Conditional neuroligin-2 knockout in adult medial prefrontal cortex links chronic changes in synaptic inhibition to cognitive impairments.

Abnormal activity in the medial prefrontal cortex (mPFC) is consistently observed in neuropsychiatric disorders, but the mechanisms involved remain unclear. Chronic aberrant excitation and/or inhibition of mPFC neurons were proposed to cause cognitive impairments. However, direct evidence for this hypothesis is lacking because it is technically challenging to control synaptic properties in a chronic and locally restricted, yet specific, manner. Here, we generated conditional knockout (cKO) mice of neuroligin-2 (Nlgn2), a postsynaptic cell-adhesion molecule of inhibitory synapses linked to neuropsychiatric disorders. cKO of Nlgn2 in adult mPFC rendered Nlgn2 protein undetectable after already 2-3 weeks, but induced major reductions in synaptic inhibition after only 6-7 weeks, and caused parallel impairments in anxiety, fear memory and social interaction behaviors. Moreover, cKO of Nlgn2 severely impaired behavioral stimulation of immediate-early gene expression in the mPFC, suggesting that chronic reduction in synaptic inhibition uncoupled the mPFC from experience-dependent inputs. Our results indicate that Nlgn2 is required for continuous maintenance of inhibitory synapses in the adult mPFC, and that chronic impairment of local inhibition disengages the mPFC from its cognitive functions by partially uncoupling the mPFC from experience-induced inputs.