The Selective 5-HT1A Agonist SR57746A Protects Intestinal Epithelial Cells and Enteric Glia Cells and Promotes Mucosal Recovery in Experimental Colitis.

BACKGROUND: Neurotrophic growth factors can stabilize the intestinal barrier by preventing the apoptosis of enteric glial cells (EGCs) and enterocytes. We reasoned that a selective 5-HT1A receptor agonist may have neuroprotective properties in the gut and that topical application of SR57746A might be an effective treatment strategy in inflammatory bowel disease (IBD). METHODS: The therapeutic potential of 5-HT1A receptor agonist SR57746A in IBD was evaluated in vitro (nontransformed NCM460 colonic epithelial cells, SW480 colorectal carcinoma cells) and in vivo (murine dextran sulfate sodium [DSS] colitis and CD4-T-cell transfer colitis). In vitro, we analyzed the effect of SR57746A on apoptosis in intestinal epithelial cells (IECs) and EGCs, and upon proliferation, migration, and intracellular signaling in IECs. In vivo, the effect of topical application of SR57746 on disease activity and on histological and endoscopic findings was compared with intraperitoneal infliximab and placebo, respectively. RESULTS: The SR57746A activates PI3-K/AKT- and ERK-signaling in IECs. Depending on ERK- and AKT activation, SR57746A potently prevents apoptosis of IECs without inducing proliferation or migration in these cells. Moreover, SR57746A prevented apoptosis in EGCs in vitro. Topical SR57746A treatment significantly reduced mucosal injury in 2 experimental murine colitis models and was as effective as intraperitoneal infliximab treatment. CONCLUSIONS: Treatment with SR57746A prevents inflammatory cell damage and apoptosis in IECs and EGCs, similar to the neurotrophic effects of EGCs on IECs. Topical treatment with SR57746A could be a candidate for clinical evaluation in the treatment of IBD.

Different Alterations of Agonist and Antagonist Binding to 5-HT1A Receptor in a Rat Model of Parkinson's Disease and Levodopa-Induced Dyskinesia: A MicroPET Study.

BACKGROUND: The gold-standard treatment for Parkinson's disease is L-DOPA, which in the long term often leads to levodopa-induced dyskinesia. Serotonergic neurons are partially responsible for this, by converting L-DOPA into dopamine leading to its uncontrolled release as a "false neurotransmitter". The stimulation of 5-HT1A receptors can reduce involuntary movements but this mechanism is poorly understood. OBJECTIVE: This study aimed to investigate the functionality of 5-HT1A receptors using positron emission tomography in hemiparkinsonian rats with or without dyskinesia induced by 3-weeks daily treatment with L-DOPA. Imaging sessions were performed "off" L-DOPA. METHODS: Each rat underwent a positron emission tomography scan with [18F]F13640, a 5-HT1AR agonist which labels receptors in a high affinity state for agonists, or with [18F]MPPF, a 5-HT1AR antagonist which labels all the receptors. RESULTS: There were decreases of [18F]MPPF binding in hemiparkinsonian rats in cortical areas. In dyskinetic animals, changes were slighter but also found in other regions. In hemiparkinsonian rats, [18F]F13640 uptake was decreased bilaterally in the globus pallidus and thalamus. On the non-lesioned side, binding was increased in the insula, the hippocampus and the amygdala. In dyskinetic animals, [18F]F13640 binding was strongly increased in cortical and limbic areas, especially in the non-lesioned side. CONCLUSION: These data suggest that agonist and antagonist 5-HT1A receptor-binding sites are differently modified in Parkinson's disease and levodopa-induced dyskinesia. In particular, these observations suggest a substantial involvement of the functional state of 5-HT1AR in levodopa-induced dyskinesia and emphasize the need to characterize this state using agonist radiotracers in physiological and pathological conditions.

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-HT1A 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-HT1A 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-HT1A receptors could prove useful in neurological or neuropsychiatric pathologies characterized by cognitive deficits consecutive to a reduced central cholinergic tone.

