Traumatic Stress-Induced Vulnerability to Addiction: Critical Role of the Dynorphin/Kappa Opioid Receptor System.

Substance use disorders (SUD) may emerge from an individual's attempt to limit negative affective states and symptoms linked to stress. Indeed, SUD is highly comorbid with chronic stress, traumatic stress, or post-traumatic stress disorder (PTSD), and treatments approved for each pathology individually often failed to have a therapeutic efficiency in such comorbid patients. The kappa-opioid receptor (KOR) and its endogenous ligand dynorphin (DYN), seem to play a key role in the occurrence of this comorbidity. The DYN/KOR function is increased either in traumatic stress or during drug use, dependence acquisition and DYN is released during stress. The behavioural effects of stress related to the DYN/KOR system include anxiety, dissociative and depressive symptoms, as well as increased conditioned fear response. Furthermore, the DYN/KOR system is implicated in negative reinforcement after the euphoric effects of a drug of abuse ends. During chronic drug consumption DYN/KOR functions increase and facilitate tolerance and dependence. The drug-seeking behaviour induced by KOR activation can be retrieved either during the development of an addictive behaviour, or during relapse after withdrawal. DYN is known to be one of the most powerful negative modulators of dopamine signalling, notably in brain structures implicated in both reward and fear circuitries. KOR are also acting as inhibitory heteroreceptors on serotonin neurons. Moreover, the DYN/KOR system cross-regulate with corticotropin-releasing factor in the brain. The sexual dimorphism of the DYN/KOR system could be the cause of the gender differences observed in patients with SUD or/and traumatic stress-related pathologies. This review underlies experimental and clinical results emphasizing the DYN/KOR system as common mechanisms shared by SUD or/and traumatic stress-related pathologies, and suggests KOR antagonist as a new pharmacological strategy to treat this comorbidity.

Insulin-like Growth Factors may be Markers of both Traumatic Brain Injury and Fear-Related Stress.

Insulin-like growth factors (IGF) are potent neurotrophic and neurorepair factors that were recently proposed as biomarkers of traumatic brain injury (TBI) and associated psychiatric comorbidities, in particular post-traumatic stress disorder (PSTD). We tested the hypothesis that the IGF system is differentially deregulated in the acute and early chronic stages of TBI, and under acute stress. Plasma and brain IGF1 and IGF2 levels were evaluated in mice 3 weeks and 3 days after a controlled cortical impact (CCI)-induced mild-to-moderate TBI. The effects of conditioned fear on IGF levels and its interaction with TBI (TBI followed, 3 weeks later, by fear-inducing procedures) were also evaluated. In the plasma, IGF1 decreased 3 weeks post-TBI only (-9%), whereas IGF2 remained unaffected. In the brain, IGF1 increased only in the cortex and hippocampus at 3 weeks post-TBI (up to +650%). At 3 days, surpringly, this increase was more diffuse and more important in sham (craniotomized) animals. Additionally, IGF2 immunostaining in brain ventricles was reorganized in TBI animals at both post-TBI stages. Conditioned fear exposure did not influence the effects of early chronic TBI on plasma IGF1 levels, but reduced plasma IGF2 (-6%) levels. It also dampened the effects of TBI on brain IGF systems, but brain IGF1 level and IGF2 tissue distribution remained statistically different from controls under these conditions. In co-exposed animals, DNA methylation increased at the hippocampal Igf1 gene promoter. These results show that blood IGF1 and IGF2 are most reduced in the early chronic phase of TBI and after exposure to a stressful event, and that the brain IGF system is up-regulated after TBI, and more so in the acute phase.

Effects of Alcohol and Cocaine in a Mutant Mouse Model of Predisposition to Post-Traumatic Stress Disorder.

