Following up on our previous post about parvalbumin interneurons, this time we look at memory consolidation and somatostatin interneurons. Another type of interneuron that express a peptide- somatostatin.
This recent paper shows how somatostatin interneurons (SST) of the hippocampus control the size of neuronal ensembles that are important for memory.
The team subjected mice to a spatial exploration task, which activates a small group of granule cells, the principal exciatory cells in the dentate gyrus of the hippocampus. The activated cells can be identified by the expression of immediate early gene– c-fos. This group of cells are referred to as the engram. The activation of this engram has been shown to be important for the retrieval of the memory it encodes. The paper discusses what determines the size of the engram.
The authors activate the granule cells using channelrhodopsin and silence them with designer channels, which can be silenced with specific drugs- DREADD (Designer receptors exclusively activated by designer drugs). They then quantify the number of activated cells by looking at cells that express c-fos, which represents the size of the engram. Surprisingly they found that if they silence the granule cells, the size of the engram increases. This seems to beneficial, as silencing the granule cells seems to strengthen the contextual memory in fear conditioning tasks.
The authors go on to show this is because of reduced lateral inhibition, which is mediated by somatostatin expressing interneurons.