Structural insights into the lipid and ligand regulation of serotonin receptors.

Serotonin, or 5-hydroxytryptamine (5-HT), is an important neurotransmitter1,2 that activates the largest subtype family of G-protein-coupled receptors3. Drugs that target 5-HT1A, 5-HT1D, 5-HT1E and other 5-HT receptors are used to treat numerous disorders4. 5-HT receptors have high levels of basal activity and are subject to regulation by lipids, but the structural basis for the lipid regulation and basal activation of these receptors and the pan-agonism of 5-HT remains unclear. Here we report five structures of 5-HT receptor-G-protein complexes: 5-HT1A in the apo state, bound to 5-HT or bound to the antipsychotic drug aripiprazole; 5-HT1D bound to 5-HT; and 5-HT1E in complex with a 5-HT1E- and 5-HT1F-selective agonist, BRL-54443. Notably, the phospholipid phosphatidylinositol 4-phosphate is present at the G-protein-5-HT1A interface, and is able to increase 5-HT1A-mediated G-protein activity. The receptor transmembrane domain is surrounded by cholesterol molecules-particularly in the case of 5-HT1A, in which cholesterol molecules are directly involved in shaping the ligand-binding pocket that determines the specificity for aripiprazol. Within the ligand-binding pocket of apo-5-HT1A are structured water molecules that mimic 5-HT to activate the receptor. Together, our results address a long-standing question of how lipids and water molecules regulate G-protein-coupled receptors, reveal how 5-HT acts as a pan-agonist, and identify the determinants of drug recognition in 5-HT receptors.

Selectivity in agonist and antagonist binding to Serotonin1A receptors via G-protein coupling.

G protein-coupled receptors (GPCRs) constitute the largest superfamily of membrane proteins in higher eukaryotes, and facilitate information transfer from the extracellular environment to the cellular interior upon activation by ligands. Their role in diverse signaling processes makes them an attractive choice as drug targets. GPCRs are coupled to heterotrimeric G-proteins which represent an important interface through which signal transduction occurs across the plasma membrane upon activation by ligands. To obtain further insight into the molecular details of interaction of G-proteins with GPCRs, in this work, we explored the selectivity of binding of specific agonists and antagonists to the serotonin1A receptor under conditions of progressive G-protein inactivation. The serotonin1A receptor is an important neurotransmitter receptor belonging to the GPCR family and is a popular drug target. By use of a number of agents to inactivate G-proteins, we show here that the serotonin1A receptor displays differential discrimination between agonist and antagonist binding. Our results show a reduction in binding sites of the receptor upon treatment with G-protein inactivating agents. In addition, G-protein coupling efficiency was enhanced when G-proteins were inactivated using urea and alkaline pH. We envision that our results could be useful in achieving multiple signaling states of the receptor by fine tuning the conditions of G-protein inactivation and in structural biology of GPCRs bound to specific ligands.

Ultra-rapid brain uptake of subcutaneous sumatriptan in the rat: Implication for cluster headache treatment.

BACKGROUND: In spite of the substantial therapeutic efficacy of triptans, their site of action is still debated. Subcutaneous sumatriptan is the most efficacious symptomatic treatment for cluster headache (CH) patients, showing therapeutic onset within a few minutes after injection even in migraine patients. However, whether subcutaneous sumatriptan is able to reach the CNS within this short time frame is currently unknown. METHODS: Here, by means of liquid chromatography/mass spectrometry, we investigated peripheral and brain distribution of subcutaneous sumatriptan soon after injection in rats at a dose equivalent to that used in patients. Tissue sumatriptan contents were compared to those of oxazepam, a prototypical lipophilic, neuroactive drug. RESULTS: We report that sumatriptan accumulated within brain regions of relevance to migraine and CH pathogenesis such as the hypothalamus and the brainstem as soon as 1 and 5 minutes after injection. Notably, sumatriptan brain distribution was faster than that of oxazepam, reaching concentrations exceeding its reported binding affinity for 5HT1B/D receptors, and in the range of those able to inhibit neurotransmitter release in vivo. CONCLUSION: Our findings indicate that sumatriptan distributes within the CNS soon after injection, and are in line with prompt pain relief by parenteral sumatriptan in CH patients.