Comorbidity between drug abuse and post-traumatic stress disorder (PTSD), a stress-related dysregulation of fear responses, is very high. While some drugs are known to increase fear and anxiety, there are only few data regarding interactions between voluntary drug consumption and fear memory. The spontaneous chronic consumption of either alcohol or cocaine under a 3-week free-choice progressive paradigm of alcohol (3/6/10%) or cocaine (0.2/0.4/0.6 mg/ml), was assessed in VGV transgenic mice, having full 5-HT2C receptor editing and displaying PTSD-like behaviors. The consequences of these drug consumptions on the potentiated contextual and cued fear conditioning responses of VGV mice were assessed. The effects of drugs on hippocampal brain-derived neurotrophic factor (Bdnf) mRNA were measured as its expression was previously found to be decreased in VGV mice. Chronic alcohol consumption was similar in WT and VGV mice. In the alcohol condition, fear acquisition was not different at the end of the learning session and cue-fear extinction was facilitated. Regarding cocaine, in contrast to WT mice, VGV mice did not increase their drug consumption along with increasing doses, an effect that might be related with enhanced drug stimuli discrimination via increased 5-HT2C receptors. Cocaine-intake VGV mice did not display the contextual fear generalization usually observed in control VGV mice. In addition, Bdnf expression was upregulated after either chronic alcohol or cocaine intake. Altogether, these results suggest that both chronic alcohol and cocaine voluntary oral consumptions can exert some therapeutic-like effects in a mutant model of PTSD predisposition.

Histological and Behavioral Evaluation after Traumatic Brain Injury in Mice: A Ten Months Follow-Up Study.

Traumatic brain injury (TBI) is a chronic pathology, inducing long-term deficits that remain understudied in pre-clinical studies. In this context, exploration, anxiety-like behavior, cognitive flexibility, and motor coordination were assessed until 5 and 10 months after an experimental TBI in the adult mouse, using two cohorts. In order to differentiate age, surgery, and remote gray and white matter lesions, three groups (unoperated, sham-operated, and TBI) were studied. TBI induced delayed motor coordination deficits at the pole test, 4.5 months after injury, that could be explained by gray and white matter damages in ipsilateral nigrostriatal structures (striatum, internal capsule) that were spreading to new structures between cohorts, at 5 versus 10 months after the injury. Further, TBI induced an enhanced exploratory behavior during stressful situations (active phase during actimetry test, object exploration in an open field), risk-taking behaviors in the elevated plus maze 5 months after injury, and a cognitive inflexibility in the Barnes maze that persisted until 9 months after the injury. These behavioral modifications could be related to the white and gray matter lesions observed in ipsi- and contralateral limbic structures (amygdala, hilus/cornu ammonis 4, hypothalamus, external capsule, corpus callosum, and cingular cortex) that were spreading to new structures between cohorts, at 5 months versus 10 months after the injury. The present study corroborates clinical findings on TBI and provides a relevant rodent chronic model which could help in validating pharmacological strategies against the chronic consequences of TBI.

[Neurotrophic mechanisms of psychedelic therapy]. / Utilisation des psychédéliques en psychiatrie : lien avec les neurotrophines.

Psychedelic drugs, often referred to as hallucinogens, are quite distinct from other classes of psychotropic drugs. Although the subjective and behavioral effects they induce are quite dramatic, they possess little addictive potential when compared to nicotine, alcohol or opiates. Since the discovery of ketamine antidepressant effects, there has been growing interest for these molecules. Serotonergic psychedelics such as psilocybin and lysergic acid diethylamide (LSD) are gaining attention as potential treatments for depression and addiction, similarly to 3,4-methylenedioxymethamphetamine (MDMA) for post-traumatic stress disorder (PTSD), and ibogaine for addiction. Although they possess distinct pharmacological profiles, their kinetics of action are quite similar: the therapeutic effects are felt within the hours following administration, and last well beyond drug elimination by the organism. This strongly suggests the induction of neurogenic and plastic mechanisms, including the involvement of trophic factors. This review will explore the literature dealing with the effects of psychedelics on neurotrophins, as well as the plastic adaptations that they induce, in an attempt to understand their surprising therapeutic potential. We will show that although ketamine and serotonergic psychedelics have affinity for very different receptors (NMDA, 5-HT2A), they ultimately initiate similar plastic adaptations in the prefrontal cortex through the involvement of the brain-derived neurotrophic factor (BDNF). We will see that although MDMA uses the same receptors as serotonergic psychedelics to alleviate PTSD symptoms, its effect on BDNF levels seem paradoxical and quite different. Finally, we show how ibogaine could exert its anti-addictive properties through a completely different neurotrophic factor than other psychedelic drugs, the glial cell line-derived neurotrophic factor (GDNF). While the current literature concerning the psychiatric applications of psychedelic therapy is encouraging, it remains to be determined whether their benefits could be obtained without their psychotomimetic effects, or concerns over potential toxicity.

