Effects of rolipram, a phosphodiesterase 4 inhibitor, in combination with imipramine on depressive behavior, CRE-binding activity and BDNF level in learned helplessness rats

Beneficial effects of rolipram in the R6/2 mouse model of Huntington's disease

We have previously showed that rolipram, a phosphodiesterase type IV inhibitor, displays a neuroprotective effect in a rat quinolinic acid model of HD [DeMarch Z., Giampa C., Patassini S., Martorana A., Bernardi G. and Fusco F.R., (2007) Beneficial effects of rolipram in a quinolinic acid model of striatal excitotoxicity. Neurobiol. Dis. 25:266-273.]. In this study, we sought to determine if rolipram exerts a neuroprotective effect in R6/2 mutant mice, which recapitulates, in many aspects, human HD [Mangiarini L., Sathasivam K., Seller M., Cozens B., Harper A., Hetherington C., Lawton M., Trottier Y., Lehrach H., Davies S.W. and Bates G.P. (1996) Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice. Cell. 87:493-506]. Transgenic mice were treated with rolipram 1.5 mg/kg daily starting from 4 weeks of age. After transcardial perfusion, histological and immunohistochemical studies were performed. We found that rolipram-treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the vehicle treated ones. Primary outcome measures such as brain volume, striatal atrophy, size and morphology of striatal neurons, neuronal intranuclear inclusions and microglial reaction confirmed a neuroprotective effect of the compound. Rolipram was effective in increasing significantly the levels of activated CREB and of BDNF the striatal spiny neurons, which might account for the beneficial effects observed in this model. Our findings show that rolipram could be considered as a valid therapeutic approach for HD.

Abeta(1-42) injection causes memory impairment, lowered cortical and serum BDNF levels, and decreased hippocampal 5-HT(2A) levels

Aggregation of the beta-amyloid protein (Abeta) is a hallmark of Alzheimer's disease (AD) and is believed to be causally involved in a neurodegenerative cascade. In patients with AD, reduced levels of serum Brain Derived Neurotrophic Factor (BDNF) and cortical 5-HT(2A) receptor binding has recently been reported but it is unknown how these changes are related to beta-amyloid accumulation. In this study we examined in rats the effect of intrahippocampal injections of aggregated Abeta(1-42) (1 microg/microl) on serum and brain BDNF or 5-HT(2A) receptor levels. A social recognition test paradigm was used to monitor Abeta(1-42) induced memory impairment. Memory impairment was seen 22 days after injection of Abeta(1-42) in the experimental group and until termination of the experiments. In the Abeta(1-42) injected animals we saw an abolished increase in serum BDNF levels that was accompanied by significant lower BDNF levels in frontal cortex and by an 8.5% reduction in hippocampal 5-HT(2A) receptor levels. A tendency towards lowered cortical 5-HT(2A) was also observed. These results indicate that the Abeta(1-42) associated memory deficit is associated with an impaired BDNF regulation, which is reflected in lower cortical BDNF levels, and changes in hippocampal 5-HT(2A) receptor levels. This suggests that the BDNF and 5-HT2A changes observed in AD are related to the presence of Abeta(1-42) deposits.

Pharmacologically diverse antidepressants rapidly activate brain-derived neurotrophic factor receptor TrkB and induce phospholipase-Cgamma signaling pathways in mouse brain

