Cortical 5-hydroxytryptamine 1A receptor biased agonist, NLX-101, displays rapid-acting antidepressant-like properties in the rat chronic mild stress model

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, like ketamine, elicit rapid-acting antidepressant activity in the rat chronic mild stress model via cortical mechanisms

BACKGROUND

The highly selective 5-HT1A serotonin receptor "biased" agonists NLX-101 and NLX-204 display, like ketamine, potent and efficacious rapid-acting antidepressant (RAAD) activity in the rat chronic mild stress (CMS) model with systemic (i.p.) administration. They rapidly (within 1 day) reverse anhedonia (i.e., CMS-induced sucrose consumption deficit), attenuate working memory deficit (novel object recognition: NOR), and decrease anxiety behavior in the elevated-plus maze (EPM).

AIMS

Here, we sought to explore the contribution of prefrontal cortex (PFC) 5-HT1A receptor activation in the RAAD activity of NLX compounds.

RESULTS/OUTCOMES

In male Wistar rats, unilateral PFC microinjections of NLX-204 and NLX-101 (16 µg), like ketamine (10 µg), reproduced the effects of their systemic administration: they reversed CMS-induced sucrose consumption deficit, attenuated anxiety (EPM), and reduced working memory deficits (NOR). In addition, unilateral PFC microinjections of the selective 5-HT1A antagonist, WAY-100,635 (2 µg), attenuated the beneficial effects of systemic NLX-204 and NLX-101 (0.16 mg/kg i.p.) in the sucrose intake and NOR models, indicating that these compounds exert their RAAD activity specifically through activation of PFC 5-HT1A receptors.

CONCLUSIONS/INTERPRETATION

These data indicate that 5-HT1A receptor biased agonists share with ketamine a common neuroanatomical site for RAAD activity, which can be obtained not only by targeting glutamatergic/NMDA neurotransmission (ketamine's primary mechanism of action) but also by activating 5-HT1A receptors, as is the case for the NLX compounds. The present observations also reinforce the notion that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve RAAD effects, with additional benefits against cognitive deficits and anxiety in depressed patients, without ketamine's troublesome side effects.

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, display ketamine-like RAAD and anti-TRD activities in rat CMS models

OBJECTIVES

NLX-101 and NLX-204 are highly selective serotonin 5-HT1A 'biased' agonists, displaying potent and efficacious antidepressant-like activity upon acute administration in models such as the forced swim test.

METHODS

we compared the effects of repeated administration of NLX-101, NLX-204 and ketamine in the chronic mild stress (CMS) model of depression, considered to have high translational potential, on sucrose consumption (anhedonia measure), novel object recognition (NOR; working memory measure) and elevated plus maze (EPM; anxiety measure) in male Wistar and Wistar-Kyoto rats (the latter being resistant to classical antidepressants).

RESULTS

in Wistar rats, NLX-204 and NLX-101 (0.08-0.16 mg/kg i.p.), like ketamine (10 mg/kg i.p.) dose-dependently reversed CMS-induced sucrose intake deficit from treatment Day 1, with nearly full reversal observed at the higher dose at Days 8 and 15. These effects persisted for 3 weeks following treatment cessation. In the NOR test, both doses of NLX-101/NLX-204, and ketamine, rescued the deficit in discrimination index caused by CMS on Days 3 and 17; all three compounds increased time spent in open arms (EPM) but only NLX-204 achieved statistical significance on Days 2 and 16. In Wistar-Kyoto rats, all 3 compounds were also active in the sucrose test and, to a lesser extent, in the NOR and EPM. In non-stressed rats (both strains), the three compounds produced no significant effects in all tests.

CONCLUSIONS

these observations further strengthen the hypothesis that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve rapid-acting/sustained antidepressant effects combined with activity against TRD, in addition to providing beneficial effects against memory deficit and anxiety in depressed patients.

