Plasticity and metaplasticity of adult rat hippocampal mossy fibers induced by neurotrophin-3.

Changes in synaptic efficacy and morphology are considered as the downstream mechanisms of consolidation of memories and other adaptive behaviors. In the last decade, neurotrophin-3 (NT-3) has emerged as one potent mediator of synaptic plasticity. In the adult brain, expression of NT-3 is largely confined to the hippocampal dentate gyrus (DG). Our previous studies show that application of high-frequency stimulation (HFS) sufficient to elicit long-term potentiation (LTP) at the DG-CA3 pathway as well as acute intrahippocampal microinfusion of brain-derived neurotrophin factor produce mossy fiber (MF) structural reorganization. Here, we show that intrahippocampal microinfusion of NT-3 induces a long-lasting potentiation of synaptic efficacy in the DG-CA3 projection accompanied by an MF structural reorganization of adult rats in vivo. It is considered that the capacity of synapses to express plastic changes is itself subject to variation depending on previous experience; taking into consideration the effects of NT-3 on MF synaptic plasticity, we thus used intrahippocampal microinfusion of NT-3 to analyse its effects on functional and structural plasticity induced by subsequent MF-HFS sufficient to induce LTP in adult rats, in vivo. Our results show that NT-3 modifies the ability of the MF pathway to present subsequent LTP by HFS, and modifies the structural reorganization pattern. The modifications in synaptic efficacy and morphology elicited by NT-3 at the MF-CA3 pathway were blocked by the presence of a Trk receptor inhibitor (K252a). These findings support the idea that NT-3 actions modify subsequent synaptic plasticity, a homeostatic mechanism thought to be essential for maintaining synapses in the adult mammalian brain.