Antidepressant-like effects of ketamine in a mouse model of serotonergic dysfunction

Investigating Resistance to Antidepressants in Animal Models

Major depressive disorder is one of the most prevalent psychiatric diseases, and up to 30-40% of patients remain symptomatic despite treatment. Novel therapies are sorely needed, and animal models may be used to elucidate fundamental neurobiological processes that contribute to human disease states. We conducted a systematic review of current preclinical approaches to investigating treatment resistance with the goal of describing a path forward for improving our understanding of treatment resistant depression. We conducted a broad literature search to identify studies relevant to the preclinical investigation of treatment resistant depression. We followed PRISMA (Preferred Reporting Items for Systemic Reviews and Meta-Analyses) guidelines and included all relevant studies. We identified 467 studies in our initial search. Of these studies, we included 69 in our systematic review after applying our inclusion/exclusion criteria. We identified 10 broad strategies for investigating treatment resistance in animal models. Stress hormone administration was the most commonly used model, and the most common behavioral test was the forced swim test. We systematically identified and reviewed current approaches for gaining insight into the neurobiology underlying treatment resistant depression using animal models. Each approach has its advantages and disadvantages, but all require careful consideration of their potential limitations regarding therapeutic translation. An enhanced understanding of treatment resistant depression is sorely needed given the burden of disease and lack of effective therapies.

Serotonergic agonism and pharmacologically-induced adolescent stress cause operant-based learning deficits in mice

BACKGROUND

The interplay between environmental stress and genetic factors is thought to play an important role in the pathogenesis and maintenance of obsessive-compulsive disorder (OCD). However, the relative contribution of these causative antecedents in the manifestation of cognitive inflexibility-a phenotype often seen in obsessive-compulsive (OC)- spectrum disorders-is not fully understood.

METHOD

In this study, we treated mice with 50 mg/L corticosterone (CORT, a glucocorticoid stress hormone) in their drinking water during adolescence. In adulthood, we assessed anxiety-like behaviour and locomotor activity; along with operant-based discrimination and reversal learning. RU-24969, a selective serotonin receptor 5-HT1A/1B receptor agonist, was used as an acute pharmacological model of OC-like behaviour. RU-24969 (5 mg/kg) was administered prior to each reversal learning testing session.

RESULTS

We found that acute treatment with 5 mg/kg RU-24969 induced stereotyped hyperlocomotion in vehicle- and CORT-treated mice. Furthermore, pre-treatment with CORT in adolescence produced subtle anxiety-like behaviour in adult mice, and also resulted in an impairment to late-stage discrimination learning and alterations to reversal learning. Finally, acute treatment with 5 mg/kg RU-24969 caused an impairment to early-stage reversal learning.

CONCLUSION

Whilst we revealed dissociable detrimental effects of adolescent CORT treatment and acute 5-HT1A/1B receptor agonism on discrimination and reversal learning, respectively, we did not find evidence of additive deleterious effects of these two treatments. We therefore suggest that while disrupted serotonergic signalling is likely to be involved in the cognitive phenotype of OC-spectrum disorders, distinct neuropathological pathways may be at play in mediating the role of stress as an antecedent in OCD and related illnesses.

Dietary fibre confers therapeutic effects in a preclinical model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder involving psychiatric, cognitive and motor deficits, as well as peripheral symptoms, including gastrointestinal dysfunction. The R6/1 HD mouse model expresses a mutant human huntingtin transgene and has been shown to provide an accurate disease model. Recent evidence of gut microbiome disruption was shown in preclinical and clinical HD. Therefore, we aimed to assess the potential role of gut microbial modulation in the treatment of HD. The R6/1 HD mice and wild-type littermate controls were randomised to receive diets containing different amounts of fibre: high-fibre (10 % fibre), control (5 % fibre), or zero-fibre (0 % fibre), from 6 to 20 weeks of age. We characterized the onset and progression of motor, cognitive and affective deficits, as well as gastrointestinal function and gut morphological changes. Faeces were collected for gut microbiome profiling using 16S rRNA sequencing, at 14 and 20 weeks of age. When compared to the control diet, high-fibre diet improved the performance of HD mice in behavioral tests of cognitive and affective function, as well as the gastrointestinal function of both HD and wild-type mice. While the diets changed the beta diversity of wild-type mice, no statistical significance was observed at 14 or 20 weeks of age within the HD mice. Analysis of Composition of Microbiomes with Bias Correction (ANCOM-BC) models were performed to evaluate microbiota composition, which identified differences, including a decreased relative abundance of the phyla Actinobacteriota, Campylobacterota and Proteobacteria and an increased relative abundance of the families Bacteroidaceae, Oscillospiraceae and Ruminococcaceae in HD mice when compared to wild-type mice after receiving high-fibre diet. PICRUSt2 revealed that high-fibre diet also decreased potentially pathogenic functional pathways in HD. In conclusion, high-fibre intake was effective in enhancing gastrointestinal function, cognition and affective behaviors in HD mice. These findings indicate that dietary fibre interventions may have therapeutic potential in Huntington's disease to delay clinical onset, and have implications for related disorders exhibiting dysfunction of the gut-brain axis.

