Biased agonism in drug discovery: Is there a future for biased 5-HT1A receptor agonists in the treatment of neuropsychiatric diseases?

5-HT1A receptors within the intermediate lateral septum modulate stress vulnerability in male mice

Chronic stress is a major risk factor for psychiatric disorders. However, certain individuals may be at higher risk due to greater stress susceptibility. Elucidating the neurobiology of stress resilience and susceptibility may facilitate the development of novel strategies to prevent and treat stress-related disorders such as depression. Mounting evidence suggests that the serotonin (5-HT) system is a major regulator of stress sensitivity. In this study, we assessed the functions of 5-HT1A and 5-HT2A receptors within the lateral septum (LS) in regulating stress vulnerability. Among a group of male mice exposed to chronic social defeat stress (CSDS), 47.2% were classified as stress-susceptible, and these mice employed more passive coping strategies during the defeat and exhibited more severe anxiety- and depression-like behaviors during the following behavioral tests. These stress-susceptible mice also exhibited elevated neuronal activity in the LS as evidenced by greater c-Fos expression, greater activity of 5-HT neurons in both the dorsal and median raphe nucleus, and downregulated expression of the 5-HT1A receptor in the intermediate LS (LSi). Finally, we found the stress-induced social withdrawal symptoms could be rapidly relieved by LSi administration of 8-OH-DPAT, a 5-HT1A receptor agonist. These results indicate that 5-HT1A receptors within the LSi play an important role in stress vulnerability in mice. Therefore, modulation of stress vulnerable via 5-HT1A receptor activation in the LSi is a potential strategy to treat stress-related psychiatric disorders.

Advances in drug design and therapeutic potential of selective or multitarget 5-HT1A receptor ligands

5-HT1A receptor (5-HT1A-R) is a serotoninergic G-protein coupled receptor subtype which contributes to several physiological processes in both central nervous system and periphery. Despite being the first 5-HT-R identified, cloned and studied, it still represents a very attractive target in drug discovery and continues to be the focus of a myriad of drug discovery campaigns due to its involvement in numerous neuropsychiatric disorders. The structure-activity relationship studies (SAR) performed over the last years have been devoted to three main goals: (i) design and synthesis of 5-HT1A-R selective/preferential ligands; (ii) identification of 5-HT1A-R biased agonists, differentiating pre- versus post-synaptic agonism and signaling cellular mechanisms; (iii) development of multitarget compounds endowed with well-defined poly-pharmacological profiles targeting 5-HT1A-R along with other serotonin receptors, serotonin transporter (SERT), D2-like receptors and/or enzymes, such as acetylcholinesterase and phosphodiesterase, as a promising strategy for the management of complex psychiatric and neurodegenerative disorders. In this review, medicinal chemistry aspects of ligands acting as selective/preferential or multitarget 5-HT1A-R agonists and antagonists belonging to different chemotypes and developed in the last 7 years (2017-2023) have been discussed. The development of chemical and pharmacological 5-HT1A-R tools for molecular imaging have also been described. Finally, the pharmacological interest of 5-HT1A-R and the therapeutic potential of ligands targeting this receptor have been considered.

N-(3-((3-(trifluoromethyl)phenyl)selanyl)prop-2-yn-1-yl) benzamide induces antidepressant-like effect in mice: involvement of the serotonergic system

RATIONALE

Major Depressive Disorder (MDD) significantly impairs the quality of life for those affected. While the exact causes of MDD are not fully understood, the deficit of monoamines, especially serotonin and noradrenaline, is widely accepted. Resistance to long-term treatments and adverse effects are often observed, highlighting the need for new pharmacological therapies. Synthetic organic compounds containing selenium have exhibited pharmacological properties, including potential antidepressant effects.

OBJECTIVE

To evaluate the antidepressant-like effect of N-(3-((3-(trifluoromethyl)phenyl)selenyl)prop-2-yn-1-yl) benzamide (CF3SePB) in mice and the involvement of the serotonergic and noradrenergic systems.