The Serotonin 1A (5-HT1A) Receptor as a Pharmacological Target in Depression

Clinical depression is a common, debilitating and heterogenous disorder. Existing treatments for depression are inadequate for a significant minority of patients and new approaches are urgently needed. A wealth of evidence implicates the serotonin 1A (5-HT1A) receptor in the pathophysiology of depression. Stimulation of the 5-HT1A receptor is an existing therapeutic target for treating depression and anxiety, using drugs such as buspirone and tandospirone. However, activation of 5-HT1A raphe autoreceptors has also been suggested to be responsible for the delay in the therapeutic action of conventional antidepressants such as selective serotonin reuptake inhibitors (SSRIs). This narrative review provides a brief overview of the 5-HT1A receptor, the evidence implicating it in depression and in the effects of conventional antidepressant treatment. We highlight that pre- and post-synaptic 5-HT1A receptors may have divergent roles in the pathophysiology and treatment of depression. To date, developing this understanding to progress therapeutic discovery has been limited, partly due to a paucity of specific pharmacological probes suitable for use in humans. The development of 5-HT1A 'biased agonism', using compounds such as NLX-101, offers the opportunity to further elucidate the roles of pre- and post-synaptic 5-HT1A receptors. We describe how experimental medicine approaches can be helpful in profiling the effects of 5-HT1A receptor modulation on the different clinical domains of depression, and outline some potential neurocognitive models that could be used to test the effects of 5-HT1A biased agonists.

NLX-101, a 5-HT1A receptor-biased agonist, improves pattern separation and stimulates neuroplasticity in aged rats

5-HT_1A serotonin receptors may play a role in cognitive function changes related to advanced age. Here, we investigated the effects of acute and repeated treatment with NLX-101 (F15599), a postsynaptic 5-HT_1A receptor-biased agonist, and F13714, a presynaptic 5-HT_1A receptor-biased agonist on spatial object pattern separation (OPS) in aged (22-24 months) rats. Neuroplasticity markers including brain-derived neurotrophic factor, PSD95, synaptophysin, and doublecortin were evaluated in the hippocampus. Unlike younger rats, aged rats were incapable of discriminating any new position of the objects in the arena, reflecting the detrimental effect of aging on pattern separation. However, aged animals treated with NLX-101 showed a significant cognitive improvement in the OPS test, accompanied by increases in hippocampal brain-derived neurotrophic factor and PSD95 protein levels. In contrast, no improvement in OPS performance was observed when aged rats received F13714. Both F13714 and NLX-101 increased the number of newborn neurons in the hippocampi of aged rats. These findings provide a rationale for targeting post-synaptic 5-HT_1A as a treatment for cognitive deficits related to aging.

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.

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.

Molecular Signaling Mechanisms for the Antidepressant Effects of NLX-101, a Selective Cortical 5-HT1A Receptor Biased Agonist

Depression is the most prevalent of the mental illnesses and serotonin (5-hydroxytryptamine, 5-HT) is considered to be the major neurotransmitter involved in its etiology and treatment. In this context, 5-HT_1A receptors have attracted interest as targets for therapeutic intervention. Notably the activation of presynaptic 5-HT_1A autoreceptors delays antidepressant effects whereas the stimulation of postsynaptic 5-HT_1A heteroreceptors is needed for an antidepressant action. NLX-101 (also known as F15599) is a selective biased agonist which exhibits preferred activation of cortical over brain stem 5-HT_1A receptors. Here, we used behavioral, neurochemical and molecular methods to examine the antidepressant-like effects in rats of a single dose of NLX-101 (0.16 mg/kg, i.p.). NLX-101 reduced immobility in the forced swim test when measured 30 min but not 24 h after drug administration. NLX-101 increased extracellular concentrations of glutamate and dopamine in the medial prefrontal cortex, but no changes were detected in the efflux of noradrenaline or 5-HT. NLX-101 also produced an increase in the activation of pmTOR, pERK1/2 and pAkt, and the expression of PSD95 and GluA1, which may contribute to its rapid antidepressant action.

