Sensitivity of N-Methyl-d-Aspartate Receptor–Mediated Excitatory Postsynaptic Potentials and Synaptic Plasticity to TCN 201 and TCN 213 in Rat Hippocampal Slices

Abstract

Whereas ifenprodil has been used as a selective GluN1/GluN2B (NR1/NR2B, B-type) receptor antagonist to distinguish between GluN2B (NR2B) and GluN2A (NR2A)-containing N-methyl-Daspartate receptors (NMDARs), Contrary to this prediction, inhibition of NMDAR EPSPs by the TCN compounds and ifenprodil were largely overlapping in the CA1 region of hippocampal slices from 30-day-old rats. After partial inhibition by ifenprodil, TCN compounds produced little further suppression of NMDAR EPSPs. Similarly, after partial inhibition by TCN compounds ifenprodil failed to further suppress NMDAR EPSPs. However, low micromolar D-2-amino-5-phosphonovalerate, a competitive NMDAR antagonist, which alone only partially inhibits NMDAR EPSPs, markedly suppresses residual NMDAR responses in the presence of ifenprodil or the TCNs, suggesting that low 2-amino-5-phosphonovalerate antagonizes both ifenprodil-and TCN-insensitive synaptic NMDARs. These observations can be most readily interpreted if ifenprodil and TCNs act on a similar population of synaptic NMDARs. Recent lines of evidence suggest that the majority of hippocampal synaptic NMDARs are triheteromers. If so, modulation of GluN2A, and not just GluN2B NMDARs, could dampen long-term depression (LTD). Indeed, both TCNs, like ifenprodil, blocked LTD, suggesting the involvement of ifenprodil-and TCN-sensitive NMDARs in LTD induction. However, the TCNs plus ifenprodil failed to inhibit long-term potentiation (LTP), suggesting that neither ifenprodil-nor TCN-sensitive NMDARs are essential for LTP induction.

Physiological activation of mGlu5 receptors supports the ion channel function of NMDA receptors in hippocampal LTD induction in vivo

“…The finding that a dose of NVP that completely inhibited LTP, given alone did not affect LFS-induced LTD, is consistent with previous in vivo findings that LTP is more sensitive to NVP than paired burst-induced LTD 38 , 39 . However, it does not rule out roles for GluN1/GluN2A diheteromers or GluN1/GluN2A/GluN2B triheteromeric NMDARs 40 , 41 , 75 – 78 . The inability of a relatively low dose of NVP to inhibit LTD when given in combination with MTEP contrasts with our finding of a lower dose requirement for Ro 25-6981 in the presence of MTEP.…”

Long-term depression is independent of GluN2 subunit composition

“…For GluN1/GluN2B receptors, ifenprodil and Ro 25-6881 antagonists are selective negative allosteric modulators that bind to the extracellular N-terminal domains (Hatton and Paoletti, 2005). Some studies have reported block of LTD by ifenprodil or Ro 25-6981 (Liu et al, 2004;Massey et al, 2004;Fox et al, 2006;Izumi et al, 2006;Gerkin et al, 2007;Ge et al, 2010;Dong et al, 2013;Izumi and Zorumski, 2015;Mizui et al, 2015;Yasuda and Mukai, 2015), though others report no effect (Hendricson et al, 2002;Bartlett et al, 2007;Li et al, 2007;Morishita et al, 2007;Kollen et al, 2008;Hanson et al, 2015;Yasuda and Mukai, 2015). However, these inhibitors display partial activitydependence and only block a fraction (~80%) of synaptic GluN1/GluN2B diheteromers (Fischer et al, 1997;Hatton and Paoletti, 2005;Gray et al, 2011), which could result in variable effects based on drug concentration, slice activity, and pre-incubation time.…”

“…Recently, more selective GluN2A inhibitors have been developed (e.g. TCN201) (Bettini et al, 2010;McKay et al, 2012) that block LTD (Izumi and Zorumski, 2015). Interestingly, these inhibitors have been shown to bind allosterically to the dimer interface between GluN1 and GluN2 (Hansen et al, 2012) which may impair conformational-based signaling.…”

Nampt is required for long-term depression and the function of GluN2B subunit-containing NMDA receptors

“…However, we recently showed that inhibition of NMDAR EPSPs by ifenprodil and TCN 213 largely overlapped in the CA1 region of hippocampal slices from 30-day-old rats [39]. We speculated that ifenprodil and TCN 213 may act on the same subset of GluN2A/GluN2B triheteromers [39]. Like ifenprodil, TCN 213 (10 µM) partially depressed NMDAR-mediated fEPSPs in all control mice tested (Fig.…”

“…TCN 213 [N-(cyclohexylmethyl)-2-[{5 [(phenylmethyl)amino]-1,3,4-thiadiazol-2-yl}thio]acetamide] was initially thought to be a selective antagonist of GluN2A–containing NMDARs [5]. However, we recently showed that inhibition of NMDAR EPSPs by ifenprodil and TCN 213 largely overlapped in the CA1 region of hippocampal slices from 30-day-old rats [39]. We speculated that ifenprodil and TCN 213 may act on the same subset of GluN2A/GluN2B triheteromers [39].…”