Frequency-dependent impairment of hippocampal LTP from NMDA receptors with reduced calcium permeability.

Changes in postsynaptic Ca2+ levels are essential for alterations in synaptic strength. At hippocampal CA3-to-CA1 synapses, the Ca2+ elevations required for LTP induction are typically mediated by NMDA receptor (NMDAR) channels but a contribution of NMDAR-independent Ca2+ sources has been implicated. Here, we tested the sensitivity of different protocols modifying synaptic strength to reduced NMDAR-mediated Ca2+ influx by employing mice genetically programmed to express in forebrain principal neurons an NR1 form that curtails Ca2+ permeability. Reduced NMDAR-mediated Ca2+ influx did not facilitate synaptic depression in CA1 neurons of these genetically modified mice. However, we observed that LTP could not be induced by pairing low frequency synaptic stimulation (LFS pairing) with postsynaptic depolarization, a protocol that induced robust LTP in wild-type mice. By contrast to LFS pairing, similar LTP levels were generated in both genotypes when postsynaptic depolarization was paired with high frequency synaptic stimulation (HFS). This indicates that the postsynaptic Ca2+ elevation also reached threshold during HFS in the mutant, probably due to summation of NMDAR-mediated Ca2+ influx. However, only in wild-type mice did repeated HFS further enhance LTP. All tested forms of LTP were blocked by the NMDAR antagonist D-AP5. Collectively, our results indicate that only NMDAR-dependent Ca2+ sources (NMDARs and Ca2+-dependent Ca2+ release from intracellular stores) mediate LFS pairing-evoked LTP. Moreover, LTP induced by the first HFS stimulus train required lower Ca2+ levels than the additional LTP obtained by repeated trains.

Mice with a fra-1 knock-in into the c-fos locus show impaired spatial but regular contextual learning and normal LTP.

The immediate early gene c-fos is part of the AP-1 transcription factor complex, which is involved in molecular mechanisms underlying learning and memory. Mice that lack c-Fos in the brain show impairments in spatial reference and contextual learning, and also exhibit a reduced long-term potentiation of synaptic transmission (LTP) at CA3-to-CA1 synapses. In the present study, we investigated mice in which c-fos was deleted and replaced by fra-1 (c-fos(fra-1) mice) to determine whether other members of the c-fos gene family can substitute for the functions of the c-fos gene. In c-fos(fra-1) mice, both CA3-to-CA1 LTP and contextual learning in a Pavlovian fear conditioning task were similar to wild-type littermates, indicating that Fra-1 expression restored the impairments caused by brain-specific c-Fos depletion. However, c-Fos-mediated learning deficits in a reference memory task of the Morris watermaze were also present in c-fos(fra-1) mice. These findings suggest that different c-Fos target genes are involved in LTP, contextual learning, and spatial reference memory formation.

Intracellular domains of NMDA receptor subtypes are determinants for long-term potentiation induction.

NMDA receptors (NMDARs) are essential for modulating synaptic strength at central synapses. At hippocampal CA3-to-CA1 synapses of adult mice, different NMDAR subtypes with distinct functionality assemble from NR1 with NR2A and/or NR2B subunits. Here we investigated the role of these NMDA receptor subtypes in long-term potentiation (LTP) induction. Because of the higher NR2B contribution in the young hippocampus, LTP of extracellular field potentials could be enhanced by repeated tetanic stimulation in young but not in adult mice. Similarly, NR2B-specific antagonists reduced LTP in young but only marginally in adult wild-type mice, further demonstrating that in mature CA3-to-CA1 connections LTP induction results primarily from NR2A-type signaling. This finding is also supported by gene-targeted mutant mice expressing C-terminally truncated NR2A subunits, which participate in synaptic NMDAR channel formation and Ca2+ signaling, as indicated by immunopurified synaptic receptors, postembedding immunogold labeling, and spinous Ca2+ transients in the presence of NR2B blockers. These blockers abolished LTP in the mutant at all ages, revealing that, without the intracellular C-terminal domain, NR2A-type receptors are deficient in LTP signaling. Without NR2B blockade, CA3-to-CA1 LTP was more strongly reduced in adult than young mutant mice but could be restored to wild-type levels by repeated tetanic stimulation. Thus, besides NMDA receptor-mediated Ca2+ influx, subtype-specific signaling is critical for LTP induction, with the intracellular C-terminal domain of the NR2 subunits directing signaling pathways with an age-dependent preference.

Impaired long-term memory and NR2A-type NMDA receptor-dependent synaptic plasticity in mice lacking c-Fos in the CNS.

The immediate early gene c-fos is part of the activator protein-1 transcription factor and has been postulated to participate in the molecular mechanisms of learning and memory. To test this hypothesis in vivo, we generated mice with a nervous system-specific c-fos knock-out using the Cre-loxP system. Adult mice lacking c-Fos in the CNS (c-fosDeltaCNS) showed normal general and emotional behavior but were specifically impaired in hippocampus-dependent spatial and associative learning tasks. These learning deficits correlated with a reduction of long-term potentiation (LTP) in hippocampal CA3-CA1 synapses. The magnitude of LTP was restored by a repeated tetanization procedure, suggesting impaired LTP induction in c-fosDeltaCNS mice. This rescue was blocked by a selective inhibitor of NR2B-type NMDA receptors. This blockade was compensated in wild-type mice by NR2A-type NMDA receptor-activated signaling pathways, thus indicating that these pathways are compromised in c-fosDeltaCNS mice. In summary, our data suggest a role for c-Fos in hippocampus-dependent learning and memory as well as in NMDA receptor-dependent LTP formation.