The selective 5-HT1A receptor agonist NLX-112 displays anxiolytic-like activity in mice

Anxiety is amongst the commonest neuropsychiatric disorders, and there is a large body of evidence to suggest that abnormalities in serotonergic function are involved in its pathogenesis. Several studies have implicated 5-HT1A receptor activation in mitigating anxiety disorders, so this study investigated the acute effects of a highly selective, potent and efficacious 5-HT1A receptor full agonist, NLX-112 (a.k.a. befiradol, F13640), in middle-aged C57bl/6 J male mice. Video tracking was used to measure several parameters including time spent in the open and closed arms of an elevated plus maze (EPM), distance travelled and thigmotaxis in an open field test (OFT). At 0.1 to 1.0 mg/kg s.c., NLX-112 markedly decreased thigmotaxis and increased exploratory behaviour in the OFT and EPM assays. Hence, at 0.3 mg/kg, NLX-112 augmented locomotor activity in the centre of an open field arena by 164% and increased the time spent in the open arms of the EPM by 119% of control. These results indicate that anxiety-like behaviours in mice are significantly diminished with low doses of NLX-112. NLX-112 may therefore possess anxiolytic properties which complement its known activity in models of movement disorders.

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.

The 5-HT1A receptor as a serotonergic target for neuroprotection in cerebral ischemia

Cerebral ischemia due to stroke or cardiac arrest greatly affects daily functioning and the quality of life of patients and has a high socioeconomic impact due to the surge in their prevalence. Advances in the identification of an effective pharmacotherapy to promote neuroprotection and recovery after a cerebral ischemic insult are, however, limited. The serotonin 1A (5-HT_1A) receptor has been implicated in the regulation of several brain functions, including mood, emotions, memory, and neuroplasticity, all of which are deleteriously affected by cerebral ischemia. This review focuses on the specific roles and mechanisms of 5-HT_1A receptors in neuroprotection in experimental models of cerebral ischemia. We present experimental evidence that 5-HT_1A receptor agonists can prevent neuronal damage and promote functional recovery induced by focal and transient global ischemia in rodents. However, indiscriminate activation of pre-and postsynaptic by non-biased 5-HT_1A receptor agonists may be a limiting factor in the anti-ischemic clinical efficacy of these compounds since 5-HT_1A receptors in different brain regions can mediate diverging or even contradictory responses. Current insights are presented into the 'biased' 5-HT_1A post-synaptic heteroreceptor agonist NLX-101 (also known as F15599), a compound that preferentially and potently stimulates postsynaptic cortical pyramidal neurons without inhibiting firing of serotoninergic neurons, as a potential strategy providing neuroprotection in cerebral ischemic conditions.

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.

Perspectives for therapy of treatment-resistant depression

A high proportion of depressed patients fail to respond to antidepressant drug treatment. Treatment-resistant depression (TRD) is a major challenge for the psychopharmacology of mood disorders. Only in the past decade have novel treatments, including deep brain stimulation (DBS) and ketamine, been discovered that provide rapid and sometimes prolonged relief to a high proportion of TRD sufferers. In this review, we consider the current status of TRD from four perspectives: the challenge of developing an appropriate regulatory framework for novel rapidly acting antidepressants; the efficacy of non-pharmacological somatic therapies; the development of an animal model of TRD and its use to understand the neural basis of antidepressant non-response; and the potential for rapid antidepressant action from targets (such as 5-HT_1A receptors) beyond the glutamate receptor.

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.

NLX-101, a cortical 5-HT1A receptor biased agonist, reverses scopolamine-induced deficit in the delayed non-matching to position model of cognition

