Consistent evidence from pharmacological and genetic studies shows that cAMP is a critical modulator of synaptic plasticity and memory formation. However, the potential of the cAMP signaling pathway as a target for memory enhancement remains unclear because of contradictory findings from pharmacological and genetic approaches. To address these issues, we have developed a novel conditional genetic system in mice based on the heterologous expression of an Aplysia octopamine receptor, a G-protein-coupled receptor whose activation by its natural ligand octopamine leads to rapid and transient increases in cAMP. We find that activation of this receptor transgenically expressed in mouse forebrain neurons induces a rapid elevation of hippocampal cAMP levels, facilitates hippocampus synaptic plasticity, and enhances the consolidation and retrieval of fear memory. Our findings clearly demonstrate that acute increases in cAMP levels selectively in neurons facilitate synaptic plasticity and memory, and illustrate the potential of this heterologous system to study cAMP-mediated processes in mammalian systems.

Histone deacetylase (HDAC) inhibitors increase histone acetylation and enhance both memory and synaptic plasticity. The current model for the action of HDAC inhibitors assumes that they alter gene expression globally and thus affect memory processes in a nonspecific manner. Here, we show that the enhancement of hippocampus-dependent memory and hippocampal synaptic plasticity by HDAC inhibitors is mediated by the transcription factor cAMP response element-binding protein (CREB) and the recruitment of the transcriptional coactivator and histone acetyltransferase CREB-binding protein (CBP) via the CREB-binding domain of CBP. Furthermore, we show that the HDAC inhibitor trichostatin A does not globally alter gene expression but instead increases the expression of specific genes during memory consolidation. Our results suggest that HDAC inhibitors enhance memory processes by the activation of key genes regulated by the CREB: CBP transcriptional complex.

The formation of many forms of long-term memory requires several molecular mechanisms including regulation of gene expression. The mechanisms directing transcription require not only activation of individual transcription factors but also recruitment of transcriptional coactivators. CBP and p300 are transcriptional coactivators that interact with a large number of transcription factors and regulate transcription through multiple mechanisms, including an intrinsic histone acetyltransferase (HAT) activity. HAT activity mediates acetylation of lysine residues on the amino-terminal tails of histone proteins, thereby increasing DNA accessibility for transcription factors to activate gene expression. CBP has been shown to play an important role in long-term memory formation. We have investigated whether p300 is also required for certain forms of memory. p300 shares a high degree of homology with CBP and has been shown to interact with transcription factors known to be critical for long-term memory formation. Here we demonstrate that conditional transgenic mice expressing an inhibitory truncated form of p300 (p300Delta1), which lacks the carboxy-terminal HAT and activation domains, have impaired long-term recognition memory and contextual fear memory. Thus, our study demonstrates that p300 is required for certain forms of memory and that the HAT and carboxy-terminal domains play a critical role.

Studies of hippocampal long-term potentiation (LTP), a cellular model of memory storage, implicate cAMP-dependent protein kinase (PKA) in presynaptic and postsynaptic mechanisms of LTP. The anchoring of PKA to AKAPs (A kinase-anchoring proteins) creates compartmentalized pools of PKA, but the roles of presynaptically and postsynaptically anchored forms of PKA in late-phase LTP are unclear. In this study, we have created genetically modified mice that conditionally express Ht31, an inhibitor of PKA anchoring, to probe the roles of anchored PKA in hippocampal LTP and spatial memory. Our findings show that at hippocampal Schaffer collateral CA3–CA1 synapses, theta-burst LTP requires presynaptically anchored PKA. In addition, a pool of anchored PKA in hippocampal area CA3 is required for spatial memory. These findings reveal a novel and significant role for anchored PKA signaling in cellular mechanisms underlying memory storage.

Synaptic plasticity, the activity-dependent change in the strength of neuronal connections, is a proposed cellular mechanism of memory storage that is critically regulated by protein kinases such as cAMP-dependent protein kinase (PKA). Despite the fact that a neuron contains thousands of synapses, the expression of synaptic plasticity can be specific to subsets of synapses. This is surprising because signal transduction pathways underlying synaptic plasticity involve diffusible second messenger molecules such as cAMP and diffusible proteins such as the catalytic subunit of PKA. One way in which this specificity can be achieved is by the localization of signal transduction molecules to specific subcellular domains. Spatial compartmentalization of PKA signaling is achieved via binding to A kinase-anchoring proteins (AKAPs). We report here that pharmacological inhibition of PKA anchoring impairs synaptically activated late-phase long-term potentiation (L-LTP) in hippocampal slices. In contrast, potentiation that is induced by the pharmacological activation of the cAMP/PKA pathway, which can potentially affect all synapses within the neuron, is not impaired by inhibition of PKA anchoring. These results suggest that PKA anchoring may be particularly important for events that occur at the synapse during the induction of L-LTP, such as synaptic tagging and capture. Indeed, our results demonstrate that blocking PKA anchoring impairs synaptic tagging and capture. Thus our data highlight the idea that PKA anchoring may allow for specific populations of synapses to change in synaptic strength in the face of plasticity-related transcription that is cell-wide.