Intracellular mechanisms responsible for serotonin-mediated reversal of metabotropic glutamate receptor-mediated long-term depression (mGluR-LTD) in wild-type and Fmr1 KO mouse hippocampus

We have shown that activation of serotonin type 7 (5-HT7) receptors prevents metabotropic glutamate receptor-mediated endocytosis of AMPA receptors and subsequent long-term depression (mGluR-LTD) in the hippocampus of wild-type (WT) and Fmr1 KO mice, a mouse model of Fragile X Syndrome (FXS) in which mGluR-LTD is abnormally enhanced (Costa et al., Biol. Psych. 2012, 72:924-933). 5-HT7 receptors are classically coupled to stimulation of adenylate cyclase and protein kinase A (PKA) but other signalling pathways have been suggested, including the kinases Akt and glycogen synthase kinase 3 (GSK3). To identify the intracellular mechanisms involved in 5-HT7-mediated reversal of mGluR-LTD, we recorded AMPA receptor-mediated excitatory post-synaptic currents (EPSCAMPA) from CA1 pyramidal neurons under patch clamp on hippocampal slices from WT mice. Application of the group-I mGluR agonist DHPG induced a long-term depression (mGluR-LTD) of EPSCAMPA. When DHPG application was followed by application of either 5-HT or 8-OH DPAT (a mixed 5-HT1A/5-HT7 agonist), DHPG-induced mGluR-LTD was completely reversed. 5-HT-mediated reversal of mGluR-LTD persisted in the presence of intracellular cAMPS-Rp (a cAMP analogue which binds PKA without activating it) and was not mimicked by 8-Br-cyclic AMP. When lithium was included in the intracellular solution at a concentration (20 mM) that inhibits GSK3, the amount of DHPG-induced mGluR-LTD was not significantly different from control conditions, but the reversal effect of 8-OH-DPAT application on mGluR-LTD was blocked. These results indicate that reversal of mGluR-LTD by 5-HT7 receptor activation is not mediated by the cAMP/PKA pathway and likely involves inhibition of GSK3 signalling. In Fmr1 KO mice, GSK3 activity is abnormally elevated and lithium treatment was shown to reverse abnormal phenotypes typical of FXS (Min et al., 2009, Neuropharmacology 56: 463-72; Guo et al., 2012, Hum Mol Genet 21: 681-91). Consistently, we suggest that activation of 5-HT7 receptors, by inhibiting GSK3 activity, might correct the pathological features in Fmr1 KO mice and might be considered as a novel therapeutic strategy for FXS.