The 5-HT3B subunit confers spontaneous channel opening and altered ligand properties of the 5-HT3 receptor

Current receptor theory suggests that there is an equilibrium between the inactive (R) and active (R*) conformations of ligand-gated ion channels and G protein-coupled receptors. The actions of ligands in both receptor types could be appropriately explained by this two-state model. Ligands such as agonists and antagonists affect receptor function by stabilizing one or both conformations. The 5-HT3 receptor is a member of the Cys-loop ligand-gated ion channel superfamily participating in synaptic transmission. Here we show that co-expression of the 5-HT3A and 5-HT3B receptor subunits in the human embryonic kidney (HEK) 293 cells results in a receptor that displays a low level of constitutive (or agonist-independent) activity. Furthermore, we also demonstrate that the properties of ligands can be modified by receptor composition. Whereas the 5-hydroxytryptamine (5-HT) analog 5-methoxyindole is a partial agonist at the 5-HT3A receptor, it becomes a "protean agonist" (functioning as an agonist and an inverse agonist at the same receptor) at the 5-HT3AB receptor (after the Greek god Proteus, who was able to change his shape and appearance at will). In addition, the 5-HT analog 5-hydroxyindole is a positive allosteric modulator for the liganded active (AR*) conformation of the 5-HT3A and 5-HT3AB receptors and a negative allosteric modulator for the spontaneously active (R*) conformation of the 5-HT3AB receptor, suggesting that the spontaneously active (R*) and liganded active (AR*) conformations are differentially modulated by 5-hydroxyindole. Thus, the incorporation of the 5-HT3B subunit leads to spontaneous channel opening and altered ligand properties.     

Characterisation of Human 5-Hydroxytryptamine2A and 5-Hydroxytryptamine2C Receptors Expressed in the Human Neuroblastoma Cell Line SH-SY5Y: Comparative Stimulation by Hallucinogenic Drugs

Abstract: Stable transfection of the human neuroblastoma cell line SH-SY5Y with the human 5-hydroxytryptamine_2A (5-HT_2A) or 5-HT_2C receptor cDNA produced cell lines demonstrating ligand affinities that correlated closely with those for the corresponding endogenous receptors in human frontal cortex and choroid plexus, respectively. Stimulation of the recombinant receptors by 5-HT induced phosphoinositide hydrolysis with higher potency but lower efficacy at the 5-HT_2C receptor (pEC₅₀ = 7.80 ± 0.06) compared with the 5-HT_2A receptor (pEC₅₀ = 7.30 ± 0.08). Activation of the 5-HT_2A receptor caused a transient fourfold increase in intracellular Ca²⁺ concentration. Whole-cell recordings of cells clamped at ?50 mV demonstrated a small inward current (2 pA) in response to 10 µM 5-HT for both receptors. There were no differences in potency or efficacy of phosphoinositide hydrolysis among four hallucinogenic [d-lysergic acid diethylamide (LSD), 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI), 5-methoxy-N,N-dimethyltryptamine, and mescaline] and three nonhallucinogenic drugs (m-chlorophenylpiperazine, quipazine, and ergotamine). Comparison of equipotent doses producing 20% of the maximal response induced by 5-HT revealed selective activation of the 5-HT_2A receptor by LSD and to a lesser degree by DOI, mescaline, and ergotamine. Quipazine and 5-methoxy-N,N-dimethyltryptamine were relatively nonselective, whereas m-chlorophenylpiperazine selectively activated the 5-HT_2C receptor. It is unlikely therefore that hallucinosis is mediated primarily by activity at the 5-HT_2C receptor, whereas activity at the 5-HT_2A receptor may represent an important but not unique mechanism associated with hallucinogenic drug action.     

Which agonist properties are important for the activation of 5-HT3A receptors?

