Anti-aggressive effects of the selective high-efficacy 'biased' 5-HT₁A receptor agonists F15599 and F13714 in male WTG rats

Serotonin 5-HT1A receptor biased agonists: The challenge of translating an innovative neuropharmacological concept into therapeutics

Serotonin 5-HT1A receptor agonists are prime candidates for CNS drug discovery due to their involvement physiological and pathological processes relevant to neurology and psychiatry. However, the lack of target specificity of many previously characterized agonists has long been a barrier to pharmacological and therapeutic progress. Some of the obstacles may be overcome through the recent concept of biased agonism, which has attracted considerable attention to the development of novel chemical entities at 5-HT, and particularly 5-HT1A receptors, by specifically targeting intracellular signalling pathways that may themselves be linked to specific brain regions and therapeutic indications. There is now abundant translational data demonstrating distinct molecular and functional pharmacological signatures between different 5-HT1A receptor agonists, opening new opportunities for research in neurology and psychiatry. Nevertheless, important limitations need to be overcome, including understanding the precise molecular basis for biased agonism, the need for improved translatable models, and the currently limited clinical data on biased agonists. Here, we review the current limits of our knowledge of 5-HT1A receptor biased agonists and the limitations of available pharmacological tools, counterbalanced by the translational possibilities and therapeutic perspectives opened by novel, highly selective 5-HT1A receptor drug-candidates. This article is part of the Special Issue on "Ligand Bias".

[Treatment of mental disorders caused or triggered by somatic and neurological diseases with the use of the multimodal antidepressant trazodone]

The purpose of this narrative review is to relate current data on the molecular mechanisms of action of trazodone with its clinical effects and applicability in mental disorders caused or triggered by somatic and neurological disease, according to available publications. In the article, the prospects for the use of the multimodal antidepressant trazodone are discussed in accordance with therapeutic targets. The latter are discussed in accordance with the typology of the mentioned above psychosomatic disorders. Trazodone is an antidepressant acting primarily due to the blockade of postsynaptic serotonin 5H2A- and 5H2C-receptors, as well as the blockade of serotonin reuptake, but also has affinity for a number of additional receptors. The drug has a favorable safety profile and a wide range of beneficial effects: antidepressive, somnolent, anxiolytic, anti-dysphoric and somatotropic. This makes it possible to influence a wide range of therapeutic targets in the structure of mental disorders caused or triggered by somatic and neurological diseases, carrying out safe and effective psychopharmacotherapy.

Buspirone, a 5-HT1A agonist attenuates social isolation-induced behavior deficits in rats: a comparative study with fluoxetine

Social isolation is a potent stressor in both humans and animals that results in increased anger-like emotion, (anger in humans), aggression and suicidal ideation in humans [suicidal trait-related behavior in rats (STRB)]. The study's purpose was to compare the effects of buspirone (BUS) and fluoxetine (Flx) on social isolation-induced behavior deficits in rats. The male Wistar rats were randomized into six groups and caged individually for 14 days except for the non stress control (nSC) group. They were then divided into the following groups, stress control (SC), Flx (30), BUS (10), BUS (20) and BUS (40) and treated from day 14 to day 28. On the last day of treatment behavior parameters were recorded. Serum cortisol, blood pressure (BP) measurement, magnetic resonance imaging (MRI) of the rat's brain and brain-derived neurotrophic factor (BDNF) expression were performed. SC group showed a significant increase in anger-like emotion, aggression, irritability score, learned helplessness, increased cortisol level and reduced BDNF. These behavioral deficits were attenuated by BUS and Flx, Both were found to be equally beneficial in preventing anger-like emotions and aggression. Flx, which has been found to promote suicidal thoughts in people, did not reduce irritability in rats, showing that it did not affect it. BUS significantly improved all behavioral traits also reduced cortisol levels, significantly increased BDNF and normalized BP. Neuroimaging studies in SC brains showed a reduction in amygdala size compared to nSC, BUS treatment mitigated this reduction. Buspirone is effective in preventing social isolation induced behavioural-deficits.

