Effect of transgenic overexpression of NR2B on NMDA receptor function and synaptic plasticity in visual cortex

The NMDA receptor (NMDAR) is a heteromer comprised of NR1 and NR2 subunits. Mice that overexpress the NR2B subunit exhibit enhanced hippocampal LTP, p rolonged NMDAR currents, and improved memory (Tang et al., 1999). In the cu rrent study, we explored visual cortex plasticity and NMDAR function in NR2 B overexpressing transgenic mice. Unlike the hippocampus, in vitro synaptic plasticity of the visual cortex was unaltered by NR2B overexpression. Cons istent with the plasticity findings, NMDAR excitatory postsynaptic current (EPSC) durations from layer 2/3 pyramidal cells were similar in wild-type ( wt) and transgenic (tg) mice. Furthermore, temporal summation of NMDAR EPSC s to 10, 20, and 40 Hz stimulation did not differ between cells from wt and tg mice. Finally, although in situ studies clearly demonstrate overexpress ion of NR2B mRNA in visual cortex, we failed to observe a significant eleva tion in the synaptic expression of NR2B protein. We conclude that the synap tic ratio of NR2B over NR2A in the NMDA receptor complex in the visual cort ex is not significantly influenced by the transgene overexpression. These d ata suggest that mRNA availability is not a limiting factor for the synthes is of NR2B protein in the visual cortex, and support the hypothesis that le vels of NR2A, rather than NR2B. normally determine the subunit composition of NMDARs in visual cortex. (C) 2001 Elsevier Science Ltd. All rights reser ved.