NLX-101 is a selective, high efficacy, biased agonist at post-synaptic cortical 5-HT_1A receptors. We have previously shown that it opposes deficits produced by blockade of NMDA receptors and has pro-cognitive activity of its own. Based on the strong interaction between 5-HT_1A receptors and the central cholinergic system, we tested NLX-101 on scopolamine-induced impairment of cognition in a delayed non-matching to position (DNMTP) model. The cholinesterase inhibitor, tacrine, was used as a comparator. In operant chambers with two retractable levers, male rats were trained to press one randomly presented lever during a "sample" phase. Following a time delay of either 1, 5 or 10 s, both levers were then presented, the rat being required to press the correct lever (i.e. the one not previously presented) to receive a food pellet reward. Scopolamine (0.16 mg/kg i.p.) significantly impaired accuracy (i.e. choice of correct lever) at 5 and 10 s delays. In contrast, NLX-101 (0.04, 0.16, 0.63 mg/kg i.p.) did not worsen accuracy, except at 0.63 mg/kg. Moreover, NLX-101 (0.04 and 0.16 mg/kg) dose-dependently and significantly opposed scopolamine-induced impairment for 5 and 10 s delays, with near-total reversal at 10 s. The acetylcholinesterase inhibitor, tacrine, also opposed scopolamine-induced impairment but was less potent and efficacious, with a single significant effect at 2.5 mg/kg and 5 s delay only. The present data suggest that biased agonism at post-synaptic, cortical 5-HT_1A receptors could prove useful in neurological or neuropsychiatric pathologies characterized by cognitive deficits consecutive to a reduced central cholinergic tone.

Correlation of distinct behaviors to the modified expression of cerebral Shank1,3 and BDNF in two autistic animal models

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder featuring altered neuronal circuitry and consequently impaired social interactions, restrictive interests plus repetitive stereotypic activities. In the present study, differentiated behaviors of valproic (VPA) and propionic (PPA) acid-mediated autism rats were correlated to cerebral scaffolding proteins (Shank1,3) and BDNF expression variations. Sprague-Dawley offspring that received VPA during pregnancy displayed a notably diminished permanence (-78 %, p < 0.01) in the light chamber of light dark (LD) test, reduced exploratory tasks, i.e. grooming (-90 %) and rearing (-65 %). Moreover, they executed extremely greater climbing intervals (+300 %, p < 0.001) in novel cage (NC) test, plus exhibited an extremely reduced (-331 %) discrimination index in novel object recognition (NOR) test when compared to controls. PPA-treated postnatal days (PND) 12-16 rats also displayed anxiety-like behaviors, although in a less evident manner, as indicated by a moderate time (+55 %; p < 0.05) spent in dark chamber along with notable and moderate decreases in digging (-78 %) plus grooming (-52 %), respectively. Contextually, VPA- more than PPA supplied opposite Shank1,3 expression changes in cerebellum (CB; -62 %; +78 %), dorsomedial prefrontal cortex (DM-PFC; +95 % -76 %), respectively, while resulting extremely upregulated in hippocampus (HIP; +125 % - +155 %). Even BDNF resulted to be substantially and notably diminished in HIP (-85 %) and DM-PFC (-72 %), respectively, of VPA rats while it was only moderately reduced (-35 % to -45 %) in these same areas of PPA rats. The early altered brain-specific expression levels accounting for different behavioral performances may provide useful diagnostic indications and constitute valuable therapeutic strategies for autistic patients.

Modulation of DNA Methylation and Gene Expression in Rodent Cortical Neuroplasticity Pathways Exerts Rapid Antidepressant-Like Effects

BACKGROUND

Stress increases DNA methylation, primarily a suppressive epigenetic mechanism catalyzed by DNA methyltransferases (DNMT), and decreases the expression of genes involved in neuronal plasticity and mood regulation. Despite chronic antidepressant treatment decreases stress-induced DNA methylation, it is not known whether inhibition of DNMT would convey rapid antidepressant-like effects.

AIM

This work tested such a hypothesis and evaluated whether a behavioral effect induced by DNMT inhibitors (DNMTi) corresponds with changes in DNA methylation and transcript levels in genes consistently associated with the neurobiology of depression and synaptic plasticity (BDNF, TrkB, 5-HT1A, NMDA, and AMPA).

METHODS

Male Wistar rats received intraperitoneal (i.p.) injection of two pharmacologically different DNMTi (5-AzaD 0.2 and 0.6 mg/kg or RG108 0.6 mg/kg) or vehicle (1 ml/kg), 1 h or 7 days before the learned helplessness test (LH). DNA methylation in target genes and the correspondent transcript levels were measured in the hippocampus (HPC) and prefrontal cortex (PFC) using meDIP-qPCR. In parallel separate groups, the antidepressant-like effect of 5-AzaD and RG108 was investigated in the forced swimming test (FST). The involvement of cortical BDNF-TrkB-mTOR pathways was assessed by intra-ventral medial PFC (vmPFC) injections of rapamycin (mTOR inhibitor), K252a (TrkB receptor antagonist), or vehicle (0.2 μl/side).