TITLE: Utilisation des psychédéliques en psychiatrie : lien avec les neurotrophines. ABSTRACT: Les psychédéliques, souvent appelés hallucinogènes, sont une classe de psychotropes très singulière. Les effets subjectifs et comportementaux qu'ils induisent sont très impressionnants, et malgré leur toxicité potentielle, le risque d'addiction est relativement faible par rapport à la nicotine, l'alcool ou les opiacés. Depuis la découverte des effets antidépresseurs de la kétamine, il existe un regain d'intérêt pour cette classe de molécules. En effet, la psilocybine et l'acide lysergique diéthylamide (LSD) gagnent de la popularité en tant que traitement pour la dépression et l'addiction, la 3,4-méthylènedioxyméthamphétamine (MDMA) pour l'état de stress post-traumatique, et l'ibogaïne pour l'addiction. Malgré des profils pharmacologiques distincts, ces différentes drogues partagent une cinétique d'action similaire : leurs effets thérapeutiques se font ressentir dans les heures suivant l'administration et perdurent au-delà de leur élimination par l'organisme. Ceci suggère des mécanismes plastiques et neurogéniques impliquant entre autres des facteurs trophiques. Cette revue explorera la littérature concernant les effets de ces différents composés sur les neurotrophines, ainsi que les adaptations plastiques qui sont mises en place dans les heures et jours suivant l'administration, afin de comprendre leur potentiel thérapeutique étonnant.

Increased 5-HT2C receptor editing predisposes to PTSD-like behaviors and alters BDNF and cytokines signaling.

Post-traumatic stress disorder (PTSD) is a trauma- and stress-related disorder with dysregulated fear responses and neurobiological impairments, notably at neurotrophic and inflammation levels. Understanding the mechanisms underlying this disease is crucial to develop PTSD models that meet behavioral and neurobiological validity criteria as well as innovative therapeutic approaches. Serotonin 2C receptors (5-HT2CR) are known for their important role in anxiety, and mice having only the fully edited VGV isoform of 5-HT2CR, which thereby overexpressed brain 5-HT2CR, are of special interest to study PTSD predisposition. Innate and conditioned fear-related behaviors were assessed in VGV and wild-type mice. mRNA expression of brain-derived neurotrophic factor (BDNF), tissue-plasminogen activator (tPA), and pro-inflammatory cytokines (IL-6, IL-1ß, and calcineurin) were measured by qRT-PCR. The effect of acute and chronic paroxetine was evaluated on both behavior and gene expression. VGV mice displayed greater fear expression, extensive fear extinction deficits, and fear generalization. Paroxetine restored fear extinction in VGV mice when administered acutely and decreased innate fear and fear generalization when administered chronically. In parallel, Bdnf, tPA, and pro-inflammatory cytokines mRNA levels were dysregulated in VGV mice. Bdnf and tPA mRNA expression was decreased in the hippocampus but increased in the amygdala, and chronic paroxetine normalized Bdnf mRNA levels both in the amygdala and the hippocampus. Amygdalar calcineurin mRNA level in VGV mice was also normalized by chronic paroxetine. VGV-transgenic mice displayed behavioral and neurobiological features that could be accessory to the investigation of PTSD and its treatment. Furthermore, these data point out to the role of 5-HT2CR in neuroplasticity and neuroinflammation.

Neuroepigenetics of Neurotrophin Signaling: Neurobiology of Anxiety and Affective Disorders.

