, 2011 and De Kloet et al., 1988). More details about the pharmacology of this behavioral test were addressed recently elsewhere (Reul, 2014). As mentioned, until recently the mechanism of action of glucocorticoid hormone in this test was completely unknown. The neuroanatomical site of hormone action however has been known since 1988 when de Kloet and colleagues reported
that micro-injection of GR antagonist specifically into the dentate gyrus of the hippocampus impaired the behavioral immobility response (De Kloet et al., 1988). We recently elucidated how glucocorticoids via GRs are implemented in this process. We discovered that in addition to GRs, dentate gyrus N-methyl d-aspartate (NMDA) receptors activating the mitogen-activated check details protein kinase (MAPK) pathway are also involved (Gutierrez-Mecinas et al., 2011 and Chandramohan et al., 2008). Forced swimming results, via a sparse activation of NMDA receptors, in the specific phosphorylation of the MAPKs extracellular signal-regulated kinase 1 and 2 (ERK1/2; also termed p42/44-MAPK). pERK1/2 subsequently phosphorylates the two downstream
chromatin-modifying kinases mitogen- and stress-activated kinases 1 and 2 (MSK1/2) and ets-like kinase 1 and 2 (Elk1/2). pMSK1/2 was shown to phosphorylate histone H3 at serine10 (S10) whereas pElk1/2, via recruitment of histone acetyl-transferases (HATs) like p300, evoke the acetylation of lysine14 (K14), thus forming the combinatorial epigenetic marks H3S10p-K14ac (Gutierrez-Mecinas SCR7 et al., 2011 and Chandramohan et al., 2008). The formation of these epigenetic marks in the promoter region of intermediate-early genes (IEGs) like c-Fos and Egr-1 (also called NGFI-A or Zif268) over facilitated the induction of these genes
(Gutierrez-Mecinas et al., 2011). Injection of a GR-occupying dose of corticosterone was ineffective in terms of H3S10p-K14ac formation and IEG induction (Chandramohan et al., 2007), indicating indeed that, in addition to GR, activation of the NMDA receptor pathway is required. Previous work has shown that the H3S10p-K14ac mark is particularly involved in the opening of silent genes, possibly through chromatin remodeling, making them accessible for transcription (Cheung et al., 2000a, Cheung et al., 2000b and Nowak and Corces, 2000). The interesting notion may be extracted that these dual histone marks tag genes that were silent before the animal was stressed. Neuroanatomically it is of interest to note that the activation of this signaling and epigenetic pathway leading to IEG induction was specifically observed in sparsely distributed mature granule neurons located in the dorsal blade of the dentate gyrus of rats and mice (Bilang-Bleuel et al., 2005, Gutierrez-Mecinas et al., 2011, Chandramohan et al., 2007 and Chandramohan et al., 2008).