Divergent effects of the 'biased' 5-HT1 A receptor agonists F15599 and F13714 in a novel object pattern separation task

Flibanserin conquers murine depressive pseudodementia by amending HPA axis, maladaptive inflammation and AKT/GSK/STAT/BDNF trajectory: Center-staging of the serotonergic/adrenergic circuitry

Depressive pseudodementia (DPD) is a debilitating cognitive dysfunction that accompanies major and/or frequent depressive attacks. DPD has gained significant research attention owing to its negative effects on the patients' quality of life and productivity. This study tested the procognitive potential of Flibanserin (FBN), the serotonin (5HT) receptor modulator, against propranolol (PRP), as β/5HT1A receptors blocker. Serving this purpose, female Wistar Albino rats were subjected to chronic unpredictable stress (CUS) and subsequently treated with FBN only (3 mg/kg/day, p.o), PRP only (10 mg/kg/day, p.o), or PRP followed by FBN, using the same doses. FBN ameliorated the behavioral/cognitive alterations and calmed the hypothalamic-pituitary-adrenal (HPA) axis storm by reducing the levels of stress-related hormones, viz, corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), corticosterone (CORT) parallel to epinephrine (EPI) hyperstimulation. The maladaptive inflammatory response, comprising of interleukin (IL)-1β/6, and tumor necrosis factor (TNF)-α, was consequently blunted. This was contemporaneous to the partial restoration of the protein kinase-B (AKT)/glycogen synthase kinase (GSK)3β/signal transducer and activator of transcription (STAT)-3 survival trajectory and the reinstatement of the levels of brain derived neurotrophic factor (BDNF). Microscopically, FBN repaired the hippocampal architecture and lessened CD68/GFAP immunoreactivity. Pre-administration of PRP partially abolished FBN effect along the estimated parameters, except for 5HT2A receptor expression and epinephrine level, to prove 5HT1A receptor as a fulcrum initiator of the investigated pathway, while its sole administration worsened the underlying condition. Ultimately, these findings highlight the immense procognitive potential of FBN, offering a new paradigm for halting DPD advancement via synchronizing adrenergic/serotonergic circuitry.

Augmentation therapy with serotonin1A receptor partial agonists on neurocognitive function in schizophrenia: A systematic review and meta-analysis

Background

In a previous meta-analysis, the use of serotonin1A(5-HT1A) receptor partial agonists of the azapirone class as an add-on therapy was associated with beneficial effects on positive symptoms and attention/processing speed in schizophrenia patients. This meta-analysis builds on that study by examining the effects of adjunctive treatment with 5-HT1A partial agonists in improving other domains of neurocognitive function in schizophrenia patients.

Methods

A literature search was performed from 1987 to May 2023 to identify randomized controlled trials. The standardized mean difference (SMD) with 95 % confidence intervals (CI) was calculated when there were two or more studies. Four studies, involving 313 patients, met the inclusion criteria and were used in the analysis.

Results

5-HT1A partial agonists (buspirone or tandospirone) did not have a significant effect on verbal learning (SMD = 0.08, 95 % CI = -0.31 to 0.47) or working memory (SMD = 0.15, 95 % CI = -0.09 to 0.39). Regarding executive functions (Wisconsin Card Sorting Test), positive but non-significant results were seen with the category number (SMD = 0.26, 95 % CI = -0.81 to 1.32), while non-significant effects were noted for percent preservation errors (SMD = -0.10, 95 % CI = -0.53 to 0.33).

Conclusions

The absence of any significant benefits in the cognitive domains studied here may have been due to the variance in the concomitant medication (typical vs atypical antipsychotic drugs), the level of cognition at baseline, or other factors. Further studies with various types of 5-HT1A agonists are warranted to examine the potential cognitive efficacy of stimulating these receptors.

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.

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 role of serotonin in declarative memory: A systematic review of animal and human research

The serotonergic system is involved in diverse cognitive functions including memory. Of particular importance to daily life are declarative memories that contain information about personal experiences, general facts, and events. Several psychiatric or neurological diseases, such as depression, attention-deficit-hyperactivity disorder (ADHD), and dementia, show alterations in serotonergic signalling and attendant memory disorders. Nevertheless, understanding serotonergic neurotransmission and its influence on memory remained a challenge until today. In this systematic review, we summarize recent psychopharmacological studies in animals and humans from a psychological memory perspective, in consideration of task-specific requirements. This approach has the advantage that comparisons between serotonin (5-HT)-related neurochemical mechanisms and manipulations are each addressing specific mnemonic circuits. We conclude that applications of the same 5-HT-related treatments can differentially affect unrelated tasks of declarative memories. Moreover, the analysis of specific mnemonic phases (e.g., encoding vs. consolidation) reveals opposing impacts of increased or decreased 5-HT tones, with low 5-HT supporting spatial encoding but impairing the consolidation of objects and verbal memories. Promising targets for protein synthesis-dependent consolidation enhancements include 5-HT₄ receptor agonists and 5-HT₆ receptor antagonists, with the latter being of special interest for the treatment of age-related decline. Further implications are pointed out as base for the development of novel therapeutic targets for memory impairment of neuropsychiatric disorders.