METHODS

Male Swiss mice were treated with CF3SePB (1-50 mg/kg, i.g.) and 30 min later the forced swimming test (FST) or tail suspension test (TST) was performed. To investigate the involvement of the serotonergic and noradrenergic systems in the antidepressant-like effect of CF3SePB, mice were pre-treated with p-CPA (a 5-HT depletor, 100 mg/kg, i.p.) or the receptor antagonists WAY100635 (0.1 mg/kg, s.c., a 5-HT1A receptor antagonist), ketanserin (1 mg/kg, i.p., a 5-HT2A/2C receptor antagonist), ondansetron (1 mg/kg, i.p., a 5-HT3 receptor antagonist), GR110838 (0.1 mg/kg, i.p., a 5-HT4 receptor antagonist), prazosin (1 mg/kg, i.p., an α1-adrenergic receptor antagonist), yohimbine (1 mg/kg, i.p., an α2-adrenergic receptor antagonist) and propranolol (2 mg/kg, i.p., a non-selective beta-adrenergic receptor antagonist) at specific times before CF3SePB (50 mg/kg, i.g.), and after 30 min of CF3SePB administration the FST was performed.

RESULTS

CF3SePB showed an antidepressant-like effect in both FST and TST and this effect was related to the modulation of the serotonergic system, specially the 5-HT1A and 5-HT3 receptors. None of the noradrenergic antagonists prevented the antidepressant-like effect of CF3SePB. The compound exhibited a low potential for inducing acute toxicity in adult female Swiss mice.

CONCLUSION

This study pointed a new compound with antidepressant-like effect, and it could be considered for the development of new antidepressants.

TC-G 1008 facilitates epileptogenesis by acting selectively at the GPR39 receptor but non-selectively activates CREB in the hippocampus of pentylenetetrazole-kindled mice

The pharmacological activation of the GPR39 receptor has been proposed as a novel strategy for treating seizures; however, this hypothesis has not been verified experimentally. TC-G 1008 is a small molecule agonist increasingly used to study GPR39 receptor function but has not been validated using gene knockout. Our aim was to assess whether TC-G 1008 produces anti-seizure/anti-epileptogenic effects in vivo and whether the effects are mediated by GPR39. To obtain this goal we utilized various animal models of seizures/epileptogenesis and GPR39 knockout mice model. Generally, TC-G 1008 exacerbated behavioral seizures. Furthermore, it increased the mean duration of local field potential recordings in response to pentylenetetrazole (PTZ) in zebrafish larvae. It facilitated the development of epileptogenesis in the PTZ-induced kindling model of epilepsy in mice. We demonstrated that TC-G 1008 aggravated PTZ-epileptogenesis by selectively acting at GPR39. However, a concomitant analysis of the downstream effects on the cyclic-AMP-response element binding protein in the hippocampus of GPR39 knockout mice suggested that the molecule also acts via other targets. Our data argue against GPR39 activation being a viable therapeutic strategy for treating epilepsy and suggest investigating whether TC-G 1008 is a selective agonist of the GPR39 receptor.

The 5-HT1A receptor biased agonist, NLX-204, shows rapid-acting antidepressant-like properties and neurochemical changes in two mouse models of depression

Activation of cortical serotonin 5-HT_1A receptors may be a promising strategy to achieve rapid-acting antidepressant (RAAD) activity. NLX-204 is a selective 5-HT_1A receptor biased agonist that, in naïve mice, robustly decreases immobility in the forced swim test (FST), and preferentially phosphorylates extracellular signal-regulated kinase (ERK1/2), involved in antidepressant activity. Here, we evaluated the properties of NLX-204 in two mouse models of depression. Male CD-1 mice were subjected to unpredictable chronic mild stress (UCMS) for 4-weeks or to repeated corticosterone (CORT, 20 mg/kg s.c./day) for 3-weeks before receiving acute administration of NLX-204 (2 mg/kg, p.o.). Depressive-like behavior was assessed in the FST, anhedonia-like behavior in the sucrose preference test (SPT) and locomotor activity was also recorded. Phosphorylation of ERK1/2 (pERK1/2) and cAMP response binding element (pCREB) were measured ex vivo in hippocampus and prefrontal cortex (PFC). UCMS or CORT treatment increased immobility in the FST, elicited a sucrose preference deficit, and decreased pERK1/2 and pCREB levels in PFC and hippocampus. NLX-204 reduced depressive-like behavior in the FST in CORT and UCMS mice, and normalized sucrose preference in CORT mice, suggesting anti-anhedonic activity. NLX-204 increased pERK1/2 levels in PFC of UCMS mice. NLX-204 also increased pCREB levels in PFC of CORT mice. These data suggest that NLX-204 has RAAD-like properties not only in naïve mice, but also in mice in a "depressive-like" state, and that these involve changes in PFC and hippocampal pERK1/2 and pCREB levels. These data provide additional evidence that activation of 5-HT_1A receptors by selective biased agonists, such as NLX-204, may constitute a promising RAAD strategy.