Nebulized and intraperitoneal ketamine have equivalent antidepressant-like effect in the forced swim and tail suspension tests in mice

Major depressive disorder (MDD) is a debilitating illness that affects millions of people worldwide. Currently available antidepressants often take weeks to months to reach their full effect, which leads to an increased risk of suicidal behavior in patients with MMD. Intranasally, esketamine has emerged as an alternative to current antidepressants because of its rapid onset and long-lasting effects in patients with MDD. Animal models are useful for the initial pharmacological screening and for a better understanding of the mechanisms underlying the effects of new drugs with potential against MDD. There is a lack of data on alternative routes of drug administration, either oral or injectable, that can be used in preclinical studies. This study aimed to test whether ketamine has antidepressant-like effects in mice when administered via nebulization using a low-cost apparatus. When mice whose depressive-like behavior was induced by corticosterone were treated with nebulized ketamine at concentrations of 1.3, 2.6, and 5.2 mg/mL, immobility was reduced by 38.6 %, 62.0 %, and 61.1 %, respectively, in the forced swimming test (FST) and 43.6 %, 42.1 %, and 57.9 %, respectively, in the tail suspension test (TST). When depression-like behavior was induced by dexamethasone, nebulization with ketamine reduced immobility by 79.7 %, 49.2 %, and 44.4 % in the FST and 80.9 %, 71.4 %, and 80.4 %, respectively, in the TST. When depression-like behavior was induced by the association between dexamethasone and unpredictable chronic mild stress (UCMS) exposure, immobility was reduced by 26.1 %, 55.3 %, and 19.1 % in FST. Mice treated with nebulized ketamine did not show significant changes in the distance covered or in the time spent moving in the open field test. The efficacy of intraperitoneal and nebulized ketamine is equivalent, which shows that nebulization can be an alternative inexpensive route of drug administration for behavioral studies in rodents.

Psilocybin as a lead candidate molecule in preclinical therapeutic studies of psychiatric disorders: A systematic review

Psilocybin is the main psychoactive compound found in hallucinogenic/magic mushrooms and can bind to both serotonergic and tropomyosin receptor kinase b (TrkB) receptors. Psilocybin has begun to show efficacy for a range of neuropsychiatric conditions, including treatment-resistant depression and anxiety disorders; however, neurobiological mechanisms are still being elucidated. Clinical research has found that psilocybin can alter functional connectivity patterns in human brains, which is often associated with therapeutic outcomes. However, preclinical research affords the opportunity to assess the potential cellular mechanisms by which psilocybin may exert its therapeutic effects. Preclinical rodent models can also facilitate a more tightly controlled experimental context and minimise placebo effects. Furthermore, where there is a rationale, preclinical researchers can investigate psilocybin administration in neuropsychiatric conditions that have not yet been researched clinically. As a result, we have systematically reviewed the knowledge base, identifying 82 preclinical studies which were screened based on specific criteria. This resulted in the exclusion of 44 articles, with 34 articles being included in the main review and another 2 articles included as Supporting Information materials. We found that psilocybin shows promise as a lead candidate molecule for treating a variety of neuropsychiatric conditions, albeit showing the most efficacy for depression. We discuss the experimental findings, and identify possible mechanisms whereby psilocybin could invoke therapeutic changes. Furthermore, we critically evaluate the between-study heterogeneity and possible future research avenues. Our review suggests that preclinical rodent models can provide valid and translatable tools for researching novel psilocybin-induced molecular and cellular mechanisms, and therapeutic outcomes.