Serotonin Receptors as Therapeutic Targets for Autism Spectrum Disorder Treatment

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders characterized by repetitive and stereotyped behaviors as well as difficulties with social interaction and communication. According to reports for prevalence rates of ASD, approximately 1~2% of children worldwide have been diagnosed with ASD. Although there are a couple of FDA (Food and Drug Administration)—approved drugs for ASD treatment such as aripiprazole and risperidone, they are efficient for alleviating aggression, hyperactivity, and self-injury but not the core symptoms. Serotonin (5-hydroxytryptamine, 5-HT) as a neurotransmitter plays a crucial role in the early neurodevelopmental stage. In particular, 5-HT has been known to regulate a variety of neurobiological processes including neurite outgrowth, dendritic spine morphology, shaping neuronal circuits, synaptic transmission, and synaptic plasticity. Given the roles of serotonergic systems, the 5-HT receptors (5-HTRs) become emerging as potential therapeutic targets in the ASD. In this review, we will focus on the recent development of small molecule modulators of 5-HTRs as therapeutic targets for the ASD treatment.

Effects of the 5-HT1A receptor agonist 8-OH-DPAT on aggressive behavior in juvenile pufferfish, Takifugu rubripes

Severe aggressive behavior of juvenile pufferfish affects economic efficiency and fish welfare in aquaculture. 5-HT plays an important role in regulating the aggressive behavior of fish in aquaculture environment. This study examined the effects of different concentrations (0, 0.25, 0.5, 1 mg/kg) of 8-OH-DPAT, a selective 5-HT_1A receptor agonist, on the aggressive behavior of juvenile pufferfish. Forty-five minutes after drug injection, the aggressive behavior of juvenile fish was recorded for 20 min, including the latency to the first attack and the frequency of aggressive behaviors. The results showed no significant differences in the latency to the first attack of juvenile fish among treatment groups. During the first 10 min of the observation period, there was no significant difference in the total aggressive acts and locomotor activity among treatment groups. Total aggressive acts and locomotor activity were the least in the 1 mg/kg 8-OH-DPAT-treated during the 20 min observation period. Both aggressive behavior and locomotor activity were negatively correlated with 8-OH-DPAT treatment overall, respectively. The above results suggested that the serotonergic system activation had suppressive effects on aggressive behavior and locomotor activity in juvenile pufferfish.

Discriminative stimulus properties of the 5-HT1A receptor biased agonists NLX-101 and F13714, in rats trained to discriminate 8-OH-DPAT from saline

NLX-101 and F13714 are selective, full efficacy, biased agonists of the serotonin (5-HT1A) receptor. NLX-101 preferentially activates cortical postsynaptic 5-HT1A receptors, whereas F13714 preferentially activates raphe nuclei presynaptic 5-HT1A receptors. We compared NLX-101 and F13714 for their efficacy and potency to substitute for the discriminative cue produced by the prototypical, nonbiased 5-HT1A receptor agonist 8-OH-DPAT (racemate). Male and female Sprague-Dawley rats were trained to discriminate 8-OH-DPAT (0.1 mg/kg i.p., 20 min pretreatment) from saline using a classical two-lever drug-discrimination procedure. 8-OH-DPAT (0.01 and 0.05 mg/kg i.p.) dose-dependently substituted for the training dose, with about 50% responding on the 8-OH-DPAT-associated lever at 0.05 mg/kg. F13714 fully and very potently substituted for the training dose of 8-OH-DPAT from 0.018 mg/kg i.p., whereas NLX-101 only achieved full substitution at 0.5 mg/kg i.p., a dose which is known to also activate presynaptic 5-HT1A receptors. The 5-HT1A receptor partial agonist, buspirone, partially substituted (~80%) at 1 and 2 mg/kg i.p., doses which also decreased response rates. F13714 decreased response rates at 0.05 mg/kg. The selective 5-HT1A receptor antagonist WAY-100 635 (1 mg/kg s.c., 40 min pretreatment) elicited almost no responding on the 8-OH-DPAT-associated lever by itself, but blocked the discriminative stimulus effects produced by administration (20 min pretreatment) of 8-OH-DPAT (0.1 mg/kg), F13714 (0.025 mg/kg), NLX-101 (0.5 mg/kg) or buspirone (1 mg/kg). These data suggest that the discriminative cue produced by 0.1 mg/kg i.p. 8-OH-DPAT results from activation of presynaptic 5-HT1A receptors. They also further demonstrate the distinct profiles in behavioral models of 5-HT1A receptor-biased agonists.

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.

