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Changes in synaptic efficacy and morphology have been proposed as mechanisms underlying learning and memory processes. In our previous studies, high frequency stimulation (HFS) sufficient to induce LTP at the hippocampal mossy fiber (MF) pathway, leads to MF synaptogenesis, in a prominent contralateral form, at the stratum oriens of hippocampal CA3 area. Recently we reported that acute intrahippocampal microinfusion of BDNF induces a lasting potentiation of synaptic efficacy at the MF projection accompanied by a structural reorganization at the CA3 area within the stratum oriens region in a prominent ipsilateral form. It is considered that the capacity of synapses to express plastic changes is itself subject to variation dependent on previous experience. Here we used intrahippocampal microinfusion of BDNF to analyze its effects on functional and structural synaptic plasticity induced by subsequent mossy fiber HFS sufficient to induce LTP in adult rats, in vivo. Our results show that BDNF modifies the ability of the MF pathway to present LTP by HFS. Moreover BDNF modified the structural reorganization pattern produced by HFS, presenting a balanced bilateral appearance. Microinfusion of K252a blocks the functional and morphological effects produced by BDNF, revealing that the BDNF modulation is dependent on its TrkB receptor activation. These findings support the idea that BDNF actions modify subsequent synaptic plasticity; a homeostatic mechanism thought to be essential for synaptic integration among prolonged temporal domains in the adult mammalian brain., (Copyright © 2010 Wiley Periodicals, Inc., Inc.)

Synaptic plasticity has been proposed as a mechanism underlying learning and memory. Synaptic reorganization of hippocampal mossy fibers has been observed after experimentally induced epilepsy, and after brief high-frequency activation inducing long-term potentiation. Furthermore, it has been suggested that synaptic changes in the hippocampus may occur after spatial learning. In this study, by using a zinc-detecting histologic technique (Timm), we demonstrate a significant increase of mossy fiber terminals in the CA3 stratum oriens region induced by training rats during 3 days in a spatial Morris water maze. In contrast, animals trained for only 1 day and animals that were just allowed to swim or were overtrained in a stress-motivated inhibitory avoidance task did not show increments of mossy fiber terminals in the stratum oriens. Electron microscopy confirmed that synaptic density of mossy 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 task induces mossy fiber synaptogenesis that could be involved in the mechanisms underlying long-term memory storage. Hippocampus 1999;9:631-636.