Biased 5-HT1A receptor agonists F13714 and NLX-101 differentially affect pattern separation and neuronal plasticity in rats after acute and chronic treatment

Pattern separation is a hippocampal process in which highly similar stimuli are recognized as separate representations, and deficits could lead to memory impairments in neuropsychiatric disorders such as schizophrenia. The 5-HT_1A receptor (5-HT_1AR) is believed to be involved in these hippocampal pattern separation processes. However, in the dorsal raphe nucleus (DRN), the 5-HT_1AR is expressed as a somatodendritic autoreceptor, negatively regulates serotonergic signaling, and could thereby counteract the effects of hippocampal postsynaptic 5-HT_1A receptors. Therefore, this study aims to identify how pre- and post-synaptic 5-HT_1AR activity affects pattern separation. Object pattern separation (OPS) performance was measured in male Wistar rats after both acute and chronic treatment (i.p.) with 5-HT_1AR biased agonists F13714 (0.0025 mg/kg acutely, 0.02 mg/kg/day chronically) or NLX-101 (0.08 mg/kg acutely, 0.32 mg/kg/day chronically), which preferentially activate autoreceptors or postsynaptic receptors respectively, for 14 days. Body temperature - a functional correlate of hypothalamic 5-HT_1AR stimulation - was measured daily. Additionally, 5-HT_1AR density (DRN) and plasticity markers (hippocampus) were assessed. Acute treatment with F13714 impaired OPS performance, whereas chronic treatment normalized this, and a drop in body temperature was found from day 4 onwards. NLX-101 enhanced OPS performance acutely and chronically, and caused an acute drop in body temperature. Chronic NLX-101 treatment increased doublecortin positive neurons in the dorsal hippocampus, while chronic treatment with F13714 resulted in a downregulation of 5-HT_1A autoreceptors, which likely reversed the acute impairment in OPS performance. Chronic treatment with NLX-101 appears to have therapeutic potential to improve brain plasticity and OPS performance.

Exercise to spot the differences: a framework for the effect of exercise on hippocampal pattern separation in humans

Exercise has a beneficial effect on mental health and cognitive functioning, but the exact underlying mechanisms remain largely unknown. In this review, we focus on the effect of exercise on hippocampal pattern separation, which is a key component of episodic memory. Research has associated exercise with improvements in pattern separation. We propose an integrated framework mechanistically explaining this relationship. The framework is divided into three pathways, describing the pro-neuroplastic, anti-inflammatory and hormonal effects of exercise. The pathways are heavily intertwined and may result in functional and structural changes in the hippocampus. These changes can ultimately affect pattern separation through direct and indirect connections. The proposed framework might guide future research on the effect of exercise on pattern separation in the hippocampus.

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.

The spontaneous location recognition task for assessing spatial pattern separation and memory across a delay in rats and mice

Keeping similar memories distinct from one another is a critical cognitive process without which we would have difficulty functioning in everyday life. Memories are thought to be kept distinct through the computational mechanism of pattern separation, which reduces overlap between similar input patterns to amplify differences among stored representations. At the behavioral level, impaired pattern separation has been shown to contribute to memory deficits seen in neuropsychiatric and neurodegenerative diseases, including Alzheimer's disease, and in normal aging. This protocol describes the use of the spontaneous location recognition (SLR) task in mice and rats to behaviorally assess spatial pattern separation ability. This two-phase spontaneous memory task assesses the extent to which animals can discriminate and remember object locations presented during the encoding phase. Using three configurations of the task, the similarity of the to-be-remembered locations can be parametrically manipulated by altering the spatial positions of objects-dissimilar, similar or extra similar-to vary the load on pattern separation. Unlike other pattern separation tasks, SLR varies the load on pattern separation during encoding, when pattern separation is thought to occur. Furthermore, SLR can be used in standard rodent behavioral facilities with basic expertise in rodent handling. The entire protocol takes ~20 d from habituation to testing of the animals on all three task configurations. By incorporating breaks between testing, and varying the objects used as landmarks, animals can be tested repeatedly, increasing experimental power by allowing for within-subjects manipulations.