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

Serotonin (5-HT) is one of the fundamental neurotransmitters that contribute to the information essential for an organism's normal, physiological function. Serotonin acts centrally and systemically. The 5-HT_1A receptor is the most widespread serotonin receptor, and participates in many brain-related disorders, including anxiety, depression, and cognitive impairments. The 5-HT_1A receptor can activate several different biochemical pathways and signals through both G protein-dependent and G protein-independent pathways. Preclinical experiments indicate that distinct signaling pathways in specific brain regions may be crucial for antidepressant-like, anxiolytic-like, and procognitive responses. Therefore, the development of new ligands that selectively target a particular signaling pathway(s) could open new possibilities for more effective and safer pharmacotherapy. This review discusses the current state of preclinical studies focusing on the concept of functional selectivity (biased agonism) regarding the 5-HT_1A receptor and its role in antidepressant-like, anxiolytic-like, and procognitive regulation. Such work highlights not only the differential effects of targeted autoreceptors, vs. heteroreceptors, but also the importance of targeting specific downstream intracellular signaling processes, thereby enhancing favorable over unfavorable signaling activation.

Pharmacological Studies on the Role of 5-HT1 A Receptors in Male Sexual Behavior of Wildtype and Serotonin Transporter Knockout Rats

Brain serotonin (5-HT) neurotransmission plays an important role in male sexual behavior and it is well established that activating 5-HT_{1 A} receptors in rats facilitate ejaculatory behavior. However, the relative contribution of 5-HT_{1 A} somatodendritic autoreceptors and heteroreceptors in this pro-sexual behavior is unclear. Moreover, it is unclear whether the contribution of somatodendritic 5-HT_{1 A} autoreceptors and postsynaptic 5-HT_{1 A} heteroreceptors alter when extracellular 5-HT levels are chronically increased. Serotonin transporter knockout (SERT^-/-) rats exhibit enhanced extracellular 5-HT levels and desensitized 5-HT_{1 A} receptors. These rats model neurochemical changes underlying chronic SSRI-induced sexual dysfunction. We want to determine the role of presynaptic versus postsynaptic 5-HT_{1 A} receptors in the pro-sexual effects of 5-HT_{1 A} receptor agonists in SERT^+/+ and in SERT^-/- rats. Therefore, acute effects of the biased 5-HT_{1 A} receptor agonists F-13714, a preferential 5-HT_{1 A} autoreceptor agonist, or F-15599, a preferential 5-HT_{1 A} heteroreceptor agonist, and S15535 a mixed 5-HT_{1 A} autoreceptor agonist/heteroreceptor antagonist, on male sexual behavior were assessed. A clear and stable genotype effect was found after training where SERT^+/+ performed sexual behavior at a higher level than SERT^-/- rats. Both F-15599 and F-13714 induced pro-sexual activity in SERT^+/+ and SERT^-/- animals. Compared to SERT^+/+, the F13714-dose-response curve in SERT^-/- rats was shifted to the right. SERT^+/+ and SERT^-/- rats responded similar to F15599. Within both SERT^+/+ and SERT^-/- rats the potency of F-13714 was much stronger compared to F-15599. S15535 had no effect on sexual behavior in either genotype. In SERT^+/+ and SERT^-/- rats that were selected on comparable low sexual activity (SERT^+/+ 3 or less ejaculations and SERT^-/- 5 or less ejaculations in 10 weeks) S15535 also did not influence sexual behavior. The two biased compounds with differential effects on 5-HT_{1 A} auto- and hetero-receptors, exerted pro-sexual activity in both SERT^+/+ and SERT^-/- rats. Applying these specific pharmacological tools has not solved whether pre- or post-synaptic 5-HT_{1 A} receptors are involved in pro-sexual activity. Moreover, the inactivity of S15535 in male sexual behavior in either genotype was unexpected. The question is whether the in vivo pharmacological profile of the different 5-HT_{1 A} receptor ligands used, is sufficient to differentiate pre- and/or post-synaptic 5-HT_{1 A} receptor contributions in male rat sexual behavior.

The stalk-eyed fly as a model for aggression - is there a conserved role for 5-HT between vertebrates and invertebrates?

