Noncompetitive Inhibition of 5-HT3 Receptors by Citral, Linalool, and Eucalyptol Revealed by Nonlinear Mixed-Effects Modeling

Phytoconstituents Targeting the Serotonin 5-HT3 Receptor: Promising Therapeutic Strategies for Neurological Disorders

The 5-hydroxytryptamine-3 receptor (5-HT3R), a subtype of serotonin receptor, is a ligand-gated ion channel crucial in mediating fast synaptic transmission in the central and peripheral nervous systems. This receptor significantly influences various neurological activities, encompassing neurotransmission, mood regulation, and cognitive processing; hence, it may serve as an innovative target for neurological disorders. Multiple studies have revealed promising results regarding the beneficial effects of these phytoconstituents and extracts on conditions such as nausea, vomiting, neuropathic pain depression, anxiety, Alzheimer's disease, cognition, epilepsy, sleep, and dyskinesia via modulation of 5-HT3R in the pathophysiology of neurological disorder. The review delves into a detailed exploration of in silico, in vitro, and in vivo studies and clinical studies that discussed phytoconstituents acting on 5-HT3R and attenuates difficulties in neurological diseases. The diverse mechanisms by which plant-derived phytoconstituents influence 5-HT3R activity offer exciting avenues for developing innovative therapeutic interventions. Besides producing an agonistic or antagonistic effect, some phytoconstituents exert modulatory effects on 5-HT3R activity through multifaceted mechanisms. These include γ-aminobutyric acid and cholinergic neuronal pathways, interactions with neurokinin (NK)-1, NK2, serotonergic, and γ-aminobutyric acid(GABA)ergic systems, dopaminergic influences, and mediation of calcium ions release and inflammatory cascades. Notably, the phytoconstituent's capacity to reduce oxidative stress has also emerged as a significant factor contributing to their modulatory role. Despite the promising implications, there is currently a dearth of exploration needed to understand the effect of phytochemicals on the 5-HT3R. Comprehensive preclinical and clinical research is of the utmost importance to broaden our knowledge of the potential therapeutic benefits associated with these substances.

Transient receptor potential channel involvement in antinociceptive effect of citral in orofacial acute and chronic pain models

This study aimed to test for the possible antinociceptive effect of the naturally occurring terpene citral in rodent models of acute and chronic orofacial pain and to test for the possible involvement of transient receptor potential (TRP) channels in this effect. Acute nociceptive behavior was induced in one series of experiments by administering formalin, cinnamaldehyde, menthol or capsaicin to the upper lip. Nociceptive behavior was assessed by orofacial rubbing, and the effects of pre-treatment with citral (0.1, 0.3 or 1.0 mg/Kg) or vehicle (control) were tested on the behavior. Nociceptive behavior was also induced by formalin injected into the temporomandibular joint or mustard oil injected into the masseter muscle, preceded by citral or vehicle (control) treatment. The chronic pain model involved infraorbital nerve transection (IONX) that induced mechanical hypersensitivity which was assessed by von Frey hair stimulation of the upper lip. Motor activity was also evaluated. Docking experiments were performed using TRPV1 and TRPM8 channels. Citral but not vehicle produced significant (p<0.01, ANOVA) antinociception on all the acute nociceptive behaviors, and these effects were attenuated by TRPV1 antagonist capsazepine, TRPM3 antagonist mefenamic acid and by TRPM8 desensitization, but not by ruthenium red and TRPA1 antagonist HC-030031. The IONX animals developed facial mechanical hypersensitivity that was significantly reduced by citral but not by vehicle. The docking experiments revealed that citral may interact with TRPV1 and TRPM8 channels. These results indicate the potential use of citral as an inhibitor of orofacial nociception in both acute and chronic pain states through TRPV1, TRPM3 and TRPM8 channels. See also Figure 1(Fig. 1).