Serotonin abnormalities in Dravet syndrome mice before and after the age of seizure onset.

Dravet syndrome (DS) is a genetic form of severe epilepsy often associated with mutation of the SCN1A gene encoding the voltage gated sodium channel Nav1.1. Typically refractive to conventional therapy, serotonin neurotransmission may be an innovative target for treatment. To further understand the role of serotonin in this disorder, in this study we examined the state of the endogenous serotonin system in an Scn1a+/- mouse model of DS. Examined at an age before seizures appear, we found the hypothermic effect of 5-HT1A receptor agonist administration was attenuated. HPLC analysis of brain monoamine content revealed modestly reduced serotonin levels in tissue samples of the midbrain that included the dorsal raphe nucleus but no changes elsewhere in the brain. The reduced sensitivity to 5-HT1A agonist administration seen at young ages reversed after the age of seizure development when mice showed an exaggerated hypothermic response. Likewise, adult DS mice showed a pronounced hypersensitivity to a 5-HT2A/2C agonist. As adults however monoamine levels were not detectably altered. Thus there are alterations in the endogenous serotonin system that both precede and follow the appearance of seizure in DS mice, most strikingly in the response to agonist administration.

Traditional and Novel Migraine Therapy in the Aging Population.

Migraine is a common disabling disorder that affects 36 million Americans. The clinical features of migraine are less typical in the people above age 60, making the diagnosis and treatment difficult in this group. In this review, we will discuss migraine-specific drugs and their use in populations about age 60 who suffer from migraine. This discussion will include an overview of traditional treatments for the acute and preventive treatment of migraine, and considerations for their use in patient populations above age 60. In addition, we will discuss newer agents that show a more promising safety profile.

Synthesis and evaluation of two new candidate high-affinity full agonist PET radioligands for imaging 5-HT1B receptors.

INTRODUCTION: The serotonin 1B receptor subtype is of interest in the pathophysiology and treatment of depression, anxiety, and migraine. Over recent years 5-HT1B receptor binding in human brain has been examined with PET using radioligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radioligand may affect imaging performance, two high-affinity full 5-HT1B receptor agonists (AZ11136118, 4; and AZ11895987, 5) were selected from a large compound library and radiolabeled for PET examination in non-human primates. METHODS: [11C]4 was obtained through Pd(0)-mediated insertion of [11C]carbon monoxide between prepared iodoarene and homochiral amine precursors. [11C]5 was obtained through N-11C-methylation of N-desmethyl precursor 6 with [11C]methyl triflate. [11C]4 and [11C]5 were studied with PET in rhesus or cynomolgus monkey. [11C]4 was studied with PET in mice and rats to measure brain uptake and specific binding. Ex-vivo experiments in rats were performed to identify whether there were radiometabolites in brain. Physiochemical parameters for [11C]4 (pKa, logD and conformational energetics) were evaluated. RESULTS: Both [11C]4 and [11C]5 were successfully produced in high radiochemical purity and in adequate amounts for PET experiments. After intravenous injection of [11C]4, brain radioactivity peaked at a low level (0.2 SUV). Pretreatment with tariquidar, an inhibitor of the brain P-gp efflux transporter, increased brain exposure four-fold whereas pretreatment with a high pharmacological dose of the 5-HT1B antagonist, AR-A000002, had no effect on the binding. Ex-vivo experiments in rats showed no radiometabolites entering brain. [11C]5 also failed to enter monkey brain under baseline conditions. CONCLUSIONS: [11C]4 and [11C]5 show too low brain uptake and specific binding to be useful PET radioligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations.

