An assessment of the effects of serotonin 6 (5-HT6) receptor antagonists in rodent models of learning

Single-Nucleus Chromatin Accessibility Landscape Reveals Diversity in Regulatory Regions Across Distinct Adult Rat Cortex

Rats have been widely used as an experimental organism in psychological, pharmacological, and behavioral studies by modeling human diseases such as neurological disorders. It is critical to identify and characterize cell fate determinants and their regulatory mechanisms in single-cell resolutions across rat brain regions. Here, we applied droplet-based single-nucleus assay for transposase-accessible chromatin using sequencing (snATAC-seq) to systematically profile the single-cell chromatin accessibility across four dissected brain areas in adult Sprague-Dawley (SD) rats with a total of 59,023 single nuclei and identified 16 distinct cell types. Interestingly, we found that different cortex regions exhibit diversity in both cellular compositions and gene regulatory regions. Several cell-type-specific transcription factors (TFs), including SPI1, KLF4, KLF6, and NEUROD2, have been shown to play important roles during the pathogenesis of various neurological diseases, such as Alzheimer's disease (AD), astrocytic gliomas, autism spectrum disorder (ASD), and intellectual disabilities. Therefore, our single-nucleus atlas of rat cortex could serve as an invaluable resource for dissecting the regulatory mechanisms underlying diverse cortex cell fates and further revealing the regulatory networks of neuropathogenesis.

Activation and blockade of 5-HT6 receptor in the medial septum-diagonal band recover working memory in the hemiparkinsonian rats

Working memory impairment is a common symptom occurred in Parkinson's disease (PD). The medial septum-diagonal band (MS-DB) complex and 5-HT₆ receptor are involved in modulation of cognition. However, their roles in working memory in PD are still unknown. Here, we used behavioral, neurochemical and immunohistochemical approaches to assess the role of MS-DB 5-HT₆ receptor in working memory in unilateral 6-hydroxydopamie (6-OHDA)-induced PD rats. Intra-MS-DB injection of 5-HT₆ receptor agonist WAY208466 (3, 6 and 12 μg/rat) enhanced working memory and increased dopamine (DA) and noradrenaline (NA) levels in the medial prefrontal cortex (mPFC) and hippocampus in sham and 6-OHDA-lesioned rats. The dose that produced significant effect on working memory in 6-OHDA-lesioned rats was lower than that in sham rats, indicating hypersensitivity of 5-HT₆ receptor after lesioning. Intra-MS-DB injection of 5-HT₆ receptor antagonist SB258585 (2, 4 and 8 μg/rat) alleviated working memory deficits and increased DA level in the mPFC and hippocampus and NA level in the mPFC in 6-OHDA-lesioned rats while having no effect in sham rats, suggesting that SB258585 did not change normal cognitive status. These results suggest that activation and blockade of MS-DB 5-HT₆ receptor recovered working memory in 6-OHDA-lesioned rats, which is probably related to changes in monoamine levels in the mPFC and hippocampus.

Procognitive profiling of a serotonin 5-HT6 receptor antagonist in a complex model system in rats: A novel translational approach for clinical prediction

INTRODUCTION

The serial clinical failures of novel cognitive enhancer candidates point out the lack of predictive power in the preceding animal experimentation. For a more predictive profiling of putative procognitive drugs in rodents, we recently elaborated a methodical approach which consists of three fundamental steps: 1. teaching various learning tasks representing different cognitive domains to the same cohort of animals with the aim to create a population with 'widespread knowledge'. 2. Applying a cognitive deficit-inducing intervention to transform this cohort of animals to a 'patient population'. 3. Testing putative procognitive drugs with a 'clinical trial-like' design on the wide spectrum of cognitive (dys)functions in the actual 'patient population'. The present study has been the first trial to test the feasibility and utility of the proposed system.

METHODS

The population with 'widespread knowledge' consisted of 2 year old male Long-Evans rats with a learning history in five-choice serial reaction time task (5-CSRTT, attentional paradigm), Morris water maze (MWM, spatial learning), a cooperative task carried out in pairs (social learning), and a skill-learning task, "pot-jumping". For inducing cognitive deficit, thus creating a 'patient population' we increased the difficulty of the tasks. For the cognitive enhancer mechanism to test in the system we chose a serotonin 5-HT₆ receptor antagonist compound, RO4368554. Animals were randomly assigned to vehicle- and drug treated groups based on their baseline learning performance and their response in a pilot test of increase in task difficulty. During the 13-day long treatment with 3 mg/kg ip. RO4368554 all the learning paradigms were repeatedly run with increased difficulty supplemented with a novel object recognition test (NOR, episodic memory).