Dietary Zinc Differentially Regulates the Effects of the GPR39 Receptor Agonist, TC-G 1008, in the Maximal Electroshock Seizure Test and Pentylenetetrazole-Kindling Model of Epilepsy

The G-protein coupled receptor 39 (GPR39) is gaining increasing attention as a target for future drugs, yet there are gaps in the understanding of its pharmacology. Zinc is an endogenous agonist or an allosteric modulator, while TC-G 1008 is a synthetic, small molecule agonist. Zinc is also a positive allosteric modulator for the activity of TC-G 1008 at GPR39. Activation of GPR39 by TC-G 1008 facilitated the development of epileptogenesis in the pentylenetetrazole (PTZ)-induced kindling model of epilepsy. Congruently, TC-G 1008 decreased the seizure threshold in the maximal electroshock seizure threshold (MEST) test. Here, we investigated the effects of TC-G 1008 under the condition of zinc deficiency. Mice were fed a zinc-adequate diet (ZnA, 50 mg Zn/kg) or a zinc-deficient diet (ZnD, 3 mg Zn/kg) for 4 weeks. Following 4 weeks of dietary zinc restriction, TC-G 1008 was administered as a single dose and the MEST test was performed. Additional groups of mice began the PTZ-kindling model during which TC-G 1008 was administered repeatedly and the diet was continued. TC-G 1008 administered acutely decreased the seizure threshold in the MEST test in mice fed the ZnD diet but not in mice fed the ZnA diet. TC-G 1008 administered chronically increased the maximal seizure severity and the percentage of fully kindled mice in those fed the ZnA diet, but not in mice fed the ZnD diet. Our data showed that the amount of zinc in a diet is a factor contributing to the effects of TC-G 1008 in vivo.

S-adenosyl-l-methionine antidepressant-like effects involve activation of 5-HT1A receptors

S-adenosyl-l-methionine (SAMe), a methyl donor, induces antidepressant effects in preclinical and clinical studies of depression. However, the mechanisms behind these effects have been poorly investigated. Since SAMe is involved in monoamine metabolism, this work aimed at 1) testing the effects induced by systemic treatment with SAMe in mice submitted to the forced swimming test (FST) and tail suspension test (TST); 2) investigating the involvement of serotonergic neurotransmission in the behavioral effects induced by SAMe. To do that, male Swiss mice received systemic injections (1 injection/day, 1 or 7 days) of imipramine (30 mg/kg), L-methionine (400, 800, 1600, and 3200 mg/kg), SAMe (10, 25, 50, 100, and 200 mg/kg), or vehicle (10 ml/kg) and were submitted to the FST or TST, 30 min after the last injection. The effect of SAMe (50 mg/kg) was further investigated in independent groups of male Swiss mice pretreated with p-chlorophenylalanine (PCPA, serotonin synthesis inhibitor, 150 mg/kg daily, 4 days) or with WAY100635 (5-HT_1A receptor antagonist, 0.1 mg/kg, 1 injection). One independent group was submitted to the FST and euthanized immediately after for collection of brain samples for neurochemical analyses. Serotonin (5-HT) and noradrenaline (NA) levels were measured in the hippocampus (HPC) and prefrontal cortex (PFC). Furthermore, to investigate if the treatments used could induce any significant exploratory/motor effect which would interfere with the FST results, the animals were also submitted to the open field test (OFT). The administration of imipramine (30 mg/kg), L-methionine (400, 800, 1600, and 3200 mg/kg), and SAMe (10 and 50 mg/kg) reduced the immobility time in the FST, an effect blocked by pretreatment with PCPA and WAY100635. None of the treatments increased the locomotion in the OFT. In conclusion, our results suggest that the antidepressant-like effects induced by SAMe treatment are dependent on serotonin synthesis and 5-HT_1A receptor activation.

