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Abstract

The ability to form precise, episodic memories develops with age, with young children only able to form gist-like memories that lack precision. The cellular and molecular events in the developing hippocampus that underlie the emergence of precise, episodic-like memory are unclear. In mice, the absence of a competitive neuronal engram allocation process in the immature hippocampus precluded the formation of sparse engrams and precise memories until the fourth postnatal week, when inhibitory circuits in the hippocampus mature. This age-dependent shift in precision of episodic-like memories involved the functional maturation of parvalbumin-expressing interneurons in subfield CA1 through assembly of extracellular perineuronal nets, which is necessary and sufficient for the onset of competitive neuronal allocation, sparse engram formation, and memory precision.

Editor’s summary

The hippocampal episodic memory system is not present at birth but develops during childhood. Ramsaran et al. examined hippocampal engrams in juvenile and adult mice and identified a cascade of events that underlie the emergence of episodic-like memory precision. The immature hippocampus forms dense engrams and imprecise memories. The ability to form sparse engrams does not emerge until the fourth postnatal week with the maturation of inhibitory circuits in the hippocampus. The maturation of perineuronal nets, extracellular matrix structures primarily ensheathing the soma and proximal dendrites of parvalbumin-containing interneurons, helps to drive inhibitory interneuron maturation in the cortex and hippocampus. —Peter Stern