Restricted and regulated expression in mice of VP16-CREB, a constitutively active form of CREB, in hippocampal CA1 neurons lowers the threshold for eliciting a persistent late phase of long-term potentiation (L-LTP) in the Schaffer collateral pathway. This L-LTP has unusual properties in that its induction is not dependent on transcription. Pharmacological and two-pathway experiments suggest a model in which VP16-CREB activates the transcription of CRE-driven genes and leads to a cell-wide distribution of proteins that prime the synapses for subsequent synapse-specific capture of L-LTP by a weak stimulus. Our analysis indicates that synaptic capture of CRE-driven gene products may be sufficient for consolidation of LTP and provides insight into the molecular mechanisms of synaptic tagging and synapse-specific potentiation.

We studied a mouse model of the haploinsufficiency form of Rubinstein-Taybi syndrome (RTS), an inheritable disorder caused by mutations in the gene encoding the CREB binding protein (CBP) and characterized by mental retardation and skeletal abnormalities. In these mice, chromatin acetylation, some forms of long-term memory, and the late phase of hippocampal long-term potentiation (L-LTP) were impaired. We ameliorated the L-LTP deficit in two ways: (1) by enhancing the expression of CREB-dependent genes, and (2) by inhibiting histone deacetyltransferase activity (HDAC), the molecular counterpart of the histone acetylation function of CBP. Inhibition of HDAC also reversed the memory defect observed in fear conditioning. These findings suggest that some of the cognitive and physiological deficits observed on RTS are not simply due to the reduction of CBP during development but may also result from the continued requirement throughout life for both the CREB co-activation and the histone acetylation function of CBP.

Whereas significant insight exists as to how LTP-related changes can contribute to the formation of long-term memory, little is known about the role of hippocampal LTD-like changes in learning and memory storage. We describe a mouse lacking the transcription factor SRF in the adult forebrain. This mouse could not acquire a hippocampus-based immediate memory for a novel context even across a few minute timespan, which led to a profound but selective deficit in explicit spatial memory. These animals were also impaired in the induction of LTD, including LTD triggered by a cholinergic agonist. Moreover, genes regulating two processes essential for LTD-calcium release from intracellular stores and phosphatase activation-were abnormally expressed in knockouts. These findings suggest that for the hippocampus to form associative spatial memories through LTP-like processes, it must first undergo learning of the context per se through exploration and the learning of familiarity, which requires LTD-like processes.

While most studies have focused on the role of long-term potentiation in behavior, far less is known about the role of long-term depression (LTD). To examine the potential involvement of LTD in learning and memory, we generated transgenic mice that express a fragment of the SV40 small t antigen known to inhibit protein phosphatase 2A (PP2A). Small t antigen expression blocked both stimulus-induced and chemically induced NMDAR-dependent LTD at Schaffer collateral synapses but did not affect potentiation, depotentiation, or mGluR-dependent LTD. This physiological phenotype was associated with deficits in behavioral flexibility in both the Morris water maze and a delayed nonmatch to place T-maze task, suggesting that NMDAR-dependent LTD is required for behavioral flexibility and may act by weakening previously encoded memory traces when new information is learned.

To investigate the role of CREB-mediated gene expression on the excitability of CA1 pyramidal neurons, we obtained intracellular recordings from pyramidal neurons of transgenic mice expressing a constitutively active form of CREB, VP16 -CREB, in a regulated and restricted manner. We found that transgene expression increased the neuronal excitability and inhibited the slow and medium afterhyperpolarization currents. These changes may contribute to the reduced threshold for LTP observed in these mice. When strong transgene expression was turned on for prolonged period of time, these mice also showed a significant loss of hippocampal neurons and sporadic epileptic seizures. These deleterious effects were dose dependent and could be halted, but not reversed by turning off transgene expression. Our experiments reveal a new role for hippocampal CREB-mediated gene expression, identify the slow afterhyperpolarization as a primary target of CREB action, provide a new mouse model to investigate temporal lobe epilepsy and associated neurodegeneration, and illustrate the risks of cell death associated to a sustained manipulation of this pathway. As a result, our study has important implications for both the understanding of the cellular bases of learning and memory and the consideration of therapies targeted to the CREB pathway.