Chronic 5-HT7R activation drives depressive phenotypes and synaptic dysfunction.

Acute and sustained effects of a metabotropic glutamate 5 receptor antagonist in the novelty-suppressed feeding test

Physiological variability in brain-derived neurotrophic factor expression predicts dendritic spine density in the mouse dentate gyrus

An emerging view of the hippocampus is that of a functionally heterogeneous structure along its longitudinal axis. Lesion studies reveal that the dorsal (septal pole) hippocampus is involved in learning and spatial memory, whereas the ventral (temporal pole) hippocampus regulates emotional and motivated behaviors (Fanselow and Dong, 2010). Anatomical connectivity and gene expression analyses support this functional dissociation. For example, serotonergic fibers provide denser input to the ventral hippocampus with a concomitant enrichment of 5-HT1A and 2C receptors ventrally (KF Tanaka and R Hen, unpublished). Efferent connectivity indicates that ventral hippocampus can modulate reward circuitry and emotional behavior through projections to nucleus accumbens, prefrontal cortex and amygdala, and stress responses by regulating the hypothalamic–pituitary–adrenal axis (Sahay and Hen, 2007). In both regions, the subgranular zone of the dentate gyrus (DG) continues to produce new neurons in adulthood. These adult-born granule cells (GCs) functionally integrate into the DG circuit, exhibit enhanced excitability, and have a significant impact on both learning and emotional behavior (Sahay and Hen, 2007). As adult neurogenesis has been implicated in both learning and mood, an exciting possibility is that adult-born GCs in the dorsal and ventral hippocampus may be functionally dissociated. Chronic antidepressant treatment increases neurogenesis in the DG, a requirement for some of their behavioral effects (Santarelli et al, 2003). Recent studies suggest that antidepressants regulate behavior by selectively increasing ventral hippocampal neurogenesis. Chronic treatment with agomelatine, a melatonin receptor agonist, and 5-HT2C receptor antagonist with robust effects in animal models as well as efficacy in human major depressive disorder increases neurogenesis selectively in the ventral DG (Banasr et al, 2006). In humans, selective serotonin reuptake inhibitors and tricyclic antidepressants increase proliferating and neuronal precursor cells more prominently in the anterior portion of the DG of patients with MDD as compared to controls and untreated MDD subjects (Boldrini et al, 2009). These two studies, although correlational, provide exciting preliminary evidence that warrants future studies aimed at selectively blocking or stimulating neurogenesis in the ventral hippocampus. In the cognitive realm, adult neurogenesis has recently been implicated in pattern separation, the ability to distinguish between similar contexts (Deng et al, 2010). Owing to their unique physiological properties, adult-born GCs may contribute to pattern separation by modulation of sparse coding in the DG. A recent report indicates that ablation of adult-born GCs increases the magnitude of spontaneous gamma bursts in the dorsal DG and enhances modulation of single unit firing by these bursts (Lacefield et al, 2010). This increase in spontaneous activity suggests that adult-born GCs may modulate network inhibition of mature GCs through either feedback inhibition or synaptic competition. Thus, young GCs may contribute to sparse coding and pattern separation by modulating inhibitory control of the mature GCs in the DG. Deficits in pattern separation may not only impact cognition, but may also contribute to anxiety disorders by impairing the ability to discriminate between safe and fearful contexts. The resulting inability to respond appropriately to similar situations that have differing emotional valence may lead to the generalization observed in certain anxiety disorders such as PTSD. As adult-born GCs have a role in sparse coding in the DG and pattern separation, it will be of great interest to examine how blocking or stimulating ventral hippocampal neurogenesis may influence emotional behavior by modulating pattern separation.

Ascorbic acid presents rapid behavioral and hippocampal synaptic plasticity effects

Increased neuroplasticity and hippocampal microglia activation in a mice model of rapid antidepressant treatment

