The effects of changing NMDA receptor subunit composition on synaptic plasticity in the hippocampus were analyzed by creating transgenic mice overexpressing NR2D, a predominantly embryonic NMDA receptor subunit. NMDA-evoked currents in the transgenic mice had smaller amplitudes and slower kinetics. The transgenics also displayed age-dependent deficits in synaptic plasticity in area CA1 of the hippocampus. Long-term depression was selectively impaired in juvenile mice when NR2D overexpression

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... moderate. In mature mice, overexpression of NR2D was associated with a reduction of both NR2B and Ca 2ϩ -independent activity of Ca 2ϩ -and calmodulin-dependent protein kinase II. These biochemical changes were correlated with a marked impairment of NMDAdependent long-term potentiation, but spatial behavior was normal in these mice. These results show that the developmental regulation of NMDA receptor subunit composition alters the frequency at which modification of synaptic responses occur after afferent stimulation. Long-term potentiation (LTP) and long-term depression (LTD) are two forms of plasticity that have been studied extensively in area CA1 of the hippocampus. Both LTP and LTD, triggered by either high-frequency stimulation (HFS) or low-frequency stimulation (L FS) of the Schaffer collateral CA1 synapses, involve calcium influx through NMDA receptors (Collingridge et al., 1983; Dudek and Bear, 1992; Mulkey and Malenka, 1992) . A recent analysis of mice genetically modified to express higher levels of the C a 2ϩ -independent form of C a 2ϩ -and calmodulindependent protein kinase II␣ (C aM K II␣) suggests that the threshold of stimulus frequency required to elicit an increase or decrease of synaptic strength can be modulated by the amount of Ca 2ϩ -independent activity of C aM K II (Mayford et al., 1995) . This work provides evidence that LTP and LTD share the same downstream pathway from the NMDA receptor activation and that C aM K II is one of the regulators of the Hebbian synaptic modification. A theoretical study has pointed out that the Ca 2ϩ increase in the postsynaptic site after HFS can be altered dramatically by several factors, including NMDA receptor channel kinetics (Gold and Bear, 1994) . This model predicts that a change in the decay constant of NMDA receptor channels of 100 msec can induce a 10-fold difference in the peak C a 2ϩ concentration in the dendritic spine. Thus, modulation of NMDA receptor properties might also be an efficient site for the frequency-dependent regulation of synaptic modification. The NMDA receptors in the brain are complexes of NR1 and NR2 subunits . The NR1 subunit is essential for NMDA receptor function and is expressed ubiquitously in the brain (Moriyoshi et al., 1991; Flint et al., 1997) . In contrast, NR2A-D subunits have distinct expression profiles that are regulated both developmentally and regionally (Kutsuwada et al., 1992; Watanabe et al., 1992; Monyer et al., 1994) . The duration of NMDA receptor-mediated excitatory postsynaptic currents becomes progressively shorter in parallel with the upregulation of the NR2A subunit in the rat cortex (Carmignoto and Vicini, 1992; Sheng et al., 1994) . These data raise the possibility that NR2 subunits regulate synaptic plasticity. In this sense, analysis of the synaptic plasticity of neurons containing the NR2D subunits is intriguing because the NR2D subunit forms channels that have much longer deactivation time and lower NMDA-mediated currents in a heterologous expression system (Monyer et al., 1994) . NR2D is abundant in the embryonic CNS and abruptly lost in the postnatal forebrain (Watanabe et al., 1992; Monyer et al., 1994) . To define the effects of inserting the NR2D subunit into the NMDA receptor complex, we made transgenic mice overexpressing NR2D in mature forebrain. CA1 pyramidal neurons of transgenic mice showed NMDA-dependent currents that were slower and smaller than in controls. In transgenic animals, Schaffer collateral CA1 synapses showed age-dependent impairment of LTP and LTD. There were no deficits in spatial memory tasks in transgenic mice. MATERIALS AND METHODS Generation of transgenic mice. An 8.5 kb upstream region of the CaMKII␣ gene was replaced with cytomegalovirus (C M V) promoter of pCMV␤ vector (C lontech, C ambridge, UK), and the rat cDNA of NMDAR2D was subcloned into the NotI site of this plasmid. The insert