Purpose: Why do anesthetics not activate excitatory ligand-gated ion channels such as 5-HT₃ receptors in contrast to inhibitory ligand-gated ion channels? This study examines the actions of structural closely-related 5-HT derivatives and 5-HT constituent parts on 5-HT_3A receptors with the aim of finding simpler if not minimal agonists and thus determining requirements for successful agonist action. Experimental approach: Responses to 5-HT derivatives of human 5-HT_3A receptors stably expressed in HEK 293 cells have been examined with the patch-clamp technique in the outside-out configuration combined with a fast solution exchange system. Results: Phenol, pyrrole and alkyl amines, constituents of 5-HT, even at high concentrations, cannot activate 5-HT_3A receptors but they can inhibit them. To date, tyramines are the smallest known agonists. However, an aromatic ring is not required for activation as acetylcholine is also an agonist of similar strength. Conclusion: Simultaneous interactions of adequate strength at two separate subsites within the 5-HT binding domain appear to be essential for successful agonist function. Anesthetics either fail to achieve this or the activation they produce is so weak that it is masked by a comparatively very strong inhibition.     

Signaling transduction regulated by 5-hydroxytryptamine 1A receptor and orexin receptor 2 heterodimers

As G-protein-coupled receptors (GPCRs), 5-hydroxytryptamine 1A receptor (5-HT1AR) and orexin receptor 2 (OX2R) regulate the levels of the cellular downstream molecules. The heterodimers of different GPCRs play important roles in various of neurological diseases. Moreover, 5-HT1AR and OX2R are involved in the pathogenesis of neurological diseases such as depression with deficiency of hippocampus plasticity. However, the direct interaction of the two receptors remains elusive. In the present study, we firstly demonstrated the heterodimer formation of 5-HT1AR and OX2R. Exchange protein directly activated by cAMP (Epac) cAMP bioluminescence resonance energy transfer (BRET) biosensor analysis revealed that the expression levels of cellular cAMP significantly increased in HEK293T cells transfected with the two receptors compared with the 5-HT1AR group. Additionally, the cellular level of calcium was upregulated robustly in HEK293T cells co-transfected with 5-HT1AR and OX2R group after agonist treatment. Furthermore, western blotting data showed that 5-HT1AR and OX2R heterodimer decreased the levels of phosphorylation of extracellular signal-regulated kinase (ERK) and cAMP-response element-binding protein (CREB). These results not only unraveled the formation of 5-HT1AR and OX2R heterodimer but also suggested that the heterodimer affected the downstream signaling pathway, which will provide new insights into the function of the two receptors in the brain.     

5-HT2 Receptors Facilitate JC Polyomavirus Entry

The human JC polyomavirus (JCPyV) causes the rapidly progressing demyelinating disease progressive multifocal leukoencephalopathy (PML). The disease occurs most often in individuals with AIDS but also occurs in individuals receiving immunomodulatory therapies for immune-related diseases such as multiple sclerosis. JCPyV infection of host cells requires the pentasaccharide lactoseries tetrasaccharide c (LSTc) and the serotonin receptor 5-hydroxytryptamine (5-HT) receptor 5-HT_2AR. While LSTc is involved in the initial attachment of virus to cells via interactions with VP1, the mechanism by which 5-HT_2AR contributes to infection is not clear. To further define the roles of serotonin receptors in infection, HEK293A cells, which are poorly permissive to JCPyV, were transfected with 14 different isoforms of serotonin receptor. Only 5-HT₂ receptors were found to support infection by JCPyV. None of the other 11 isoforms of serotonin receptor supported JCPyV infection. Expression of 5-HT₂ receptors did not increase binding of JCPyV to cells, but this was not unexpected, given that the cells uniformly expressed the major attachment receptor, LSTc. Infection of these cells remained sensitive to inhibition with soluble LSTc, confirming that LSTc recognition is required for JCPyV infection. Virus internalization into HEK293A cells was significantly and specifically enhanced when 5HT₂ receptors were expressed. Taken together, these data confirm that the carbohydrate LSTc is the attachment receptor for JCPyV and that the type 2 serotonin receptors contribute to JCPyV infection by facilitating entry.     

