Schnell_2012_PLoS.One_7_e48045

Adult-born dentate granule cells integrate into the hippocampal network extend neurites and form synapses in otherwise mature tissue Excitatory and inhibitory inputs innervate these new granule cells in a stereotyped temporally segregated manner which presents a unique opportunity to study synapse development in the adult brain To examine the role of neuroligins as synapse-inducing molecules in vivo we infected dividing neural precursors in adult mice with a retroviral construct that increased neuroligin-1 levels during granule cell differentiation By 21 days post-mitosis exogenous neuroligin-1 was expressed at the tips of dendritic spines and increased the number of dendritic spines Neuroligin-1-overexpressing cells showed a selective increase in functional excitatory synapses and connection multiplicity by single afferent fibers as well as an increase in the synaptic AMPA/NMDA receptor ratio In contrast to its synapse-inducing ability in vitro neuroligin-1 overexpression did not induce precocious synapse formation in adult-born neurons However the dendrites of neuroligin-1-overexpressing cells did have more thin protrusions during an early period of dendritic outgrowth suggesting enhanced filopodium formation or stabilization Our results indicate that neuroligin-1 expression selectively increases the degree but not the onset of excitatory synapse formation in adult-born neurons.