Clinical and preclinical studies report the implication of 5-hydroxytryptamine 4 receptors (5-HT4Rs) in depression and anxiety. Here, we tested whether the absence of 5-HT4Rs influences the response to the antidepressant fluoxetine in mice subjected to chronic corticosterone administration, an animal model of depression and anxiety. Therefore, the effects of chronic administration of fluoxetine in corticosterone-treated wild-type (WT) and 5-HT4R knockout (KO) mice were evaluated in the open-field and novelty suppressed feeding tests. As 5-HT1A receptor (5-HT1AR) and brain-derived neurotrophic factor (BDNF) are critically involved in depression and anxiety, we further evaluated 5-HT1A receptor functionality by [35S]GTPγS autoradiography and BDNF mRNA expression by in situ hybridization techniques. We found that 5-HT4R KO and WT mice displayed anxiety- and depressive-like behavior following chronic administration of corticosterone, as evidenced in the open-field and novelty suppressed feeding tests. In the open-field, a decreased central activity was observed in naïve and corticosterone-treated mice of both genotypes following chronic fluoxetine administration. In the novelty suppressed feeding test, a predictive paradigm of antidepressant activity, chronic treatment with fluoxetine reverted the latency to eat in both genotypes. The antidepressant also potentiated the corticosterone-induced desensitization of the 5-HT1AR in the dorsal raphe nucleus. Further, chronic fluoxetine increased BDNF mRNA expression in the dentate gyrus of the hippocampus in corticosterone-treated mice of both genotypes. Therefore, our findings indicate that the behavioral effects of fluoxetine in the corticosterone model of depression and anxiety appear not to be dependent on 5-HT4Rs.

Modulation of serotonergic signaling by treatment with selective serotonin reuptake inhibitors (SSRIs) has been suggested to mitigate amyloid-β (Aβ) pathology in Alzheimer's disease, in addition to exerting an anti-depressant action. To investigate the efficacy of chronic treatment with the SSRI paroxetine, in mitigating Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of 18-month-old APPswe/PS1ΔE9 mice. Plaque-bearing APPswe/PS1ΔE9 and wildtype mice were treated with paroxetine per os at a dose of 5 mg/kg/day, from 9 to 18 months of age. The per os treatment was monitored by recording of the body weights and serum paroxetine concentrations, and by assessment of the serotonin transporter occupancy by [3H]DASB-binding in wildtype mice. Additionally, 5,7-dihydroxytryptamine was administered to 9-month-old APPswe/PS1ΔE9 mice, to examine the effect of serotonin depletion on Aβ pathology. Aβ pathology was evaluated by Aβ plaque load estimation and the Aβ42/Aβ40 ratio by ELISA. Paroxetine treatment led to > 80% serotonin transporter occupancy. The treatment increased the body weight of wildtype mice, but not of APPswe/PS1ΔE9 mice. The treatment had no effect on the Aβ plaque load (p = 0.39), the number and size of plaques, or the Aβ plaque-induced increases in microglial numbers in the dentate gyrus. Three months of serotonin depletion did not significantly impact the Aβ plaque load or Aβ42/Aβ40 ratio in APPswe/PS1ΔE9 mice at 12 months. Our results show that chronic treatment with the SSRI paroxetine does not mitigate Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of APPswe/PS1ΔE9 mice.

A current hypothesis to explain the limited recovery following brain and spinal cord trauma stems from the dogma that neurons in the mammalian central nervous system lack the ability to regenerate their axons after injury. Serotonin (5-HT) neurons in the adult brain are a notable exception in that they can slowly regrow their axons following chemical or mechanical lesions. This process of regrowth occurs without intervention over several months and results in anatomical recovery that approximates the preinjured state. During development, serotonin is a trophic factor, playing a role in both cell survival and axon growth. Additionally, some studies have shown that stroke patients treated after injury with serotonin selective reuptake inhibitors (SSRIs) appeared to have improved recovery. To test the hypothesis that serotonin can influence the regrowth of 5-HT axons, mice received a high dose of para-chloroamphetamine (PCA) to induce widespread retrograde degeneration of 5-HT axons. Then, after a short rest period to avoid any interaction with the acute injury phase, SSRIs were administered daily for 6 or 10 weeks. Using immunohistochemistry in 5-HT transporter-GFP BAC transgenic mice, we determined that while PCA led to a rapid initial decrease in total 5-HT axon length in the somatosensory cortex, visual cortex, or area CA1 of the hippocampus, treatment with either fluoxetine or sertraline (two different SSRIs) did not affect the recovery of axon length. These results suggest that chronic SSRI treatment does not affect the regrowth of 5-HT axons and argue against SSRIs as a potential therapy following brain injury.

