Excitatory and inhibitory inputs converge on single neurons and are integrated into a coherent output. Although much is known about short-term integration, little is known about how neurons sum opposing signals for long-term synaptic plasticity and memory storage. In Aplysia, we find that when a sensory neuron simultaneously receives inputs from the facilitatory transmitter 5-HT at one set of synapses and the inhibitory transmitter FMRFamide at another, long-term facilitation is blocked and synapse-specific long-term depression dominates. Chromatin immunoprecipitation assays show that 5-HT induces the downstream gene C/EBP by activating CREB1, which recruits CBP for histone acetylation, whereas FMRFa leads to CREB1 displacement by CREB2 and recruitment of HDAC5 to deacetylate histones. When the two transmitters are applied together, facilitation is blocked because CREB2 and HDAC5 displace CREB1-CBP, thereby deacetylating histones.

Synapse-specific facilitation requires rapamycin-dependent local protein synthesis at the activated synapse. In Aplysia, rapamycin-dependent local protein synthesis serves two functions: (1) it provides a component of the mark at the activated synapse and thereby confers synapse specificity and (2) it stabilizes the synaptic growth associated with long-term facilitation. Here we report that a neuron-specific isoform of cytoplasmic polyadenylation element binding protein (CPEB) regulates this synaptic protein synthesis in an activity-dependent manner. Aplysia CPEB protein is upregulated locally at activated synapses, and it is needed not for the initiation but for the stable maintenance of long-term facilitation. We suggest that Aplysia CPEB is one of the stabilizing components of the synaptic mark.

Neuroligin-1 is a potent trigger for the de novo formation of synaptic connections, and it has recently been suggested that it is required for the maturation of functionally competent excitatory synapses. Despite evidence for the role of neuroligin-1 in specifying excitatory synapses, the underlying molecular mechanisms and physiological consequences that neuroligin-1 may have at mature synapses of normal adult animals remain unknown. By silencing endogenous neuroligin-1 acutely in the amygdala of live behaving animals, we have found that neuroligin-1 is required for the storage of associative fear memory. Subsequent cellular physiological studies showed that suppression of neuroligin-1 reduces NMDA receptor-mediated currents and prevents the expression of long-term potentiation without affecting basal synaptic connectivity at the thalamo-amygdala pathway. These results indicate that persistent expression of neuroligin-1 is required for the maintenance of NMDAR-mediated synaptic transmission, which enables normal development of synaptic plasticity and long-term memory in the amygdala of adult animals.synaptic plasticity ͉ neuroligin ͉ autism S everal studies have found that synaptically localized cell adhesion molecules not only trigger synapse formation but also play a major role in regulating both basal synaptic transmission and synaptic plasticity (1, 2). Among them, neurexins and neuroligins (NLs), which undergo a heterophilic interaction with each other, have emerged as important organizers of de novo synapse formation (3). Moreover, modifying the interaction of neuroligin-1 and PSD-95 alters the balance of neuronal excitation and inhibition required for normal brain function (4). The indispensable role of neuroligins for proper neuronal connectivity is further supported by the genetic linkage of neuroligin mutations with autism, a disease that is thought to be a disorder in social cognition that critically involves the amygdala (5, 6).Because neuroligins are present both during development and throughout adulthood (7,8), it is likely that neuroligins play roles other than that of an inducer of synaptogenesis in the adult brain. Indeed, a recent study of knockout (KO) mice deficient in neuroligin-1 demonstrated that neuroligin-1 regulates excitatory synaptic responses (9). Although neuroligin-1 has been suggested to be essential for maintaining normal N-methyl-D-aspartate (NMDA)-type glutamate receptor-mediated currents (9), the underlying mechanism and its physiological consequence remain to be identified. Furthermore, because the regulation of NMDA receptor (NMDAR) is critical for long-term synaptic modification (10), alterations of NMDAR-dependent currents regulated by neuroligin-1 are likely to have effects on synaptic plasticity and long-term memory in adult animals.To address the functional role of neuroligin-1 at existing mature synapses, we used virus-mediated RNA interference to deplete endogenous neuroligin-1 in the lateral nucleus of the amygdala (LA) of adult animals. We investigated the actions...

The Alzheimer's disease-related ␤-amyloid precursor protein (␤-APP) is metabolized to a number of potentially amyloidogenic peptides that are believed to be pathogenic. Application of relatively low concentrations of the soluble forms of these peptides has previously been shown to block high-frequency stimulation-induced long-term potentiation (LTP) of glutamatergic transmission in the hippocampus. The present experiments examined how these peptides affect low-frequency stimulationinduced long-term depression (LTD) and the reversal of LTP (depotentiation). We discovered that ␤-amyloid peptide (A␤1-42) and the A␤-containing C -terminus of ␤-APP (CT) facilitate the induction of LTD in the CA1 area of the intact rat hippocampus. The LTD was frequency-and NMDA receptor-dependent. Thus, although low-frequency stimulation alone was ineffective, after intracerebroventricular injection of A␤1-42, it induced an LTD that was blocked by D-(Ϫ)-2-amino-5-phosphonopentanoic acid. Furthermore, an NMDA receptor-dependent depotentiation was induced in a time-dependent manner, being evoked by injection of CT 10 min, but not 1 hr, after LTP induction. These use-and time-dependent effects of the amyloidogenic peptides on synaptic plasticity promote long-lasting reductions in synaptic strength and oppose activity-dependent strengthening of transmission in the hippocampus. This will result in a profound disruption of information processing dependent on hippocampal synaptic plasticity.

SUMMARY The time course of the requirement for local protein synthesis in the stabilization of learning-related synaptic growth and the persistence of long-term memory was examined using Aplysia bifurcated sensory neuron-motor neuron cultures. We find that following repeated pulses of serotonin (5-HT) the local perfusion of emetine, an inhibitor of protein synthesis, or a TAT-AS oligonucleotide directed against ApCPEB blocks long-term facilitation (LTF) at either 24 hr or 48 hr and leads to a selective retraction of newly formed sensory neuron varicosities induced by 5-HT. By contrast, later inhibition of local protein synthesis, at 72 hr after 5-HT, has no effect on either synaptic growth or LTF. These results define a specific stabilization phase for the storage of long-term memory during which newly formed varicosities are labile and require sustained CPEB-dependent local protein synthesis to acquire the more stable properties of mature varicosities required for the persistence of LTF.