Previous studies suggest that brain-derived neurotrophic factor and its receptor TrkB are critically involved in the therapeutic actions of antidepressant drugs. We have previously shown that the antidepressants imipramine and fluoxetine produce a rapid autophosphorylation of TrkB in the rodent brain. In the present study, we have further examined the biochemical and functional characteristics of antidepressant-induced TrkB activation in vivo. We show that all the antidepressants examined, including inhibitors of monoamine transporters and metabolism, activate TrkB rapidly in the rodent anterior cingulate cortex and hippocampus. Furthermore, the results indicate that acute and long-term antidepressant treatments induce TrkB-mediated activation of phospholipase-Cgamma1 (PLCgamma1) and increase the phosphorylation of cAMP-related element binding protein, a major transcription factor mediating neuronal plasticity. In contrast, we have not observed any modulation of the phosphorylation of TrkB Shc binding site, phosphorylation of mitogen-activated protein kinase or AKT by antidepressants. We also show that in the forced swim test, the behavioral effects of specific serotonergic antidepressant citalopram, but not those of the specific noradrenergic antidepressant reboxetine, are crucially dependent on TrkB signaling. Finally, brain monoamines seem to be critical mediators of antidepressant-induced TrkB activation, as antidepressants reboxetine and citalopram do not produce TrkB activation in the brains of serotonin- or norepinephrine-depleted mice. In conclusion, our data suggest that rapid activation of the TrkB neurotrophin receptor and PLCgamma1 signaling is a common mechanism for all antidepressant drugs.

PDE inhibitors in psychiatry--future options for dementia, depression and schizophrenia?

Phosphodiesterases are key enzymes in cellular signalling pathways. They degrade cyclic nucleotides and their inhibition via specific inhibitors offers unique 'receptor-independent' opportunities to modify cellular function. An increasing number of in vitro and animal model studies point to innovative treatment options in neurology and psychiatry. This review critiques a selection of recent studies and developments with a focus on dementia/neuroprotection, depression and schizophrenia. Despite increased interest among the clinical neurosciences, there are still no approved PDE inhibitors for clinical use in neurology or psychiatry. Adverse effects are a major impediment for clinical approval. It is therefore necessary to search for more specific inhibitors at the level of different PDE sub-families and isoforms.

Learned helplessness is independent of levels of brain-derived neurotrophic factor in the hippocampus

Reduced levels of brain-derived neurotrophic factor (BDNF) in the hippocampus have been implicated in human affective disorders and behavioral stress responses. The current studies examined the role of BDNF in the behavioral consequences of inescapable stress, or learned helplessness. Inescapable stress decreased BDNF mRNA and protein in the hippocampus of sedentary rats. Rats allowed voluntary access to running wheels for either 3 or 6 weeks prior to exposure to stress were protected against stress-induced reductions of hippocampal BDNF protein. The observed prevention of stress-induced deceases in BDNF, however, occurred in a time course inconsistent with the prevention of learned helplessness by wheel running, which is evident following 6 weeks, but not 3 weeks, of wheel running. BDNF suppression in physically active rats was produced by administering a single injection of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg) just prior to stress. Despite reduced levels of hippocampal BDNF mRNA following stress, physically active rats given the combination of fluoxetine and stress remained resistant against learned helplessness. Sedentary rats given both fluoxetine and stress still demonstrated typical learned helplessness behaviors. Fluoxetine by itself reduced BDNF mRNA in sedentary rats only, but did not affect freezing or escape learning 24 h later. Finally, bilateral injections of BDNF (1 mug) into the dentate gyrus prior to stress prevented stress-induced reductions of hippocampal BDNF but did not prevent learned helplessness in sedentary rats. These data indicate that learned helplessness behaviors are independent of the presence or absence of hippocampal BDNF because blocking inescapable stress-induced BDNF suppression does not always prevent learned helplessness, and learned helplessness does not always occur in the presence of reduced BDNF. Results also suggest that the prevention of stress-induced hippocampal BDNF suppression is not necessary for the protective effect of wheel running against learned helplessness.

Dopamine D2-like receptors and the antidepressant response

Converging lines of evidence suggest a role for the mesolimbic dopamine system in the response to somatic antidepressant therapies. Here, we review evidence suggesting that antidepressant treatments of different types share the effect of increasing the sensitivity of dopamine D2-like receptors in the nucleus accumbens, clinical studies suggesting that activation of these receptors has antidepressant efficacy, as well as relevant imaging and genetic data on the role of this system in the antidepressant response. We then attempt to reconcile this data with evidence of a common target of antidepressant drugs in the cyclic adenosine monophosphate (cAMP) response element binding protein-brain-derived neurotrophic factor (CREB-BDNF) pathway in a model that suggests potential directions for future inquiry.