This review examines the epigenetic of neurotrophin signaling in anxiety, affective disorders and related symptoms associated with drug addiction, in particular alcoholism. It is first important to understand the epigenetics of aversion memories, as they are so central to anxiety and affective disorders symptomology. The crucial role of neurotrophins in memory formation, in particular the brain-derived nerve growth factor (BDNF), is explored at the physiological and behavioral levels. Numerous studies describe how various epigenetic phenomena, mainly histone acetylation, histone methylation, DNA methylation, but also other less known epigenetic phenomena such as histone poly[ADP]-ribosylation and 5-HT2C receptor pre-mRNA editing, exert significant regulatory roles in aversion memory and fear extinction memory formation. Other models of anxiety and affective disorders, that use stress or transgenic constructs directed at elements of the stress axis or the serotonergic system, are then explored in relation with the epigenetic of neurotrophin signaling. Epigenetic marks differentially change according to brain areas. In the hippocampus for example, anxious or chronically stressed animals tend to show epigenetic changes that are at the opposite of those observed after memory consolidation following a brief aversive stimulus. Behaviorally relevant epigenetic changes have been found to be reversible by drug treatments. Surprisingly, moderate alcohol consumption may trigger, on the long term, changes of BDNF expression and of its epigenetic elements that are somehow similar to those produced by antidepressant drugs. However, alcohol withdrawal associated with anxiety symptoms has not yet been very well explored. Overall, it appears that a multidisciplinary view on the epigenetics of neurotrophin secretion might bring innovative treatments to psychiatric diseases involving stress and fear memories.

Tetrahydrobiopterin improves hippocampal nitric oxide-linked long-term memory.

Tetrahydrobiopterin (BH4) is synthesized by the combined action of three metabolic pathways, namely de novo synthesis, recycling, and salvage pathways. The best-known function of BH4 is its mandatory action as a natural cofactor of the aromatic amino acid hydroxylases and nitric oxide synthases. Thus, BH4 is essential for the synthesis of nitric oxide, a retrograde neurotransmitter involved in learning and memory. We investigated the effect of BH4 (4-4000 pmol) intracerebroventricular administration on aversive memory, and on BH4 metabolism in the hippocampus of rodents. Memory-related behaviors were assessed in Swiss and C57BL/6 J mice, and in Wistar rats. It was consistently observed across all rodent species that BH4 facilitates aversive memory acquisition and consolidation by increasing the latency to step-down in the inhibitory avoidance task. This effect was associated with a reduced threshold to generate hippocampal long-term potentiation process. In addition, two inhibitors of memory formation (N(ω)-nitro-L-arginine methyl ester - L-Name - and dizocilpine - MK-801 -) blocked the enhanced effect of BH4 on memory, while the amnesic effect was not rescue by the co-administration of BH4 or a cGMP analog (8-Br-cGMP). The data strongly suggest that BH4 enhances aversive memory by activating the glutamatergic neurotransmission and the retrograde activity of NO. It was also demonstrated that BH2 can be converted into BH4 by activating the BH4 salvage pathway under physiological conditions in the hippocampus. This is the first evidence showing that BH4 enhances aversive memory and that the BH4 salvage pathway is active in the hippocampus.

Response of Htr3a knockout mice to antidepressant treatment and chronic stress.

BACKGROUND AND PURPOSE: It has recently been suggested that 5-HT3 receptor blockade enhances the efficacy of selective 5-HT (serotonin) reuptake inhibitor (SSRI) antidepressants and may reverse stress-induced deficits in rodents. EXPERIMENTAL APPROACH: To further explore this hypothesis, we used mice lacking the 5-HT3 receptor (Htr3a KO) and their wild-type (WT) controls to assess their response in behavioural paradigms relevant to anxiety and depression. Mice were studied under basal, antidepressant treatments and chronic social defeat stress (CSDS) conditions. KEY RESULTS: In basal conditions, Htr3a KO mice displayed anxiolytic- and antidepressant-like behaviours in the elevated plus maze, the social interaction and the forced swim tests (FST), but behaved as WT mice in response to acute citalopram in the FST. However, the effects of fluoxetine were blunted in Htr3a KO mice in these same tests. In an in vitro electrophysiological paradigm, a low-dose citalopram treatment triggered 5-HT1A receptor desensitization only in the dorsal raphe nucleus of Htr3a KO, although a high dose desensitized 5-HT1A autoreceptor function equally in Htr3a KO and WT mice, suggesting that citalopram may become effective at lower doses when 5-HT3 receptors are inactivated. In addition, Htr3a deletion blocked CSDS-induced modification in the cortical expression of two genes involved in oxidative stress, CaMKIIa and SOD1. CONCLUSIONS AND IMPLICATIONS: Taken together, these data show that Htr3a deletion promotes SSRI efficacy and prevents the occurrence of stress-induced deleterious effects, suggesting that the 5-HT3 receptor may represent an interesting target for the treatment of stress-related disorders.