Acute administration of the NMDA receptor antagonists ketamine and MK-801 reveals dysregulation of glutamatergic signalling and sensorimotor gating in the Sapap3 knockout mouse model of compulsive-like behaviour

Obsessive-compulsive disorder (OCD) is characterised by excessive intrusive thoughts that may cause an individual to engage in compulsive behaviours. Frontline pharmacological treatments (i.e., selective serotonin reuptake inhibitors (SSRIs)) leave approximately 40% of patients refractory to treatment. To investigate the possibility of novel pharmacological therapies for OCD, as well as the potential mechanisms underlying its pathology, we used the Sapap3 knockout (KO) mouse model of OCD, which exhibits increased anxiety and compulsive grooming behaviours. Firstly, we investigated whether administration of the NMDA receptor (NMDAR) antagonist ketamine (30 mg/kg), would reduce anxiety and grooming behaviour in Sapap3 KO mice. Anxiety-like behaviour was measured via time spent in the light component of the light-dark box test. Grooming behaviour was recorded and scored in freely moving mice. In line with previous works conducted in older animals (i.e. typically between 6 and 9 months of age), we confirmed here that Sapap3 KO mice exhibit an anxious, compulsive grooming, hypolocomotive and reduced body weight phenotype even at a younger age (i.e., 2-3 months of age). However, we found that acute administration of ketamine did not cause a reduction in anxiety or grooming behaviour. We then investigated in vivo glutamatergic function via the administration of a different NMDAR antagonist, MK-801 (0.25 mg/kg), prior to locomotion and prepulse inhibition assays. We found evidence of altered functional NMDAR activity, as well as sexually dimorphic prepulse inhibition, a measure of sensorimotor gating, in Sapap3 KO mice. These results are suggestive of in vivo glutamatergic dysfunction and their functional consequences, enabling future research to further investigate novel treatments for OCD.

Areca catechu L. ameliorates chronic unpredictable mild stress-induced depression behavior in rats by the promotion of the BDNF signaling pathway

OBJECTIVES

In this study, we have investigated the anti-depressant effects of the fruit Areca catechu L. (ACL) and elucidated its potential underlying mechanism using a rat model of chronic unpredictable mild stress (CUMS).

METHODS

CUMS was induced in rats to establish a depression animal model for 28 days. According to the baseline sucrose preference, the male rats were divided into 6 different groups. They were treated with paroxetine hydrochloride, ACL, and water once a day until the behavioral tests were performed. The levels of corticosterone (CORT), malondialdehyde (MDA), catalase (CAT), and total superoxide dismutase (T-SOD) in serum were detected using a commercial kit, and the concentrations of 5-hydroxytryptamine (5-HT) and dopamine (DA) monoamine neurotransmitters in the brain tissues were detected by liquid chromatography-tandem mass spectrometry. doublecortin (DCX) expression in the hippocampal dentate gyrus (DG) was determined by immunofluorescence, and the relative abundance of brain-derived neurotrophic factor (BDNF), TrkB, PI3K, p-AKT/AKT, PSD-95, and p-GSK-3β/GSK-3β of brain tissues were assayed by western blot.

RESULTS

ACL markedly increased sucrose preference, decreased the immobility time, and shortened the feeding latency of CUMS-induced rats. CUMS induction resulted in marked changes in the contents of the monoamine neurotransmitters (5-HT and DA) in the hippocampus and cortex of brain tissues and the levels of CORT, MDA, CAT, and T-SOD in serum, whereas ACL administration alleviated these considerable changes. ACL promoted DCX expression in DG and increased the protein levels of BDNF, TrkB, PI3K, p-AKT/AKT, PSD-95, and p-GSK-3β/GSK-3β in the brains of CUMS-induced rats.

CONCLUSIONS

Our results indicated that ACL may improve depression-like behaviors in CUMS-induced rats by decreasing the hyperfunction and oxidative stress of the hypothalamic-pituitary-adrenal axis, stimulating hippocampal neurogenesis, and activating the BDNF signaling pathway.