Chronic nicotine doses down-regulate PDE4 isoforms that are targets of antidepressants in adolescent female rats

BACKGROUND

Previous data in humans and animal models has suggested connections between anxiety, depression, smoking behavior, and nicotine dependence. The importance of these connections has been confirmed by clinical studies that led to the recent FDA approval of an anti-depressant (Zyban) for use in human smoking cessation programs. Other anti-depressants (such as rolipram) specifically inhibit PDE4 phosphodiesterases.

METHODS

We used DNA microarrays to discover gene expression changes in adolescent female rats following chronic nicotine treatments, and real-time PCR assays to confirm and extend those results.

RESULTS

We found a consistent decrease in the mRNA levels encoded by the Pde4b gene in nucleus accumbens, prefrontal cortex, and hippocampus of adolescent female rats treated with .24 mg/day nicotine, and in prefrontal cortex of adolescent female rats treated with .12 mg/day nicotine. We further show that each of these brain areas produced a different profile of Pde4b isoforms.

CONCLUSIONS

Chronic nicotine treatments produce a dose-dependent down-regulation of Pde4b, which may have an antidepressant effect. This is the first report of a link between nicotine dependence and phosphodiesterase gene expression. Our results also add to the complex interrelationships between smoking and schizophrenia, because mutations in the PDE4B gene are associated with schizophrenia.

Beneficial effects of rolipram in a quinolinic acid model of striatal excitotoxicity

Activity of c-AMP responsive element-binding protein (CREB) is decreased in Huntington's disease (HD). Such decrease was also described by our group in the quinolinic acid lesion model of striatal excitotoxicity. The phosphodiesterase type IV inhibitor rolipram increases CREB phosphorylation. Such drug has a protective effect in global ischaemia and embolism in rats. In this study, we sought to determine whether rolipram displays a neuroprotective effect in our rat model of HD. Animals were surgically administered QA and subsequently treated with rolipram daily up to 2 and 8 weeks respectively. After these time points, rats were sacrificed and immunohistochemical studies were performed in the striata. In the rolipram-treated animals, striatal lesion size was about 62% smaller that in the vehicle-treated ones at 2 weeks time point. Moreover, the surviving cell number was several times higher in the rolipram-treated animals than in the vehicle group at both time points. Rolipram also showed to be effective in increasing significantly the levels of activated CREB in the striatal spiny neurons, which accounts mostly for its beneficial effect in our rodent model of excitotoxicity. Our findings show that rolipram could be considered as a valid therapeutic approach for HD.

Cyclic AMP response element binding protein and brain-derived neurotrophic factor: molecules that modulate our mood?

Depression is the major psychiatric ailment of our times, afflicting approximately 20% of the population. Despite its prevalence, the pathophysiology of this complex disorder is not well understood. In addition, although antidepressants have been in existence for the past several decades, the mechanisms that underlie their therapeutic effects remain elusive. Building evidence implicates a role for the plasticity of specific neuro-circuitry in both the pathophysiology and treatment of depression. Damage to limbic regions is thought to contribute to the etiology of depression and antidepressants have been reported to reverse such damage and promote adaptive plasticity. The molecular pathways that contribute to the damage associated with depression and antidepressant-mediated plasticity are a major focus of scientific enquiry. The transcription factor cyclic AMP response element binding protein (CREB) and the neurotrophin brain-derived neurotrophic factor (BDNF) are targets of diverse classes of antidepressants and are known to be regulated in animal models and in patients suffering from depression. Given their role in neuronal plasticity, CREB and BDNF have emerged as molecules that may play an important role in modulating mood. The purpose of this review is to discuss the role of CREB and BDNF in depression and as targets/mediators of antidepressant action.