Neopterin acts as an endogenous cognitive enhancer.

Neopterin is found at increased levels in biological fluids from individuals with inflammatory disorders. The biological role of this pteridine remains undefined; however, due to its capacity to increase hemeoxygenase-1 content, it has been proposed as a protective agent during cellular stress. Therefore, we investigated the effects of neopterin on motor, emotional and memory functions. To address this question, neopterin (0.4 and/or 4pmol) was injected intracerebroventricularly before or after the training sessions of step-down inhibitory avoidance and fear conditioning tasks, respectively. Memory-related behaviors were assessed in Swiss and C57BL/6 mice, as well as in Wistar rats. Moreover, the putative effects of neopterin on motor and anxiety-related parameters were addressed in the open field and elevated plus-maze tasks. The effects of neopterin on cognitive performance were also investigated after intraperitoneal lipopolysaccharide (LPS) administration (0.33mg/kg) in interleukin-10 knockout mice (IL-10(-/-)). It was consistently observed across rodent species that neopterin facilitated aversive memory acquisition by increasing the latency to step-down in the inhibitory avoidance task. This effect was related to a reduced threshold to generate the hippocampal long-term potentiation (LTP) process, and reduced IL-6 brain levels after the LPS challenge. However, neopterin administration after acquisition did not alter the consolidation of fear memories, neither motor nor anxiety-related parameters. Altogether, neopterin facilitated cognitive processes, probably by inducing an antioxidant/anti-inflammatory state, and by facilitating LTP generation. To our knowledge, this is the first evidence showing the cognitive enhancer property of neopterin.

5-HT2C receptor desensitization moderates anxiety in 5-HTT deficient mice: from behavioral to cellular evidence.

BACKGROUND: Desensitization and blockade of 5-HT2C receptors (5-HT2CR) have long been thought to be central in the therapeutic action of antidepressant drugs. However, besides behavioral pharmacology studies, there is little in vivo data documenting antidepressant-induced 5-HT2CR desensitization in specific brain areas. METHODS: Mice lacking the 5-HT reuptake carrier (5-HTT(-/-)) were used to model the consequences of chronic 5-HT reuptake inhibition with antidepressant drugs. The effect of this mutation on 5-HT2CR was evaluated at the behavioral (social interaction, novelty-suppressed feeding, and 5-HT2CR-induced hypolocomotion tests), the neurochemical, and the cellular (RT-qPCR, mRNA editing, and c-fos-induced expression) levels. RESULTS: Although 5-HTT(-/-) mice had an anxiogenic profile in the novelty-suppressed feeding test, they displayed less 5-HT2CR-mediated anxiety in response to the agonist m-chlorophenylpiperazine in the social interaction test. In addition, 5-HT2CR-mediated inhibition of a stress-induced increase in 5-HT turnover, measured in various brain areas, was markedly reduced in 5-HTT(-/-) mutants. These indices of tolerance to 5-HT2CR stimulation were associated neither with altered levels of 5-HT2CR protein and mRNA nor with changes in pre-mRNA editing in the frontal cortex. However, basal c-fos mRNA production in cells expressing 5-HT2CR was higher in 5-HTT(-/-) mutants, suggesting an altered basal activity of these cells following sustained 5-HT reuptake carrier inactivation. Furthermore, the increased c-fos mRNA expression in 5-HT2CR-like immune-positive cortical cells observed in wild-type mice treated acutely with the 5-HT2CR agonist RO-60,0175 was absent in 5-HTT(-/-) mutants. CONCLUSIONS: Such blunted responsiveness of the 5-HT2CR system, observed at the cell signaling level, probably contributes to the moderation of the anxiety phenotype in 5-HTT(-/-) mice.