The Effects of Acute and Repeated Administration of Ketamine on Memory, Behavior, and Plasma Corticosterone Levels in Female Mice

Ketamine is an anesthetic drug that has recently been approved for the treatment of treatment-resistant depression. Females are diagnosed with Major Depressive Disorder at higher rates than males, yet most of the pre-clinical research on ketamine has been conducted in male subjects. Additionally, the literature on the acute and long-term behavioral and cognitive effects of ketamine shows conflicting results. It is important to examine the acute and long-term cognitive and behavioral effects of ketamine exposure at lower sub-anesthetic doses, as the recreational use of the drug at higher doses is associated with cognitive and memory impairments. The current study examined the effects of acute and repeated ketamine exposure on anxiety-like behavior, novel object recognition memory, depression-like behavior, and plasma corticosterone levels in 20 adult female C57BL/6J mice. Mice were exposed acutely or repeatedly for 10 consecutive days to saline or 15 mg/kg ketamine and behavior was measured in the open field test, novel object recognition test, and the Porsolt forced swim test. Plasma corticosterone levels were measured following behavioral testing. Acute ketamine exposure decreased locomotor activity and increased anxiety-like behavior in the open field test compared to controls, while repeated ketamine exposure impaired memory in the novel object recognition test. There were no effects of acute or repeated ketamine exposure on depression-like behavior in the Porsolt forced swim test or on plasma corticosterone levels. These findings suggest that a subanesthetic dose of ketamine alters behavior and cognition in female mice and the effects are dependent on the duration of exposure.

Novel Antidepressant-Like Properties of the Iron Chelator Deferiprone in a Mouse Model of Depression

Depressed individuals who carry the short allele for the serotonin-transporter-linked promotor region of the gene are more vulnerable to stress and have reduced response to first-line antidepressants such as selective serotonin reuptake inhibitors. Since depression severity has been reported to correlate with brain iron levels, the present study aimed to characterise the potential antidepressant properties of the iron chelator deferiprone. Using the serotonin transporter knock-out (5-HTT KO) mouse model, we assessed the behavioural effects of acute deferiprone on the Porsolt swim test (PST) and novelty-suppressed feeding test (NSFT). Brain and blood iron levels were also measured following acute deferiprone. To determine the relevant brain regions activated by deferiprone, we then measured c-Fos expression and applied network-based analyses. We found that deferiprone reduced immobility time in the PST in 5-HTT KO mice and reduced latency to feed in the NSFT in both genotypes, suggesting potential antidepressant-like effects. There was no effect on brain or blood iron levels following deferiprone treatment, potentially indicating an acute iron-independent mechanism. Deferiprone reversed the increase in c-Fos expression induced by swim stress in 5-HTT KO mice in the lateral amygdala. Functional network analyses suggest that hub regions of activity in mice treated with deferiprone include the caudate putamen and prefrontal cortex. The PST-induced increase in network modularity in wild-type mice was not observed in 5-HTT KO mice. Altogether, our data show that the antidepressant-like effects of deferiprone could be acting via an iron-independent mechanism and that these therapeutic effects are underpinned by changes in neuronal activity in the lateral amygdala.

Partial resistance to citalopram in a Wistar-Kyoto rat model of depression: An evaluation using resting-state functional MRI and graph analysis

Wistar-Kyoto (WKY) rats as an endogenous depression model partially lack a response to classic selective serotonin reuptake inhibitors (SSRIs). Thus, this strain has the potential to be established as a model of treatment-resistant depression (TRD). However, the SSRI resistance in WKY rats is still not fully understood. In this study, WKY and control rats were subjected to a series of tests, namely, a forced swim test (FST), a sucrose preference test (SPT), and an open field test (OFT), and were scanned in a 7.0-T MRI scanner before and after three-week citalopram or saline administration. Behavioral results demonstrated that WKY rats had increased immobility in the FST and decreased sucrose preference in the SPT and central time spent in the OFT. However, citalopram did not improve immobility in the FST. The amplitude of low-frequency fluctuation (ALFF) analysis showed regional changes in the striatum and hippocampus of WKY rats. However, citalopram partially reversed the ALFF value in the dorsal part of the two regions. Functional connectivity (FC) analysis showed that FC strengths were decreased in WKY rats compared with controls. Nevertheless, citalopram partially increased FC strengths in WKY rats. Based on FC, global graph analysis demonstrated decreased network efficiency in WKY + saline group compared with control + saline group, but citalopram showed weak network efficiency improvement. In conclusion, resting-state fMRI results implied widely affected brain function at both regional and global levels in WKY rats. Citalopram had only partial effects on these functional changes, indicating a potential treatment resistance mechanism.

