Stimulation of protein synthesis by optogenetic and chemical induction of excitatory synaptic plasticity in hippocampal somatostatin interneurons.

Somatostatin-expressing interneurons (SOM-INs) are a major subpopulation of GABAergic cells in CA1 hippocampus that receive excitation from pyramidal cells (PCs) and provide feedback control of synaptic inputs onto PC dendrites. Excitatory synapses from PCs onto SOM-INs (PC-SOM synapses) exhibit long-term potentiation (LTP) mediated by type 1a metabotropic glutamate receptors (mGluR1a). LTP at PC-SOM synapses translates in lasting regulation of metaplasticity of entorhinal and CA3 synaptic inputs on PCs and contributes to hippocampus-dependent learning. A persistent form of PC-SOM synapse LTP lasting hours is prevented by blockers of transcription and translation, and a more transient form of PC-SOM synapse LTP lasting tens of minutes requires mTORC1-signaling, suggesting an involvement of protein synthesis. However, the role of protein synthesis in these forms of plasticity has not been directly demonstrated. Here we use the SUrface SEnsing of Translation (SUnSET) assay of protein synthesis to directly show that the induction protocols for both forms of LTP at PC-SOM synapses stimulate protein synthesis in SOM-INs. Moreover, protein synthesis stimulated by persistent LTP induction was prevented in mice with a SOM-IN conditional knock-out of Raptor, an essential component of mTORC1, indicating a critical role of mTORC1 in the control of translation in PC-SOM synapse plasticity. Moreover, protein synthesis induced by both forms of LTP may share common mechanisms as transient LTP induction occluded further stimulation of protein synthesis by persistent LTP induction. Our findings highlight a crucial role of protein synthesis and its control by mTORC1 in SOM-INs that is important for hippocampus-dependent memory function.