Behavioural adaptation in response to stressful events may involve gene transcription associated epigenetic mechanisms in the brain. We have previously shown that forced swimming (FS)-induced phosphorylation and phospho-acetylation of histone H3 in adult dentate gyrus (DG) granule neurons is associated with the acquisition of the behavioural immobility response [1]. However, the intracellular molecular mechanisms underlying this learned behavioural response have hardly been investigated. Therefore, we investigated the signalling pathways that mediate the FS-induced increase in histone H3 phospho-acetylation and the acquisition of the immobility response. In particular, we examined the role of the NMDA receptor (NMDAR), the mitogen-activated protein kinase kinase (MEK) and mitogen- and stress-activated protein kinase (MSK). Male Wistar rats were pre-treated with the NMDAR antagonist MK-801 (100µg/kg i.p.; n=6) or the MEK inhibitor SL327 (50mg/kg i.p.; n=6) before subjecting them to a FS session (15 min in 25°C water). We also subjected wild-type (n=6) and MSK1/2 double knock-out (KO; n=6) mice to FS. Respective control groups were not forced to swim. Animals were then killed after 2 h for immunohistochemical determination of histone H3 phospho-acetylation in dentate neurons (the numbers of positive neurons were counted and expressed as number/DG/10µm section/animal. In other experiments, animals were pre-treated as above, subjected to a FS session (15 min in 25°C water) followed by a second FS session (5 min in 25°C water) 24 h later to examine the behavioural immobility response. The immobility, swimming and struggling behaviour of the animal was scored in the test and the re-test. Our results show that FS induces significant increases in histone H3 phospho-acetylation in the DG of rats (vehicle/control: 10.5±0.8 (n=6); vehicle/FS 20.5±0.5 (n=6)) and wild-type mice (control: 4.6±0.3 (n=6); FS: 8.7±0.8 (n=6)), P<0.05, post-hoc Bonferroni tests. The FS-induced histone H3 modifications could be blocked by pre-treatment with MK-801 (MK-801/control: 8.2±0.7 (n=6) ; MK-801/FS 12.3±1.0 (n=6)), and SL327 (SL327/ control: 5.0±0.9 (n=6); SL327/FS: 9.4±0.8 (n=6)), as well as double genetic deletion of MSK1/2 (KO/control: 0.1±0.1 (n=6); KO/FS: 0.2±0.1 (n=6), P<0.05). Moreover, blockade of the forced swimming-induced epigenetic response resulted in an impaired acquisition of behavioural immobility as measured in the re-test (mean immobility scores (arbitrary units): control: 19.6±1.0; MK-801: 9.9±1.7; SL327: 15.7±0.6; wild-type: 24.7±1.4; KO: 14.5±1.0; all manipulations: P<0.05). Thus, acquisition of behavioural immobility after FS appears to involve phospho-acetylation of histone H3 in DG neurons via activation of the NMDA/MAPK/ERK/MSK pathway.