Selective phosphodiesterase (PDE)-4 inhibitors: a novel approach to treating memory deficit?

Phosphodiesterase-4 (PDE4) belongs to an important family of proteins that regulates the intracellular level of cyclic adenosine monophosphate (cAMP). Several lines of evidence indicate that targeting PDE4 with selective inhibitors may offer novel strategies in the treatment of age-related memory impairment and Alzheimer's disease. The rationale for such an approach stems from preclinical studies indicating that PDE4 inhibitors can counteract deficits in long-term memory caused by pharmacological agents, aging or overexpression of mutant forms of human amyloid precursor proteins. In addition to their pro-cognitive and pro-synaptic plasticity properties, PDE4 inhibitors are potent neuroprotective, neuroregenerative and anti-inflammatory agents. Based on the fact that Alzheimer's disease is a progressive neurodegenerative disorder that is characterised by cognitive impairment, and that neuroinflammation is now recognised as a prominent feature in Alzheimer's pathology, we have concluded that targeting PDE4 with selective inhibitors may offer a novel therapy aimed at slowing progression, prevention and, eventually, therapy of Alzheimer's disease.

Postulated vasoactive neuropeptide autoimmunity in fatigue-related conditions: a brief review and hypothesis

Disorders such as chronic fatigue syndrome (CFS) and gulf war syndrome (GWS) are characterised by prolonged fatigue and a range of debilitating symptoms of pain, intellectual and emotional impairment, chemical sensitivities and immunological dysfunction. Sudden infant death syndrome (SIDS) surprisingly may have certain features in common with these conditions. Post-infection sequelae may be possible contributing factors although ongoing infection is unproven. Immunological aberration may prove to be associated with certain vasoactive neuropeptides (VN) in the context of molecular mimicry, inappropriate immunological memory and autoimmunity. Adenylate cyclase-activating VNs including pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP) act as hormones, neurotransmitters, neuroregulators, immune modulators and neurotrophic substances. They and their receptors are potentially immunogenic. VNs are widely distributed in the body particularly in the central and peripheral nervous systems and have been identified in the gut, adrenal gland, blood cells, reproductive system, lung, heart and other tissues. They have a vital role in maintaining cardio-respiratory function, thermoregulation, memory, concentration and executive functions such as emotional responses including social cues and appropriate behaviour. They are co-transmitters for a number of neurotransmitters including acetylcholine and gaseous transmitters, are potent immune regulators with primarily anti-inflammatory activity, and have a significant role in protection of the nervous system against toxic assault as well as being important in the maintenance of homeostasis. This paper describes a biologically plausible mechanism for the development of certain fatigue-related syndromes based on loss of immunological tolerance to these VNs or their receptors following infection, other events or de novo resulting in significant pathophysiology possibly mediated via CpG fragments and heat shock (stress) proteins. These conditions extend the public health context of autoimmunity and VN dysregulation and have implications for military medicine where radiological, biological and chemical agents may have a role in pathogenesis. Possible treatment and prevention options are considered.