Serotonin (5-HT) has largely been accepted to be inhibitory to vertebrate aggression, whereas an opposing stimulatory role has been proposed for invertebrates. Herein, we argue that critical gaps in our understanding of the nuanced role of 5-HT in invertebrate systems drove this conclusion prematurely, and that emerging data suggest a previously unrecognized level of phylogenetic conservation with respect to neurochemical mechanisms regulating the expression of aggressive behaviors. This is especially apparent when considering the interplay among factors governing 5-HT activity, many of which share functional homology across taxa. We discuss recent findings using insect models, with an emphasis on the stalk-eyed fly, to demonstrate how particular 5-HT receptor subtypes mediate the intensity of aggression with respect to discrete stages of the interaction (initiation, escalation and termination), which mirrors the complex behavioral regulation currently recognized in vertebrates. Further similarities emerge when considering the contribution of neuropeptides, which interact with 5-HT to ultimately determine contest progression and outcome. Relative to knowledge in vertebrates, much less is known about the function of 5-HT receptors and neuropeptides in invertebrate aggression, particularly with respect to sex, species and context, prompting the need for further studies. Our Commentary highlights the need to consider multiple factors when determining potential taxonomic differences, and raises the possibility of more similarities than differences between vertebrates and invertebrates with regard to the modulatory effect of 5-HT on aggression.

Novel Aryloxyethyl Derivatives of 1-(1-Benzoylpiperidin-4-yl)methanamine as the Extracellular Regulated Kinases 1/2 (ERK1/2) Phosphorylation-Preferring Serotonin 5-HT1A Receptor-Biased Agonists with Robust Antidepressant-like Activity

Novel 1-(1-benzoylpiperidin-4-yl)methanamine derivatives were designed as "biased agonists" of serotonin 5-HT_1A receptors. The compounds were tested in signal transduction assays (ERK1/2 phosphorylation, cAMP inhibition, Ca²⁺ mobilization, and β-arrestin recruitment) which identified ERK1/2 phosphorylation-preferring aryloxyethyl derivatives. The novel series showed high 5-HT_1A receptor affinity, >1000-fold selectivity versus noradrenergic α₁, dopamine D₂, serotonin 5-HT_2A, histamine H₁, and muscarinic M₁ receptors, and favorable druglike properties (CNS-MPO, Fsp³, LELP). The lead structure, (3-chloro-4-fluorophenyl)(4-fluoro-4-(((2-(pyridin-2-yloxy)ethyl)amino)methyl)piperidin-1-yl)methanone (17, NLX-204), displayed high selectivity in the SafetyScreen44 panel (including hERG channel), high solubility, metabolic stability, and Caco-2 penetration and did not block CYP3A4, CYP2D6 isoenzymes, or P-glycoprotein. Preliminary in vivo studies confirmed its promising pharmacokinetic profile. 17 also robustly stimulated ERK1/2 phosphorylation in rat cortex and showed highly potent (MED = 0.16 mg/kg) and efficacious antidepressant-like activity, totally eliminating immobility in the rat Porsolt test. These data suggest that the present 5-HT_1A receptor-biased agonists could constitute promising antidepressant drug candidates.

Activation of somatodendritic 5-HT1A autoreceptors reduces the acquisition and expression of cued fear in the rat fear-potentiated startle test

RATIONALE

Fear conditioning is an important factor in the etiology of anxiety disorders. Previous studies have demonstrated a role for serotonin (5-HT)_1A receptors in fear conditioning. However, the relative contribution of somatodendritic 5-HT_1A autoreceptors and post-synaptic 5-HT_1A heteroreceptors in fear conditioning is still unclear.

OBJECTIVE

To determine the role of pre- and post-synaptic 5-HT_1A receptors in the acquisition and expression of cued and contextual conditioned fear.

METHODS

We studied the acute effects of four 5-HT_1A receptor ligands in the fear-potentiated startle test. Male Wistar rats were injected with the 5-HT_1A receptors biased agonists F13714 (0-0.16 mg/kg, IP), which preferentially activates somatodendritic 5-HT_1A autoreceptors, or F15599 (0-0.16 mg/kg, IP), which preferentially activates cortical post-synaptic 5-HT_1A heteroreceptors, with the prototypical 5-HT_1A receptor agonist R(+)8-OH-DPAT (0-0.3 mg/kg, SC) or the 5-HT_1A receptor antagonist WAY100,635 (0-1.0 mg/kg, SC).

RESULTS

F13714 (0.16 mg/kg) and R(+)-8-OH-DPAT (0.03 mg/kg) injected before training reduced cued fear acquisition. Pre-treatment with F15599 or WAY100,635 had no effect on fear learning. In the fear-potentiated startle test, F13714 (0.04-0.16 mg/kg) and R(+)-8-OH-DPAT (0.1-0.3 mg/kg) reduced the expression of cued and contextual fear, whereas F15599 had no effect. WAY100,635 (0.03-1.0 mg/kg) reduced the overall startle response.