Characterization of 5-HT1A/1B−/− mice: An animal model sensitive to anxiolytic treatments

In animal models of addiction, repeated exposure to drugs of abuse, such as cocaine, amphetamines, and nicotine, increases the number of dendritic spines (panel A) in the nucleus accumbens (NAc), a key brain region involved in reward, pleasure, and motivation. Dendritic spines are the primary anatomical sites of excitatory synapses. Many scientists deem that this long-lasting increase in synaptic connectivity in the NAc may underlie the similarly persistent behaviors of drug taking and drug seeking associated with drug addiction and relapse. Although this concept is appealing, the functional relationship between increased synaptic connectivity in the NAc and addiction-related behaviors is poorly understood. Our recent findings shed some important light on this issue. We recently found that cocaine regulates myocyte enhancer factor 2 (MEF2) transcription factors in the NAc to control the observed increase in synaptic connectivity. During development, MEF2 sculpts mature brain circuits by promoting activity-dependent synapse elimination. Excitatory synaptic activity and subsequent calcium signaling events stimulate MEF2 activity. However, after chronic cocaine exposure or activation of dopamine receptor signaling, we observed a decrease in MEF2 activity in the striatum and found that decreasing MEF2 activity in the NAc promoted an increase in dendritic spine density. We also found that chronic cocaine and dopamine signaling reduced MEF2 activity in the striatum through novel signaling pathways involving the regulator of calmodulin signaling (RCS), calcineurin (CaN), and Cdk5 (panel B). Reducing MEF2 is required for cocaine-induced dendritic spine plasticity in vivo. On the other hand, we observed that expression of a hyperactive MEF2 in the NAc, the condition that blocked dendritic spine changes, increased behavioral responses to repeated cocaine exposure. This suggests that the increased spine density in the NAc is not required for cocaineinduced behavioral plasticity and that increased synaptic connections during chronic drug use may actually limit behavioral changes associated with drug addiction rather than support them. Together with the previous literature, our findings suggest that the increased density of synapses may be a compensatory response to decreased activity of these brain reward regions. Therefore, a focus on enhancing synaptic contacts and neuronal activity in the NAc may be therapeutic in drug addiction.

Despite the efficacy of selective serotonin reuptake inhibitors (SSRIs) in the treatment of major depression, a significant number of patients show partial or no remission of symptoms. Some evidence suggests that psychostimulant augmentation may be helpful in treating patients with residual symptoms of depression. The efficacy of modafinil in augmenting SSRIs in depressed patients with residual fatigue or excessive daytime sleepiness has yet to be systematically investigated. In a series of 25 patients with major depressive disorder, according to the criteria of the Diagnostic and Statistical Manual of Mental Disorders, 4th edition, who showed significant residual symptoms after an adequate SSRI trial (12 wk) and who were evaluated according to the Fatigue Severity Scale (FSS), subjects with scores > or = 4 were given open-label modafinil augmentation for a minimum of an additional 6 wk. Treatment response was assessed prospectively with the FSS, the Epworth Sleepiness Scale (ESS), and the Hamilton Rating Scale for Depression (HAM-D) during the first visit and at the second and sixth weeks. Twenty-one of 25 patients in this series who were treated with modafinil and SSRIs completed the 6-wk augmentation trial. At end-point assessment, all patients showed significant improvement in fatigue and sleepiness in FSS and ESS scores, as well as in HAM-D scores (P<.01). In the second week, 29.4% of patients had a HAM-D score <7, which was defined as remission; this rate was 64.7% in the sixth week. The rate of patients whose HAM-D score dropped by more than 50%, defined as responders to treatment, was 41.1% and 76.4% in the second and sixth weeks, respectively. Results of this preliminary, open-label trial suggest that modafinil may be effective in augmenting ongoing SSRI treatment for a portion of patients with major depression who have residual fatigue and sleepiness. Larger, placebo-controlled trials appear warranted to investigate the clinical efficacy and tolerability of modafinil augmentation of SSRI treatment in these patients.