Role of 5-HT1A-mediated upregulation of brain indoleamine 2,3 dioxygenase 1 in the reduced antidepressant and antihyperalgesic effects of fluoxetine during maintenance treatment

The reduced antidepressant and antihyperalgesic effects of selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine during maintenance treatment has been reported, but little is known about the molecular mechanism of this phenomenon. In three comorbid pain and depression animal models (genetic predisposition, chronic social stress, arthritis), we showed that the fluoxetine's antidepressant and antihyperalgesic effects were reduced during the maintenance treatment. Fluoxetine exposure induced upregulation of the 5-hydroxytryptamine 1A (5-HT1A) auto-receptor and indoleamine 2,3 dioxygenase 1 (IDO1, a rate-limiting enzyme of tryptophan metabolism) in the brainstem dorsal raphe nucleus (DRN), which shifted the tryptophan metabolism away from the 5-HT biosynthesis. Mechanistically, IDO1 upregulation was downstream to fluoxetine-induced 5-HT1A receptor expression because 1) antagonism of the 5-HT1A receptor with WAY100635 or 5-HT1A receptor knockout blocked the IDO1 upregulation, and 2) inhibition of IDO1 activity did not block the 5-HT1A receptor upregulation following fluoxetine exposure. Importantly, inhibition of either the 5-HT1A receptor or IDO1 activity sustained the fluoxetine's antidepressant and antihyperalgesic effects, indicating that 5-HT1A-mediated IDO1 upregulation in the brainstem DRN contributed to the reduced antidepressant and antihyperalgesic effects of fluoxetine. These results suggest a new strategy to improving the therapeutic efficacy of SSRI during maintenance treatment.

Beneficial effects of atypical antipsychotics on object recognition deficits after adolescent toluene exposure in mice: involvement of 5-HT1A receptors

Background: Inhalant (e.g. toluene) misuse by adolescents has been linked to psychosis and persistent cognitive deficits. Identifying effective strategies to improve cognitive deficits following chronic toluene misuse is critical. 5-HT_1A receptor has been proposed as a target for the treatment of cognitive deficits.Objectives: We compared the effects of antipsychotics on recognition deficits after adolescent toluene exposure in mice and elucidated the role of 5-HT_1A receptors in the cognition-improving effects of antipsychotics.Methods: Male NMRI mice (n = 279) received one injection per day of either toluene (750 mg/kg) or corn oil at postnatal days 35-39 and 42-46. Thereafter, the acute and subchronic effects of haloperidol, aripiprazole, or clozapine on toluene-induced recognition deficits were evaluated by novel object recognition test.Results: Acute administration of aripiprazole (p < .05) and clozapine (p < .01), but not haloperidol, significantly attenuated the toluene-induced recognition deficits. Pretreatment with 5-HT_1A receptor antagonist WAY -100,635 (p < .05) blocked their beneficial effects. Moreover, 5-HT_1A receptor agonist buspirone (p < .01) ameliorated the toluene-induced recognition deficits, which was reversed by WAY -100,635 (p < .001). Finally, after repeated treatment with clozapine, aripiprazole, and buspirone daily for 14 days, the impaired object recognition in toluene-exposed mice was significantly improved (p < .05) and the beneficial effects lasted for at least 2 weeks (p < .05).Conclusions: The results indicate that clozapine and aripiprazole, which display 5-HT_1A agonist properties, restored cognitive deficits in mice induced by adolescent toluene exposure. These findings suggest that these antipsychotics should be further explored as a potential treatment option for cognitive deficits in patients with psychosis associated with toluene exposure.

Survey of Patients Employing Cannabigerol-Predominant Cannabis Preparations: Perceived Medical Effects, Adverse Events, and Withdrawal Symptoms

Introduction: Cannabigerol (CBG), and its precursor before decarboxylation, cannabigerolic acid is sometimes labeled the "mother of all cannabinoids." The purpose of the present study was to investigate reasons for use and self-reported therapeutic effects in CBG-predominant cannabis users. Usage patterns and adverse effects, including withdrawal symptoms were also explored. Methods: Cannabidiol-predominant cannabis users were recruited online to complete an online survey assessing CBG use patterns, conditions treated with CBG-predominant cannabis (containing >50% CBG), perceived efficacy, associated adverse events, and withdrawal symptoms. One hundred twenty-seven eligible participants (U.S. residents ages 21+ who reported using CBG-predominant cannabis in the past 6 months) completed the survey. Results: Most of the samples (n=65; 51.2%) reported use of CBG-predominant products solely for medical purposes (n=46; 36.2% reported use for medical and recreational purposes; n=8; 6.3% reported recreational use only, and n=8 were missing). The most common conditions the complete sample reported using CBG to treat were anxiety (51.2%), chronic pain (40.9%), depression (33.1%), and insomnia/disturbed sleep (30.7%). Efficacy was highly rated, with the majority reporting their conditions were "very much improved" or "much improved" by CBG. Furthermore, 73.9% claimed superiority of CBG-predominant cannabis over conventional medicines for chronic pain, 80% for depression, 73% for insomnia, and 78.3% for anxiety. Forty-four percent of CBG-predominant cannabis users reported no adverse events, with 16.5% noting dry mouth, 15% sleepiness, 11.8% increased appetite, and 8.7% dry eyes. Around 84.3% reported no withdrawal symptoms, with sleep difficulties representing the most frequently endorsed withdrawal symptom (endorsed by two respondents). Conclusions: This is the first patient survey of CBG-predominant cannabis use to date, and the first to document self-reported efficacy of CBG-predominant products, particularly for anxiety, chronic pain, depression, and insomnia. Most respondents reported greater efficacy of CBG-predominant cannabis over conventional pharmacotherapy, with a benign adverse event profile and negligible withdrawal symptoms. This study establishes that humans are employing CBG and suggests that CBG-predominant cannabis-based medicines should be studied in randomized controlled trials.