5-HT(2A) and D(2) receptor blockade increases cortical DA release via 5-HT(1A) receptor activation: a possible mechanism of atypical antipsychotic-induced cortical dopamine release

Atypical antipsychotic drugs (APDs), all of which are relatively more potent as serotonin (5-HT)(2A) than dopamine D(2) antagonists, may improve negative symptoms and cognitive dysfunction in schizophrenia, in part, via increasing cortical dopamine release. 5-HT(1A) agonism has been also suggested to contribute to the ability to increase cortical dopamine release. The present study tested the hypothesis that clozapine, olanzapine, risperidone, and perhaps other atypical APDs, increase dopamine release in rat medial prefrontal cortex (mPFC) via 5-HT(1A) receptor activation, as a result of the blockade of 5-HT(2A) and D(2) receptors. M100907 (0.1 mg/kg), a 5-HT(2A) antagonist, significantly increased the ability of both S:(-)-sulpiride (10 mg/kg), a D(2) antagonist devoid of 5-HT(1A) affinity, and R:(+)-8-OH-DPAT (0.05 mg/kg), a 5-HT(1A) agonist, to increase mPFC dopamine release. These effects of M100907 were abolished by WAY100635 (0.05 mg/kg), a 5-HT(1A) antagonist, which by itself has no effect on mPFC dopamine release. WAY100635 (0.2 mg/kg) also reversed the ability of clozapine (20 mg/kg), olanzapine (1 mg/kg), risperidone (1 mg/kg), and the R:(+)-8-OH-DPAT (0.2 mg/kg) to increase mPFC dopamine release. Clozapine is a direct acting 5-HT(1A) partial agonist, whereas olanzapine and risperidone are not. These results suggest that the atypical APDs via 5-HT(2A) and D(2) receptor blockade, regardless of intrinsic 5-HT(1A) affinity, may promote the ability of 5-HT(1A) receptor stimulation to increase mPFC DA release, and provide additional evidence that coadministration of 5-HT(2A) antagonists and typical APDs, which are D(2) antagonists, may facilitate 5-HT(1A) agonist activity.     

Long-term Stress with Hyperglucocorticoidemia-induced Hepatic Steatosis with VLDL Overproduction Is Dependent on both 5-HT2 Receptor and 5-HT Synthesis in Liver

Hepatic triglycerides production and adipose lipolysis are pivotal for long-term stress (LTS) or hyperglucocorticoidemia-induced insulin resistance. 5-hydroxytryptamine (5-HT) has been demonstrated to induce hepatic lipid metabolic abnormality by activating mammalian target of rapamycin (mTOR). In present study, we explored whether 5-HT is involved in LTS effects in liver using restraint stress-exposed rats and cultured primary rat hepatocytes and HepG2 cells. LTS with hyperglucocorticoidemia induced hepatic 5-HT synthetic increase with tryptophan hydroxylase 1 (Tph1) up-regulation, and 5-HT2 receptor (5-HT₂R, including 5-HT2A, 2B receptor) up-regulation in liver and visceral adipose, as well as hepatic mTOR activation with triglycerides and VLDL overproduction with steatosis, and visceral adipose lipolytic increase with high blood free fatty acids (FFAs) level. 5-HT exposure exhibited LTS-like effects in both tissues, and both LTS and 5-HT effects could be abolished significantly by blocking 5-HT₂R. In HepG2 cells dexamethasone or palmitate-induced mTOR activation with triglycerides and VLDL overproduction were accompanied by up-regulations of 5-HT synthesis and 5-HT₂R, which were significantly abolished by gene silencing Tph1 or 5-HT₂R and were almost fully abolished by co-silencing of both, especially on VLDL overproduction. Chemical inhibition of Tph1 or/and 5-HT₂R in both hepatocytes exhibited similar abolishment with genetic inhibition on dexamethason-induced effects. 5-HT-stimulated effects in both hepatocytes were fully abolished by blocking 5-HT₂R, while 5-HT itself also up-regulated 5-HT₂R. In conclusion, up-regulated hepatic 5-HT synthesis and 5-HT₂R induced by both glucocorticoid and FFAs are crucial for LTS-induced hepatic steatosis with VLDL overproduction, while 5-HT by acting on 5-HT₂R mediates mTOR activation in liver.     