RESULTS

In the 5-CSRTT, reducing the stimulus duration from 1 s to 0.25 s caused a significant decrease in the percentage of correct responses (from 52 % to 31 % in the control group) which was not affected by the 5-HT₆ receptor antagonist treatment (correct responses decreased from 58 % to 31 %). In the MWM, replacing the escape platform to a new location did not mean a hard challenge for the rats. Members of both groups could find it within a relatively short time: mean escape latencies were 83 s and 65 s at the first replacement trial and 58 s and 74 s at the second one in the control and drug-treated groups, respectively. In the cooperation paradigm, where the rats had to perform simultaneous nose-pokes to get a reward, task difficulty was increased by requiring two consecutive simultaneous nose-poking from the animals. This caused a fall in the percentage of successful trials in both groups (from 48 % to 12 % and from 50 % to 20 % in the saline - and drug-treated group, respectively), however, by the end of the treatment RO4368554-treated animals showed significantly higher performance (29 %) than saline treated rats (2%). The NOR test, carried out with a 5 -h delay, revealed poor recognition memory in both groups (discrimination index (DI) values were 0.13 and 0.06 for saline and RO4368554, respectively). Performance in the pot jumping test was also not improved by the drug-treatment.

CONCLUSIONS

The applied study design allowed parallel measurements of the action of the test compound on several cognitive functions and to follow its time course. RO4368554 did not show notable effects on impaired attention and visual recognition; nor did it affect spatial and procedural learning, but it exerted beneficial effect on cooperative behaviour. The revealed activity pattern highlight the cognitive domain most sensitive to the particular drug effect and may give hints for further target validating and clinical studies.

Calbindin Deficits May Underlie Dissociable Effects of 5-HT6 and mGlu7 Antagonists on Glutamate and Cognition in a Dual-Hit Neurodevelopmental Model for Schizophrenia

Despite several compounds entering clinical trials for the negative and cognitive symptoms of schizophrenia, few have progressed beyond phase III. This is partly attributed to a need for improved preclinical models, to understand disease and enable predictive evaluation of novel therapeutics. To this end, one recent approach incorporates "dual-hit" neurodevelopmental insults like neonatal phencyclidine plus isolation rearing (PCP-Iso). Glutamatergic dysfunction contributes to schizophrenia pathophysiology and may represent a treatment target, so we used enzyme-based microsensors to evaluate basal- and drug-evoked glutamate release in hippocampal slices from rats that received neonatal PCP and/or isolation rearing. 5-HT₆ antagonist-evoked glutamate release (thought to be mediated indirectly via GABAergic disinhibition) was reduced in PCP-Iso, as were cognitive effects of a 5-HT₆ antagonist in a hippocampal glutamate-dependent novel object discrimination task. Yet mGlu₇ antagonist-evoked glutamatergic and cognitive responses were spared. Immunohistochemical analyses suggest these findings (which mirror the apparent lack of clinical response to 5-HT₆ antagonists in schizophrenia) are not due to reduced hippocampal 5-HT input in PCP-Iso, but may be explained by reduced calbindin expression. This calcium-binding protein is present in a subset of GABAergic interneurons receiving preferential 5-HT innervation and expressing 5-HT₆ receptors. Its loss (in schizophrenia and PCP-Iso) would be expected to reduce interneuron firing and potentially prevent further 5-HT₆ antagonist-mediated disinhibition, without impacting on responses of VIP-expressing interneurons to mGlu₇ antagonism. This research highlights the importance of improved understanding for selection of appropriate preclinical models, especially where disease neurobiology impacts on cells mediating the effects of potential therapeutics.

Characterization of Behavioral, Signaling and Cytokine Alterations in a Rat Neurodevelopmental Model for Schizophrenia, and Their Reversal by the 5-HT6 Receptor Antagonist SB-399885

Post-weaning social isolation of rats produces neuroanatomical, neurochemical and behavioral alterations resembling some core features of schizophrenia. This study examined the ability of the 5-HT₆ receptor antagonist SB-399885 to reverse isolation-induced cognitive deficits, then investigated alterations in hippocampal cell proliferation and hippocampal and frontal cortical expression of selected intracellular signaling molecules and cytokines. Male Lister hooded rats (weaned on post-natal days 21-24 and housed individually or in groups of 3-4) received six i.p. injections of vehicle (1% Tween 80, 1 mL/kg) or SB-399885 (5 or 10 mg/kg) over a 2-week period starting 40 days post-weaning, on the days that locomotor activity, novel object discrimination (NOD), pre-pulse inhibition of acoustic startle and acquisition, retention and extinction of a conditioned freezing response (CFR) were assessed. Tissue was collected 24 h after the final injection for immunohistochemistry, reverse-phase protein microarray and western blotting. Isolation rearing impaired NOD and cue-mediated CFR, decreased cell proliferation within the dentate gyrus, and elevated hippocampal TNFα levels and Cdc42 expression. SB-399885 reversed the NOD deficit and partially normalized CFR and cell proliferation. These effects were accompanied by altered expression of several members of the c-Jun N-terminal Kinase (JNK) and p38 MAPK signaling pathways (including TAK1, MKK4 and STAT3). Although JNK and p38 themselves were unaltered at this time point hippocampal TAK1 expression and phosphorylation correlated with visual recognition memory in the NOD task. Continued use of this neurodevelopmental model could further elucidate the neurobiology of schizophrenia and aid assessment of novel therapies for drug-resistant cognitive symptoms.