Antispasmodic Effect of Bergamot Essential Oil on Rat Isolated Gut Tissues

Preclinical data indicate that bergamot essential oil (BEO) can modulate the synaptic functions within the central nervous system (CNS). Particularly, several data shows that essential oil is endowed with reproducible analgesic and anxiolytic effects that may derived from the ability to modulate the excitatory and inhibitory neurotransmission in the CNS. Although there are differences in the functional complexity of the enteric nervous system (ENS), it is likely that the phytocomplex has biological properties in gut superimposable to those showed in the CNS. Accordingly, the aim of this study was to investigate ex-vivo the effect of bergamot essential oil and its main constituents on the contractile activity of rat isolated colon, jejunum and ileum induced by different muscle stimulants such as acetylcholine (10⁻⁶ M) and potassium chloride (80 mM). Our present data demonstrate that BEO inhibits cholinergically- and non cholinergically-mediated contractions in rat isolated gut and that linalool is the most active component. These results suggest that the phytocomplex might be useful in the treatment of spastic disorders in ENS mainly characterized by the presence of pain; incidentally, irritable bowel syndrome (IBS) is a painful condition in which a role for neurotransmitter dysfunction has been envisaged. More investigation is required for clinical translation of the present data.

A Review of the Potential Use of Pinene and Linalool as Terpene-Based Medicines for Brain Health: Discovering Novel Therapeutics in the Flavours and Fragrances of Cannabis

"Medicinal cannabis" is defined as the use of cannabis-based products for the treatment of an illness. Investigations of cannabis compounds in psychiatric and neurological illnesses primarily focus on the major cannabinoids, cannabidiol (CBD) and Δ9-tetrahydrocannabinol (Δ9-THC), which are hypothesised to benefit multiple illnesses manifesting cognitive impairment, neurodegeneration and neuro-inflammation, as well as chronic pain, epilepsy and post-traumatic stress disorder, respectively. The cannabis plant contains >500 compounds, including terpenes responsible for the flavour and fragrance profiles of plants. Recently, research has begun providing evidence on the potential use of certain plant-derived terpenes in modern medicine, demonstrating anti-oxidant, anti-inflammatory, and neuroprotective effects of these compounds. This review examined the effects of two key terpenes, pinene and linalool, on parameters relevant to neurological and psychiatric disorders, highlighting gaps in the literature and recommendations for future research into terpene therapeutics. Overall, evidence is mostly limited to preclinical studies and well-designed clinical trials are lacking. Nevertheless, existing data suggests that pinene and linalool are relevant candidates for further investigation as novel medicines for illnesses, including stroke, ischemia, inflammatory and neuropathic pain (including migraine), cognitive impairment (relevant to Alzheimer's disease and ageing), insomnia, anxiety, and depression. Linalool and pinene influence multiple neurotransmitter, inflammatory and neurotrophic signals as well as behaviour, demonstrating psycho-activity (albeit non-intoxicating). Optimising the phytochemical profile of cannabis chemovars to yield therapeutic levels of beneficial terpenes and cannabinoids, such as linalool, pinene and CBD, could present a unique opportunity to discover novel medicines to treat psychiatric and neurological illnesses; however, further research is needed.

GABA- and Glycine-Mimetic Responses of Linalool on the Substantia Gelatinosa of the Trigeminal Subnucleus Caudalis in Juvenile Mice: Pain Management through Linalool-Mediated Inhibitory Neurotransmission

Linalool, a major odorous constituent in essential oils extracted from lavender, is known to have a wide range of physiological effects on humans including pain management. The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) is involved in transmission of orofacial nociceptive responses through thin myelinated A[Formula: see text] and unmyelinated C primary afferent fibers. Up to date, the orofacial antinociceptive mechanism of linalool concerning SG neurons of the Vc has not been completely clarified yet. To fill this knowledge gap, whole-cell patch-clamp technique was used in this study to examine how linalool acted on SG neurons of the Vc in mice. Under a high chloride pipette solution, non-desensitizing and repeatable linalool-induced inward currents were preserved in the presence of tetrodotoxin (a voltage-gated Na[Formula: see text]channel blocker), CNQX (a non-NMDA glutamate receptor antagonist), and DL-AP5 (an NMDA receptor antagonist). However, linalool-induced inward currents were partially suppressed by picrotoxin (a GABA[Formula: see text] receptor antagonist) or strychnine (a glycine receptor antagonist). These responses were almost blocked in the presence of picrotoxin and strychnine. It was also found that linalool exhibited potentiation with GABA- and glycine-induced responses. Taken together, these data show that linalool has GABA- and glycine-mimetic effects, suggesting that it can be a promising target molecule for orofacial pain management by activating inhibitory neurotransmission in the SG area of the Vc.