Insights into the Promising Prospect of G Protein and GPCR-Mediated Signaling in Neuropathophysiology and Its Therapeutic Regulation

G protein-coupled receptors (GPCRs) are intricately involved in the conversion of extracellular feedback to intracellular responses. These specialized receptors possess a crucial role in neurological and psychiatric disorders. Most nonsensory GPCRs are active in almost 90% of complex brain functions. At the time of receptor phosphorylation, a GPCR pathway is essentially activated through a G protein signaling mechanism via a G protein-coupled receptor kinase (GRK). Dopamine, an important neurotransmitter, is primarily involved in the pathophysiology of several CNS disorders; for instance, bipolar disorder, schizophrenia, Parkinson's disease, and ADHD. Since dopamine, acetylcholine, and glutamate are potent neuropharmacological targets, dopamine itself has potential therapeutic effects in several CNS disorders. GPCRs essentially regulate brain functions by modulating downstream signaling pathways. GPR6, GPR52, and GPR8 are termed orphan GPCRs because they colocalize with dopamine D1 and D2 receptors in neurons of the basal ganglia, either alone or with both receptors. Among the orphan GPCRs, the GPR52 is recognized for being an effective psychiatric receptor. Various antipsychotics like aripiprazole and quetiapine mainly target GPCRs to exert their actions. One of the most important parts of signal transduction is the regulation of G protein signaling (RGS). These substances inhibit the activation of the G protein that initiates GPCR signaling. Developing a combination of RGS inhibitors with GPCR agonists may prove to have promising therapeutic potential. Indeed, several recent studies have suggested that GPCRs represent potentially valuable therapeutic targets for various psychiatric disorders. Molecular biology and genetically modified animal model studies recommend that these enriched GPCRs may also act as potential therapeutic psychoreceptors. Neurotransmitter and neuropeptide GPCR malfunction in the frontal cortex and limbic-related regions, including the hippocampus, hypothalamus, and brainstem, is likely responsible for the complex clinical picture that includes cognitive, perceptual, emotional, and motor symptoms. G protein and GPCR-mediated signaling play a critical role in developing new treatment options for mental health issues, and this study is aimed at offering a thorough picture of that involvement. For patients who are resistant to current therapies, the development of new drugs that target GPCR signaling cascades remains an interesting possibility. These discoveries might serve as a fresh foundation for the creation of creative methods for pharmacologically useful modulation of GPCR function.

New investigational agents for the treatment of major depressive disorder

INTRODUCTION

Pharmacotherapy of depression is characterized by the delayed onset of action, chronic treatment requirements, and insufficient effectiveness. Ketamine, with its rapid action and long-lasting effects, represents a breakthrough in the modern pharmacotherapy of depression.

AREAS COVERED

: The current review summarizes the latest findings on the mechanism of the antidepressant action of ketamine and its enantiomers and metabolites. Furthermore, the antidepressant potential of psychedelics, non-hallucinogenic serotonergic modulators and metabotropic glutamate receptor ligands was discussed.

EXPERT OPINION

Recent data indicated that to achieve fast and long-acting antidepressant-like effects, compounds must induce durable effects on the architecture and density of dendritic spines in brain regions engaged in mood regulation. Such mechanisms underlie the actions of ketamine and psychedelics. These compounds trigger hallucinations; however, it is thought that these effects might be essential for their antidepressant action. Behavioral studies with serotonergic modulators affecting 5-HT1A (biased agonists), 5-HT4 (agonists), and 5-HT-7 (antagonists) receptors exert rapid antidepressant-like activity, but they seem to be devoid of this effects. Another way to avoid psychomimetic effects and achieve the desired rapid antidepressant-like effects is combined therapy. In this respect, ligands of metabotropic receptors show some potential.