Serotonin Signaling Trough Prelimbic 5-HT1A Receptors Modulates CSDS-Induced Behavioral Changes in Adult Female Voles.

BACKGROUND: Most previous studies have focused on the effects of social defeat in male juvenile individuals. Whether chronic social defeat stress in adulthood affects female emotion and the underlying mechanisms remains unclear. METHODS: Using highly aggressive adult female mandarin voles (Microtus mandarinus), the present study aimed to determine the effects of chronic social defeat stress on anxiety- and depression-like behaviors in adult female rodents and investigate the neurobiological mechanisms underlying these effects. RESULTS: Exposure of adult female voles to social defeat stress for 14 days reduced the time spent in the central area of the open field test and in the open arms of the elevated plus maze and lengthened the immobility time in the tail suspension and forced swimming tests, indicating increased anxiety- and depression-like behaviors. Meanwhile, defeated voles exhibited increased neural activity in the prelimbic cortex of the medial prefrontal cortex. Furthermore, chronic social defeat stress reduced serotonin projections and levels of serotonin 1A receptors in the medial prefrontal cortex-prelimbic cortex. Intra-prelimbic cortex microinjections of the serotonin 1A receptor agonist 8-OH-DPAT reversed the alterations in emotional behaviors, whereas injections of the serotonin 1A receptor antagonist WAY-100635 into the prelimbic cortex of control voles increased the levels of anxiety- and depression-like behaviors. CONCLUSIONS: Taken together, our results demonstrated that chronic social defeat stress increased anxiety- and depression-like behaviors in adult female voles, and these effects were mediated by the action of serotonin on the serotonin 1A receptors in the prelimbic cortex. The serotonin system may be a promising target to treat emotional disorders induced by chronic social defeat stress.

In vivo biased agonism at 5-HT1A receptors: characterisation by simultaneous PET/MR imaging.

In neuropharmacology, the recent concept of 'biased agonism' denotes the capacity of certain agonists to target-specific intracellular pathways of a given receptor in specific brain areas. In the context of serotonin pharmacotherapy, 5-HT1A receptor-biased agonists could be of great interest in several neuropsychiatric disorders. The aim of this study was to determine whether biased agonists could be differentiated in terms of regional targeting by use of simultaneous functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) brain imaging. We compared two 5-HT1A-biased agonists, NLX-112 and NLX-101, injected at three different doses in anaesthetised cats (n = 4). PET imaging was acquired for 90 min after bolus administration followed by constant infusion of the 5-HT1A radiotracer, [18F]MPPF. Drug occupancy was evaluated after injection at 50 min and BOLD fMRI was simultaneously acquired to evaluate subsequent brain activation patterns. 5-HT1A receptor occupancy was found to be dose-dependent for both agonists, but differed in magnitude and spatial distribution at equal doses with distinct BOLD patterns. Functional connectivity, as measured by BOLD signal temporal correlations between regions, was also differently modified by NLX-112 or NLX-101. Voxel-based correlation analyses between PET and fMRI suggested that NLX-112 stimulates both 5-HT1A autoreceptors and post-synaptic receptors, whereas NLX-101 preferentially stimulates post-synaptic cortical receptors. In cingulate cortex, the agonists induced opposite BOLD signal changes in response to receptor occupancy. These data constitute the first simultaneous exploration of 5-HT1A occupancy and its consequences in terms of brain activation, and demonstrates differential signalling by two 5-HT1A-biased agonists. Combined PET/fMRI represents a powerful tool in neuropharmacology, and opens new ways to address the concept of biased agonism by translational approaches.

Cryo-EM structure of the serotonin 5-HT1B receptor coupled to heterotrimeric Go.