Effect of genetic and pharmacological blockade of GABA receptors on the 5-HT2C receptor function during stress.

Serotonin (5-HT)2C receptors play a role in psychoaffective disorders and often contribute to the antidepressant and anxiolytic effects of psychotropic drugs. During stress, activation of these receptors exerts a negative feedback on 5-HT release, probably by increasing the activity of GABAergic interneurons. However, to date, the GABA receptor types that mediate the 5-HT2C receptor-induced feedback inhibition are still unknown. To address this question, we assessed the inhibition of 5-HT turnover by a 5-HT2C receptor agonist (RO 60-0175) at the hippocampal level and under conditions of stress, after pharmacological or genetic inactivation of either GABA-A or GABA-B receptors in mice. Neither the GABA-B receptor antagonist phaclofen nor the specific genetic ablation of either GABA-B1a or GABA-B1b subunits altered the inhibitory effect of RO 60-0175, although 5-HT turnover was markedly decreased in GABA-B1a knock-out mice in both basal and stress conditions. In contrast, the 5-HT2C receptor-mediated inhibition of 5-HT turnover was reduced by the GABA-A receptor antagonist bicuculline. However, a significant effect of 5-HT2C receptor activation persisted in mutant mice deficient in the α3 subunit of GABA-A receptors. It can be inferred that non-α3 subunit-containing GABA-A receptors, but not GABA-B receptors, mediate the 5-HT2C -induced inhibition of stress-induced increase in hippocampal 5-HT turnover in mice.

Controversies on the role of 5-HT(2C) receptors in the mechanisms of action of antidepressant drugs.

Evidence from the various sources indicates alterations in 5-HT2C receptor functions in anxiety, depression and suicide, and other stress-related disorders treated with antidepressant drugs. Although the notion of a 5-HT2C receptor desensitization following antidepressant treatments is rather well anchored in the literature, this concept is mainly based on in vitro assays and/or behavioral assays (hypolocomotion, hyperthermia) that have poor relevance to anxio-depressive disorders. Our objective herein is to provide a comprehensive overview of the studies that have assessed the effects of antidepressant drugs on 5-HT2C receptors. Relevant molecular (second messengers, editing), neurochemical (receptor binding and mRNA levels), physiological (5-HT2C receptor-induced hyperthermia and hormone release), behavioral (5-HT2C receptor-induced changes in feeding, anxiety, defense and motor activity) data are summarized and discussed. Setting the record straight about drug-induced changes in 5-HT2C receptor function in specific brain regions should help to determine which pharmacotherapeutic strategy is best for affective and anxiety disorders.

Six weeks of voluntary exercise don't protect C57BL/6 mice against neurotoxicity of MPTP and MPP(+).

Exercise improves the central nervous system (CNS) functions and is widely recommended for neurological patients with, e.g., Alzheimer's and Parkinson's disease (PD). However, exercise-induced neuroprotection is an open discussion. Here, the intranasal administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 65 mg/kg) caused death of dopaminergic neurons in the substantia nigra pars compacta and depletion of dopamine in the striatum of C57BL/6 mice. 1-Methyl-4-phenylpyridinium, the active metabolite of MPTP, also inhibited complex-I activity of mitochondria isolated from the CNS of mice. However, 6 weeks of exercise on voluntary running wheels did not protect against nigrostriatal neurodegeneration or mitochondrial inhibition, suggesting that benefits of exercise for PD may not be associated with neuroprotection. The literature presents other candidates, such as neurotrophins or increased antioxidant defenses.

