The synthesis and structure−activity relationship of a novel series of compounds with combined effects on 5-HT3A and 5-HT1A receptors and on the serotonin (5-HT) transporter (SERT) are described. ...Compound 5m (Lu AA21004) was the lead compound, displaying high affinity for recombinant human 5-HT1A (K i = 15 nM), 5-HT1B (K i = 33 nM), 5-HT3A (K i = 3.7 nM), 5-HT7 (K i = 19 nM), and noradrenergic β1 (K i = 46 nM) receptors, and SERT (K i = 1.6 nM). Compound 5m displayed antagonistic properties at 5-HT3A and 5-HT7 receptors, partial agonist properties at 5-HT1B receptors, agonistic properties at 5-HT1A receptors, and potent inhibition of SERT. In conscious rats, 5m significantly increased extracellular 5-HT levels in the brain after acute and 3 days of treatment. Following the 3-day treatment (5 or 10 (mg/kg)/day) SERT occupancies were only 43% and 57%, respectively. These characteristics indicate that 5m is a novel multimodal serotonergic compound, and 5m is currently in clinical development for major depressive disorder.
Serotonin, or 5-hydroxytryptamine (5-HT), is an important neurotransmitter
that activates the largest subtype family of G-protein-coupled receptors
. Drugs that target 5-HT
, 5-HT
, 5-HT
and other ...5-HT receptors are used to treat numerous disorders
. 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-HT
in the apo state, bound to 5-HT or bound to the antipsychotic drug aripiprazole; 5-HT
bound to 5-HT; and 5-HT
in complex with a 5-HT
- and 5-HT
-selective agonist, BRL-54443. Notably, the phospholipid phosphatidylinositol 4-phosphate is present at the G-protein-5-HT
interface, and is able to increase 5-HT
-mediated G-protein activity. The receptor transmembrane domain is surrounded by cholesterol molecules-particularly in the case of 5-HT
, 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-HT
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.
Abstract The present study investigated interactions between the novel psychoactive tryptamines DiPT, 4-OH-DiPT, 4-OH-MET, 5-MeO-AMT, and 5-MeO-MiPT at monoamine receptors and transporters compared ...with the classic hallucinogens lysergic acid diethylamide (LSD), psilocin, N , N -dimethyltryptamine (DMT), and mescaline. We investigated binding affinities at human monoamine receptors and determined functional serotonin (5-hydroxytryptamine 5-HT) 5-HT2A and 5-HT2B receptor activation. Binding at and the inhibition of human monoamine uptake transporters and transporter-mediated monoamine release were also determined. All of the novel tryptamines interacted with 5-HT2A receptors and were partial or full 5-HT2A agonists. Binding affinity to the 5-HT2A receptor was lower for all of the tryptamines, including psilocin and DMT, compared with LSD and correlated with the reported psychoactive doses in humans. Several tryptamines, including psilocin, DMT, DiPT, 4-OH-DiPT, and 4-OH-MET, interacted with the serotonin transporter and partially the norepinephrine transporter, similar to 3,4-methylenedioxymethamphetamine but in contrast to LSD and mescaline. LSD but not the tryptamines interacted with adrenergic and dopaminergic receptors. In conclusion, the receptor interaction profiles of the tryptamines predict hallucinogenic effects that are similar to classic serotonergic hallucinogens but also MDMA-like psychoactive properties.
Drugs may cause serotonin toxicity by a number of different mechanisms including inhibition of serotonin uptake and metabolism, increased serotonin synthesis and release, activation of serotonin ...receptors, and inhibition of cytochrome P450 oxidases. Some drug interactions involving opioids can increase intrasynaptic levels of serotonin, and opioid analgesic drugs are now recognized as being involved in some cases of serotonin toxicity especially if administered in conjunction with other serotonergic medications including monoamine oxidase inhibitors, selective serotonin reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors, and tricyclic antidepressants. In March 2016, the FDA issued a Drug Safety Communication concerning the association of the entire class of opioid pain medicines with serotonin toxicity. Reports of the involvement of individual opioids particularly tramadol, tapentadol, meperidine, methadone, oxycodone, fentanyl, and dextromethorphan are reviewed. While relevance to human serotonin toxicity of animal models, including many studies on rat brain synaptosomes, is questionable, important insights have recently been forthcoming from research utilizing 5-HT receptors, serotonin transporter (SERT), and knockout mice. In studies with human SERT-transfected human HEK293 cells, the synthetic opioids tramadol, meperidine, methadone, tapentadol, and dextromethorphan inhibited SERT, but fentanyl and a number of phenanthrenes including morphine and hydromorphone did not. Receptor ligand-binding assays revealed interaction of fentanyl with 5-HT
1A
receptors and interaction of meperidine, methadone, and fentanyl with 5-HT
2A
receptors. Although the opioids most often associated with serotonin toxicity in humans inhibit human SERT in vitro, fentanyl and oxycodone are not inhibitory even though their clinical involvement has been reported. This suggests some SERT-independent effects on the serotonin system in vivo. Heightened clinician awareness of the possibility of serotonin toxicity among patients taking opioids and serotonergic antidepressants is called for.