G-protein-coupled receptors (GPCRs) form the largest family of receptors encoded by the human genome (around 800 genes). They transduce signals by coupling to a small number of heterotrimeric G proteins (16 genes encoding different α-subunits). Each human cell contains several GPCRs and G proteins. The structural determinants of coupling of Gs to four different GPCRs have been elucidated1-4, but the molecular details of how the other G-protein classes couple to GPCRs are unknown. Here we present the cryo-electron microscopy structure of the serotonin 5-HT1B receptor (5-HT1BR) bound to the agonist donitriptan and coupled to an engineered Go heterotrimer. In this complex, 5-HT1BR is in an active state; the intracellular domain of the receptor is in a similar conformation to that observed for the ß2-adrenoceptor (ß2AR) 3 or the adenosine A2A receptor (A2AR) 1 in complex with Gs. In contrast to the complexes with Gs, the gap between the receptor and the Gß-subunit in the Go-5-HT1BR complex precludes molecular contacts, and the interface between the Gα-subunit of Go and the receptor is considerably smaller. These differences are likely to be caused by the differences in the interactions with the C terminus of the Go α-subunit. The molecular variations between the interfaces of Go and Gs in complex with GPCRs may contribute substantially to both the specificity of coupling and the kinetics of signalling.

Functional Characterization of 5-HT1B Receptor Drugs in Nonhuman Primates Using Simultaneous PET-MR.

In the present study, we used a simultaneous PET-MR experimental design to investigate the effects of functionally different compounds (agonist, partial agonist, and antagonist) on 5-HT1B receptor (5-HT1BR) occupancy and the associated hemodynamic responses. In anesthetized male nonhuman primates (n = 3), we used positron emission tomography (PET) imaging with the radioligand [11C]AZ10419369 administered as a bolus followed by constant infusion to measure changes in 5-HT1BR occupancy. Simultaneously, we measured changes in cerebral blood volume (CBV) as a proxy of drug effects on neuronal activity. The 5-HT1BR partial agonist AZ10419369 elicited a dose-dependent biphasic hemodynamic response that was related to the 5-HT1BR occupancy. The magnitude of the response was spatially overlapping with high cerebral 5-HT1BR densities. High doses of AZ10419369 exerted an extracranial tissue vasoconstriction that was comparable to the less blood-brain barrier-permeable 5-HT1BR agonist sumatriptan. By contrast, injection of the antagonist GR127935 did not elicit significant hemodynamic responses, even at a 5-HT1BR cerebral occupancy similar to the one obtained with a high dose of AZ10419369. Given the knowledge we have of the 5-HT1BR and its function and distribution in the brain, the hemodynamic response informs us about the functionality of the given drug: changes in CBV are only produced when the receptor is stimulated by the partial agonist AZ10419369 and not by the antagonist GR127935, consistent with low basal occupancy by endogenous serotonin.SIGNIFICANCE STATEMENT We here show that combined simultaneous positron emission tomography and magnetic resonance imaging uniquely enables the assessment of CNS active compounds. We conducted a series of pharmacological interventions to interrogate 5-HT1B receptor binding and function and determined blood-brain barrier passage of drugs and demonstrate target involvement. Importantly, we show how the spatial and temporal effects on brain hemodynamics provide information about pharmacologically driven downstream CNS drug effects; the brain hemodynamic response shows characteristic dose-related effects that differ depending on agonistic or antagonistic drug characteristics and on local 5-HT1B receptor density. The technique lends itself to a comprehensive in vivo investigation and understanding of drugs' effects in the brain.

Sphingolipids modulate the function of human serotonin1A receptors: Insights from sphingolipid-deficient cells.