Design and synthesis of novel N-sulfonyl-2-indoles that behave as 5-HT6 receptor ligands with significant selectivity for D3 over D2 receptors

All clinically-used antipsychotics display similar affinity for both D₂ (D2R) and D₃ (D3R) receptors, and they likewise act as 5-HT_2A receptor antagonists. They provide therapeutic benefit for positive symptoms, but no marked or consistent improvement in neurocognitive, social cognitive or negative symptoms. Since blockade of D₃ and 5-HT₆ (5-HT6R) receptors enhances neurocognition and social cognition, and potentially improves negative symptoms, a promising approach for improved treatment for schizophrenia would be to develop drugs that preferentially act at D3R versus D2R and likewise recognize 5-HT6R. Starting from the high affinity 5-HT6R ligands I and II, we identified compounds 11a and 14b that behave as 5-HT6R ligands with significant selectivity for D3R over D2R.     

Dynamic modulation of FGFR1–5-HT1A heteroreceptor complexes. Agonist treatment enhances participation of FGFR1 and 5-HT1A homodimers and recruitment of β-arrestin2

New findings show that neurotrophic and antidepressant effects of 5-HT in brain can, in part, be mediated by activation of the 5-HT1A receptor protomer in the hippocampal and raphe FGFR1–5-HT1A heteroreceptor complexes enhancing the FGFR1 signaling. The dynamic agonist modulation of the FGFR1–5-HT1A heteroreceptor complexes and their recruitment of β-arrestin is now determined in cellular models with focus on its impact on 5-HT1AR and FGFR1 homodimerization in the heteroreceptor complexes based on BRET² assays. The findings show that coagonist treatment with 8-OH-DPAT and FGF2 but not treatment with the 5-HT1A agonist alone markedly increases the BRETmax values and significantly reduces the BRET50 values of 5HT1A homodimerization. The effects of FGF2 or FGF20 with or without the 5-HT1A agonist were also studied on the FGFR1 homodimerization of the heteroreceptor complexes. FGF2 produced a marked and rapid increase in FGFR1 homodimerization which partially declined over a 10 min period. Cotreatment with FGF2 and 5-HT1A agonist blocked this decline in FGFR1 homodimerization. Furthermore, FGF2 alone produced a small increase in the BRET² signal from the 5-HT1A-β-arrestin2 receptor–protein complex which was additive to the marked effect of 8-OH-DPAT alone. Taken together, the participation of 5-HT1A and FGFR1 homodimers and recruitment of β-arrestin2 was demonstrated in the FGFR1–5-HT1A heteroreceptor complexes upon agonist treatments.     

Hallucinogens recruit specific cortical 5-HT(2A) receptor-mediated signaling pathways to affect behavior

Hallucinogens, including mescaline, psilocybin, and lysergic acid diethylamide (LSD), profoundly affect perception, cognition, and mood. All known drugs of this class are 5-HT(2A) receptor (2AR) agonists, yet closely related 2AR agonists such as lisuride lack comparable psychoactive properties. Why only certain 2AR agonists are hallucinogens and which neural circuits mediate their effects are poorly understood. By genetically expressing 2AR only in cortex, we show that 2AR-regulated pathways on cortical neurons are sufficient to mediate the signaling pattern and behavioral response to hallucinogens. Hallucinogenic and nonhallucinogenic 2AR agonists both regulate signaling in the same 2AR-expressing cortical neurons. However, the signaling and behavioral responses to the hallucinogens are distinct. While lisuride and LSD both act at 2AR expressed by cortex neurons to regulate phospholipase C, LSD responses also involve pertussis toxin-sensitive heterotrimeric G(i/o) proteins and Src. These studies identify the long-elusive neural and signaling mechanisms responsible for the unique effects of hallucinogens.     

