining in a spatial learning tas k induces mossy fiber synaptogenesis that could be involved in the mechanis ms underlying long-term memory storage. (C) 1999 Wiley-Liss, Inc.