Phosphodiesterase-4 (PDE4) plays an important role in mediating memory via the control of intracellular cAMP signaling; inhibition of PDE4 enhances memory. However, development of PDE4 inhibitors as memory enhancers has been hampered by their major side effect of emesis. PDE4 has four subtypes (PDE4A-D) consisting of 25 splice variants. Mice deficient in PDE4D displayed memory enhancement in radial arm maze, water maze, and object recognition tests. These effects were mimicked by repeated treatment with rolipram in wild-type mice. In addition, similarly as rolipram-treated wild-type mice, PDE4D-deficient mice also displayed increased hippocampal neurogenesis and phosphorylated cAMP response element-binding protein (pCREB). Furthermore, microinfusion of lentiviral vectors that contained microRNAs (miRNAs) targeting long-form PDE4D isoforms into bilateral dentate gyri of the mouse hippocampus downregulated PDE4D4 and PDE4D5, enhanced memory, and increased hippocampal neurogenesis and pCREB. Finally, while rolipram and PDE4D deficiency shortened ␣2 adrenergic receptor-mediated anesthesia, a surrogate measure of emesis, miRNA-mediated PDE4D knock-down in the hippocampus did not. The present results suggest that PDE4D, in particular long-form PDE4D, plays a critical role in the mediation of memory and hippocampal neurogenesis, which are mediated by cAMP/CREB signaling; reduced expression of PDE4D, or at least PDE4D4 and PDE4D5, in the hippocampus enhances memory but appears not to cause emesis. These novel findings will aid in the development of PDE4 subtype-or variant-selective inhibitors for treatment of disorders involving impaired cognition, including Alzheimer's disease.

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Pharmacological inhibition of type 4 cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase (PDE4) produces antidepressant-like effects in animals; however, it is not known which of the four PDE4 subtypes mediates these actions. In the present study, immunoblot analysis showed loss of phosphodiesterase 4D (PDE4D) expression in the cerebral cortex and hippocampus of PDE4D knockout (PDE4D-/-) mice, but unchanged PDE4A and PDE4B expression, relative to the wild type (PDE4D+/+) and heterozygous knockout (PDE4D+/-) mice. This reduced expression was accompanied by a reduction in PDE4 activity, while non-PDE4 activity was unchanged. PDE4D-/- mice exhibited decreased immobility in tail-suspension and forced-swim tests, which is indicative of an antidepressant-like effect on behavior. Desipramine and fluoxetine produced similar antidepressant-like effects in all three genotypes, even though their behavioral baselines differed markedly. By contrast, the PDE4 inhibitor rolipram only produced antidepressant-like effects in PDE4D+/+ mice. Consistent with this, rolipram potentiated isoproterenol-induced cyclic AMP formation only in the PDE4D+/+ mice. These results suggest that PDE4D is an essential mediator of the antidepressant-like effects of rolipram, and that PDE4D-regulated cyclic adenosine monophosphate signaling may play a role in the pathophysiology and pharmacotherapy of depression.

Phosphodiesterase-4 (PDE4), an enzyme that catalyzes the hydrolysis of cyclic AMP and plays a critical role in controlling its intracellular concentration, has been implicated in depression-and anxiety-like behaviors. However, the functions of the four PDE4 subfamilies (PDE4A, PDE4B, PDE4C, and PDE4D) remain largely unknown. In animal tests sensitive to anxiolytics, antidepressants, memory enhancers, or analgesics, we examined the behavioral phenotype of mice deficient in PDE4B (PDE4BÀ/À). Immunoblot analysis revealed loss of PDE4B expression in the cerebral cortex and amygdala of PDE4BÀ/À mice. The reduction of PDE4B expression was accompanied by decreases in PDE4 activity in the brain regions of PDE4BÀ/À mice. Compared to PDE4B + / + littermates, PDE4BÀ/À mice displayed anxiogenic-like behavior, as evidenced by decreased head-dips and time spent in head-dipping in the holeboard test, reduced transitions and time on the light side in the light-dark transition test, and decreased initial exploration and rears in the open-field test. Consistent with anxiogenic-like behavior, PDE4BÀ/À mice displayed increased levels of plasma corticosterone. In addition, these mice also showed a modest increase in the proliferation of neuronal cells in the hippocampal dentate gyrus. In the forced-swim test, PDE4BÀ/À mice exhibited decreased immobility; however, this was not supported by the results from the tail-suspension test. PDE4BÀ/À mice did not display changes in memory, locomotor activity, or nociceptive responses. Taken together, these results suggest that the PDE4B subfamily is involved in signaling pathways that contribute to anxiogenic-like effects on behavior.