Pharmacogenetic-based management of depression: Role of traditional Persian medicine

Depression is one of the most common mental disorders worldwide. The genetic factors are linked to depression and anti-depressant outcomes. Traditional Persian medicine (TPM) manuscripts have provided various anti-depressant remedies, which may be useful in depression management. This review has studied the bioactive compounds, underlying mechanisms, and treatment outcomes of the medicinal plants traditionally mentioned effective for depression from "The storehouse of medicament" (a famous pharmacopeia of TPM) to merge those with the novel genetics science and serve new scope in depression prevention and management. This review paper has been conducted in two sections: (1) Collecting medicinal plants and their bioactive components from "The storehouse of medicament," "Physician's Desk Reference (PDR) for Herbal Medicines," and "Google scholar" database. (2) The critical key factors and genes in depression pathophysiology, prevention, and treatment were clarified. Subsequently, the association between bioactive components' underlying mechanism and depression treatment outcomes via considering polymorphisms in related genes was derived. Taken together, α-Mangostin, β-carotene, β-pinene, apigenin, caffeic acid, catechin, chlorogenic acid, citral, ellagic acid, esculetin, ferulic acid, gallic acid, gentiopicroside, hyperoside, kaempferol, limonene, linalool, lycopene, naringin, protocatechuic acid, quercetin, resveratrol, rosmarinic acid, and umbelliferone are suitable for future pharmacogenetics-based studies in the management of depression.

Myorelaxant Effect of the Dysphania ambrosioides Essential Oil on Sus scrofa domesticus Coronary Artery and Its Toxicity in the Drosophila melanogaster Model

PURPOSE

Alternative methods for the use of animals in research have gained increasing importance, due to assessments evaluating the real need for their use and the development of legislation that regulates the subject. The principle of the 3R's (replacement, reduction and refinement) has been an important reference, such that in vitro, ex vivo and cord replacement methods have achieved a prominent place in research.

METHODS

Therefore, due to successful results from studies developed with these methods, the present study aimed to evaluate the myorelaxant effect of the Dysphania ambrosioides essential oil (EODa) using a Sus scrofa domesticus coronary artery model, and the toxicity of both the Dysphania ambrosioides essential oil and its major constituent, α-terpinene, against Drosophila melanogaster in toxicity and negative geotaxis assays.

RESULTS

The EODa relaxed the smooth muscle of swine coronary arteries precontracted with K⁺ and 5-HT in assays using Sus scrofa domesticus coronary arteries. The toxicity results presented LC₅₀ values of 1.546 mg/mL and 2.282 mg/mL for the EODa and α-terpinene, respectively, thus showing the EODa and α-terpinene presented toxicity to these dipterans, with the EODa being more toxic.

CONCLUSIONS

Moreover, the results reveal the possibility of using the EODa in vascular disease studies since it promoted the relaxation of the Sus scrofa domesticus coronary smooth muscle.

Neural and subjective effects of inhaled N,N-dimethyltryptamine in natural settings

BACKGROUND

N,N-dimethyltryptamine is a short-acting psychedelic tryptamine found naturally in many plants and animals. Few studies to date have addressed the neural and psychological effects of N,N-dimethyltryptamine alone, either administered intravenously or inhaled in freebase form, and none have been conducted in natural settings.

AIMS

Our primary aim was to study the acute effects of inhaled N,N-dimethyltryptamine in natural settings, focusing on questions tuned to the advantages of conducting field research, including the effects of contextual factors (i.e. "set" and "setting"), the possibility of studying a comparatively large number of subjects, and the relaxed mental state of participants consuming N,N-dimethyltryptamine in familiar and comfortable settings.

METHODS

We combined state-of-the-art wireless electroencephalography with psychometric questionnaires to study the neural and subjective effects of naturalistic N,N-dimethyltryptamine use in 35 healthy and experienced participants.

RESULTS

We observed that N,N-dimethyltryptamine significantly decreased the power of alpha (8-12 Hz) oscillations throughout all scalp locations, while simultaneously increasing power of delta (1-4 Hz) and gamma (30-40 Hz) oscillations. Gamma power increases correlated with subjective reports indicative of some features of mystical-type experiences. N,N-dimethyltryptamine also increased global synchrony and metastability in the gamma band while decreasing those measures in the alpha band.