The role of CREB in depression and antidepressant treatment

Major depressive disorder is a severe clinical problem across the globe, with a lifetime risk of 10%-30% for women and 7%-15% for men. The World Health Organization ranks major depression at the top of the list in terms of disease burden, and this burden is expected to rise in the next decade as the prevalence of the disorder grows. Since the late 1950s, a wide range of antidepressant medications targeting the monoamine systems has been available to alleviate the symptoms of major depressive disorder. Although widely prescribed, such antidepressant medications are accompanied by a delay in effectiveness, as well as varied side effects. Therefore, further characterization of the biological mechanisms behind their function is crucial for the development of new and more effective treatments. One protein that could serve as a convergence point for multiple classes of antidepressant drugs is the transcription factor CREB (cyclic adenosine monophosphate response element binding protein). CREB is upregulated by chronic antidepressant treatment, and increasing CREB levels in rodent models results in antidepressant-like behaviors. Furthermore, postmortem studies indicate that CREB levels are increased in subjects taking antidepressants at the time of death. However, not all antidepressants increase CREB levels and/or activity, and reducing CREB levels in some brain regions also results in antidepressant-like behaviors. This review attempts to consolidate the information relevant to the structure and function of the CREB protein and describe how this relates to the mechanism of antidepressant drugs. Animal models in which CREB function is enhanced, by overexpression of the protein, or reduced, by expression of mutant forms of the protein or through gene deletion experiments, are summarized in terms of identifying a role for CREB in behavioral responses in depression tests that were originally designed to evaluate antidepressant efficacy. Human postmortem and genetic studies that implicate CREB in depression and antidepressant efficacy are also discussed.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Impairment of the spatial learning and memory induced by learned helplessness and chronic mild stress

Increasing evidences indicate the concurrence and interrelationship of depression and cognitive impairments. The present study was undertaken to investigate the effects of two depressive animal models, learned helplessness (LH) and chronic mild stress (CMS), on the cognitive functions of mice in the Morris water maze task. Our results demonstrated that both LH and CMS significantly decreased the cognitive performance of stressed mice in the water maze task. The escaping latency to the platform was prolonged and the probe test percentage in the platform quadrant was reduced. These two models also increased the plasma corticosterone concentration and decreased the brain derived neurotrophic factor (BDNF) and cAMP-response element-biding protein (CREB) messenger ribonucleic acid (mRNA) levels in hippocampus, which might cause the spatial cognition deficits. Repeated treatment with antidepressant drugs, imipramine (Imi) and fluoxetine (Flu), significantly reduced the plasma corticosterone concentration and enhanced the BDNF and CREB levels. Furthermore, antidepressant treated animals showed an ameliorated cognitive performance compared with the vehicle treated stressed animals. These data suggest that both LH and CMS impair the spatial cognitive function and repeated treatment with antidepressant drugs decreases the prevalence of cognitive impairments induced by these two animal models. Those might in part be attributed to the reduced plasma corticosterone and enhanced hippocampal BDNF and CREB expressions. This study provided a better understanding of molecular mechanisms underlying interactions of depression and cognitive impairments, although animal models used in this study can mimic only some aspects of depression or cognition of human.

Subchronic rolipram delivery activates hippocampal CREB and arc, enhances retention and slows down extinction of conditioned fear

Rolipram, a type IV-specific phosphodiesterase inhibitor, is known to improve memory under various learning tasks. Moreover, Rolipram treatments have been shown to increase expression and phosphorylation of a key factor for hippocampal memory consolidation, the cAMP-dependent response element-binding protein, CREB. However, the exact correlation between hippocampal CREB phosphorylation and memory improvement induced by Rolipram has not yet been determined in a CREB-dependent type of hippocampal-related learning in normogenic, intact rodents. Here, we report that subchronic Rolipram delivery by using osmotic minipumps increased the basal rat hippocampal expression and phosphorylation of CREB, as well as the expression of the cAMP-dependent, memory-related protein, Arc. In parallel, the same treatment improved memory consolidation of conditioned fear. Furthermore, the increase of CREB phosphorylation and Arc expression consequent to the learning experience was enhanced in Rolipram-treated rats, compared to controls. By evaluating the time course of memory extinction over 10 days after the initial learning test, we also observed significant slowing down of the memory extinction rate in Rolipram-treated rats. This effect could be attributed to CREB phosphorylation and memory having been initially higher, as osmotic minipumps stopped to release Rolipram the first day after the initial learning test. Our data define the conditions through which the pharmacological manipulation of hippocampal CREB expression and activation result in memory amelioration in normogenic, intact animals. These results are relevant for the study of molecular correlates of memory, and may also be important in view of the efforts to design new pharmacological treatments, targeting the CREB pathway and leading to enhancement of learning and memory, even in the absence of patent neuropathology.