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.

Serotonin 5-HT6 Receptor Antagonists in Alzheimer's Disease: Therapeutic Rationale and Current Development Status

Alzheimer's disease (AD) is the most common cause of dementia in elderly people. Because of the lack of effective treatments for this illness, research focused on identifying compounds that restore cognition and functional impairments in patients with AD is a very active field. Since its discovery in 1993, the serotonin 5-HT₆ receptor has received increasing attention, and a growing number of studies supported 5-HT₆ receptor antagonism as a target for improving cognitive dysfunction in AD. This article reviews the rationale behind investigations into the targeting of 5-HT₆ receptors as a symptomatic treatment for cognitive and/or behavioral symptoms of AD. In addition to describing the available clinical evidence, this article also describes the purported biochemical and neurochemical mechanisms of action by which 5-HT₆ receptor antagonists could influence cognition, and the preclinical data supporting this therapeutic approach to AD. A large number of publications describing the development of ligands for this receptor have come to light and preclinical data indicate the procognitive efficacy of 5-HT₆ receptor antagonists. Subsequently, the number of patents protecting 5-HT₆ chemical entities has continuously grown. Some of these compounds have successfully undergone phase I clinical studies and have been further evaluated in clinical phase II trials with variable success. Phase II studies have also revealed the potential of combining 5-HT₆ receptor antagonism and cholinesterase inhibition. Two of these antagonists, idalopirdine and RVT-101, have been further developed into ongoing phase III clinical trials. Overall, 5-HT₆ receptor antagonists can reasonably be regarded as potential drug candidates for the treatment of AD.

Effects of serotonin in the hippocampus: how SSRIs and multimodal antidepressants might regulate pyramidal cell function

The hippocampus plays an important role in emotional and cognitive processing, and both of these domains are affected in patients with major depressive disorder (MDD). Extensive preclinical research and the notion that modulation of serotonin (5-HT) neurotransmission plays a key role in the therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs) support the view that 5-HT is important for hippocampal function in normal and disease-like conditions. The hippocampus is densely innervated by serotonergic fibers, and the majority of 5-HT receptor subtypes are expressed there. Furthermore, hippocampal cells often co-express multiple 5-HT receptor subtypes that can have either complementary or opposing effects on cell function, adding to the complexity of 5-HT neurotransmission. Here we review the current knowledge of how 5-HT, through its various receptor subtypes, modulates hippocampal output and the activity of hippocampal pyramidal cells in rodents. In addition, we discuss the relevance of 5-HT modulation for cognitive processing in rodents and possible clinical implications of these results in patients with MDD. Finally, we review the data on how SSRIs and vortioxetine, an antidepressant with multimodal activity, affect hippocampal function, including cognitive processing, from both a preclinical and clinical perspective.

AVN-211, Novel and Highly Selective 5-HT6 Receptor Small Molecule Antagonist, for the Treatment of Alzheimer's Disease

Within the past decade several novel targets have been indicated as key players in Alzheimer-type dementia and associated conditions, including a "frightening" memory loss as well as severe cognitive impairments. These proteins are deeply implicated in crucial cell processes, e.g., autophagy, growth and progression, apoptosis, and metabolic equilibrium. Since recently, 5-HT6R has been considered as one of the most prominent biological targets in AD drug therapy. Therefore, we investigated the potential procognitive and neuroprotective effects of our novel selective 5-HT6R antagonist, AVN-211. During an extensive preclinical evaluation the lead compound demonstrated a relatively high therapeutic potential and improved selectivity toward 5-HT6R as compared to reference drug candidates. It was thoroughly examined in different in vivo behavioral models directly related to AD and showed evident improvements in cognition and learning. In many cases, the observed effect was considerably greater than that determined for the reported drugs and drug candidates, including memantine, SB-742457, and Lu AE58054, evaluated under the same conditions. In addition, AVN-211 showed a similar or better anxiolytic efficacy than fenobam, rufinamide, lorazepam, and buspirone in an elevated plus-maze model, elevated platform, and open field tests. The compound demonstrated low toxicity and no side effects in vivo, an appropriate pharmacokinetic profile, and stability. In conclusion, AVN-211 significantly delayed or partially halted the progressive decline in memory function associated with AD, which makes it an interesting drug candidate for the treatment of neurodegenerative and psychiatric disorders. Advanced clinical trials are currently under active discussion and in high priority.