Serotonin 4 receptor (5-HT4R) internalization is isoform-specific: Effects of 5-HT and RS67333 on isoforms A and B

Serotonin 4 receptors (5-HT4Rs) are particularly abundant within the limbic system, where they constitute potential targets for the development of novel, rapid acting antidepressants. However, the population of limbic 5-HT4Rs is not homogenous, comprising various isoforms of which 5-HT4(a) and 5-HT4(b) are among the most abundant variants. Sequence divergence at their C-termini is predictive of specificity in isoform signalling and regulation, but the differences, if any, remain ill-defined. The present study compared isoforms 5-HT4(a) and 5-HT4(b) in their ability to undergo endocytic regulation following exposure to 5-HT and to the putatively fast acting antidepressant RS67333. Both ligands differed in their ability to induce internalization of either isoform, 5-HT being more effective than RS67333 in HEK293 cells and in neurons. In contrast, trafficking induced by 5-HT was isoform-specific. In particular, while PKC, GRK2 and βarrestin were necessary for 5-HT4(a)R internalization, sequestration of 5-HT4(b)Rs required PKC but not GRK2 and relied significantly less on βarrestin. After endocytosis, isoform (b) appeared scattered throughout the intracellular compartment and efficiently recycled to the membrane upon agonist removal. Isoform (a) accumulated in the perinuclear compartment and displayed little recycling. Isoform-specific subcellular distribution was present in HEK293 cells and in neurons. In neurons, where internalization by RS67333 was more pronounced than in HEK293 cells, receptors internalized by this ligand followed the same distribution pattern as observed with 5-HT. These results point to isoform-related differences in the way that 5-HTRs respond to different ligands. Such diversity should be taken into account when developing therapeutic agents that target 5-HT4Rs.     

Constitutive dimerization of human serotonin 5-HT4 receptors in living cells

Serotonin 5-HT4 receptor isoforms are G protein-coupled receptors (GPCRs) with distinct pharmacological properties and may represent a valuable target for the treatment of many human disorders. Here, we have explored the process of dimerization of human 5-HT4 receptor (h5-HT4R) by means of co-immunoprecipitation and bioluminescence resonance energy transfer (BRET). Constitutive h5-HT4(d)R dimer was observed in living cells and membrane preparation of CHO and HEK293 cells. 5-HT4R ligands did not influence the constitutive energy transfer of the h5-HT4(d)R splice variant in intact cells and isolated plasma membranes. In addition, we found that h5-HT4(d)R and h5-HT4(g)R which structurally differ in the length of their C-terminal tails were able to form constitutive heterodimers independently of their activation state. Finally, we found that coexpression of h5-HT4R and beta2-adrenergic receptor (beta2AR) led to their heterodimerization. Given the large number of h5-HT4R isoforms which are coexpressed in a same tissue, our results points out the complexity by which this 5-HTR sub-type mediates its biological effects.     

Functional characterization of the novel antipsychotic iloperidone at human D2, D3, alpha 2C, 5-HT6, and 5-HT1A receptors

Iloperidone has demonstrated an interesting monoamine receptor profile in radioligand binding studies, with nanomolar affinity for certain noradrenaline, dopamine, and serotonin receptors. In this study, the agonist/antagonist activity of iloperidone was determined in cell lines expressing recombinant human D(2A), D(3), alpha(2C), 5-HT(1A), or 5-HT(6) receptors. With the exception of 5-HT(6) receptors, these receptors are negatively coupled to cyclase. Thus, after stimulation with forskolin, the agonists dopamine (at D(2A) and D(3)), noradrenaline (at alpha(2C)), or 8-OH-DPAT (at 5-HT(1A)) induced a reduction in cAMP accumulation. Conversely, activation of the 5-HT(6) receptor by 5-HT led to an increase in cAMP accumulation. Iloperidone alone was devoid of significant agonist activity but inhibited the agonist response in all 5 cell lines in a surmountable and concentration-dependent fashion. Iloperidone was most potent at D(3) receptors (pK(B) 8.59 +/- 0.20; n = 6), followed by alpha(2C) (pK(B) 7.83 +/- 0.06; n = 15), 5-HT(1A) (pK(B) 7.69 +/- 0.18; n = 10), D(2A) (pK(B) 7.53 +/- 0.04; n = 11) and 5-HT(6) (pK(B) 7.11 +/- 0.08; n = 11) receptors.     

