Conditional ablation of neuroligin-1 in CA1 pyramidal neurons blocks LTP by a cell-autonomous NMDA receptor-independent mechanism

Neuroligins are postsynaptic cell-adhesion molecules implicated in autism and other neuropsychiatric disorders. Despite extensive work, the role of neuroligins in synapse function and plasticity, especially NMDA receptor (NMDAR)-dependent LTP, remains unclear. To establish which synaptic functions unequivocally require neuroligins, we analyzed single and triple conditional knockout (cKO) mice for all three major neuroligin isoforms (NL1-NL3). We inactivated neuroligins by stereotactic viral expression of Cre-recombinase in hippocampal CA1 region pyramidal neurons at postnatal day 0 (P0) or day 21 (P21), and measured synaptic function, synaptic plasticity, and spine numbers in acute hippocampal slices 2–3 weeks later. Surprisingly, we find that ablation of neuroligins in newborn or juvenile mice only modestly impaired basal synaptic function in hippocampus, and caused no alteration in postsynaptic spine numbers. However, triple cKO of NL1-NL3 or single cKO of NL1 impaired NMDAR-mediated excitatory postsynaptic currents (NMDAR EPSCs), and abolished NMDAR-dependent LTP. Strikingly, the NL1 cKO also abolished LTP elicited by activation of L-type Ca(2+)-channels during blockade of NMDARs. These findings demonstrate that neuroligins are generally not essential for synapse formation in CA1 pyramidal neurons but shape synaptic properties and that NL1 specifically is required for LTP induced by postsynaptic Ca(2+)-elevations, a function which may contribute to the pathophysiological role of neuroligins in brain disorders.