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.

5-HT7 receptor activation: procognitive and antiamnesic effects

RATIONALE

The serotonin (5-hydroxytryptamine (5-HT)) 5-HT7 receptor is localized in brain areas mediating memory; however, the role of this receptor on memory remains little explored.

OBJECTIVE

First, demonstrating the associative nature of Pavlovian/instrumental autoshaping (P/I-A) task, rats were exposed (three sessions) to CS-US (Pavlovian autoshaping), truly random control, free operant, and presentations of US or CS, and they were compared with rats trained-tested for one session to the P/I-A procedure. Also, effects of the 5-HT7 receptor agonist LP-211 administered intraperitoneally after training was determined on short- (1.5 h) and long-term memory 24 and 48 h) and on scopolamine-induced memory impairment and cAMP production.

METHODS

Autoshaping and its behavioral controls were studied. Other animals were subjected to an autoshaping training session and immediately afterwards were given (intraperitoneal) vehicle or LP-211 (0.1-10 mg/kg) and/or scopolamine (0.2 mg/kg) and tested for short-term memory (STM) and long-term memory (LTM); their brains were extracted for the cAMP ELISA immunoassay.

RESULTS

P/I-A group produced the higher %CR. LP-211 did not affect STM; nonetheless, at 0.5 and 1.0 mg/kg, it improved LTM. The 5-HT7 receptor antagonist SB-269970 (SB; 10.0 mg/kg) alone had no effect; nevertheless, the LP-211 (1.0 mg/kg) LTM facilitation was reversed by SB. The scopolamine (0.2 mg/kg) induced-decrement in CR was accompanied by significant increased cAMP production. The scopolamine-induced decrement in CR and increments in cAMP were significantly attenuated by LP-211.

CONCLUSIONS

Autoshaping is a reliable associative learning task whose consolidation is facilitated by the 5-HT7 receptor agonist LP-211.

Ro 04-6790-induced cognitive enhancement: no effect in trace conditioning and novel object recognition procedures in adult male Wistar rats

The evidence for cognitively enhancing effects of 5-hydroxytryptamine6 (5-HT6) receptor antagonists such as Ro 04-6790 is inconsistent and seems to depend on the behavioral test variant in use. Trace conditioning holds promise as a behavioral assay for hippocampus-dependent working memory function. Accordingly, Experiment 1 assessed the effect of Ro 04-6790 (5 and 10mg/kg i.p.) on associating a noise conditioned stimulus paired with foot shock (unconditioned stimulus) at a 3 or 30s trace interval in adult male Wistar rats. Contextual conditioning was measured as suppression to the contextual cues provided by the experimental chambers and as suppression to a temporally extended light background stimulus which provided an experimental context. Experiment 2 assessed the effect of Ro 04-6790 (5 and 10mg/kg i.p.) on recognition memory as tested by the exploration of novel relative to familiar objects in an open arena. In Experiment 1, Ro 04-6790 (5 and 10mg/kg) was without effect on trace and contextual conditioning. In Experiment 2, there was no indication of the expected improvement under Ro 04-6790 at the same doses previously found to enhance recognition memory as measured in tests of novel object exploration. Thus, there was no evidence that treatment with the 5-HT6 receptor antagonist Ro 04-6790 acted as a cognitive enhancer in either trace conditioning or object recognition procedures. We cannot exclude the possibility that the experimental procedures used in the present study would have been sensitive to the cognitive enhancing effects of Ro 04-6790 in a different dose range, behavioral test variant, or in a different strain of rat. Nonetheless the drug treatment was not ineffective in that object exploration was reduced under 10mg/kg Ro 04-6790.

Potentiation of GluN2C/D NMDA receptor subtypes in the amygdala facilitates the retention of fear and extinction learning in mice

NMDA receptors are glutamate receptor ion channels that contribute to synaptic plasticity and are important for many forms of learning and memory. In the amygdala, NMDA receptors are critical for the acquisition, retention, and extinction of classically conditioned fear responses. Although the GluN2B subunit has been implicated in both the acquisition and extinction of conditioned fear, GluN2C-knockout mice show reduced conditioned fear responses. Moreover, D-cycloserine (DCS), which facilitates fear extinction, selectively enhances the activity of GluN2C-containing NMDA receptors. To further define the contribution of GluN2C receptors to fear learning, we infused the GluN2C/GluN2D-selective potentiator CIQ bilaterally into the basolateral amygdala (3, 10, or 30 μg/side) following either fear conditioning or fear extinction training. CIQ both increased the expression of conditioned fear 24 h later and enhanced the extinction of the previously conditioned fear response. These results support a critical role for GluN2C receptors in the amygdala in the consolidation of learned fear responses and suggest that increased activity of GluN2C receptors may underlie the therapeutic actions of DCS.