Parkin-knockout mice did not display increased vulnerability to intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

The loss of nigral dopaminergic neurons in Parkinson's disease (PD) is believed to result from interactions between genetic susceptibility and environmental factors. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, the hybrid 129Sv-C57BL/6 parkin-deficient mice did not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to neurotoxicity induced by 6-hydroxydopamine (6-OHDA) or intraperitoneal 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. We aimed to re-evaluate the role of parkin in a pure C57BL/6 background after an acute intranasal (i.n.) MPTP administration, a new route of toxin delivery to the brain that mimics environmental exposure to neurotoxins. We found that the deficiency of parkin gene modifies the D-amphetamine-induced locomotion in saline-treated animals. Intranasal MPTP induced Parkinsonism in parkin⁺/⁺ mice, through depletion of striatal dopamine, decreased number of dopaminergic neurons in the substantia nigra, and decreased D-amphetamine-induced hyperlocomotion. Additionally, the deletion of the parkin gene in a pure C57BL/6 background did not lead to increased vulnerability to i.n. MPTP-induced neurotoxicity. Moreover, the i.n. MPTP induced nigral astrogliosis predominantly in the pars reticulata in wild type and parkin⁻/⁻ mice. Taken together, these results showed that the absence of parkin did not modify the vulnerability of nigrostriatal dopaminergic pathway after i.n. MPTP intoxication, suggesting that independently of mouse strain, the endogenous parkin is not required for protection of this system. These findings also suggest that the development of familial parkin-linked PD is not associated with exposure to environmental factors that specifically affects the dopaminergic system.

Biological rhythms and melatonin in mood disorders and their treatments.

Affective disorders such as major depression, bipolar disorders and seasonal affective disorders have been described as alterations of various neuronal systems. In addition to the classical monoaminergic hypotheses that have been long proposed to explain the pathophysiology of these disorders, a strong association between circadian rhythms and mood regulation has been suggested in the light of several clinical and preclinical findings. In this review, we summarize the different hypotheses on pathophysiology mechanisms underlying depressive disorders and put a special emphasis on the alterations of melatonin secretion and associated changes in biological rhythms that characterize mood disorders. Causal relationships between alterations in circadian rhythms and mood disorders are strongly supported by the antidepressant efficacy of innovative pharmacological treatments aimed at resynchronizing endogenous rhythms in depressed patients. Genetic, epigenetic and environmental factors generating desynchronization between endogenous biological rhythms and exogenous rhythms driven by environmental and societal constraints are very probably involved in the vulnerability to mood disorders. Further investigations of the molecular/cellular bases of the relationships between stress axis dysfunctions, endogenous biological rhythm dysregulations and associated functional and anatomical brain alterations should allow important progress in the knowledge of pathophysiological mechanisms of affective disorders and the downstream development of innovative, more effective and better tolerated, therapies.

Beyond the monoaminergic hypothesis: neuroplasticity and epigenetic changes in a transgenic mouse model of depression.

The monoamine hypothesis of depression has dominated our understanding of both the pathophysiology of depression and the action of pharmacological treatments for the last decades, and it has led to the production of several generations of antidepressant agents. However, there are serious limitations to the current monoamine theory, and additional mechanisms, including hypothalamic-pituitary-adrenal (HPA) axis dysfunctions, as well as neurodegenerative and inflammatory alterations, are potentially associated with the pathogenesis of mood disorders. Moreover, new data have recently indicated that epigenetic mechanisms such as histone modifications and DNA methylation could affect diverse pathways leading to depression-like behaviours in animal models. In a transgenic mouse model of depression, in which a downregulation of glucocorticoid receptors (GR) causes a deficit in the HPA axis feedback control, besides alterations in monoamine neurotransmission and neuroplasticity, we found modifications in the expression of many proteins involved in epigenetic regulation, as well as clock genes, in the hippocampus and the frontal cortex, that might be central in the genesis of depressive-like behaviours.

Severe serotonin depletion after conditional deletion of the vesicular monoamine transporter 2 gene in serotonin neurons: neural and behavioral consequences.

