Rapid homeostatic downregulation of LTP by extrasynaptic GluN2B receptors.

Although the activation of extrasynaptic GluN2B-containing N-methyl-d-aspartate (NMDA) receptors has been implicated in neurodegenerative diseases, such as Alzheimer's and Huntington's disease, their physiological function remains unknown. In this study, we found that extrasynaptic GluN2B receptors play a homeostatic role by antagonizing long-term potentiation (LTP) induction under conditions of prolonged synaptic stimulation. In particular, we have previously found that brief theta-pulse stimulation (5 Hz for 30 s) triggers robust LTP, whereas longer stimulation times (5 Hz for 3 min) have no effect on basal synaptic transmission in the hippocampal CA1 region. Here, we show that prolonged stimulation blocked LTP by activating extrasynaptic GluN2B receptors via glutamate spillover. In addition, we found that this homeostatic mechanism was absent in slices from the SAP102 knockout, providing evidence for a functional coupling between extrasynaptic GluN2B and the SAP102 scaffold protein. In conclusion, we uncovered a rapid homeostatic mechanism that antagonizes LTP induction via the activation of extrasynaptic GluN2B-containing NMDA receptors. NEW & NOTEWORTHY Although long-term potentiation (LTP) is an attractive model for memory storage, it tends to destabilize neuronal circuits because it drives synapses toward a maximum value. Unless opposed by homeostatic mechanisms operating through negative feedback rules, cumulative LTP could render synapses unable to encode additional information. In this study, we uncovered a rapid homeostatic mechanism that antagonizes LTP induction under conditions of prolonged synaptic stimulation via the activation of an extrasynaptic GluN2B-SAP102 complex.