5-HT6 receptors and Alzheimer's disease

During the past 20 years, the 5-HT6 receptor has received increasing attention and become a promising target for improving cognition. Several studies with structurally different compounds have shown that not only antagonists but also 5-HT6 receptor agonists improve learning and memory in animal models. A large number of publications describing the development of ligands for this receptor have come to light, and it is now quite evident that 5-HT6 receptors have great pharmaceutical potential in terms of related patents. However, 5-HT6 receptor functionality is much more complex than initially defined. According to the existing data, different cellular pathways may be activated, depending on the drug being used. This article reviews preclinical and clinical evidence of the effects that 5-HT6 receptor compounds have on cognition. In addition, the biochemical and neurochemical mechanisms of action through which 5-HT6 receptor compounds can influence cognition will be described. Overall, several 5-HT6-targeted compounds can reasonably be regarded as powerful drug candidates for the treatment of Alzheimer's disease.

5-HT(6) receptor agonists and antagonists enhance learning and memory in a conditioned emotion response paradigm by modulation of cholinergic and glutamatergic mechanisms

BACKGROUND AND PURPOSE

5-HT(6) receptors are abundant in the hippocampus, nucleus accumbens and striatum, supporting their role in learning and memory. Selective 5-HT(6) receptor antagonists produce pro-cognitive effects in several learning and memory paradigms while 5-HT(6) receptor agonists have been found to enhance and impair memory.

EXPERIMENTAL APPROACH

The conditioned emotion response (CER) paradigm was validated in rats. Then we examined the effect of the 5-HT(6) receptor antagonist, EMD 386088 (10 mg·kg(-1) , i.p.), and agonists, E-6801 (2.5 mg·kg(-1) , i.p.) and EMD 386088 (5 mg·kg(-1) , i.p.) on CER-induced behaviour either alone or after induction of memory impairment by the muscarinic receptor antagonist, scopolamine (0.3 mg·kg(-1) , i.p) or the NMDA receptor antagonist, MK-801 (0.1 mg·kg(-1) , i.p).

KEY RESULTS

Pairing unavoidable foot shocks with a light and tone cue during CER training induced a robust freezing response, providing a quantitative index of contextual memory when the rat was returned to the shock chamber 24 h later. Pretreatment (-20 min pre-training) with scopolamine or MK-801 reduced contextual freezing 24 h after CER training, showing production of memory impairment. Immediate post-training administration of 5-HT(6) receptor antagonist, SB-270146, and agonists, EMD 386088 and E-6801, had little effect on CER freezing when given alone, but all significantly reversed scopolamine- and MK-801-induced reduction in freezing. CONCLUSION AND IMPLICATIONS Both the 5-HT(6) receptor agonists and antagonist reversed cholinergic- and glutamatergic-induced deficits in associative learning. These findings support the therapeutic potential of 5-HT(6) receptor compounds in the treatment of cognitive dysfunction, such as seen in Alzheimer's disease and schizophrenia.

The effects of PRX-07034, a novel 5-HT6 antagonist, on cognitive flexibility and working memory in rats

RATIONALE

Accumulating evidence indicates that schizophrenia and autism spectrum disorder patients are marked by cognitive deficits in working memory and strategy switching. There is accumulating evidence that 5-hydroxytryptamine (5-HT)(6) receptors may serve as a useful target to improve cognitive functioning.

OBJECTIVES

In the present experiments, the novel 5-HT(6) antagonist, PRX-07034, was examined for its selectivity of the 5-HT(6) receptor, as well as its effect on delayed spontaneous alternation and strategy switching.

METHODS

The binding affinity of PRX-07034 to the 5-HT(6) receptor, other 5-HT receptors, as well as other G-protein coupled receptors, ion channels, and transporters was evaluated. Cyclic AMP production was measured from transfected HEK-293 cells. In separate behavioral experiments, rats received different doses of PRX-07034 (0.1, 1, or 3 mg/kg, i.p.) 30 min prior to delayed spontaneous alternation testing or prior to the acquisition and switch phases in a place-response switch test.

RESULTS

The results indicated that PRX-07034 is both a potent (Ki = 4-8 nM) and highly selective 5-HT(6) receptor antagonist (≥100-fold selectivity for the 5-HT(6) receptor compared to 68 other GPCRs, ion channels, and transporters, except D(3) (Ki = 71 nM) and 5-HT(1B) (Ki = 260 nM) receptors. For cyclic AMP quantification, PRX-07034 demonstrated antagonist activity (IC(50) = 19 nM) without an effect on basal levels and did not show any agonist activity up to 10 μM. PRX-07034 at 1 and 3 mg/kg (but not 0.1 mg/kg) significantly enhanced delayed spontaneous alternation. The drug at 1 and 3 mg/kg also enhanced switching between a place and response strategy, but did not affect initial learning of either a place or response discrimination.

