Ex vivo depotentiation of conditioning-induced potentiation at thalamic input synapses onto the lateral amygdala requires GluN2B-containing NMDA receptors

Abstract

We have previously characterized the ex vivo depotentiation (depotentiationex vivo) of conditioning-induced synaptic potentiation at thalamic input synapses onto the lateral amygdala (T-LA synapses) as a potential cellular substrate for fear extinction: both depotentiationex vivo and fear extinction require NMDA receptors, mitogen-activated protein kinases, metabotropic glutamate receptor 1, de novo protein synthesis and AMPA receptor internalization in the amygdala. Surprisingly, as shown in our and other previous studies, ifenprodil, an antagonist of GluN2B-containing NMDA receptors, fails to inhibit depotentiationex vivo at a saturating concentration (10μM), although it has been suggested that GluN2B-containing NMDA receptors are required for fear extinction. Because ifenprodil is also known to act on other molecular targets in addition to GluN2B-containing NMDA receptors, especially at high concentrations (i.e., ≥10μM), the ineffectiveness of 10μM of ifenprodil may be due to its side effects. Therefore, in the present study, we tested Ro25-6981, a more specific antagonist of GluN2B-containing NMDA receptors, and a lower concentration (3μM) of ifenprodil, which may reduce any possible side effects. Ro25-6981 (3μM) blocked both depotentiationex vivo and late-phase long-term potentiation at T-LA synapses. While 10μM ifenprodil failed to inhibit depotentiationex vivo, a lower concentration (3μM) of ifenprodil blocked depotentiationex vivo. Together, our findings suggest that depotentiationex vivo requires GluN2B-containing NMDA receptors.