CONCLUSIONS

The current findings indicate that activation of somatodendritic 5-HT_1A autoreceptors reduces cued fear learning, whereas 5-HT_1A receptors seem not involved in contextual fear learning. Moreover, activation of somatodendritic 5-HT_1A autoreceptors may reduce cued and contextual fear expression, whereas we found no evidence for the involvement of cortical 5-HT_1A heteroreceptors in the expression of conditioned fear.

Linking organismal growth, coping styles, stress reactivity, and metabolism via responses against a selective serotonin reuptake inhibitor in an insect

Evidence suggests that brain serotonin (5-HT) is one of the central mediators of different types of animal personality. We tested this assumption in field crickets Gryllus integer using a selective serotonin reuptake inhibitor (SSRI). Crickets were selected for slow and rapid development and tested for their coping styles under non-stressful conditions (time spent exploring a novel object). Resting metabolic rate, maximum metabolic rate and latency to resume activity were measured under stressful conditions (stress reactivity). Measurements were taken (i) before and (ii) during the SSRI treatment. Before the SSRI treatment, a strong negative correlation was observed between coping style and stress reactivity, which suggests the existence of a behavioral syndrome. After the SSRI treatment, the syndrome was no longer evident. The results of this study show that 5-HT may be involved in regulating behavior not only along a stress reactivity gradient but also along a coping styles axis. The relationship between personality and the strength and direction of 5-HT treatment on observed behaviors indicates trait-like individual differences in 5-HT signaling. Overall, these findings do not support recent ideas arising from the pace-of-life syndrome (POLS) hypothesis, which predict higher exploration and metabolic rates in rapidly developing bold animals.

Effects of para-methoxyamphetamine (PMA) on agonistic encounters between male mice

Para-methoxyamphetamine (PMA) is a synthetic drug chemically similar to the recreational drug 3,4-methylenedioxy-methamphetamine (MDMA or "ecstasy") and often replaces MDMA in tablets that show an "ecstasy" logo. PMA displays a higher toxic potential than MDMA, but the behavioral profile of PMA has been scarcely studied in animal models. Here we evaluated the effects of PMA (2, 4, 8, and 12 mg/kg, i.p.) on agonist encounters between male mice using an ethopharmacological approach, the isolation-induced aggression model. Likewise, since PMA and MDMA share common mechanisms of action, we compared the behavioral profile of PMA with that induced by MDMA (8 mg/kg, i.p.) which behavioral effects in this model are well characterized. Individually housed mice were exposed to anosmic standard opponents 30 min after drug administration. The encounters were videotaped and evaluated using an ethologically based analysis. PMA (all doses) significantly reduced offensive behaviors (threat and attack), however, a detailed behavioral analysis suggests that the observed antiaggressive effect seems to be unspecific, showing a complex dose-dependent behavioral profile. Thus, antiaggresive actions observed after the administration of the lowest dose were accompanied by increases in social investigation, avoidance/flee behaviors and non-social explorations, together with a reduction of digging behavior. This pattern reflects both approach-contact behaviors and avoidance-flee behaviors. From 4 mg/kg to 12 mg/kg, the increase in social investigation previously observed disappears, and there is a slight increase in immobility, together with a different behavioral pattern that suggests anxiogenic effects of PMA, similar to those reported after the administration of MDMA. The higher doses of PMA exhibit a behavioral profile very similar to that observed in animals treated with MDMA, with the exception of the immobility produced by PMA. These findings show for the first time the non-specific antiaggressive profile of PMA in the model of aggression induced by isolation in male mice.

Marmoset Serotonin 5-HT1A Receptor Mapping with a Biased Agonist PET Probe 18F-F13714: Comparison with an Antagonist Tracer 18F-MPPF in Awake and Anesthetized States

BACKGROUND

In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. ¹⁸F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their "active state," but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors.

METHODS

Positron emission tomography scans were conducted in a cohort of common marmosets (n=4) using the serotonin 1A receptor biased agonist radiotracer, ¹⁸F-F13714, compared with a well-characterized ¹⁸F-labeled antagonist radiotracer, ¹⁸F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions.

RESULTS

¹⁸F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the ¹⁸F-MPPF. Whereas ¹⁸F-MPPF showed highest binding in hippocampus and amygdala, ¹⁸F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of ¹⁸F-F13714 were about one-third of those observed with ¹⁸F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions.

CONCLUSIONS

These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as ¹⁸F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.