CONCLUSIONS

These findings demonstrate that PRX-07034 is a selective 5-HT(6) receptor antagonist that may represent a novel treatment for enhancing working memory and cognitive flexibility.

Serotonin receptors and heart valve disease--it was meant 2B

Carcinoid heart disease was one of the first valvular pathologies studied in molecular detail, and early research identified serotonin produced by oncogenic enterochromaffin cells as the likely culprit in causing changes in heart valve tissue. Researchers and physicians in the mid-1960s noted a connection between the use of several ergot-derived medications with structures similar to serotonin and the development of heart valve pathologies similar to those observed in carcinoid patients. The exact serotonergic target that mediated valvular pathogenesis remained a mystery for many years until similar cases were reported in patients using the popular diet drug Fen-Phen in the late 1990s. The Fen-Phen episode sparked renewed interest in serotonin-mediated valve disease, and studies led to the identification of the 5-HT(2B) receptor as the likely molecular target leading to heart valve tissue fibrosis. Subsequent studies have identified numerous other activators of the 5-HT(2B) receptor, and consequently, the use of many of these molecules has been linked to heart valve disease. Herein, we: review the molecular properties of the 5-HT(2B) receptor including factors that differentiate the 5-HT(2B) receptor from other 5-HT receptor subtypes, discuss the studies that led to the identification of the 5-HT(2B) receptor as the mediator of heart valve disease, present current efforts to identify potential valvulopathogens by screening for 5-HT(2B) receptor activity, and speculate on potential therapeutic benefits of 5-HT(2B) receptor targeting.

5-HT(6) receptor and cognition

Since its discovery in 1993 and subsequent development of selective antagonists, a growing number of studies support the use of serotonin 5-HT(6) receptor antagonism as a promising mechanism for treating cognitive dysfunction. Lately, several studies with structurally different compounds have shown that not only antagonists, but also 5-HT(6) receptor agonists improve learning and memory in animal models. There is even an antagonist, SB-742457, that has completed phase II trials for the treatment of Alzheimer's disease. In addition to describe preclinical and clinical evidence of the effect of 5-HT(6) receptor compounds on cognition, this article will also focus on the purported biochemical and neurochemical mechanisms of action by which 5-HT(6) receptor compounds could influence cognition in health and disease.

E-6801, a 5-HT6 receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission in the rat

RATIONALE AND OBJECTIVES

In rats, 5-hydroxytryptamine(6) (5-HT(6)) receptor antagonists improve learning and memory, but the effects of agonists are poorly defined. This study investigated the effects of 5-HT(6) receptor agonists and antagonists on a rodent model of recognition memory.

METHODS

Selective 5-HT(6) receptor agonists and antagonists were administered either alone, after a scopolamine-induced impairment, or combined with sub-effective doses of the acetylcholinesterase inhibitor, donepezil, or the glutamate NMDA receptor antagonist, memantine, in a novel object discrimination paradigm in adult rats.

RESULTS

After a 4-h inter-trial delay to induce natural forgetting, vehicle-treated rats spent an equivalent time exploring novel and familiar objects during the choice trial. The 5-HT(6) receptor agonists, E-6801 (1.25-10 mg/kg i.p.) and EMD-386088 (5-10 mg/kg i.p.), and antagonists, SB-271046 and Ro 04-6790 (5 and 10 mg/kg), along with donepezil (0.1-3 mg/kg) and memantine (5-20 mg/kg) all produced significant and mostly dose-dependent increases in novel object exploration, indicative of memory enhancement. Furthermore, sub-effective doses of E-6801 (1 mg/kg) when co-administered with either SB-271046 (3 mg/kg), donepezil (0.1 mg/kg) or memantine (5 mg/kg), and EMD-386088 (2 mg/kg) co-administered with SB-271046 (3 mg/kg) also significantly enhanced object-recognition memory. Additionally, using a 1-min inter-trial delay, E-6801 (2.5 and 5 mg/kg) was as effective as donepezil (0.3 and 1 mg/kg) in reversing a scopolamine-induced (0.5 mg/kg) impairment in object recognition.

CONCLUSIONS

This is the first study to demonstrate that E-6801, a potent 5-HT(6) receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission.

One-pot synthesis of new substituted 1,2,3,4-tetrahydrocarbazoles via Petasis reaction

The one-pot synthesis of a new substituted 1,2,3,4-tetrahydrocarbazoles has been described via Petasis reactions. These tetrahydrocarbazoles exhibits various medicinal importance and might be suitable for elaboration into larger peptides at carboxy termini. The scope and limitations of this method have been examined.