A systematic review of studies investigating the acute effects of N-methyl-D-aspartate receptor antagonists on behavioural despair in normal animals suggests poor predictive validity

The ability of the N-methyl-D-aspartate receptor antagonist ketamine to induce a rapid and sustained antidepressant effect has led to a surge in pre-clinical studies investigating underlying mechanisms and seeking novel treatments. Animal models are key to this research as they can provide a behavioural readout linking underlying mechanisms to clinical benefits. However, quantifying depression-related behaviours in rodents represents a major challenge with the validity of traditional methods such as models of behavioural despair (forced swim test and tail suspension test) a topic of debate. While there is good evidence to support the value of using these behavioural readouts to study the effects of stress, these approaches have largely failed to detect reliable phenotypic effects in other disease models. In this systematic review, we identified publications which had tested N-methyl-D-aspartate receptor antagonists in normal animals using either the forced swim test or tail suspension test. We compared findings for different doses and time points and also drugs with different clinical profiles to investigate how well the outcomes in the rodent model predicted their effects in the clinic. Despite clear evidence that N-methyl-D-aspartate receptor antagonists reduce immobility time and hence exhibit an antidepressant profile in these tasks, we found similar effects with both clinically effective drugs as well as those which have failed to show efficacy in clinical trials. These findings suggest that behavioural despair tests in normal animals do not provide a good method to predict clinical efficacy of N-methyl-D-aspartate receptor antagonists.

Loss-of-function and gain-of-function studies refute the hypothesis that tau protein is causally involved in the pathogenesis of Huntington's disease

Tau is a microtubule-associated protein, whose abnormal phosphorylation and deposition in the brain characterizes a range of neurodegenerative diseases called tauopathies. Recent clinical (post-mortem) and pre-clinical evidence suggests that Huntington's disease (HD), an autosomal dominant neurodegenerative disorder, could be considered as a tauopathy. Studies have found the presence of hyperphosphorylated tau, altered tau isoform ratio and aggregated tau in HD brains. However, little is known about the implication of tau in the development of HD pathophysiology, which includes motor, cognitive and affective symptoms. To shine a light on the involvement of tau in HD, our present study aimed at (i) knocking out tau expression and (ii) expressing a transgene encoding mutant human tau in the R6/1 mouse model of HD. We hypothesized that expression of the mutant human tau transgene in HD mice would worsen the HD phenotype, while knocking out endogenous mouse tau in HD mice would improve some behavioural deficits display by HD mice. Our data suggests that neither the expression of a tau transgene nor the ablation of tau expression impacted the progression of the HD motor, cognitive and affective phenotypes. Supporting these behavioural findings, we also found that modulating tau expression had no effect on brain weights in HD mice. We also report that expression of the tau transgene increased the weight of WT and HD male mice, whereas tau ablation increased the weight of HD females only. Together, our results indicate that tau might not be as important in regulating the progression of HD symptomatology.

The effectiveness of (R)-ketamine and its mechanism of action differ from those of (S)-ketamine in a chronic unpredictable mild stress model of depression in C57BL/6J mice

(S)-ketamine has been approved as a rapid-acting antidepressant drug (RAAD). Although ketamine has an advantage over classic antidepressants (ADs) due to its rapid action, it remains a controversial drug due to its undesirable effects. Behavioral studies indicate that another enantiomer of ketamine, namely, (R)-ketamine, has been proposed as a safer but still effective RAAD. However, these conclusions have not been confirmed in any model of depression based on chronic environmental stress, which effectively reflects the core symptoms of this disease. Thus, we decided to compare the effects of (R)- and (S)-ketamine on chronic unpredictable mild stress (CUMS) in mice. Behavioral studies showed that (R)-ketamine induced anti-anhedonic and anti-apathetic efficacy up to seven days after administration, while the (S)-ketamine effect persisted up to 24 h or 3 days after injection. The behavioral effects of (R)-ketamine depended on the activation of TrkB receptors, while the (S)-ketamine effects did not. Western blot analyses showed that (S)-ketamine action might be related to both mTOR and ERK pathway activation and to the increased expression of GluA1 protein in the prefrontal cortex (PFC). In contrast, (R)-ketamine did not change ERK phosphorylation in the PFC, while it increased mTOR expression. (S)-Ketamine produced behavioral effects indicative of possible side effects in the dose range studied, while (R)-ketamine did not. This indicates that (R)-ketamine may be more effective, have a longer-lasting effect, and be safer to use than (S)-ketamine.