Psychostimulant drugs of abuse increase dendritic spine density in reward centers of the brain. However, little is known about their effects in the hippocampus, where activity-dependent changes in the density of dendritic spine are associated with learning and memory. Recent reports suggest that Cdk5 plays an important role in drug addiction, but its role in psychostimulant’s effects on dendritic spines in hippocampus remain unknown. We used in vivo and in vitro approaches to demonstrate that amphetamine increases dendritic spine density in pyramidal neurons of the hippocampus. Primary cultures and organotypic slice cultures were used for cellular, molecular, pharmacological and biochemical analyses of the role of Cdk5/p25 in amphetamine-induced dendritic spine formation. Amphetamine (two-injection protocol) increased dendritic spine density in hippocampal neurons of thy1-green fluorescent protein (GFP) mice, as well as in hippocampal cultured neurons and organotypic slice cultures. Either genetic or pharmacological inhibition of Cdk5 activity prevented the amphetamine–induced increase in dendritic spine density. Amphetamine also increased spine density in neurons overexpressing the strong Cdk5 activator p25. Finally, inhibition of calpain, the protease necessary for the conversion of p35 to p25, prevented amphetamine’s effect on dendritic spine density. We demonstrate, for the first time, that amphetamine increases the density of dendritic spine in hippocampal pyramidal neurons in vivo and in vitro. Moreover, we show that the Cdk5/p25 signaling and calpain activity are both necessary for the effect of amphetamine on dendritic spine density. The identification of molecular mechanisms underlying psychostimulant effects provides novel and promising therapeutic approaches for the treatment of drug addiction.

In a recent study, utilizing single cell recording techniques, we have shown that administration of 5-HT1A receptor antagonists, e.g. (S)-UH-301, to rats concomitantly treated, acute or chronically, with the selective serotonin reuptake inhibitor (SSRI) citalopram significantly increases the activity of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN). Here we report correlative experiments using microdialysis in freely moving animals to measure extracellular levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex, a major projection area for DRN-5-HT neurons. Acute administration of (S)-UH-301 (2.5 mg/kg s.c.) or citalopram (2.0 mg/kg s.c.) increased 5-HT concentrations with a maximum of about 70% and 185%, respectively, above baseline. However, when (S)-UH-301 was administered 30 min before citalopram the maximal increase in 5-HT levels was approximately 400%. In rats chronically treated with citalopram (20 mg/kg/day i.p. for 14 days) basal 5-HT concentrations in the frontal cortex were significantly increased and 5-HIAA concentrations were decreased when measured 10-12 h, but not 18-20 h, after the last injection of citalopram, as compared to basal 5-HT and 5-HIAA concentrations in chronic saline-treated rats. When (S)-UH-301 (2.5 mg/kg s.c.) was administered 12 h, but not 20 h, after the last dose of citalopram it produced a significantly larger increase in extracellular concentrations of 5-HT than in control rats. However, in rats pretreated with a single, very high dose of citalopram, 20 mg/kg i.p., administration of (S)-UH-301 at 12 h after citalopram did not increase 5-HT levels. The augmentation by (S)-UH-301 of the increase in brain 5-HT output produced by acute administration of citalopram is probably due to antagonism of the citalopram induced feedback inhibition of 5-HT cells in the DRN, as previously suggested. However, the capacity of (S)-UH-301 to further increase the already elevated extracellular concentrations of 5-HT in brain in animals maintained on a chronic citalopram regimen, in which significant tolerance to the initial feedback inhibition of DRN-5-HT cells and developed, represents a novel finding. Generally, the reduced feedback inhibition of 5-HT neurons obtained with chronic citalopram treatment, and the associated elevation of brain 5-HT concentrations, may be related to functional desensitization of somatodendritic 5-HT1A autoreceptors in the DRN. This phenomenon may also largely explain the larger increase in 5-HT output produced by (S)-UH-301 in chronic citalopram treated animals as compared to its effect in control animals. Yet, a contributory factor may be a slight, remaining feedback inhibition of the 5-HT cells caused by residual citalopram at 12, but not 20 h after its last administration. Previous clinical studies suggest that addition of a 5-HT1A receptor antagonist to an SSRI in the treatment of depression may accelerate the onset of clinical effects. Moreover, in therapy-resistant cases maintained on SSRI treatment, addition of a 5-HT1A receptor antagonist may improve clinical efficacy. Since the therapeutic effect of SSRIs in depression has been found to be critically linked to the availability of 5-HT in brain, our experiments results support, in principle, both of the above clinically based notions.

Chronic unpredictable mild stress-induced behavioral changes are coupled with dopaminergic hyperfunction and serotonergic hypofunction in mouse models of depression

Chronic treatment with fluvoxamine desensitizes 5-HT 2C receptor-mediated hypolocomotion in rats