Novel, flexible, and conformationally defined analogs of gepirone: synthesis and 5-HT1A, 5-HT2A, and D2 receptor activity

Novel, flexible arylpiperazine gepirone analogs (1a-3a) with a mixed 5-HT1A/5-HT2A receptor profile, low D2 receptor affinity, and agonistic (2a) or partial agonistic (1a, 3a) activity toward 5-HT1A receptor sites were synthesized. Their conformationally restricted counterparts (1b-3b) were selective 5-HT1A ligands (over 5-HT2A and D2 receptors), which turned out to be agonists (2b, 3b), or partial agonist (1b) of 5-HT1A receptors.     

5-HT receptors on identified Lymnaea neurones in culture. Pharmacological characterization of 5-HT1A receptors

The ability of the selective 5-HT_1A receptor agonist R(+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) to bind with 5-HT receptor(s) on cultured, identified neurones in Lymnaea stagnalis was examined. The identified neurones studied were from the buccal ganglia and the serotonin-containing cerebral giant cells (CGCs). 5-HT and its agonists were applied from puffer pipettes, whilst 5-HT antagonists were applied in the bathing medium. At 10⁻³ M, the 5-HT_1A agonist, always produced paroxysmal depolarizing shifts (PDS) while at a lower concentration (10⁻⁴ M), it always mimicked the effects of 10⁻³ M 5-HT. 8-OH-DPAT (10⁻⁴ M) and 5-HT 10⁻³ M produced dose-dependent increases in the responses they evoked. At 10⁻⁴ M, the 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide hydrochloride (m-CPBG), failed to hyperpolarize most of the neurones hyperpolarized by 5-HT. At 10⁻⁴ M, the antagonists ketanserin (5-HT₂), MDL 72222 (5-HT₃), and pindobind-5-HT_1A (5-HT_1A) consistently abolished spike generation ii spontaneously active neurones. Both ketanserin and MDL 72222 failed to block the actions of 8-OH-DPAT and only partially blocked those of 5-HT, but pindobind-5-HT_1A completely, but reversibly,blocked the 8-OH-DPAT effects while greatly reducing those of 5-HT. These results suggest that 5-HT_1A receptor subtypes might be involved in the hyperpolarizing responses of the CGCs and their follower motor neurones in the buccal ganglia of Lymnaea stagnalis to 5-HT. The presence of 5-HT_1A receptors on these neurones can be considered to correspond with those found in mammals because their pharmacological responses resemble those of mammalian 5-HT_1A receptors.     

ALEPH-2, a suspected anxiolytic and putative hallucinogenic phenylisopropylamine derivative, is a 5-HT2a and 5-HT2c receptor agonist

To assess the pharmacodynamic profile of ALEPH-2, a phenylisopropylamine derivative with alleged anxiolytic and hallucinogenic properties, Xenopus laevis oocytes were microinjected with either of the rat cRNA for the 5-HT2A or the 5-HT2C receptor. Concentration-response curves were obtained following the exposure of the oocytes to varying concentrations of either ALEPH-2 or 5-hydroxytryptamine (5-HT) for 10 s. ALEPH-2 is a partial agonist on the 5-HT2A receptor with a similar potency to 5-HT. In contrast, ALEPH-2 is a full 5-HT2C receptor agonist and is about 15-fold less potent than 5-HT. Pre-application of 1 microM ritanserin antagonized the responses induced by 5-HT and ALEPH-2 to the same extent; however, the 5-HT2A receptor is more sensitive to ritanserin blockade than the 5-HT2C receptor.     

Dopamine D2 receptor-mediated Akt/PKB signalling: initiation by the D2S receptor and role in quinpirole-induced behavioural activation