RESULTS

We found that both 5-AzaD and RG108 acutely and 7 days before the test decreased escape failures in the LH. LH stress increased DNA methylation and decreased transcript levels of BDNF IV and TrkB in the PFC, effects that were not significantly attenuated by RG108 treatment. The systemic administration of 5-AzaD (0.2 mg/kg) and RG108 (0.2 mg/kg) induced an antidepressant-like effect in FST, which was, however, attenuated by TrkB and mTOR inhibition into the vmPFC.

CONCLUSION

These findings suggest that acute inhibition of stress-induced DNA methylation promotes rapid and sustained antidepressant effects associated with increased BDNF-TrkB-mTOR signaling in the PFC.

NLX-101, a highly selective 5-HT1A receptor biased agonist, mediates antidepressant-like activity in rats via prefrontal cortex 5-HT1A receptors

NLX-101 (also known as F15599) exhibits nanomolar affinity, exceptional selectivity and biased agonist activation of serotonin 5-HT_1A receptors. Given systemically, it displays antidepressant-like activity in the rat forced swim test (FST), and preferentially activates 5-HT_1A post-synaptic heteroreceptors in the prefrontal cortex (PFC), a brain region involved in the control of mood. Here, we assessed the ability of NLX-101 to produce antidepressant-like activity in the FST following in-situ PFC unilateral microinjection. (+)8-OH-DPAT and F13714, two 5-HT_1A receptor agonists that do not display cortical biased agonism, were tested as comparators. NLX-101 decreased time spent in immobility in a bi-modal manner, with a first MED of 0.25 μg (immobility reduced from 160 to 80 s) but immobility returned to control levels at the next dose (1 μg). At higher doses, immobility decreased monotonically, with a second MED of 16 μg and a maximal effect (36 s) at 32 μg. (+)8-OH-DPAT and F13714 also diminished immobility but, unlike NLX-101, they did so in a unimodal manner, with MEDs of 1 and 4 μg, and maximal responses of 31 and 4 s, for (+)8-OH-DPAT and F13714, respectively. The effects of (+)8-OH-DPAT (16 μg) and of both active doses of NLX-101 (0.25 and 16 μg) were prevented by the 5-HT_1A receptor antagonist WAY-100,635 (0.63 mg/kg s.c.). In conclusion, activation of 5-HT_1A receptors in the PFC by NLX-101 produces robust antidepressant-like effects in the rat FST, with a distinctive bimodal dose-response pattern. These data suggest that NLX-101 may target specific 5-HT_1A receptor subpopulations in PFC, likely located on GABAergic and/or glutamatergic neurons.

Discovery of Novel pERK1/2- or β-Arrestin-Preferring 5-HT1A Receptor-Biased Agonists: Diversified Therapeutic-like versus Side Effect Profile

Novel 1-(1-benzoylpiperidin-4-yl)methanamine derivatives with high affinity and selectivity for serotonin 5-HT_1A receptors were obtained and tested in four functional assays: ERK1/2 phosphorylation, adenylyl cyclase inhibition, calcium mobilization, and β-arrestin recruitment. Compounds 44 and 56 (2-methylaminophenoxyethyl and 2-(1H-indol-4-yloxy)ethyl derivatives, respectively) were selected as biased agonists with highly differential “signaling fingerprints” that translated into distinct in vivo profiles. In vitro, 44 showed biased agonism for ERK1/2 phosphorylation and, in vivo, it preferentially exerted an antidepressant-like effect in the Porsolt forced swimming test in rats. In contrast, compound 56 exhibited a first-in-class profile: it preferentially and potently activated β-arrestin recruitment in vitro and potently elicited lower lip retraction in vivo, a component of “serotonergic syndrome”. Both compounds showed promising developability properties. The presented 5-HT_1A receptor-biased agonists, preferentially targeting various signaling pathways, have the potential to become drug candidates for distinct central nervous system pathologies and possessing accentuated therapeutic activity and reduced side effects.