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

The role of 5-HT metabolism in cancer

Serotonin (5-hydroxytryptamine, 5-HT) metabolism has long been linked to tumorigenesis and tumor progression. Numerous studies have shown the functions of 5-HT and its metabolites in the regulation of tumor biological processes like cell proliferation, invasion, metastasis, tumor angiogenesis and immunomodulatory through multi-step complex mechanisms. Reprogramming of 5-HT metabolism has been revealed in various tumors paving way for development of drugs that target enzymes, metabolites or receptors involved in 5-HT metabolic pathway. However, information on the role of 5-HT metabolism in cancer is scanty. This review briefly describes the main metabolic routes of 5-HT, the role of 5-HT metabolism in cancer and systematically summarizes the most recent advances in 5-HT metabolism-targeted cancer therapy.

The selective 5-HT1A receptor biased agonists, F15599 and F13714, show antidepressant-like properties after a single administration in the mouse model of unpredictable chronic mild stress

RATIONALE

The prevalence of depression is ever-increasing throughout the population. However, available treatments are ineffective in around one-third of patients and there is a need for more effective and safer drugs.

OBJECTIVES

The antidepressant-like and procognitive effects of the "biased agonists" F15599 (also known as NLX-101) which preferentially targets postsynaptic 5-HT1A receptors and F13714, which targets 5-HT1A autoreceptors, were investigated in mice.

METHODS

Antidepressant-like properties of the compounds and their effect on cognitive functions were assessed using the forced swim test (FST) and the novel object recognition (NOR), respectively. Next, we induced a depressive-like state by an unpredictable chronic mild stress (UCMS) procedure to test the compounds' activity in the depression model, followed by measures of sucrose preference, FST, and locomotor activity. Levels of phosphorylated cyclic AMP response element-binding protein (p-CREB) and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) were also determined.

RESULTS

F15599 reduced immobility time in the FST over a wider dose-range (2 to 16 mg/kg po) than F13714 (2 and 4 mg/kg po), suggesting accentuated antidepressant-like properties in mice. F15599 did not disrupt long-term memory consolidation in the NOR at any dose tested, while F13714 impaired memory formation, notably at higher doses (4-16 mg/kg). In UCMS mice, a single administration of F15599 and F13714 was sufficient to robustly normalize depressive-like behavior in the FST but did not rescue disrupted sucrose preference. Both F15599 and F13714 rescued cortical and hippocampal deficits in p-ERK1/2 levels of UCMS mice but did not influence the p-CREB levels.

CONCLUSIONS

Our studies showed that 5-HT1A receptor biased agonists such as F13714 and especially F15599, due to its less pronounced side effects, might have potential as fast-acting antidepressants.

Synthesis and Evaluation of the Antidepressant-like Properties of HBK-10, a Novel 2-Methoxyphenylpiperazine Derivative Targeting the 5-HT1A and D2 Receptors

The increasing number of patients reporting depressive symptoms requires the design of new antidepressants with higher efficacy and limited side effects. As our previous research showed, 2-methoxyphenylpiperazine derivatives are promising candidates to fulfill these criteria. In this study, we aimed to synthesize a novel 2-methoxyphenylpiperazine derivative, HBK-10, and investigate its in vitro and in vivo pharmacological profile. After assessing the affinity for serotonergic and dopaminergic receptors, and serotonin transporter, we determined intrinsic activity of the compound at the 5-HT_1A and D₂ receptors. Next, we performed behavioral experiments (forced swim test, tail suspension test) to evaluate the antidepressant-like activity of HBK-10 in naïve and corticosterone-treated mice. We also assessed the safety profile of the compound. We showed that HBK-10 bound strongly to 5-HT_1A and D₂ receptors and presented antagonistic properties at these receptors in the functional assays. HBK-10 displayed the antidepressant-like effect not only in naïve animals, but also in the corticosterone-induced mouse depression model, i.e., chronic administration of HBK-10 reversed corticosterone-induced changes in behavior. Moreover, the compound’s sedative effect was observed at around 26-fold higher doses than the antidepressant-like ones. Our study showed that HBK-10 displayed a favorable pharmacological profile and may represent an attractive putative treatment candidate for depression.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.