CONCLUSIONS

Our results are consistent with previous studies of psychedelic action in the human brain, while at the same time the results suggest potential electroencephalography markers of mystical-type experiences in natural settings, thus highlighting the importance of investigating these compounds in the contexts where they are naturally consumed.

Monoterpenes as a perspective for the treatment of seizures: A Systematic Review

BACKGROUND

Epilepsy affects more than 65 million people worldwide. Treatment for epileptic seizures is ineffective and has many adverse effects. For this reason, the search for new therapeutic options capable of filling these limitations is necessary.

HYPOTHESIS/PURPOSE

In this sense, natural products, such as monoterpenes, have been indicated as a new option to control neurological disorders such as epilepsy.

STUDY DESIGN

Therefore, the objective of this study was to review the monoterpenes that have anticonvulsive activity in animal models.

METHODS

The searches were performed in the PubMed, Web of Science and Scopus databases in September, 2020 and compiled studies using monoterpenes as an alternative to seizure. Two independent reviewers performed the study selection, data extraction and methodological quality assessment using the Syrcle tool.

RESULTS

51 articles that described the anticonvulsant activity of 35 monoterpenes were selected with action on the main pharmacological target, including GABAA receptors, glutamate, calcium channels, sodium and potassium. In addition, these compounds are capable of reducing neuronal inflammation and oxidative stress caused by seizure.

CONCLUSION

These compounds stand out as a promising alternative for acting through different pharmacological mechanisms, which may not only reduce seizure, but also promote neuroprotective effect by reducing toxicity in brain regions. However, further studies are needed to determine the mechanism of action and safety assessment of these compounds.

Network pharmacology-based therapeutic mechanism of Kuanxiong aerosol for angina pectoris

ETHNOPHARMACOLOGICAL RELEVANCE

Kuanxiong aerosol has been reported to be an effective and safe clinical treatment for angina pectoris (AP).

AIM OF THE STUDY

To explore the potential pharmacological mechanism of Kuanxiong aerosol by combined methods of network pharmacology prediction and experimental verification.

MATERIALS AND METHODS

Networks of Kuanxiong aerosol-associated targets and AP-related genes were constructed through STRING database. Potential targets and pathway enrichment analysis related to the therapeutic efficacy of Kuanxiong aerosol were identified using Cytoscape and Database for Annotation, Visualization and Integrated Discovery (DAVID). To explore the mechanism of action of Kuanxiong aerosol, its in vitro effects on myocardial hypoxia, inflammatory cytokines, and oxidative injury, and its in vivo pharmacological effects on myocardial ischemia and cardiac fibrosis were studied in rat models.

RESULTS

Network pharmacology analysis revealed that the potential targets mainly include the Fas ligand (FASLG), interleukin 4 (IL4), and catalase (CAT), which mediated the processes of apoptosis, and cellular responses to hypoxia, lipopolysaccharide (LPS), reactive oxygen species (ROS), and mechanical stimulus. Multiple pathways, such as the hypoxia-inducible factor 1 (HIF1) and tumor necrosis factor (TNF) pathways were found to be closely related to the pharmacological protective mechanism of Kuanxiong aerosol against AP. In addition, Kuanxiong aerosol suppressed the hypoxia, LPS, and hydrogen peroxide (H₂O₂)-induced injuries of H9c2 cardiomyocytes through the regulation of HIF1A, suppressed expression of IL6 and TNF, and antioxidant property. In the rat model of myocardial ischemia, Kuanxiong aerosol was found to lower the creatine kinase (CK), creatine kinase-myocardial band (CK-MB), and lactate dehydrogenase (LDH) levels, without altering the hemodynamic function. Kuanxiong aerosol was capable of attenuating cardiac fibrosis and improving cardiac function in a cardiac fibrosis rat model.

CONCLUSIONS

This study revealed that the pharmacological mechanisms of Kuanxiong aerosol for AP therapy were related to anti-myocardial ischemia, anti-inflammation, and anti-oxidation via a non-hemodynamic manner, indicating that Kuanxiong aerosol is a preferable drug clinically for AP treatment due to its both preventive and protective effects.