TRPA1 involvement in depression- and anxiety-like behaviors in a progressive multiple sclerosis model in mice

Progressive multiple sclerosis (PMS) is a neurological disease associated with the development of depression and anxiety, but treatments available are unsatisfactory. The transient receptor potential ankyrin 1 (TRPA1) is a cationic channel activated by reactive compounds, and the blockage of this receptor can reduce depression- and anxiety-like behaviors in naive mice. Thus, we investigated the role of TRPA1 in depression- and anxiety-like behaviors in a PMS model in mice. PMS model was induced in C57BL/6 female mice by the experimental autoimmune encephalomyelitis (EAE). Nine days after the PMS-EAE induction, behavioral tests (tail suspension and elevated plus maze tests) were performed to verify the effects of sertraline (positive control), selective TRPA1 antagonist (A-967,079), and antioxidants (α-lipoic acid and apocynin). The prefrontal cortex and hippocampus were collected to evaluate biochemical and inflammatory markers. PMS-EAE induction did not cause locomotor changes but triggered depression- and anxiety-like behaviors, which were reversed by sertraline, A-967,079, α-lipoic acid, or apocynin treatments. The neuroinflammatory markers (AIF1, GFAP, IL-1β, IL-17, and TNF-α) were increased in mice's hippocampus. Moreover, this model did not alter TRPA1 RNA expression levels in the hippocampus but decrease TRPA1 levels in the prefrontal cortex. Moreover, PMS-EAE induced an increase in NADPH oxidase and superoxide dismutase activities and TRPA1 endogenous agonist levels (hydrogen peroxide and 4-hydroxynonenal). TRPA1 plays a fundamental role in depression- and anxiety-like behaviors in a PMS-EAE model; thus, it could be a possible pharmacological target for treating these symptoms in PMS.

The group II mGlu receptor antagonist LY341495 induces a rapid antidepressant-like effect and enhances the effect of ketamine in the chronic unpredictable mild stress model of depression in C57BL/6J mice

Ketamine produces a rapid antidepressant effect, but its use can be associated with serious side effects. Hence, other therapeutic options that will allow us to obtain a quick and safe antidepressant effect by modulating glutamatergic transmission are needed. Antagonists of mGlu2/3 receptors, which share some mechanisms of action with ketamine, may be good candidates to obtain this effect. Here, we show that the metabotropic glutamate (mGlu) 2/3 receptor antagonist LY341495 induced a dose-dependent antidepressant-like effect in the chronic unpredictable mild stress (CUMS) model of depression in C57BL/6J mice after both single and subchronic (three-day) administration. Furthermore, a noneffective dose of LY341495 (0.3 mg/kg) given jointly with a noneffective dose of ketamine (3 mg/kg) reversed the CUMS-induced behavioral effects, indicating that coadministration of ketamine with an mGlu2/3 receptor antagonist might allow its therapeutically effective dose to be lowered. Western blot results indicate that mTOR pathway activation might be involved in the mechanism of action of this drug combination. Moreover, the combined doses of both substances did not produce undesirable behavioral effects characteristic of a higher dose of ketamine (10 mg/kg) commonly used in rodent studies to induce antidepressant effects. Coadministration of low doses of ketamine and LY341495 did not induce the hyperactivity typical of NMDA channel blockers, did not disturb short-term memory in the novel object recognition (NOR) test, and did not disturb motor coordination in the rotarod test. Our research not only confirmed the earlier data on the rapid antidepressant effect of mGlu2/3 receptor antagonists but also indicated that such compounds can safely lower the effective dose of ketamine.

Serotonin research: Crossing scales and boundaries

Nearly 100 years after the discovery of serotonin, its role remains elusive. Modulation of serotonin transmission is considered in numerous central nervous system (CNS) diseases including depression, anxiety, schizophrenia, obsessive-compulsive disorders, addiction, Parkinson's disease, and Alzheimer's disease. The therapeutic strategies based on serotonin systems have evolved thanks to better identification of the involvement of serotonin in various diseases, the better use of animal models, a better understanding of the molecular environment of serotonin receptors, and ultimately the better understanding of the interaction of serotonin neurotransmission with other biological systems. Some 5-HT receptors are still the object of numerous investigations including 5-HT_1A, 5-HT_2A, and 5-HT₆ receptor subtypes. It is noteworthy that the direction of research is moving towards a simultaneous action at multiple targets either through different 5-HT targets or the consideration of both 5-HT and other targets to achieve better therapeutic responses.