Inhibition of phosphodiesterase-4 (PDE4), an enzyme that catalyzes the hydrolysis of cyclic AMP (cAMP), increases phosphorylation of cAMP-response element binding protein (pCREB) and hippocampal neurogenesis, and produces antidepressant-like effects on behavior; however, causal links among these have not been established. In the present study, chronic administration of rolipram produced antidepressant- and anxiolytic-like effects on behavior in mice. It also increased cAMP and pCREB levels in the hippocampus and prefrontal cortex, but increased Sox2, a marker for mitotic progenitor cells, only in the hippocampus. Chronic rolipram treatment also increased hippocampal neurogenesis, as evidenced by increased bromodeoxyuridine (BrdU)-positive cells in the hippocampal dentate gyrus. Methylazoxymethanol (MAM), which is toxic to proliferating cells, reversed rolipram-induced increases in BrdU-positive cells and pCREB in the hippocampus and partially blocked its behavioral effects. Approximately 84% of BrdU-positive cells became newborn neurons, 93% of which co-expressed pCREB; these proportions were not altered by rolipram or MAM, either alone or in combination. Finally, three weeks following the end of MAM treatment, when neurogenesis was no longer inhibited, rolipram again increased hippocampal pCREB, with its antidepressant- and anxiolytic-like effects resumed. Overall, the present results suggest that rolipram produces its effects on behavior in a manner that at least partially depends on its neurogenic action in the hippocampus, targeting mitotic progenitor cells rather than newborn or mature neurons; cAMP/CREB signaling in hippocampal newborn neurons is critical for neurogenesis and contributes to the behavioral effects of rolipram.

, a selective inhibitor of type 4 cyclic AMP phosphodiesterase (PDE4), completely reversed the amnesic effects of MK-801 on working and reference memory (F[4,64] ϭ 11.10; p Ͻ .0001 and F[4,64] N-methyl-D-aspartate (NMDA) receptors are widely distributed in the brain; their density is highest in the hippocampal CA1 subregion (Monaghan and Cotman 1985; Monyeret al. 1994;Boyer et al. 1998). It has been shown that NMDA receptors in this area are very important in the regulation of synaptic plasticity and the process of learning and memory, especially long-term memory (Kesner and Dakis 1995;Morris et al. 1986;Nguyen and Kandel 1996;Kawabe et al. 1998). Meanwhile, NMDA elicits an increase in cAMP in the hippocampal CA1 area that is antagonized by the competitive antagonist DL-2-amino-5-phosphonovaleric acid (AP5) or removal of extracellular Ca 2 ϩ (Chetkovich et al. 1991). Antagonism of NMDA receptors not only blocks NMDA-induced increases in cAMP, but also impairs learning and memory (Morris et al. 1986;Nguyen and Kandel 1996;Kawabe et al. 1998;Chetkovich et al. 1991;Meehan 1996). These results indicate that cAMP is involved in the NMDA receptor antagonist-induced impairment of learning and memory.Rolipram, a selective inhibitor of type 4 cAMP-specific phosphodiesterase (PDE4), produces an increase in brain cAMP levels via the inhibition of its degradation Rolipram Antagonizes MK-801-Induced Memory Deficits 199 (Schneider 1984;Ilien et al. 1982). Behavioral studies show that rolipram inhibits locomotor activity and rearing induced by methaphetamine and produces biphasic effects on schedule-controlled behavior, increasing response rate at lower doses and decreasing response rate at higher doses (Iyo et al. 1995;O'Donnell and Frith 1999). It elicits a morphine-withdrawal-like behavioral syndrome characterized by head twitches, forepaw shaking, grooming and hypoactivity, which are related to a high level of cAMP (Wachtel 1982;Wachtel 1983). Rolipram also exhibits antidepressant-like effects in animal models and in patients with depressive disorders (O'Donnell and Frith 1999;Hebenstreit et al. 1989;O'Donnell 1993).Recently, rolipram has been shown to reverse the impairment of either working memory or reference memory induced by the muscarinic receptor antagonist scopolamine (Egawa et al. 1997;Imanishi et al. 1997;Zhang and O'Donnell 2000). Furthermore, PDE4 has been shown to be involved in NMDA receptor-mediated signal transduction mechanisms. Chronic treatment with rolipram up-regulates NMDA receptors in the rat hippocampus ; rolipram also attenuates the expression of the heat shock protein HSP-70 induced by the NMDA receptor antagonist MK-801 (Hashimoto et al. 1997). Thus, although there is no direct evidence linking the effect of rolipram to NMDA receptors, the results described above suggest that rolipram will reverse the amnesic effect of the NMDA receptor antagonist MK-801. Such a finding would suggest an important role for PDE4 and cAMP in signal transduction mechanisms for NMDA receptors that are involved in ...