Orthosteric and Allosteric Activation of Human 5-HT3A Receptors

The serotonin type 3 receptor (5-HT₃) is a ligand-gated ion channel that converts the binding of the neurotransmitter serotonin (5-HT) into a transient cation current that mediates fast excitatory responses in peripheral and central nervous systems. Information regarding the activation and modulation of the human 5-HT₃ type A receptor has been based only on macroscopic current measurements because of its low ion conductance. By constructing a high-conductance human 5-HT₃A receptor, we here revealed mechanistic information regarding the orthosteric activation by 5-HT and by the partial agonist tryptamine, and the allosteric activation by the terpenoids, carvacrol, and thymol. Terpenoids potentiated macroscopic currents elicited by the orthosteric agonist and directly elicited currents with slow-rising phases and submaximal amplitudes. At the single-channel level, activation by orthosteric and allosteric agonists appeared as openings in quick succession (bursts) that showed no ligand concentration dependence. Bursts were grouped into long-duration clusters in the presence of 5-HT and even longer in the presence of terpenoids, whereas they remained isolated in the presence of tryptamine. Kinetic analysis revealed that allosteric and orthosteric activation mechanisms can be described by the same scheme that includes transitions of the agonist-bound receptor to closed intermediate states before opening (priming). Reduced priming explained the partial agonism of tryptamine; however, equilibrium constants for gating and priming were similar for 5-HT and terpenoid activation. Thus, our kinetic analysis revealed that terpenoids are efficacious agonists for 5-HT₃A receptors. These findings not only extend our knowledge about the human 5-HT₃A molecular function but also provide novel insights into the mechanisms of action of allosteric ligands, which are of increasing interest as therapeutic drugs in all the superfamily.

Recent advances with 5-HT3 modulators for neuropsychiatric and gastrointestinal disorders

Serotonin (5-hydroxytryptophan [5-HT]) is a biologically active amine expressed in platelets, in gastrointestinal (GI) cells and, to a lesser extent, in the central nervous system (CNS). This biogenic compound acts through the activation of seven 5-HT receptors (5-HT_1-7 Rs). The 5-HT₃ R is a ligand-gated ion channel belonging to the Cys-loop receptor family. There is a wide variety of 5-HT₃ R modulators, but only receptor antagonists (known as setrons) have been used clinically for chemotherapy-induced nausea and vomiting and irritable bowel syndrome treatment. However, since the discovery of the setrons in the mid-1980s, a large number of studies have been published exploring new potential applications due their potency in the CNS and mild side effects. The results of these studies have revealed new potential applications, including the treatment of neuropsychiatric disorders such as schizophrenia, depression, anxiety, and drug abuse. In this review, we provide information related to therapeutic potential of 5-HT₃ R antagonists on GI and neuropsychiatric disorders. The major attention is paid to the structure, function, and pharmacology of novel 5-HT₃ R modulators developed over the past 10 years.

Inhibition of Neuromuscular Contractions of Human and Rat Colon by Bergamot Essential Oil and Linalool: Evidence to Support a Therapeutic Action

Bergamot essential oil (BEO) added to food and drink promotes a citrus flavour. Folklore suggests benefits on gastrointestinal functions but with little supporting evidence. BEO and major constituents (linalool, limonene, linalyl acetate) were therefore examined for any ability to influence neuromuscular contractions of human and rat colon. Circular muscle strips (macroscopically-normal human colon obtained following ethical approval at cancer surgery; Sprague-Dawley rats) were suspended in baths (Krebs solution; 37 °C; 5% CO₂ in O₂) for measurement of neuronally-mediated contractions (prevented by tetrodotoxin or atropine) evoked by electrical field stimulation (5 Hz, 0.5 ms pulse width, 10s/minute, maximally-effective voltage), or contractions evoked by KCl (submaximally-effective concentrations). BEO and each constituent concentration dependently inhibited neuronally-mediated and KCl-induced contractions. In human: apparent pIC₅₀ for BEO (volume/volume Krebs), respectively, 3.8 ± 0.3 and 4.4 ± 0.3; I_max 55.8% ± 4.2% and 37.5% ± 4.2%. For the constituents, the rank order of potency differed in human (linalool > limonene >> linalyl-acetate) and rat colon (linalyl-acetate > limonene = linalool), but rank order of efficacy was similar (linalool >> (BEO) = linalyl-acetate >> limonene). Thus, linalool had high efficacy but greater potency in human colon (I_max 76.8% ± 6.9%; pIC₅₀ 6.7 ± 0.2; n = 4) compared with rat colon (I_max 75.3% ± 1.9%; pIC₅₀ 5.8 ± 0.1; n = 4). The ability of BEO and linalool to inhibit human colon neuromuscular contractility provides a mechanism for use as complementary treatments of intestinal disorders.

