Mice carrying a truncated form of cAMP-responsive element binding protein (CREB)-binding protein (CBP) show several developmental abnormalities similar to patients with Rubinstein-Taybi syndrome (RTS). RTS patients suffer from mental retardation, whereas long-term memory formation is defective in mutant CBP mice. A critical role for cAMP signaling during CREB-dependent long-term memory formation appears to be evolutionarily conserved. From this observation, we reasoned that drugs that modulate CREB function by enhancing cAMP signaling might yield an effective treatment for the memory defect(s) of CBP ؉/؊ mice. To this end, we designed a cell-based drug screen and discovered inhibitors of phosphodiesterase 4 (PDE4) to be particularly effective enhancers of CREB function. We extend previous behavioral observations by showing that CBP ؉/؊ mutants have impaired long-term memory but normal learning and short-term memory in an object recognition task. We demonstrate that the prototypical PDE4 inhibitor, rolipram, and a novel one (HT0712) abolish the long-term memory defect of CBP ؉/؊ mice. Importantly, the genetic lesion in CBP acts specifically to shift the dose sensitivity for HT0712 to enhance memory formation, which conveys molecular specificity on the drug's mechanism of action. Our results suggest that PDE4 inhibitors may be used to treat the cognitive dysfunction of RTS patients.

We injected small interfering RNAs (siRNAs) directly into the hippocampus of wild-type mice, knocking down expression of cyclic AMP responsive element-binding protein (CREB) and disrupting long-term, but not short-term, memory after both contextual and trace fear conditioning. In contrast, similar knockdown of siRNA for protein phosphatase 1 (PP1) was sufficient to enhance contextual and temporal memory formation, thereby demonstrating with such a gain-of-function effect a lack of any general deleterious effect for this method of RNAi-mediated gene knockdown. Our findings clearly confirm that contextual memory formation involves CREB and PP1 as positive and negative regulators, respectively, and show for the first time that temporal memory formation shares this mechanism. More generally, we establish that direct injection of siRNA into identified adult brain regions yields specific gene knockdowns, which can be used to validate in vivo candidate genes involved in behavioral plasticity.