As miRNAs generally bind to numerous target mRNAs, and many mRNAs are regulated by multiple miRNA species, the possibilities for fine orchestration of translation are enormous. Primary miRNA transcripts are processed in the nucleus by the RNAase III endonuclease Drosha to generate short-hairpin precursors of ∼70–100 nucleotides, which are exported from the nucleus and further processed by another RNAase family enzyme, Dicer, to produce a mature miRNA of ∼22 nucleotides in length. The activity of miRNAs may therefore be modulated at multiple steps in the biogenesis pathway as

well as through regulation of the miRNA-bound RISC (Ashraf et al., 2006; Kosik, 2006; Presutti et al., 2006; Kye et al., 2007; Winter et al., 2009). miRNAs play coordinating roles in a variety of cellular selleck kinase inhibitor processes, selleck including cell specification and apoptosis (Bartel, 2004; Chang et al., 2007). In neurons, recent studies have established roles for specific miRNAs in neurogenesis and dendritic spine morphogenesis (Vo et al., 2005; Krichevsky et al., 2006; Schratt et al., 2006; Cao et al., 2007; Fiore et al., 2009; Siegel et al., 2009). In the marine snail Aplysia, expression of miR-124 is linked to synapse-specific long-term sensitization (Rajasethupathy et al., 2009). In flies, degradation of the protein Armitage, a component of the miRNA-RISC,

promotes synaptic protein synthesis during long-term memory. Despite the advances in understanding neuronal miRNAs, little is known about miRNA regulation during activity-dependent synaptic plasticity in the adult mammalian brain. We therefore examined miRNA expression following induction of long-term potentiation (LTP) by high-frequency stimulation (HFS) of the perforant path input to the dentate gyrus of anesthetized rats. Using

miRNA expression profiling and quantitative oxyclozanide reverse transcription polymerase chain reaction (RT-PCR), we identified mature miRNAs with significantly increased (miR-132, miR-212) or decreased (miR-219) expression during LTP. Analysis of the primary and precursor transcripts demonstrated massive metabotropic glutamate receptor (mGluR)-dependent transcription of miR-132 and -212 in dentate granule cells that is functionally correlated with depotentiation rather than LTP. In contrast, activation of N-methyl-d-aspartate receptors (NMDAR) during LTP induction selectively downregulated mature miR-132, -212 and -219 levels, indicating stimulation of mature miRNA turnover. Animal experiments were carried out in accordance with the European Community Council Directive of 24 November 1986 (86/609/EEC) and approved by the Norwegian Committee for Animal Research. Experiments were performed on 45 adult male Sprague–Dawley rats. The electrophysiological procedures have been detailed elsewhere (Messaoudi et al., 2002; Panja et al., 2009).