ENVIRONMENTAL ENRICHMENT PROMOTES SPARSE CODING IN HIPPOCAMPUS VIA INCREASED DENDRITIC INHIBITION

Environmental enrichment, defined by a combination of cognitive, sensory and physical stimulation, has been shown to improve learning and memory. To understand this effect, the focus was set to a larger extent on studying excitatory transmission in hippocampal circuits of enriched animals. However, the effect of enrichment on hippocampal inhibition remains unclear.
Using cFos labeling, we first showed that spatial exploration leads to increased cFos activity in hippocampal principal neurons in mice housed under both standard and enrichment conditions, with remarkably lower number of cFos-expressing cells observed in enriched mice. In vivo Ca²⁺ imaging from CA1 pyramidal cells (PCs) using miniature microscope in mice during exploration revealed lower firing frequency and lower number of active PCs in enriched mice. Furthermore, place cell firing was more sparse and spatially more selective. Next, we found that enrichment leads to increased activation of somatostatin-expressing (SOM) interneurons during exploration due to larger number of glutamatergic synapses onto SOM interneurons. To understand how it translates in vivo, we performed simultaneous imaging from CA1 PCs and optogenetic silencing of SOM interneurons in mice during exploration. As a result, we found that SOM interneurons exhibit stronger control over the activity of CA1 PCs in enriched mice.
Taken together, our data show that environmental enrichment decreases the size of hippocampal cell assemblies via enhanced recruitment of dendrite-targeting SOM interneurons. It leads to sparse coding, which increases memory capacity and minimize interference between memory items, providing an advantage for hippocampal processing.