The serotonin transporter (SERT) regulates neurotransmitter homeostasis through the sodium- and chloride-dependent recycling of serotonin into presynaptic neurons
. Major depression and anxiety ...disorders are treated using selective serotonin reuptake inhibitors-small molecules that competitively block substrate binding and thereby prolong neurotransmitter action
. The dopamine and noradrenaline transporters, together with SERT, are members of the neurotransmitter sodium symporter (NSS) family. The transport activities of NSSs can be inhibited or modulated by cocaine and amphetamines
, and genetic variants of NSSs are associated with several neuropsychiatric disorders including attention deficit hyperactivity disorder, autism and bipolar disorder
. Studies of bacterial NSS homologues-including LeuT-have shown how their transmembrane helices (TMs) undergo conformational changes during the transport cycle, exposing a central binding site to either side of the membrane
. However, the conformational changes associated with transport in NSSs remain unknown. To elucidate structure-based mechanisms for transport in SERT we investigated its complexes with ibogaine, a hallucinogenic natural product with psychoactive and anti-addictive properties
. Notably, ibogaine is a non-competitive inhibitor of transport but displays competitive binding towards selective serotonin reuptake inhibitors
. Here we report cryo-electron microscopy structures of SERT-ibogaine complexes captured in outward-open, occluded and inward-open conformations. Ibogaine binds to the central binding site, and closure of the extracellular gate largely involves movements of TMs 1b and 6a. Opening of the intracellular gate involves a hinge-like movement of TM1a and the partial unwinding of TM5, which together create a permeation pathway that enables substrate and ion diffusion to the cytoplasm. These structures define the structural rearrangements that occur from the outward-open to inward-open conformations, and provide insight into the mechanism of neurotransmitter transport and ibogaine inhibition.
Rationale
Accumulating evidence indicates that the mixed serotonin and dopamine receptor agonist lysergic acid diethylamide (LSD) induces an altered state of consciousness that resembles dreaming.
...Objectives
This study aimed to test the hypotheses that LSD produces dreamlike waking imagery and that this imagery depends on 5-HT2A receptor activation and is related to subjective drug effects.
Methods
Twenty-five healthy subjects performed an audiorecorded guided mental imagery task 7 h after drug administration during three drug conditions: placebo, LSD (100 mcg orally) and LSD together with the 5-HT2A receptor antagonist ketanserin (40 mg orally). Cognitive bizarreness of guided mental imagery reports was quantified as a standardised formal measure of dream mentation. State of consciousness was evaluated using the Altered State of Consciousness (5D-ASC) questionnaire.
Results
LSD, compared with placebo, significantly increased cognitive bizarreness (
p
< 0.001). The LSD-induced increase in cognitive bizarreness was positively correlated with the LSD-induced loss of self-boundaries and cognitive control (
p
< 0.05). Both LSD-induced increases in cognitive bizarreness and changes in state of consciousness were fully blocked by ketanserin.
Conclusions
LSD produced mental imagery similar to dreaming, primarily via activation of the 5-HT2A receptor and in relation to loss of self-boundaries and cognitive control. Future psychopharmacological studies should assess the differential contribution of the D2/D1 and 5-HT1A receptors to cognitive bizarreness.
Abstract
Introduction
Serotoninergic dorsal raphe neurons (DRSert) are CO2 responsive, and mice lacking serotonin have impaired arousal to CO2. We showed that the neurons in external lateral ...parabrachial nucleus containing calcitonin gene related peptide (PBelCGRP), are required for CO2-arousal. PBelCGRP neurons also receive serotoninergic innervation from the DRSert. 5HT2A agonist restores CO2 responsiveness in mice lacking serotonin, suggesting that DRSert may modulate CO2 arousal by acting on 5HT2A receptors possibly on the PBel neurons.
Methods
We used serotonin transporter (Sert)-Cre mice to optogenetically inhibit DRSert neurons and their terminals in the PBel. We injected AAV-FLEX-ArchT into the DR and implanted an optical fiber just above it in one set of Sert-Cre mice and bilaterally in the PBel in another set. All mice were instrumented for sleep and optogenetics and were tested for EEG arousals to 10% CO2. Latencies of arousal were compared with optogenetic inhibition of either the DR neurons or their terminals in the PBel with a 593nm laser light. We further tested whether a 5HT2A agonist (TCB-2) can reverse blockade of CO2 arousal in mice where DRSert terminals in PBel were inhibited. Finally, TCB-2 was injected in mice with PBelCGRP deletions and arousal latency to CO2 was compared.