Sphingolipids are essential components of eukaryotic cell membranes and are known to modulate a variety of cellular functions. It is becoming increasingly clear that membrane lipids play a crucial role in modulating the function of integral membrane proteins such as G protein-coupled receptors (GPCRs). In this work, we utilized LY-B cells, that are sphingolipid-auxotrophic mutants defective in sphingolipid biosynthesis, to monitor the role of cellular sphingolipids in the function of an important neurotransmitter receptor, the serotonin1A receptor. Serotonin1A receptors belong to the family of GPCRs and are implicated in behavior, development and cognition. Our results show that specific ligand binding and G-protein coupling of the serotonin1A receptor exhibit significant enhancement under sphingolipid-depleted conditions, which reversed to control levels upon replenishment of cellular sphingolipids. In view of the reported role of sphingolipids in neuronal metabolism and pathogenesis of several neuropsychiatric disorders, exploring the role of serotonin1A receptors under conditions of defective sphingolipid metabolism assumes relevance, and could contribute to our overall understanding of such neuropsychiatric disorders. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Neurochemical and behavioural effects of hypidone hydrochloride (YL-0919): a novel combined selective 5-HT reuptake inhibitor and partial 5-HT1A agonist.

BACKGROUND AND PURPOSE: Our previous studies revealed that hypidone hydrochloride (YL-0919), which acts as a selective 5-HT (serotonin) reuptake inhibitor (SSRI) and displays partial 5-HT1A receptor agonist properties, exerts a significant antidepressant effect in various animal models. The aim of present research was to further investigate the pharmacology of YL-0919. EXPERIMENTAL APPROACH: We first investigated the target profile of YL-0919 using [35 S]-GTPγS binding and microdialysis. To determine whether the 5-HT or noradrenergic systems are involved in the antidepressant-like effect of YL-0919, the 5-hydroxytryptophan (5-HTP)-induced head-twitch test and antagonism with a high dose of apomorphine were performed. Using the learned helplessness paradigm, the novelty suppressed feeding test, the Vogel-type conflict and elevated plus-maze test, we further verified the antidepressant-like and anxiolytic-like effects of YL-0919. The effects of YL-0919 on hippocampal long-term potentiation (LTP) and sexual behaviour were also evaluated. KEY RESULTS: Data from the present study demonstrated that YL-0919 displays partial 5-HT1A receptor agonist properties, producing a greater impact on extracellular 5-HT levels than a conventional SSRI (fluoxetine), as well as significant antidepressant and anxiolytic effects. Furthermore, YL-0919 treatment rapidly influenced the synaptic plasticity (enhancing LTP) of rats. Finally, at doses close to those producing antidepressant-like effects, YL-0919 did not result in a marked inhibition of sexual function. CONCLUSIONS AND IMPLICATIONS: These data suggest that YL-0919 is probably a fast-onset potent antidepressant with few side effects.

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.

Mechanistic Studies on the Stereoselectivity of the Serotonin 5-HT1A Receptor.

G-protein-coupled receptors (GPCRs) are involved in a wide range of physiological processes, and they have attracted considerable attention as important targets for developing new medicines. A central and largely unresolved question in drug discovery, which is especially relevant to GPCRs, concerns ligand selectivity: Why do certain molecules act as activators (agonists) whereas others, with nearly identical structures, act as blockers (antagonists) of GPCRs? To address this question, we employed all-atom, long-timescale molecular dynamics simulations to investigate how two diastereomers (epimers) of dihydrofuroaporphine bind to the serotonin 5-HT1A receptor and exert opposite effects. By using molecular interaction fingerprints, we discovered that the agonist could mobilize nearby amino acid residues to act as molecular switches for the formation of a continuous water channel. In contrast, the antagonist epimer remained firmly stabilized in the binding pocket.

Agonist and antagonist bind differently to 5-HT1A receptors during Alzheimer's disease: A post-mortem study with PET radiopharmaceuticals.