Signalling pathways associated with 5-HT6 receptors: relevance for cognitive effects

A growing body of evidence supports the use of serotonin 5-HT6 receptor antagonists as a promising mechanism for treating cognitive dysfunction. We evaluated 5-HT6 receptor expression and associated biochemical mechanisms in the hippocampus of rats that had been trained in the Morris water maze (MWM), a spatial learning task. Training in the MWM induces a down-regulation of 5-HT6 receptor protein and mRNA receptor expression. The learning procedure or the administration of the selective 5-HT6 receptor antagonist SB-271046 induced an increase in pCREB1 levels while CREB2 levels were significantly reduced. However, although SB-271046 was able to improve retention in the MWM, no further changes in pCREB1 or CREB2 levels were found to be associated with the presence of the 5-HT6 receptor antagonist during the learning procedure. The MWM procedure significantly increased pERK1/2 levels and interestingly, further increases were seen when treating with SB-271046 during the MWM. These results suggest that, in the hippocampus, biochemical pathways associated with pERK1/2 expression, and not with the CREB family of transcription factors, seem to be related to the cognitive-enhancing properties of 5-HT6 receptor antagonists.

Selective 5-HT6 receptor blockade improves spatial recognition memory and reverses age-related deficits in spatial recognition memory in the mouse

Studies have recently suggested that blockade of 5-HT6 receptors (5-HT6R) improves memory processes. As episodic memory alteration is one of the first deficits observed during normal aging and in neurological and neuropsychiatric disorders (Alzheimer's disease, schizophrenia), the present study sought to characterize the effects of 5-HT6R blockade on spatial recognition memory, which can be considered as 'episodic-like' memory, in rodents. We quantified the effects of the selective 5-HT6R antagonist SB-271046 (10 mg/kg, i.p.), using the two-trial place recognition task in the Y-maze, on acquisition, consolidation, and retrieval of spatial recognition memory in young adult mice (6-week-old; intertrial intervals (ITIs) 30, 60, 120, 240, and 360 min) and on the consolidation of spatial recognition memory in aged mice (3-, 12-, 18-, and 21-month-old; ITI 60 and 240 min). SB-271046-treated young adult mice explored the new arm more after a 240-min (pre-acquisition) and 360-min (post-acquisition) ITI, whereas vehicle-treated animals failed to discriminate the new arm when the ITI exceeded 120 min (pre-acquisition) or 240 min (post-acquisition). Aged mice, which expressed spatial memory deficits, explored the new arm more after a 60-min ITI (21-month-old) and a 240-min ITI (18- and 21-month-old) when treated with SB-271046. Consequently, 5-HT6R blockade improves spatial recognition memory in adult mice and reverses age-related consolidation deficits of episodic-like memory. This study provides further support for the use of 5-HT6R antagonists in the treatment of episodic memory disorders related to aging as well as neurological disorders such as Alzheimer's disease and schizophrenia.

Increased impulsivity and disrupted attention induced by repeated phencyclidine are not attenuated by chronic quetiapine treatment

Atypical antipsychotic medications differ in how effectively they attenuate cognitive and other deficits in schizophrenia. The present study aimed to explore whether quetiapine, an atypical antipsychotic medication, would reverse disruptions of performance in the 5-choice serial reaction time task (5-CSRTT), a test of attention and impulsivity, induced by repeated administration of the psychotomimetic phencyclidine (PCP). In confirmation of previous findings, repeated PCP administration (2 mg/kg, s.c., 30 min before behavioral testing, for 2 consecutive days, followed by a 2-week PCP-free period and then 5 consecutive days of PCP treatment) increased premature responding (impulsivity), decreased accuracy (attention), and increased response latencies (processing speed) and timeout responding (impulsivity/cognitive inflexibility). Chronic quetiapine (5 or 10 mg/kg/day, s.c.) did not attenuate these PCP-induced disruptions in performance, while at the highest dose used, quetiapine disrupted 5-CSRTT performance in the absence of PCP treatment and tended to exacerbate the PCP-induced increase in premature responding. Considering that clozapine, another atypical antipsychotic, was shown previously to reverse PCP-induced deficits in the same task [Amitai N, Semenova S, Markou A. Cognitive-disruptive effects of the psychotomimetic phencyclidine and attenuation by atypical antipsychotic medications in rats. Psychopharmacology (Berl) 2007;193:521-37], the present findings demonstrate differences between clozapine and quetiapine in their effectiveness on schizophrenia-like cognitive deficits and impulsivity that may be attributable to their different receptor affinity profiles.