Results
Compared to the control (Laser-OFF) condition, arousal latency to CO2 was significantly increased by photoinhibition of either the DRSert neurons (n=6; latency- 40.9 ± 6.4 vs. 13.81± 0.69 sec; F3, 17= 11.5; P< 0.001) or their terminals in PBel (n=8; latency-34.9 ± 2.3 sec vs. 16.62 ± 0.97sec, F1, 14= 56.9; P< 0.001). This was reversed by the 5HT2A agonist TCB-2 (5mg/kg), as it reduced the latency to CO2 arousal in mice with photoinhibition of terminals in PBel from 35.48 ± 7.31 sec to 16.24 ± 1.06 sec (F3, 9= 8.05; P= 0.006), but had no effect in mice with PBelCGRP neurons deletions.
Conclusion
The serotonin system modulate CO2-arousals by the DRSert input to the PBel. TCB-2 reversed the effect of inhibition of DRSert terminals in the PBel, but not in mice with PBelCGRP deletions, suggests that DRSert modulate PBelCGRP neurons through 5HT2a receptors.
Support
NIH- 2P01 HL095491 and NS112175
The main class of atypical antipsychotic drugs (APDs) in current use includes the protypical atypical APD, clozapine, as well as aripiprazole, asenapine, iloperidone, lurasidone, olanzapine, ...quetiapine, risperidone, and ziprasidone. At clinically effective doses, these agents produce extensive blockade of serotonin (5-HT)2A receptors, direct or indirect stimulation of 5-HT1A receptors, and to a lesser extent, reduction in dopamine (DA) D2 receptor-mediated neurotransmission. This contrasts with typical APDs, for example haloperidol and perphenazine, which are mainly DA D2/ D3 receptor antagonists and have weaker, if any, potency as 5-HT2A receptor antagonists. Some, but not all, atypical APDs are also effective 5-HT2C receptor inverse agonists or neutral antagonists, 5-HT6 or 5-HT7 receptor antagonists. This diverse action on 5-HT receptors may contribute to significant differences in efficacy and tolerability among the atypical APDs. There is considerable preclinical and some clinical evidence that effects on 5-HT receptors contribute to the low risk of producing extrapyramidal side effects, which is the defining characteristic of an atypical APD, the lack of elevation in plasma prolactin levels (with risperidone and 9-hydroxyrisperidone being exceptions), antipsychotic action, and ability to improve some domains of cognition in patients with schizophrenia. The serotonergic actions of the atypical APDs, especially 5-HT2A receptor antagonism, are particularly important to the differential effects of typical and atypical APDs to overcome the effects of acute or subchronic administration of N-methyl- d -aspartate (NMDA) receptor antagonists, such as phencyclidine, ketamine, and dizocipline (MK-801). 5-HT1A receptor stimulation and 5-HT6 and 5-HT7 receptor antagonism may contribute to beneficial effects of these agents on cognition. In particular, 5-HT7 receptor antagonism may be the basis for the pro-cognitive effects of the atypical APD, amisulpride, a D2 /D3 receptor antagonist, which has no effect on other 5-HT receptor. 5-HT2C receptor antagonism appears to contribute to the weight gain produced by some atypical APDs and may also affect cognition and psychosis via its influence on cortical and limbic dopaminergic activity.
Previous attempts to identify a unified theory of brain serotonin function have largely failed to achieve consensus. In this present synthesis, we integrate previous perspectives with new and older ...data to create a novel bipartite model centred on the view that serotonin neurotransmission enhances two distinct adaptive responses to adversity, mediated in large part by its two most prevalent and researched brain receptors: the 5-HT1A and 5-HT2A receptors. We propose that passive coping (i.e. tolerating a source of stress) is mediated by postsynaptic 5-HT1AR signalling and characterised by stress moderation. Conversely, we argue that active coping (i.e. actively addressing a source of stress) is mediated by 5-HT2AR signalling and characterised by enhanced plasticity (defined as capacity for change). We propose that 5-HT1AR-mediated stress moderation may be the brain’s default response to adversity but that an improved ability to change one’s situation and/or relationship to it via 5-HT2AR-mediated plasticity may also be important – and increasingly so as the level of adversity reaches a critical point. We propose that the 5-HT1AR pathway is enhanced by conventional 5-HT reuptake blocking antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), whereas the 5-HT2AR pathway is enhanced by 5-HT2AR-agonist psychedelics. This bipartite model purports to explain how different drugs (SSRIs and psychedelics) that modulate the serotonergic system in different ways, can achieve complementary adaptive and potentially therapeutic outcomes.
The original version of this article contained an error in the labelling of Figures 2 and 3. While the captions and figures themselves are correct, in order to correspond with the in-text references, ...they have now been re-numbered in both the PDF and HTML versions of the article.
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