The short and long isoforms of the dopamine D2 receptor (D2S and D2L respectively) are highly expressed in the striatum. Functional D2 receptors activate an intracellular signalling pathway that includes a cAMP-independent route involving Akt/GSK3 (glycogen synthase kinase 3). To investigate the Akt/GSK3 response to the seldom-studied D2S receptor, we established a rat D2S receptor-expressing cell line [HEK (human embryonic kidney)-293/rD2S]. We found that in HEK-293/rD2S cells, the D2/D3 agonists bromocriptine and quinpirole significantly induced Akt and GSK3 phosphorylation, as well as ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. The D2S receptor-induced Akt signals were profoundly inhibited by the internalization blockers monodansyl cadaverine and concanavalin A. Activation of the D2S receptor in HEK-293/rD2S cells appeared to trigger Akt/phospho-Akt translocation to the cell membrane. In addition to our cell culture experiments, we studied D2 receptor-dependent Akt in vivo by systemic administration of the D2/D3 agonist quinpirole. The results show that quinpirole evoked Akt-Ser⁴⁷³ phosphorylation in the ventral striatum. Furthermore, intra-accumbens administration of wortmannin, a PI3K (phosphoinositide 3-kinase) inhibitor, significantly suppressed the quinpirole-evoked behavioural activation. Overall, we demonstrate that activation of the dopamine D2S receptor stimulates Akt/GSK3 signalling. In addition, in vivo Akt activity in the ventral striatum appears to play an important role in systemic D2/D3 agonist-induced behavioural activation.     

Serotonin and its 5-HT2 receptor agonist DOI hydrochloride inhibit the oxidative burst in total leukocytes but not in isolated neutrophils

AimsSerotonin (5-HT) is capable of reducing the oxidative burst of professional phagocytes. In this study, we investigated whether 5-HT mediates this modulation via 5-HT receptors (5-HTR) or whether this is due instead to 5-HT antioxidative properties.Main methodsThe leukocytes or polymorphonuclear leukocytes (PMNL) were isolated from human blood, and their ability to produce reactive oxygen species (ROS) after 5-HT or its agonist treatment was tested by luminol-enhanced chemiluminescence (CL) analysis.Key findingsIt was found that 5-HTR₂ agonist DOI hydrochloride does not have any antioxidative properties, despite its ability to inhibit the CL response of activated human total leukocytes. On the other hand, DOI hydrochloride was unable to inhibit the CL response of activated human PMNL. It seems that the reduction of the oxidative burst of professional phagocytes was evoked by the activation of 5-HTR not on the neutrophil surface but on the surface of different leukocytes, which produced anti-inflammatory cytokines with NADPH oxidase activity modulating properties.SignificancePlatelets and activated PMNL are in tight contact at sites of inflammation. 5-HT released from platelets might have a protective function against PMNL-derived oxidative stress and oxidative damages.     

Inotropic effects of prokinetic agents with 5-HT(4) receptor agonist actions on human isolated myocardial trabeculae

AimsBesides acting as gastrointestinal prokinetic agents, 5-hydroxytryptamine₄ (5-HT₄) receptor agonists can induce positive inotropism in human isolated atrium, but not in ventricles. We pharmacologically evaluated the gastroprokinetic 5-HT₄ receptor agonists tegaserod, prucalopride, R199715, cisapride, the cisapride metabolite norcisapride, and the 5-HT₃ receptor agonist MKC773 on human isolated myocardial trabeculae, and compared their effects with those induced by 5-HT and 5-methoxytryptamine (5-MeOT).Main methodsAtrial and ventricular trabeculae were paced and changes in contractile force were studied in the absence or presence of the 5-HT₄ receptor antagonist GR113808. Partial agonism was assessed using 5-HT₄ receptor agonists as antagonists against 5-HT. To test the contribution of L-type calcium channels, the inotropic responses to 5-HT and 5-MeOT were studied in the absence or presence of verapamil.Key findingsLike 5-HT and 5-MeOT, cisapride and tegaserod, but not prucalopride, R19971 and MKC­733, induced concentration-dependent positive inotropic responses on atrial trabeculae, which were abolished by GR113808. The L-type calcium channel blocker verapamil attenuated inotropic responses to 5-HT and 5-MeOT. None of the agonists affected the contraction of left ventricular trabeculae. Concentration response curves to 5-HT were shifted to the right in the presence of prucalopride, cisapride, tegaserod and R199715, but not MKC-773.SignificanceWe conclude that (i) inotropic responses to 5-HT and 5-MeOT seem to depend on L-type calcium channels, (ii) tegaserod and cisapride behave as partial 5-HT₄ receptor agonists, while prucalopride, norcisapride and MKC-733 cause no significant effects on human atrial trabeculae, (iii) R199715 seems to behave as a 5-HT₄ receptor antagonist.