Adult Neural Stem Cells as Promising Targets in Psychiatric Disorders

The development of new therapies for psychiatric disorders is of utmost importance, given the enormous toll these disorders pose to society nowadays. This should be based on the identification of neural substrates and mechanisms that underlie disease etiopathophysiology. Adult neural stem cells (NSCs) have been emerging as a promising platform to counteract brain damage. In this perspective article, we put forth a detailed view of how NSCs operate in the adult brain and influence brain homeostasis, having profound implications at both behavioral and functional levels. We appraise evidence suggesting that adult NSCs play important roles in regulating several forms of brain plasticity, particularly emotional and cognitive flexibility, and that NSC dynamics are altered upon brain pathology. Furthermore, we discuss the potential therapeutic value of utilizing adult endogenous NSCs as vessels for regeneration, highlighting their importance as targets for the treatment of multiple mental illnesses, such as affective disorders, schizophrenia, and addiction. Finally, we speculate on strategies to surpass current challenges in neuropsychiatric disease modeling and brain repair.

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.

Improving cognitive functioning in major depressive disorder with psychedelics: A dimensional approach

The high symptomatic and biological heterogeneity of major depressive disorder (MDD) makes it very difficult to find broadly efficacious treatments that work against all symptoms. Concentrating on single core symptoms that are biologically well understood might consist of a more viable approach. The Research Domain Criteria (RDoC) framework is a trans-diagnostic dimensional approach that focuses on symptoms and their underlying neurobiology. Evidence is accumulating that psychedelics may possess antidepressant activity, and this can potentially be explained through a multi-level (psychobiological, circuitry, (sub)cellular and molecular) analysis of the cognitive systems RDoC domain. Cognitive deficits, such as negative emotional processing and negativity bias, often lead to depressive rumination. Psychedelics can increase long-term cognitive flexibility, leading to normalization of negativity bias and reduction in rumination. We propose a theoretical model that explains how psychedelics can reduce the negativity bias in depressed patients. At the psychobiological level, we hypothesize that the negativity bias in MDD is due to impaired pattern separation and that psychedelics such as psilocybin help in depression because they enhance pattern separation and hence reduce negativity bias. Pattern separation is a mnemonic process that relies on adult hippocampal neurogenesis, where similar inputs are made more distinct, which is essential for optimal encoding of contextual information. Impairment in this process may underlie the negative cognitive bias in MDD by, for example, increased pattern separation of cues with a negative valence that can lead to excessive deliberation on aversive outcomes. On the (sub) cellular level, psychedelics stimulate hippocampal neurogenesis as well as synaptogenesis, spinogenesis and dendritogenesis in the prefrontal cortex. Together, these effects help restoring resilience to chronic stress and lead to modulation of the major connectivity hubs of the prefrontal cortex, hippocampus, and amygdala. Based on these observations, we propose a new translational framework to guide the development of a novel generation of therapeutics to treat the cognitive symptoms in MDD.

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.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

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.

Activation of 5-HT1A postsynaptic receptors by NLX-101 results in functional recovery and an increase in neuroplasticity in mice with brain ischemia

Pharmacological interventions that selectively activate serotonin 5-hydroxytryptramine-1A (5-HT_1A) heteroreceptors may prevent or attenuate the consequences of brain ischemic episodes. The present study investigated whether the preferential 5-HT_1A postsynaptic receptor agonist NLX-101 (a.k.a. F15599) mitigates cognitive and emotional impairments and affects neuroplasticity in mice that are subjected to the bilateral common carotid artery occlusion (BCCAO) model of brain ischemia. The selective serotonin reuptake inhibitor escitalopram (Esc) was used for comparative purposes because it is able to decrease morbidity and improve recovery in stroke patients and ischemic rodents. Sham and BCCAO mice received daily doses of NLX-101 (0.32 mg/kg, i.p) or Esc (20 mg/kg, i.p) for 28 days. During this period, they were evaluated for locomotor activity, anxiety- and despair-related behaviors and hippocampus-dependent cognitive function, using the open field, elevated zero maze, forced swim test and object location test, respectivelly. The mice's brains were processed for biochemical and histological analyses. BCCAO mice exhibited high anxiety and despair-like behaviors and performed worse than controls in the cognitive assessment. BCCAO induced neuronal and dendritic spine loss and decreases in the protein levels of neuronal plasticity markers, including brain-derived neurotrophic factor (BDNF), synaptophysin (SYN), and postsynaptic density protein-95 (PSD-95), in prefrontal cortex (PFC) and hippocampus. NLX-101 and Esc attenuated cognitive impairments and despair-like behaviors in BCCAO mice. Only Esc decreased anxiety-like behaviors due to brain ischemia. Both NLX-101 and Esc blocked the increase in plasma corticosterone levels and, restored BDNF, SYN and PSD-95 protein levels in the hippocampus. Moreover, both compounds impacted positively dentritic remodeling in the hippocampus and PFC of ischemic mice. In the PFC, NLX-101 increased the BDNF protein levels, while Esc in turn, attenuated the decrease in the PSD-95 protein levels induced by BCCAO. The present results suggest that activation of post-synaptic 5-HT_1A receptors is the molecular mechanism for serotonergic protective effects in BCCAO. Moreover, post-synaptic biased agonists such as NLX-101 might constitute promising therapeutics for treatment of functional and neurodegenerative outcomes of brain ischemia.