PET imaging studies using 5-HT1A receptor radiotracers show a decreased density of this receptor in hippocampi of patients with Alzheimer's disease (AD) at advanced stages. However, current 5-HT1A receptor radiopharmaceuticals used in neuroimaging are antagonists, thought to bind to 5-HT1A receptors in different functional states (i.e., both the one which displays high affinity for agonists and is thought to mediate receptor activation, as well as the state which has low affinity for agonists). Comparing the PET imaging obtained using an agonist radiotracer, which binds selectively to functional receptors, with the PET imaging obtained using an antagonist radiotracer would therefore provide original information on 5-HT1A receptor impairment during AD. Quantitative autoradiography using [(18)F]F13640 and [(18)F]MPPF, a 5-HT1A agonist and antagonist, respectively, was measured in hippocampi of patients with AD (n = 25, at different Braak stages) and control subjects (n = 9). The neuronal density was measured in the same tissues by NeuN immunohistochemistry. The specific binding of both radiotracers was determined by addition of WAY-100635, a selective 5-HT1A receptor antagonist. The autoradiography distribution of both 5-HT1A PET radiotracers varied across hippocampus regions. The highest binding density was in the pyramidal layer of CA1. Incubation with Gpp(NH)p, a non-hydrolysable analogue of GTP, reduced significantly [(18)F]F13640 binding in hippocampal regions, confirming its preferential interaction with G-coupled receptors, and slightly increased [(18)F]MPPF binding. In the CA1 subfield, [(18)F]F13640 binding was significantly decreased at Braak stages I/II (-19%), Braak stages III/IV (-23%), and Braak stages V/VI (-36%) versus control. In contrast, [(18)F]MPPF binding was statistically reduced only at the most advanced Braak stages V/VI compared to control (-33%). Since [(18)F]F13640 and [(18)F]MPPF can be used in vivo in humans, this neuropharmacological paradigm supports testing the concept of functional imaging using agonist radiopharmaceuticals in future clinical studies.

OCD is associated with an altered association between sensorimotor gating and cortical and subcortical 5-HT1b receptor binding.

Obsessive-compulsive disorder (OCD) is characterized by impaired sensorimotor gating, as measured using prepulse inhibition (PPI). This effect may be related to abnormalities in the serotonin (5-HT) system. 5-HT1B agonists can impair PPI, produce OCD-like behaviors in animals, and exacerbate OCD symptoms in humans. We measured 5-HT1B receptor availability using (11)C-P943 positron emission tomography (PET) in unmedicated, non-depressed OCD patients (n=12) and matched healthy controls (HC; n=12). Usable PPI data were obtained from 20 of these subjects (10 from each group). There were no significant main effects of OCD diagnosis on 5-HT1B receptor availability ((11)C-P943 BPND); however, the relationship between PPI and (11)C-P943 BPND differed dramatically and significantly between groups. 5-HT1B receptor availability in the basal ganglia and thalamus correlated positively with PPI in controls; these correlations were lost or even reversed in the OCD group. In cortical regions there were no significant correlations with PPI in controls, but widespread positive correlations in OCD patients. Positive correlations between 5-HT1B receptor availability and PPI were consistent across diagnostic groups only in two structures, the orbitofrontal cortex and the amygdala. Differential associations of 5-HT1B receptor availability with PPI in patients suggest functionally important alterations in the serotonergic regulation of cortical/subcortical balance in OCD.

Design, synthesis and evaluation of benzo[a]thieno[3,2-g]quinolizines as novel l-SPD derivatives possessing dopamine D1, D2 and serotonin 5-HT1A multiple action profiles.

A novel scaffold derived from l-SPD with a substituted thiophene group in the D ring were designed, synthesized, and evaluated for their binding affinities at dopamine (D1, D2 and D3) and serotonin (5-HT1A and 5-HT2A) receptors. Most of the tetracyclic compounds exhibited higher affinities for D2 and 5-HT1A receptors than l-SPD, while compound 23 e showed the highest Ki value of 7.54 nM at D2 receptor which was 14 times more potent than l-SPD. Additionally, compounds 23 d and 23 e were more potent than l-SPD at D3 receptor. According to the functional assays, 23 d and 23 e were demonstrated as full antagonists at D1 and D2 receptors and full agonists at 5-HT1A receptor. Since the combination of D2 antagonism and 5-HT1A agonism is considered effective in treating both the positive and negative symptoms of schizophrenia, these novel compounds are implicated as potential therapeutic agents.