The vesicular monoamine transporter type 2 gene (VMAT2) has a crucial role in the storage and synaptic release of all monoamines, including serotonin (5-HT). To evaluate the specific role of VMAT2 in 5-HT neurons, we produced a conditional ablation of VMAT2 under control of the serotonin transporter (slc6a4) promoter. VMAT2(sert-cre) mice showed a major (-95%) depletion of 5-HT levels in the brain with no major alterations in other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2(sert-cre) mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT(1A) autoreceptor coupling to G protein, as assessed with agonist-stimulated [(35)S]GTP-γ-S binding, was observed in the raphe area, indicating an adaptive change to reduced 5-HT transmission. Behavioral evaluation in adult VMAT2(sert-cre) mice showed an increase in escape-like reactions in response to tail suspension and anxiolytic-like response in the novelty-suppressed feeding test. In an aversive ultrasound-induced defense paradigm, VMAT2(sert-cre) mice displayed a major increase in escape-like behaviors. Wild-type-like defense phenotype could be rescued by replenishing intracellular 5-HT stores with chronic pargyline (a monoamine oxidase inhibitor) treatment. Pargyline also allowed some form of 5-HT release, although in reduced amounts, in synaptosomes from VMAT2(sert-cre) mouse brain. These findings are coherent with the notion that 5-HT has an important role in anxiety, and provide new insights into the role of endogenous 5-HT in defense behaviors.

Mice with genetic deletion of the heparin-binding growth factor midkine exhibit early preclinical features of Parkinson's disease.

There is considerable evidence showing that the neurodegenerative processes that lead to sporadic Parkinson's disease (PD) begin many years before the appearance of the characteristic motor symptoms and that impairments in olfactory, cognitive and motor functions are associated with time-dependent disruption of dopaminergic neurotransmission in different brain areas. Midkine is a 13-kDa retinoic acid-induced heparin-binding growth factor involved in many biological processes in the central nervous system such as cell migration, neurogenesis and tissue repair. The abnormal midkine expression may be associated with neurochemical dysfunction in the dopaminergic system and cognitive impairments in rodents. Here, we employed adult midkine knockout mice (Mdk(-/-)) to further investigate the relevance of midkine in dopaminergic neurotransmission and in olfactory, cognitive and motor functions. Mdk(/-) mice displayed pronounced impairments in their olfactory discrimination ability and short-term social recognition memory with no gross motor alterations. Moreover, the genetic deletion of midkine decreased the expression of the enzyme tyrosine hydroxylase in the substantia nigra reducing partially the levels of dopamine and its metabolites in the olfactory bulb and striatum of mice. These findings indicate that the genetic deletion of midkine causes a partial loss of dopaminergic neurons and depletion of dopamine, resulting in olfactory and memory deficits with no major motor impairments. Therefore, Mdk(-/-) mice may represent a promising animal model for the study of the early stages of PD and for testing new therapeutic strategies to restore sensorial and cognitive processes in PD.

5-HT2C receptor activation prevents stress-induced enhancement of brain 5-HT turnover and extracellular levels in the mouse brain: modulation by chronic paroxetine treatment.

Stress is known to activate the central 5-hydroxytryptamine (5-HT) system, and this is probably part of a coping response involving several 5-HT receptors. Although 5-HT(2C) receptors are well known to be implicated in anxiety, their participation in stress-induced changes had not been investigated in parallel at both behavioral and neurochemical levels. We show here that the preferential 5-HT(2C) receptor agonist, m-chlorophenylpiperazine, as well as restraint stress increased anxiety in the mouse social interaction test. The selective 5-HT(2C) receptor antagonist, SB 242,084, prevented both of these anxiogenic effects. Restraint stress increased 5-HT turnover in various brain areas, and this effect was prevented by the 5-HT(2B/2C) receptor agonist RO 60-0175 (1 mg/kg), but not the preferential 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1 mg/kg), and in contrast potentiated by SB 242,084 (1 mg/kg), which also blocked the effect of RO 60-0175. Using microdialysis, RO 60-0175 was shown to inhibit cortical 5-HT overflow in stressed mice when 5-HT reuptake was blocked locally. Chronic paroxetine prevented both the anxiogenic effect of m-chlorophenylpiperazine and the inhibitory effect of RO 60-0175 on locomotion and stress-induced increase in 5-HT turnover. The anxiolytic action of chronic paroxetine might be associated with an enhancement of 5-HT neurotransmission caused by a decreased 5-HT(2C) receptor-mediated inhibition of stress-induced increase in 5-HT release.