Phosphodiesterase inhibitors roflumilast and vardenafil prevent sleep deprivation-induced deficits in spatial pattern separation

Sleep deprivation (SD) is known to impair hippocampus‐dependent memory processes, in part by stimulating the phosphodiesterase (PDE) activity. In the present study, we assessed in mice whether SD also affects spatial pattern separation, a cognitive process that specifically requires the dentate gyrus (DG) subregion of the hippocampus. Adult male mice were trained in an object pattern separation (OPS) task in the middle of the light phase and then tested 24 hr thereafter. In total, we conducted three studies using the OPS task. In the first study, we validated the occurrence of pattern separation and tested the effects of SD. We found that 6 hr of SD during the first half of the light phase directly preceding the test trial impaired the spatial pattern separation performance. As a next step, we assessed in two consecutive studies whether the observed SD‐induced performance deficits could be prevented by the systemic application of two different PDE inhibitors that are approved for human use. Both the PDE4 inhibitor roflumilast and PDE5 inhibitor vardenafil successfully prevented SD‐induced deficits in spatial pattern separation. As a result, these PDE inhibitors have clinical potential for the prevention of memory deficits associated with loss of sleep.

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.

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

BACKGROUND

NLX-101 (also known as F15599) is a highly selective and efficacious 'biased' agonist at cortical 5-hydroxytryptamine 1A (5-HT_1A) heteroreceptors. In rodents, it possesses marked antidepressant-like activity, potently and completely abolishing immobility in the forced swim test (FST) with extended duration of action.

METHODS

We investigated the antidepressant-like activity of NLX-101 using the rat chronic mild stress (CMS) model of depression, considered to have a higher translational potential than the FST, as it possesses construct, face and predictive validity. The effects of CMS and repeated NLX-101 treatment were tested using sucrose consumption (a measure of anhedonia), novel object recognition (NOR; a measure of working memory) and elevated plus maze (EPM; a measure of anxiety) tests.

RESULTS

NLX-101 reversed the CMS-induced decrease of sucrose intake on day 1 of testing, with full reversal observed at the dose of 0.16 mg/kg and a less pronounced but still significant effect at 0.04 mg/kg, both given twice a day intraperitoneally. The effects of NLX-101 were maintained over the 2 week treatment period and persisted for four weeks following cessation of treatment. In the NOR test, both doses of NLX-101 rescued the deficit in discrimination index caused by CMS, without any effect on locomotor activity. However, NLX-101 had no effect on the reduction of open-arms entries produced by CMS in the EPM model. In control, non-stressed rats, NLX-101 produced non-significant effects in all three models.

CONCLUSIONS

NLX-101 displayed efficacious activity in the CMS test, with more rapid (1 day) antidepressant-like effects than pharmacological compounds tested previously under the same experimental conditions. These observations suggest that biased agonist targeting of cortical 5-HT_1A receptors constitutes a promising strategy to achieve rapid-acting and sustained antidepressant effects.

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.