Myorelaxant action of the Dysphania ambrosioides (L.) Mosyakin & Clemants essential oil and its major constituent α-terpinene in isolated rat trachea

This study aimed to investigate the myorelaxant action of the Dysphania ambrosioides essential oil (EODa) and its major constituent α-terpinene on tracheal smooth muscle isolated from rats. In tracheal smooth muscle ex vivo, in organ baths, isometric contractions recordings were done in order to evaluated the effect of EODa (1-1000 μg/mL) and α-terpinene (1-3000 μg/mL) on the following parameters: basal tone, contractions evoked by potassium (KCl 60 mM), acetylcholine (ACh 10 μM) or serotonin (5-HT 10 μM). The EODa and its major constituent α-terpinene, did not statistically alter basal tone; however, they induced myorelaxant effects on top of contractions induced by KCl, ACh and 5-HT. EODa and α-terpinene also inhibited the contractions induced by barium in presence of High [K+] (80 mM). The data suggest that the relaxation induced by these agents is caused by the inhibition of L-type VGCC, inhibiting the inward Ca2+ current through these channels, but does not exclude the possibility of participation of other mechanisms. Results from this study also suggest the EODa, due to their efficacy on relaxation of the respiratory tract, posses a therapeutic potential as a antispasmodic agent for respiratory tract.

Targets and pathways involved in the antitumor activity of citral and its stereo-isomers

This review provides a comprehensive analysis of the anticancer potential of the natural product citral (CIT) found in many plants and essential oils, and extensively used in the food and cosmetic industry. CIT is composed of two stereoisomers, the trans-isomer geranial being a more potent anticancer compound than the cis-isomer neral. CIT inhibits cancer cell proliferation and induces cancer cell apoptosis. Its pluri-factorial mechanism of anticancer activity is essentially based on three pillars: (i) a drug-induced accumulation of reactive oxygen species in cancer cells leading to an oxidative burst and DNA damages, (ii) a colchicine-like inhibition of tubulin polymerization and promotion of microtubule depolymerization, associated with an inhibition of the microtubule affinity-regulating kinase MARK4, and (iii) a potent inhibition of the aldehyde dehydrogenase isoform ALDH1A3 which is associated with cancer stem cell proliferation and chemoresistance. This unique combination of targets and pathways confers a significant anticancer potential. However, the intrinsic potency of CIT is limited, mainly because the drug is not very stable and has a low bioavailability and it does not present a high selectivity for cancer cells versus non-tumor cells. Stable formulations of CIT, using cyclodextrins, biodegradable polymers, or various nano-structured particles have been designed to enhance the bioavailability, to increase the effective doses window and to promote the anticancer activity. The lack of tumor cell selectivity is more problematic and limits the use of the drug in cancer therapy. Nevertheless, CIT offers interesting perspectives to design more potent analogues and drug combinations with a reinforced antitumor potential.

5-HT3 Receptor Antagonists in Neurologic and Neuropsychiatric Disorders: The Iceberg Still Lies beneath the Surface

5-HT₃ receptor antagonists, first introduced to the market in the mid-1980s, are proven efficient agents to counteract chemotherapy-induced emesis. Nonetheless, recent investigations have shed light on unappreciated dimensions of this class of compounds in conditions with an immunoinflammatory component as well as in neurologic and psychiatric disorders. The promising findings from multiple studies have unveiled several beneficial effects of these compounds in multiple sclerosis, stroke, Alzheimer disease, and Parkinson disease. Reports continue to uncover important roles for 5-HT₃ receptors in the physiopathology of neuropsychiatric disorders, including depression, anxiety, drug abuse, and schizophrenia. This review addresses the potential of 5-HT₃ receptor antagonists in neurology- and neuropsychiatry-related disorders. The broad therapeutic window and high compliance observed with these agents position them as suitable prototypes for the development of novel pharmacotherapeutics with higher efficacy and fewer adverse effects.

