br Materials methods br Results br Discussion As
Materials & methods
Discussion As shown by qPCR analysis, Bdnf1 expression was up-regulated by approximately fifty percent after injection of RG108. A possible explanation is that the overall methylation in promoter of exon I is decreased by the compound, therefore facilitating its gene expression. The rodent Bdnf gene has a complex structure consisting of eight (I-VIII) 5′ non-coding exons, each driven by a specific promoter, and one 3′ coding exon (IX). Several splice variants have been described, all consisting of the 3′ coding exons and differing in the number of 5′ non-coding exons. Interestingly, multiple promoters are postulated to allow for spatio-temporal regulation of BDNF transcripts in the CNS (Pruunsild, Kazantseva, Aid, Palm, & Timmusk, 2007). Distinct sub-cellular distribution of the different splice variants and, subsequently, the BDNF protein, is achieved by restricted regulation of Bdnf mRNA trafficking (Chiaruttini, Sonego, Baj, Simonato, & Tongiorgi, 2008). For example, BDNF1 was shown to be expressed in the soma and dendrites of neuronal chloroquine phosphate receptor (Chiaruttini et al., 2009, Pattabiraman et al., 2005), where it contributes to the synthesis of neurotransmitters (Loudes, Petit, Kordon, & Faivre-Bauman, 1999) and the local synthesis of BDNF (Kang et al., 1996, Tongiorgi et al., 1997), respectively. Considering the important role of BDNF in neuronal functioning, we could speculate that the observed increase in Bdnf1 expression after treatment with RG108 could account for the pro-cognitive effect of the latter in short-term OPS. The observed increase in Bdnf1 mRNA occurred 1 h after treatment indicates participation of BDNF in early plasticity and subsequently mnemonic processes. Most of the evidence regarding the importance of BDNF for memory formation derived from electrophysiological studies that utilized the molecular correlate of memory, referred to as long-term potentiation (LTP) (Chen et al., 2010, Cunha et al., 2010, Dixon, 1959). LTP consists of a labile early phase (E-LTP), lasting 1 to 3 h, followed by a more stable late phase (L-LTP) characterized by protein synthesis (Reymann & Frey, 2007). Although there is a plethora of studies establishing the importance of BDNF signaling for the L-LTP and subsequently long-term memory (Cunha et al., 2010, Edelmann et al., 2014, Reymann and Frey, 2007), it has been shown that BDNF is also important for the early phase of LTP. Considering the time point that we measure gene expression, our results underscore the involvement of BDNF in the early phase of memory formation. There is a growing body of evidence indicating the importance of pre- or post-synaptic BNDF signaling for E-LTP and early memory processes (for a review see Edelmann et al., 2014). The differential findings regarding the site of BDNF action could be the result of different stimulation protocols or stimulation of different areas in the hippocampus. A study from Mohajerani et al. reported a distinct role of BDNF signaling in the different phases of LTP (Mohajerani, Sivakumaran, Zacchi, Aguilera, & Cherubini, 2007). Specifically, it was shown that during E-LTP, BDNF acts at the presynaptic cell to enhance neurotransmitter release, while at the L-LTP its action is mainly located at the postsynaptic site, where it promotes protein synthesis. Another study showed that the transcription of Bdnf gene occurs within thirty minutes upon stimulation of cortical cell culture neurons (Tao, Finkbeiner, Arnold, Shaywitz, & Greenberg, 1998). The above observation regarding immediate transcription of Bdnf in vitro was subsequently confirmed in a memory paradigm in rats. In more detail, it was shown that Bdnf is immediately transcribed in the CA1 region of the hippocampus during contextual learning in the fear conditioning test (Hall, Thomas, & Everitt, 2000). Although the role of BDNF in mnemonic processes is well established, there is scarcity of evidence associating BDNF with pattern separation. Nevertheless, a recent study has shown that BDNF in the DG has an eminent role in the early memory processes of pattern separation (Bekinschtein et al., 2013). Specifically, intra-DG blockage of BDNF, either before or after the acquisition phase of the pattern separation task, impaired rats’ ability to separate similar representations. Importantly, biochemical analysis showed an increase in BDNF protein levels in the DG when animals were sacrificed within 1 h after learning the pattern separation task. Finally, intrahippocampal injection of human recombinant BDNF, after the acquisition phase of the pattern separation task, enhanced discrimination of similar environmental cues. These findings provide ground evidence for the involvement of BDNF in pattern separation memory (Bekinschtein et al., 2013). Also the role of BDNF during pattern separation processes was shown to be mediated by interaction of BDNF with adult-born immature cells in the DG (Bekinschtein et al., 2014). Our study corroborates the above findings regarding the role of BDNF in pattern separation and provides further evidence for the involvement of Bdnf1 during the early memory processes of this mnemonic task.