Medial PFC AMPA receptor and BDNF signaling are required for the rapid and sustained antidepressant-like effects of 5-HT1A receptor stimulation

We previously reported that the serotonergic system is important for the antidepressant-like effects of ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist, which produces rapid and long-lasting antidepressant effects in patients with major depressive disorder (MDD). In particular, selective stimulation of the 5-HT_1A receptor in the medial prefrontal cortex (mPFC), as opposed to the somatic 5-HT_1A autoreceptor, has been shown to play a critical role in the antidepressant-like actions of ketamine. However, the detailed mechanisms underlying mPFC 5-HT_1A receptor-mediated antidepressant-like effects are not fully understood. Here we examined the involvement of the glutamate AMPA receptor and brain-derived neurotrophic factor (BDNF) in the antidepressant-like effects of 5-HT_1A receptor activation in the mPFC. The results show that intra-mPFC infusion of the 5-HT_1A receptor agonist 8-OH-DPAT induces rapid and long-lasting antidepressant-like effects in the forced swim, novelty-suppressed feeding, female urine sniffing, and chronic unpredictable stress tests. In addition, the results demonstrate that the antidepressant-like effects of intra-mPFC infusion of 8-OH-DPAT are blocked by co-infusion of an AMPA receptor antagonist or an anti-BDNF neutralizing antibody. In addition, mPFC infusion of 8-OH-DPAT increased the phosphorylation of signaling proteins downstream of BDNF, including mTOR, ERK, 4EBP1, and p70S6K. Finally, selective stimulation of the 5-HT_1A receptor increased levels of synaptic proteins and synaptic function in the mPFC. Collectively, these results indicate that selective stimulation of 5-HT_1A receptor in the mPFC exerts rapid and sustained antidepressant-like effects via activation of AMPA receptor/BDNF/mTOR signaling in mice, which subsequently increase synaptic function in the mPFC, and provide evidence for the 5-HT_1A receptor as a target for the treatment of MDD.

From Receptor Selectivity to Functional Selectivity: The Rise of Biased Agonism in 5-HT1A Receptor Drug Discovery

Despite extensive efforts to design serotonin 5-HT1A receptor compounds, there are currently no clinically available selective agonists to explore the therapeutic potential of activating this receptor. Commonly used drugs targeting 5-HT1A receptors, such as buspirone or other azapirone compounds, possess only limited selectivity over cross-reacting sites, act as partial agonists for 5-HT1A receptor activation, and are metabolically labile, generating active metabolites. In addition, drug discovery has been hampered by the multiplicity of 5-HT1A receptor subpopulations, expressed in different brain regions, that are coupled to distinct molecular signaling mechanisms and mediate a wide variety of physiological responses, both desired and undesired. In this context, advances in 5-HT1A receptor drug discovery have attracted attention of novel 'biased agonists' that are selective, efficacious and preferentially target the brain regions that mediate therapeutic activity without triggering side effects. The prototypical first-in-class compound NLX-101 (a.k.a. F15599; 3-chloro-4-fluorophenyl-[4-fluoro-4-[[(5-methylpyrimidin-2-ylmethyl)amino]methyl]piperidin- 1-yl]methanone), preferentially activates 5-HT1A receptors in cortical regions and exhibits potent, rapidacting and sustained antidepressant-like and procognitive properties in animal models. Here the background has been reviewed that led to the discovery of the class of 1-(1-benzoylpiperidin-4- yl)methanamine derivatives, including NLX-101, as well as recent advances in discovery of novel 5-HT1A receptor biased agonists, notably aryloxyethyl derivatives of 1‑(1-benzoylpiperidin-4yl)methanamine which show promising pharmacological activity both in vitro and in vivo. Overall, the results suggest that opportunities exist for innovative drug discovery of selective 5-HT1A receptor biased agonists that may open new avenues for the treatment of CNS disorders involving dysfunction of serotonergic neurotransmission.