Evidence for an effect of receptor density on ligand occupancy and agonist EC50

Drug-receptor interaction theory predicts that proportional receptor occupancy is a function of ligand concentration as defined by a ligand-receptor affinity constant, and is independent of receptor density. However, we previously observed that the EC₅₀ of 5-HT reduced as the density of 5-HT₃ receptors increased, suggesting an effect of receptor density on occupancy. The current study was designed to maximise variability in experimentally observed currents and confirm this apparent contradiction prospectively. Xenopus oocytes were injected with RNA encoding 5-HT₃A receptors under conditions designed to achieve varying receptor expression levels and 5-HT-evoked currents measured using two electrode voltage clamp. Results from 99 oocytes showed that as the maximal peak current increased from 0.05 µA to 12.1 µA there was a 3.7-fold reduction in EC₅₀. Since occupancy and conductance are directly related in this system, this indicates that for a given concentration of 5-HT, proportional occupancy increases with increased receptor density. We conclude that normalising data masks this correlation, and can result in reduced accuracy of pharmacological measurements. We propose a mechanistic explanation for our observations.

An odorant receptor from Anopheles gambiae that demonstrates enantioselectivity to the plant volatile, linalool

Insects express chemical receptors within sensory neurons that are activated by specific cues in the environment, thereby influencing the acquisition of critical resources. A significant gap in our current understanding of insect chemical ecology is defining the molecular mechanisms that underlie sensitivity to plant-emitted volatiles. Linalool is a commonly-occurring monoterpene that has various effects on insect behavior, either acting as an attractant or a repellent, and existing in nature as one of two possible stereoisomers, (R)-(-)-linalool and (S)-(+)-linalool. In this study, we have used a cell-based functional assay to identify linalool and structurally-related compounds as ligands of Odorant receptor 29, a labellum-expressed receptor in the malaria vector mosquito, Anopheles gambiae (AgamOr29). While (R)-(-)-linalool activates AgamOr29, a mixture of the (R) and (S) stereoisomers activates the receptor with higher potency, implying enantiomeric selectivity. Orthologs of Or29 are present in the genomes of Anophelines within the Cellia subgenus. The conservation of this receptor across Anopheline lineages suggests that this ecologically important compound might serve as an attraction cue for nectar-seeking mosquitoes. Moreover, the characterization of a mosquito terpene receptor could serve as a foundation for future ligand-receptor studies of plant volatiles and for the discovery of compounds that can be integrated into push-pull vector control strategies.

Tropisetron inhibits sepsis by repressing hyper-inflammation and regulating the cardiac action potential in rat models

OBJECTIVE

The objective of the present investigation was to explore the possible effect of the 5-HT3 receptor antagonist tropisetron on the expression levels of the inflammatory factors interleukin 6 (IL-6), creatine kinase isoenzyme (CK-MB), soluble growth stimulating gene 2 protein (sST2) and immunoglobulin E (IgE), as well as the cardiac action potential in septic rats.

METHODS

The cecal ligation and perforation (CLP) method was utilized to construct abdominal infarction in rats. A total of 68 male adult Sprague Dawley rats were used, including 40 for assessing survival and 28 for detecting the expression levels of IL-6 and IgE, myocardial injury, cardiac dysfunction and the cardiac action potential. These 28 rats were divided into the sham (6 rats), sham + Tropisetron (6 rats), CLP (8 rats) and CLP + Tropisetron (8 rats) groups. Twenty-four hours after establishment of the sepsis rat model, immunohistochemistry was used to analyze 5-HT3 receptor protein expression, and enzyme-linked immunosorbent assay (ELISA) was employed to monitor the serum levels of IL-6, CKMB, sST2 and IgE. Furthermore, the structure of the myocardium in various groups was examined by H&E staining.

RESULTS

The levels of IL-6, CK-MB, sST2 and IgE in the sepsis group were significantly higher than those of the sham group (P <  0.01). Furthermore, the heart rate in the sepsis group was lower than that of the sham group (P <  0.01), and the time of atrial ventricular action potential in the sepsis group was longer than that of the sham group (P <  0.05). In addition, immunohistochemical analyses showed that the area, intensity and index of 5-HT3 receptor in the sepsis group were significantly lower than those of the sham group (P <  0.01). Importantly, the 5-HT3 receptor antagonist Tropisetron exhibited significant inhibitory effects IL-6, CK-MB, sST2 and IgE expression levels, and inductive effects on atrial ventricular action potential in the sepsis group.

