Role of glutamate and advantages of combining memantine with a 5HT6 ligand in a model of depression

Age-Related Change in 5-HT6 Receptor Availability in Healthy Male Volunteers Measured with 11C-GSK215083 PET

Serotonin receptor 6 (5-hydroxytrypamine-6, or 5-HT₆) is a potential therapeutic target given its distribution in brain regions that are important in depression, anxiety, and cognition. This study sought to investigate the effects of age on 5-HT₆ receptor availability using ¹¹C-GSK215083, a PET ligand with affinity for 5-HT₆ in the striatum and 5-HT_2A in the cortex. Methods: Twenty-eight healthy male volunteers (age range, 23-52 y) were scanned with ¹¹C-GSK215083 PET. Time-activity curves in regions of interest were fitted using a multilinear analysis method. Nondisplaceable binding potential (BP_ND) was calculated using the cerebellum as the reference region and corrected for partial-volume effects. Results: In 5-HT₆-rich areas, regional ¹¹C-GSK215083 showed a negative correlation between BP_ND and age in the caudate (r = -0.41, P = 0.03) (14% change per decade) and putamen (r = -0.30, P = 0.04) (11% change per decade) but not in the ventral striatum or pallidum. A negative correlation with age was also seen in cortical regions (r = -0.41, P = 0.03) (7% change per decade), consistent with the literature on 5-HT_2A availability. Conclusion: To our knowledge, this was the first in vivo study on humans to examine the effect of age on 5-HT₆ receptor availability. The study demonstrated a significant age-related decline in 5-HT₆ availability (BP_ND) in the caudate and putamen.

Characterization of Behavioral, Signaling and Cytokine Alterations in a Rat Neurodevelopmental Model for Schizophrenia, and Their Reversal by the 5-HT6 Receptor Antagonist SB-399885

Post-weaning social isolation of rats produces neuroanatomical, neurochemical and behavioral alterations resembling some core features of schizophrenia. This study examined the ability of the 5-HT₆ receptor antagonist SB-399885 to reverse isolation-induced cognitive deficits, then investigated alterations in hippocampal cell proliferation and hippocampal and frontal cortical expression of selected intracellular signaling molecules and cytokines. Male Lister hooded rats (weaned on post-natal days 21-24 and housed individually or in groups of 3-4) received six i.p. injections of vehicle (1% Tween 80, 1 mL/kg) or SB-399885 (5 or 10 mg/kg) over a 2-week period starting 40 days post-weaning, on the days that locomotor activity, novel object discrimination (NOD), pre-pulse inhibition of acoustic startle and acquisition, retention and extinction of a conditioned freezing response (CFR) were assessed. Tissue was collected 24 h after the final injection for immunohistochemistry, reverse-phase protein microarray and western blotting. Isolation rearing impaired NOD and cue-mediated CFR, decreased cell proliferation within the dentate gyrus, and elevated hippocampal TNFα levels and Cdc42 expression. SB-399885 reversed the NOD deficit and partially normalized CFR and cell proliferation. These effects were accompanied by altered expression of several members of the c-Jun N-terminal Kinase (JNK) and p38 MAPK signaling pathways (including TAK1, MKK4 and STAT3). Although JNK and p38 themselves were unaltered at this time point hippocampal TAK1 expression and phosphorylation correlated with visual recognition memory in the NOD task. Continued use of this neurodevelopmental model could further elucidate the neurobiology of schizophrenia and aid assessment of novel therapies for drug-resistant cognitive symptoms.

5-HT6 Receptor Antagonist as an Adjunct Treatment Targeting Residual Symptoms in Patients With Schizophrenia: Unexpected Sex-Related Effects (Double-Blind Placebo-Controlled Trial)

BACKGROUND

Treating patients who experience residual psychotic symptoms during remission of schizophrenia remains one of the most challenging problems. The mechanisms underlying these symptoms differ from those of acute hallucinations and delusions. 5-HT6 receptor antagonists have been considered promising agents in treatment of residual psychotic symptoms and cognitive dysfunction. The aim of the study was to assess the efficacy of a selective 5-HT6 inhibitor Avisetron in the reduction of residual psychotic symptoms in patients with schizophrenia on stable antipsychotic therapy.

METHODS

Eighty clinically stable outpatient subjects with schizophrenia with residual psychotic symptoms were randomized in a double-blind manner to 6 weeks of Avisetron or placebo at 1:1 ratio. Subjects received 8 mg of Avisetron or placebo on top their stable antipsychotic treatment. Standard clinical scales and cognitive tests were used for endpoint assessment. The primary efficacy endpoint was the mean reduction of total Positive and Negative Syndrome Scale score after 6 weeks of treatment.

RESULTS

No significant differences in the primary and secondary endpoints were found between the groups. However, based on the subgroup analysis, the significant improvement of total Positive and Negative Syndrome Scale score and residual psychotic symptoms was observed in female patients.

CONCLUSIONS

It was a negative study with unexpected benefits of the drug only in females. We hypothesized that the role of patients' sex can impact the treatment response to serotonergic drugs in general. We suggest a possible synergistic interaction between estrogen and Avisetron by means of modulating the effect of estrogens on the serotonergic system. Future studies targeting the sex-related effects of serotonergic drugs are warranted.

[Pharmacological properties of vortioxetine and its pre-clinical consequences]

Selective Serotonin Reuptake Inhibitors (SSRIs) are extensively used for the treatment of major depressive disorder (MDD). SSRIs are defined as indirect receptor agonists since the activation of postsynaptic receptors is a consequence of an increase in extracellular concentrations of serotonin (5-HT) mediated by the blockade of serotonin transporter. The activation of some serotoninergic receptors (5-HT1A, post-synaptic, 5-HT1B post-synaptic, 5-HT2B, and 5-HT4), but not all (5-HT1A, pre-synaptic, 5-HT1B pre-synaptic, 5-HT2A, 5-HT2C, 5-HT3, and probably 5-HT6), induces anxiolytic/antidepressive - like effects. Targetting specifically some of them could potentially improve the onset of action and/or efficacy and/or prevent MD relapse. Vortioxetine (Brintellix, 1- [2-(2,4-dimethylphenyl-sulfanyl)-phenyl]-piperazine) is a novel multi-target antidepressant drug approved by the Food and Drug Administration (FDA) and by European Medicines Agency. Its properties are markedly different from the extensively prescribed SSRIs. Compared to the SSRIs, vortioxetine is defined as a multimodal antidepressant drug since it is not only a serotonin reuptake inhibitor, but also a 5-HT1D, 5-HT3, 5-HT7 receptor antagonist, 5-HT1B receptor partial agonist and 5-HT1A receptor agonist. This specific pharmacological profile enables vortioxetine to affect not only the serotoninergic and noradrenergic systems, but also the histaminergic, cholinergic, gamma-butyric acid (GABA) ergic and glutamatergic ones. Thus, vortioxetine not only induces antidepressant-like or anxiolytic-like activity but also improves cognitive parameters in several animal models. Indeed, vortioxetine was shown to improve working memory, episodic memory, cognitive flexibility and spatial memory in young adult rodents and also in old animal models. These specific effects of the vortioxetine are of interest considering that cognitive dysfunction is a common comorbidity to MDD. Altogether, even though this molecule still needs to be investigated further, especially in the insufficient-response to antidepressant drugs, vortioxetine is already an innovative therapeutic option for the treatment of major depression.