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

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".

Trends in research on novel antidepressant treatments

Mood disorders, such as major depressive disorder and bipolar disorder, are among the most common mental illnesses and a leading cause of disability worldwide. Key symptoms of these conditions include a depressed mood or anhedonia, sleep and psychomotor disturbances, changes in appetite or weight, and fatigue or loss of energy. Prolonged cognitive disturbances further impair the ability to think or concentrate and are often accompanied by persistent feelings of worthlessness or excessive guilt. Collectively, these symptoms underscore depression as a serious, long-term global health issue. In addition, clinical studies indicate a growing number of patients experiencing difficulties in responding to treatment, even in the long term. This phenomenon poses significant challenges for healthcare professionals, families, and patients alike. As a result, there is an urgent need for therapies that are both rapid-acting and safe. This review aims to summarize the prevailing trends in research on novel antidepressants, emphasizing their diversity and multi-directional mechanisms of action. The development of rapid-acting drugs is increasingly focused on achieving high efficacy, particularly for treatment-resistant depression. Such advances offer the potential for rapid therapeutic effects without the prolonged and often tedious administration of older generation antidepressants. Findings from studies using animal models of depression continue to play a crucial role in predicting and designing new therapeutic strategies. These models remain indispensable for understanding the physiological effects of newly developed compounds, thereby guiding the creation of innovative treatments.

The antidepressant-like activity of ketamine in the rat chronic mild stress model requires activation of cortical 5-HT1A receptors

Ketamine displays efficacious rapid-acting antidepressant (RAAD) activity in the rat chronic mild stress (CMS) model. It rapidly reverses anhedonia (CMS-induced sucrose consumption deficit) and attenuates working memory deficit (novel object recognition: NOR) following both systemic (intraperitoneal, i.p.) administration or local administration in the prefrontal cortex (PFC). However, the receptor mechanisms underlying these effects remain to be clarified and may involve activation of serotonin 5-HT1A receptors, as previously found in experiments using the forced swim test. The present study explored the contribution of PFC 5-HT1A receptors in ketamine's RAAD activity in the CMS model. Ketamine (10 mg/kg i.p.) reversed CMS-induced sucrose consumption and working memory (NOR test) deficits. Notably, unilateral PFC microinjections of a 5-HT1A receptor antagonist, WAY-100635 (2 µg), prevented the antidepressant-like and pro-cognitive activity of systemic ketamine on sucrose consumption and working memory deficits. These data indicate that the RAAD activity of ketamine in the rat CMS model requires activation of PFC 5-HT1A receptors. They also reinforce the notion that drugs that directly activate PFC 5-HT1A receptors could constitute an alternative to ketamine as a promising strategy to achieve RAAD effects, with additional benefits against cognitive deficits in depressed patients, but without ketamine's troublesome side-effects and requirements for in-patient supervision.

Rapid reorganization of serotonin projections and antidepressant response to 5-HT1A-biased agonist NLX-101 in fluoxetine-resistant cF1ko mice

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) like fluoxetine remain a first-line treatment for major depression, but are effective in less than half of patients and can take 4-8 weeks to show results. In this study, we examined cF1ko mice with genetically induced upregulation of 5-HT1A autoreceptors that reduces 5-HT neuronal activity. These mice display anxiety- and depression-related behaviors that did not respond to chronic fluoxetine treatment. We examined treatment with NLX-101, a biased agonist that preferentially targets 5-HT1A heteroreceptors. By testing different doses of NLX-101, we found that a dose of 0.2 mg/kg was effective in reducing depression-related behavior in cF1ko mice without causing hypothermia, a 5-HT1A autoreceptor-mediated response. After 1 h, this dose activated dorsal raphe 5-HT neurons and cells in the medial prefrontal cortex (mPFC), increasing nuclear c-fos labelling in cF1ko mice. In cF1ko mice but not wild-type littermates, 0.2 mg/kg NLX-101 administered 1 h prior to each behavioral test for two weeks reduced depressive behavior in the forced swim test, but increased anxiety-related behaviors in the open field, elevated plus maze, and novelty suppressed feeding tests. During this treatment, NLX-101 induced widespread increases in the density of 5-HT axons, varicosities, and especially synaptic and triadic structures, particularly in depression-related brain regions including mPFC, hippocampal CA1 and CA2/3, amygdala and nucleus accumbens of cF1ko mice. Overall, NLX-101 was rapid and effective in reducing depressive behavior in SSRI-resistant mice, but also induced anxiety-related behaviors. The increase in serotonin innervation induced by intermittent NLX-101 may contribute to its behavioral actions.

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.

Validation of Wistar-Kyoto rats kept in solitary housing as an animal model for depression using voxel-based morphometry

Major depressive disorder is a common psychiatric condition often resistant to medication. The Wistar-Kyoto (WKY) rat has been suggested as an animal model of depression; however, it is still challenging to translate results from animal models into humans. Solitary housing is a mild stress paradigm that can simulate the environment of depressive patients with limited social activity due to symptoms. We used voxel-based morphometry to associate the solitary-housed WKY (sWKY) rat model with data from previous human studies and validated our results with behavioural studies. As a result, atrophy in sWKY rats was detected in the ventral hippocampus, caudate putamen, lateral septum, cerebellar vermis, and cerebellar nuclei (p < 0.05, corrected for family-wise error rate). Locomotor behaviour was negatively correlated with habenula volume and positively correlated with atrophy of the cerebellar vermis. In addition, sWKY rats showed depletion of sucrose consumption not after reward habituation but without reward habituation. Although the application of sWKY rats in a study of anhedonia might be limited, we observed some similarities between the regions of brain atrophy in sWKY rats and humans with depression, supporting the translation of sWKY rat studies to humans.

Preclinical models of treatment-resistant depression: challenges and perspectives

Treatment-resistant depression (TRD) is a subgroup of major depressive disorder in which the use of classical antidepressant treatments fails to achieve satisfactory treatment results. Although there are various definitions and grading models for TRD, common criteria for assessing TRD have still not been established. However, a common feature of any TRD model is the lack of response to at least two attempts at antidepressant pharmacotherapy. The causes of TRD are not known; nevertheless, it is estimated that even 60% of TRD patients are so-called pseudo-TRD patients, in which multiple biological factors, e.g., gender, age, and hormonal disturbances are concomitant with depression and involved in antidepressant drug resistance. Whereas the phenomenon of TRD is a complex disorder difficult to diagnose and successfully treat, the search for new treatment strategies is a significant challenge of modern pharmacology. It seems that despite the complexity of the TRD phenomenon, some useful animal models of TRD meet the construct, the face, and the predictive validity criteria. Based on the literature and our own experiences, we will discuss the utility of animals exposed to the stress paradigm (chronic mild stress, CMS), and the Wistar Kyoto rat strain representing an endogenous model of TRD. In this review, we will focus on reviewing research on existing and novel therapies for TRD, including ketamine, deep brain stimulation (DBS), and psychedelic drugs in the context of preclinical studies in representative animal models of TRD.