Phosphodiesterase-4 (PDE4), one of eleven PDE enzyme families, specifically catalyzes hydrolysis of cyclic AMP (cAMP); it has four subtypes (PDE4A-D) with at least 25 splice variants. PDE4 plays a critical role in the control of intracellular cAMP concentrations. PDE4 inhibitors produce antidepressant actions in both animals and humans via enhancement of cAMP signaling in the brain. However, their clinical utility has been hampered by side effects, in particular nausea and emesis. While there is still a long way to go before PDE4 inhibitors with high therapeutic indices are available for treatment of depressive disorders, important advances have been made in the development of PDE4 inhibitors as antidepressants. First, limited, but significant studies point to PDE4D as the major PDE4 subtype responsible for antidepressant-like effects of PDE4 inhibitors, although the role of PDE4A cannot be excluded. Second, PDE4D may contribute to emesis, the major side effect of PDE4 inhibitors. For this reason, identification of roles of PDE4D splice variants in mediating antidepressant activity is particularly important. Recent studies using small interfering RNAs (siRNAs) have demonstrated the feasibility to identify cellular functions of individual PDE4 variants. Third, mixed inhibitors of PDE4 and PDE7 or PDE4 and serotonin reuptake have been developed and may be potential antidepressants with minimized side effects. Finally, relatively selective inhibitors of one or two PDE4 subtypes have been synthesized using structure- and scaffold-based design. This review also discusses the relationship between PDE4 and antidepressant activity based on structures, brain distributions, and pharmacological properties of PDE4 and its isoforms.

The endocannabinoid (eCB) system regulates mood, emotion, and stress coping, and dysregulation of the eCB system is critically involved in pathophysiology of depression. The eCB ligand 2-arachidonoylglycerol (2-AG) is inactivated by monoacylglycerol lipase (MAGL). Using chronic unpredictable mild stress (CUS) as a mouse model of depression, we examined how 2-AG signaling in the hippocampus was altered in depressive-like states and how this alteration contributed to depressive-like behavior. We report that CUS led to impairment of depolarization-induced suppression of inhibition (DSI) in mouse hippocampal CA1 pyramidal neurons, and this deficiency in 2-AG-mediated retrograde synaptic depression was rescued by MAGL inhibitor JZL184. CUS induced depressive-like behaviors and decreased mammalian target of rapamycin (mTOR) activation in the hippocampus, and these biochemical and behavioral abnormalities were ameliorated by chronic JZL184 treatments. The effects of JZL184 were mediated by cannabinoid CB1 receptors. Genetic deletion of mTOR with adeno-associated viral (AAV) vector carrying the Cre recombinase in the hippocampus of mTORf/f mice recapitulated depressive-like behaviors induced by CUS and abrogated the antidepressant-like effects of chronic JZL184 treatments. Our results suggest that CUS decreases eCB-mTOR signaling in the hippocampus, leading to depressive-like behaviors, whereas MAGL inhibitor JZL184 produces antidepressant-like effects through enhancement of eCB-mTOR signaling.