CONCLUSIONS

Sepsis leads to systemic inflammatory reaction, resulting in myocardial injury, structural changes and immune imbalance. The inhibitory effect of tropisetron on inflammation, and the regulatory inflammatory disorder by the efferent vagus nerve may be one of the important mechanisms leading to cardiac electrophysiological changes in sepsis.

Natural Negative Allosteric Modulators of 5-HT₃ Receptors

Chemotherapy-induced nausea and vomiting (CINV) remain the most common and devastating side-effects associated with cancer chemotherapy. In recent decades, several lines of research emphasize the importance of 5-hydroxytryptamine3 (5-HT3; serotonin) receptors in the pathogenesis and treatment of CINV. 5-HT₃ receptors are members of ligand-gated ion channels that mediate the rapid and transient membrane-depolarizing effect of 5-HT in the central and peripheral nervous system. These receptors play important roles in nausea and vomiting, as well as regulation of peristalsis and pain transmission. The development of antagonists for 5-HT₃ receptor dramatically improved the treatment of CINV in cancer patients. In fact, the most common use of 5-HT₃ receptor antagonists to date is the treatment of nausea and vomiting. In recent years, there has been an increasing tendency to use natural plant products as important therapeutic entities in the treatment of various diseases. In this article, we examined the results of earlier studies on the actions of natural compounds on the functional properties of 5-HT₃ receptors. It is likely that these natural modulators of 5-HT₃ receptors can be employed as lead structures for the synthesis of therapeutic agents for treating CINV in future clinical studies.

Advancements in drug development for diarrhea-predominant irritable bowel syndrome

INTRODUCTION

Diarrhea-predominant irritable bowel syndrome (IBS-D) is a common disorder characterized by a complex pathophysiology hampering optimal targeted drug development. Recent advances in our understanding of key underlying mechanisms prompted novel therapeutics including novel pharmacological approaches.

AREAS COVERED

This review summarizes the latest advancements in the pipeline of IBS-D drugs focusing on new pharmacological targets, efficacy and safety of medicinal products considering the recent harmonization of regulatory requirements by the FDA and the EMA.

EXPERT OPINION

The new 5-HT₃ receptor antagonist ramosetron appears a promising therapeutic approach devoid of significant adverse events, although it is presently unavailable in Western countries, most likely because of the precautionary approach taken by regulatory agencies with this drug class. New pharmacological concepts on full agonists/antagonists, mixed-receptor activity and novel drug targets may streamline the present drug pipeline along with the adherence on new regulatory guidelines on outcome measures. Eluxadoline can be taken as an example of this paradigm shift. It has now been granted marketing authorization for IBS-D on both sides of the Atlantic, but it is still considered as a second-line agent by the NICE. There is still much work to be done to fully cover clinical needs of patients with IBS-D.

Citalopram inhibits platelet function independently of SERT-mediated 5-HT transport

Citalopram prevents serotonin (5-HT) uptake into platelets by blocking the serotonin reuptake transporter (SERT). Although some clinical data suggest that selective serotonin reuptake inhibitors (SSRIs) may affect haemostasis and thrombosis, these poorly-characterised effects are not well understood mechanistically and useful in vitro data is limited. We sought to determine whether the inhibitory effects of citalopram on platelets are mediated via its pharmacological inhibition of 5-HT transport. We quantified the inhibitory potency of (RS)-, (R)- and (S)-citalopram on platelet function. If SERT blockade is the primary mechanism for citalopram-mediated platelet inhibition, these potencies should show quantitative congruence with inhibition of 5-HT uptake. Our data show that citalopram inhibits platelet aggregation, adhesion and thromboxane production with no difference in potency between (R)- and (S)-isomers. By contrast, citalopram had a eudysmic ratio of approximately 17 (S > R) for SERT blockade. Furthermore, nanomolar concentrations of citalopram inhibited 5-HT uptake into platelets but had no effect on other platelet functions, which were inhibited by micromolar concentrations. Our data indicate that citalopram-induced inhibition of platelets in vitro is not mediated by blockade of 5-HT transport. This raises a new question for future investigation: by what mechanism(s) does citalopram inhibit platelets?