Enhanced synaptic plasticity four weeks after birth of new neurons in adult hippocampus support memory

Even though the very first observations suggesting that birth of new neurons, neurogenesis, does take place in the adult brain were documented as early as the 1960s, it was not until the early 1990s that converging lines of evidence confirmed this to be true. The functional role of neurogenesis has been less clear, and while results have been reported suggesting that hippocampal neurogenesis supports memory and learning, the underlying synaptic mechanisms have remained to a large extent an open question.

In their recent study Gu et al. (2012), by combining retroviral and optogenetic methods in adult mice, determined the time of birth of hippocampal dental granule cells, tracked the timescale over which these cells formed functional synaptic connections, and tested whether reversible inactivation of these cells adversely impacted memory performance. It was observed that adult-born neurons formed functional synapses onto area CA3 neurons two weeks after birth and that these projections became stable at about four weeks after birth. Reversible silencing these neurons at four weeks (but not two or eight weeks) after birth disrupted recall of a task that was learned while the neurons were intact and part of the hippocampal circuitry, suggesting that there is a specific time window within which adult-born hippocampal neurons support memory.

These highly exciting results shed light on the synaptic mechanisms that take place during neurogenesis, and suggest that there is a specific time window during which newborn neurons become integrated to hippocampal neuronal circuits and support learning and subsequent recall of a task. The methods used by the authors in their study constitute an astounding example of the possibilities that have become available for elucidation of the functionality of neuronal networks at the level of individual neurons and synaptic connections.

Reference: Gu Y, Arruda-Carvalho M, Wang J, Janoschka SR, Josselyn SA, Frankland PW, Ge S. Optical controlling reveals time-dependent roles for adult-born dentate granule cells. Nature Neuroscience (2012) advance online publication. http://dx.doi.org/10.1038/nn.3260