Is there a role for the endocannabinoid system in the etiology and treatment of melancholic depression?

With advances in basic and clinical neuroscience, many gaps have appeared in the traditional monoamine theory of depression that have led to reformulation of the hypotheses concerning the neurobiology of depression. The more recent hypotheses suggest that melancholic depression is characterized by central glucocorticoid resistance that results in hypercortisolemia, which in turn leads to down-regulation of neurotrophins and subsequent neurodegeneration. Examining the neurobiology of depression from this perspective suggests that the endocannabinoid system may play a role in the etiology of melancholic depression. Specifically, pharmacological and genetic blockade of the cannabinoid CB1 receptor induces a phenotypic state that is analogous to melancholic depression, including symptoms such as reduced food intake, heightened anxiety, increased arousal and wakefulness, deficits in extinction of aversive memories and supersensitivity to stress. These similarities between melancholic depression and an endocannabinoid deficiency become more interesting in light of recent findings that endocannabinoid activity is down-regulated by chronic stress and possibly increased by some antidepressant regimens. We propose that an endocannabinoid deficiency may underlie some of the symptoms of melancholic depression, and that enhancement of this system may ultimately be a novel form of pharmacotherapy for treatment-resistant depression.

Antidepressant action: to the nucleus and beyond

After decades of effort, the field of depression research is far from understanding how antidepressant drugs mediate their clinical effects. The time lag of 2-6 weeks of therapy that is necessary to obtain antidepressant efficacy indicates a requirement for long-term regulation of molecules activated by drug treatment. The focus of antidepressant research has thus expanded from examining acute monoamine-mediated mechanisms to include long-term transcriptional regulators such as cAMP response element-binding protein (CREB) and trophic factors such as brain-derived nerve growth factor and insulin-like growth factor. In addition, the recent discovery of antidepressant-induced neurogenesis provides another avenue by which antidepressants might exert their effects. Current efforts are aimed at understanding how CREB and trophic factor signaling pathways are coupled to neurogenic effects and how alterations in behavioral, molecular and cellular endpoints are related to the alleviation of the symptoms of depression.

Effects of estrogen treatment on expression of brain-derived neurotrophic factor and cAMP response element-binding protein expression and phosphorylation in rat amygdaloid and hippocampal structures

Clinical studies indicate an effect of estrogen (E2) on affect and cognition, which may be mediated by the cAMP response element-binding protein (CREB) pathway and CREB-related gene target brain-derived neurotrophic factor (BDNF). We investigated the effect of E2 on CREB expression and phosphorylation and BDNF expression in the amygdala and hippocampus, areas involved in emotional processing. Ovariectomized rats were given 10 microg 17beta-estradiol or vehicle for 14 days and expression of components of the CREB signaling pathway, i.e., CREB, phosphorylated CREB (pCREB), and BDNF in amygdala and hippocampus were investigated using immunogold labeling. Levels of BDNF mRNA were determined by in situ reverse-transcriptase polymerase chain reaction. We also examined the effect of E2 on calcium/calmodulin kinase (CaMK IV) immunolabeling in the hippocampus. E2 increased immunolabeling and mRNA levels of BDNF in the medial and basomedial amygdala and CA1 and CA3 regions of the hippocampus, but not in any other amygdaloid or hippocampal regions examined. E2 increased immunolabeling of CREB and pCREB in the medial and basomedial, but not central or basolateral amygdala. E2 also increased CaMK IV and pCREB immunolabeling in the CA1 and CA3 regions, but not CA2 region or dentate gyrus, of the hippocampus. There was no change in immunolabeling of CREB in any hippocampal region. These data identify a signaling pathway through which E2 increases BDNF expression that may underlie some actions of E2 on affective behavior and indicate neuroanatomical heterogeneity in the E2 effect within the amygdala and hippocampus.