Effects of 5-HT6 receptor antagonism and cholinesterase inhibition in models of cognitive impairment in the rat

BACKGROUND AND PURPOSE

The beneficial effect of 5-HT6 receptor antagonism in cognition remains controversial. This study has been undertaken to reassess the cognition enhancing properties of acute vs subchronic treatment with the selective 5-HT6 receptor antagonist SB-271046 in unimpaired rats, as well as against scopolamine (cholinergic-) or MK-801 (glutamatergic-mediated) deficits.

EXPERIMENTAL APPROACH

The Morris water maze was used, measuring behaviour acquisition and retention, and swim speed. Other behavioural measures included yawning and motor activity. SB-271046 was given acutely before each trial or subchronically for 7 days before the trials. The AChE inhibitor galanthamine was also used alone or in combination with SB-271046.

KEY RESULTS

Subchronic treatment with SB-271046 improved acquisition in the Morris water maze, while the acute treatment only improved retention. Neither acute nor subchronic SB-271046 treatment reversed scopolamine-induced learning deficits. MK-801 induced learning impairment associated with a behavioural syndrome, reversed by acute, but not subchronic, SB-271046 treatment. Interestingly, combined treatment with galanthamine and SB-271046 reversed the scopolamine- or MK-801-induced learning impairments. Subchronic treatment with SB-271046 did not modify motor activity or the increased number of yawns, a cholinergic-mediated behaviour, induced by single administration of SB-271046.

CONCLUSIONS AND IMPLICATIONS

These data suggest a potential therapeutic role of 5-HT6 receptor antagonists such as SB-271046, alone or in combination with galanthamine, in the treatment of cognitive dysfunction, such as those seen in Alzheimer's disease and schizophrenia.

An update on the role of the 5-hydroxytryptamine6 receptor in cognitive function

As the 5-hydroxytryptamine(6) (5-HT(6)) receptor is almost exclusively expressed in the CNS, particularly in areas associated with learning and memory, many studies have examined its role in cognitive function in the rodent, as reviewed herein. Most studies, in healthy adult rats, report that 5-HT(6) receptor antagonists enhance retention of spatial learning in the Morris water maze, improve consolidation in autoshaping tasks and reverse natural forgetting in object recognition. Antagonists appear to facilitate both cholinergic and glutamatergic neurotransmission, reversing scopolamine- and NMDA receptor antagonist-induced memory impairments. Recent reports show that the 5-HT(6) receptor antagonist, PRX-07034, restores the impairment of novel object recognition produced in rats reared in social isolation, a neurodevelopmental model producing behavioural changes similar to several core symptoms seen in schizophrenia. The 5-HT(6) receptor antagonist, Ro 04-6790, modestly improved reversal learning in isolation reared but not group-housed controls in the water maze. Ro 04-6790 also improved novel object discrimination both in adult rats that received chronic intermittent phencyclidine and drug-naïve 18-month-old rats. However, more information on their effect in animal models of schizophrenia and Alzheimer's disease is required. Several selective high-affinity 5-HT(6) receptor agonists developed recently also improve object discrimination and extra-dimensional set-shifting behaviour. Thus both 5-HT(6) receptor agonist and antagonist compounds show promise as pro-cognitive agents in pre-clinical studies but the explanation for their paradoxical analogous effect is currently unclear, and is discussed in this article.

5-HT6 receptor antagonists as novel cognitive enhancing agents for Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurological condition characterized by a progressive decline in cognitive performance accompanied by behavioral and psychological syndromes, such as depression and psychosis. The neurochemical correlates of these clinical manifestations now appear to involve dysfunctions of multiple neurotransmitter pathways. Because of the extensive serotonergic denervation that has been observed in the AD brain and the important role played by serotonin (5-HT) in both cognition and behavioral control, this neurotransmitter system has become a focus of concerted research efforts to identify new treatments for AD. 5-HT exerts its diverse physiological and pharmacological effects through actions on multiple receptor subtypes. One of the newest members of this family is the 5-HT6 receptor, a subtype localized almost exclusively in the CNS, predominating in brain regions associated with cognition and behavior. With the subsequent development of selective 5-HT6 receptor antagonists, preclinical studies in rodents and primates have elucidated the function of this receptor subtype in more detail. It is increasingly clear that blockade of 5-HT6 receptors leads to an improvement of cognitive performance in a wide variety of learning and memory paradigms and also results in anxiolytic and antidepressant-like activity. These actions are largely underpinned by enhancements of cholinergic, glutamatergic, noradrenergic, and dopaminergic neurotransmission, together with learning-associated neuronal remodeling. A preliminary report that the cognitive enhancing properties of a 5-HT6 receptor antagonist (namely, SB-742457) extends into AD sufferers further highlights the therapeutic promise of this mechanistic approach.