A second wind for the cholinergic system in Alzheimer's therapy

Notwithstanding tremendous research efforts, the cause of Alzheimer's disease (AD) remains elusive and there is no curative treatment. The cholinergic hypothesis presented 35 years ago was the first major evidence-based hypothesis on the etiology of AD. It proposed that the depletion of brain acetylcholine was a primary cause of cognitive decline in advanced age and AD. It relied on a series of observations obtained in aged animals, elderly, and AD patients that pointed to dysfunctions of cholinergic basal forebrain, similarities between cognitive impairments induced by anticholinergic drugs and those found in advanced age and AD, and beneficial effects of drugs stimulating cholinergic activity. This review revisits these major results to show how this hypothesis provided the drive for the development of anticholinesterase inhibitor-based therapies of AD, the almost exclusively approved treatment in use despite transient and modest efficacy. New ideas for improving cholinergic therapies are also compared and discussed in light of the current revival of the cholinergic hypothesis on the basis of two sets of evidence from new animal models and refined imagery techniques in humans. First, human and animal studies agree in detecting signs of cholinergic dysfunctions much earlier than initially believed. Second, alterations of the cholinergic system are deeply intertwined with its reactive responses, providing the brain with efficient compensatory mechanisms to delay the conversion into AD. Active research in this field should provide new insight into development of multitherapies incorporating cholinergic manipulation, as well as early biomarkers of AD enabling earlier diagnostics. This is of prime importance to counteract a disease that is now recognized to start early in adult life.

Frameworking memory and serotonergic markers

The evidence for neural markers and memory is continuously being revised, and as evidence continues to accumulate, herein, we frame earlier and new evidence. Hence, in this work, the aim is to provide an appropriate conceptual framework of serotonergic markers associated with neural activity and memory. Serotonin (5-hydroxytryptamine [5-HT]) has multiple pharmacological tools, well-characterized downstream signaling in mammals’ species, and established 5-HT neural markers showing new insights about memory functions and dysfunctions, including receptors (5-HT1A/1B/1D, 5-HT2A/2B/2C, and 5-HT3-7), transporter (serotonin transporter [SERT]) and volume transmission present in brain areas involved in memory. Bidirectional influence occurs between 5-HT markers and memory/amnesia. A growing number of researchers report that memory, amnesia, or forgetting modifies neural markers. Diverse approaches support the translatability of using neural markers and cerebral functions/dysfunctions, including memory formation and amnesia. At least, 5-HT1A, 5-HT4, 5-HT6, and 5-HT7receptors and SERT seem to be useful neural markers and therapeutic targets. Hence, several mechanisms cooperate to achieve synaptic plasticity or memory, including changes in the expression of neurotransmitter receptors and transporters.

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.

Selective serotonin 5-HT1A receptor biased agonists elicitdistinct brain activation patterns: a pharmacoMRI study

Serotonin 1A (5-HT1A) receptors are involved in several physiological and pathological processes and constitute therefore an important therapeutic target. The recent pharmacological concept of biased agonism asserts that highly selective agonists can preferentially direct receptor signaling to specific intracellular responses, opening the possibility of drugs targeting a receptor subtype in specific brain regions. The present study brings additional support to this concept thanks to functional magnetic resonance imaging (7 Tesla-fMRI) in anaesthetized rats. Three 5-HT1A receptor agonists (8-OH-DPAT, F13714 and F15599) and one 5-HT1A receptor antagonist (MPPF) were compared in terms of influence on the brain blood oxygen level-dependent (BOLD) signal. Our study revealed for the first time contrasting BOLD signal patterns of biased agonists in comparison to a classical agonist and a silent antagonist. By providing functional information on the influence of pharmacological activation of 5-HT1A receptors in specific brain regions, this neuroimaging approach, translatable to the clinic, promises to be useful in exploring the new concept of biased agonism in neuropsychopharmacology.

Serotonin, neural markers, and memory

Diverse neuropsychiatric disorders present dysfunctional memory and no effective treatment exits for them; likely as result of the absence of neural markers associated to memory. Neurotransmitter systems and signaling pathways have been implicated in memory and dysfunctional memory; however, their role is poorly understood. Hence, neural markers and cerebral functions and dysfunctions are revised. To our knowledge no previous systematic works have been published addressing these issues. The interactions among behavioral tasks, control groups and molecular changes and/or pharmacological effects are mentioned. Neurotransmitter receptors and signaling pathways, during normal and abnormally functioning memory with an emphasis on the behavioral aspects of memory are revised. With focus on serotonin, since as it is a well characterized neurotransmitter, with multiple pharmacological tools, and well characterized downstream signaling in mammals' species. 5-HT_1A, 5-HT₄, 5-HT₅, 5-HT₆, and 5-HT₇ receptors as well as SERT (serotonin transporter) seem to be useful neural markers and/or therapeutic targets. Certainly, if the mentioned evidence is replicated, then the translatability from preclinical and clinical studies to neural changes might be confirmed. Hypothesis and theories might provide appropriate limits and perspectives of evidence.