Serotonin-induced calcium signaling via 5-HT1A receptors in human leukemia (K 562) cells

Human taste thresholds are modulated by serotonin and noradrenaline

Circumstances in which serotonin (5-HT) and noradrenaline (NA) are altered, such as in anxiety or depression, are associated with taste disturbances, indicating the importance of these transmitters in the determination of taste thresholds in health and disease. In this study, we show for the first time that human taste thresholds are plastic and are lowered by modulation of systemic monoamines. Measurement of taste function in healthy humans before and after a 5-HT reuptake inhibitor, NA reuptake inhibitor, or placebo showed that enhancing 5-HT significantly reduced the sucrose taste threshold by 27% and the quinine taste threshold by 53%. In contrast, enhancing NA significantly reduced bitter taste threshold by 39% and sour threshold by 22%. In addition, the anxiety level was positively correlated with bitter and salt taste thresholds. We show that 5-HT and NA participate in setting taste thresholds, that human taste in normal healthy subjects is plastic, and that modulation of these neurotransmitters has distinct effects on different taste modalities. We present a model to explain these findings. In addition, we show that the general anxiety level is directly related to taste perception, suggesting that altered taste and appetite seen in affective disorders may reflect an actual change in the gustatory system.

Effects of an intense training on functional activity of 5-HT(1B) receptors in human peripheral blood lymphocytes

Serotonin (5-HT) is a neurotransmitter and an immune modulator. At the periphery, the serotonergic system appears to possess a regulatory activity via 5-HT 1B receptors. The present study investigated the effects of a 5-day military course following 3 weeks of combat training on the functional activity of 5-HT 1B/1D receptors in peripheral blood lymphocytes of male soldiers. The results of [35S]GTPgammaS binding assays showed that h5-HT 1B/1D receptors were desensitized after the training program, although serum 5-HT was unchanged. These data suggest the existence of a control on T cells mediated through h5-HT 1B/1D receptors leading cytokine production modulation after a physical training.

Coupling of 5-HT1A autoreceptors to inhibition of mitogen-activated protein kinase activation via G beta gamma subunit signaling

The 5-HT1A receptor is expressed presynaptically as the primary somatodendritic autoreceptor on serotonergic raphe neurons, and postsynaptically in several brain regions. Signaling of the 5-HT1A autoreceptor was studied in RN46A cells, a model of serotonergic raphe neurons that express endogenous 5-HT1A receptors. In undifferentiated RN46A cells stably transfected with the wild-type 5-HT1A receptor, 5-HT1A receptor activation inhibited forskolin-induced cyclic adenosine monophosphate (cAMP) formation (by 50%), increased [Ca2+]i, and induced a novel inhibition (up to 60%) of phospho-p42/p44-mitogen-activated protein kinase (MAPK). Upon differentiation of non-transfected or 5-HT1A-transfected RN46A cells, agonist-mediated inhibition of MAPK was enhanced. These actions were blocked by pretreatment with pertussis toxin indicating mediation via Gi/Go proteins and the calcium response was blocked by preactivation of protein kinase C (PKC). In cells overexpressing the G beta gamma scavenger carboxyl-terminal domain of G protein receptor kinase 2 (GRK-CT), 5-HT1A receptor activation inhibited cAMP formation, but coupling to calcium mobilization and inhibition of MAPK was abolished. The activity of 5-HT1A receptors containing mutations of PKC sites in the second (i2: T149A) or third intracellular loop (i3: T229A/S253G/T343A) was tested. At comparable levels of receptor expression, the signaling of the 5-HT1A i3 mutant was similar to the 5-HT1A wild-type receptor, while the i2 and quadruple (i2/i3) mutants failed to couple to G beta gamma-mediated increase in [Ca2+]i or inhibition of MAPK, but did couple to G alpha i-mediated inhibition of cAMP. Thus, the i2-domain of the 5-HT1A autoreceptor is crucial for coupling to G beta gamma subunits and their subsequent responses (e.g. calcium mobilization and inhibition of MAPK activity).

Immunohistochemical study of serotonin in lesions of chronic eczema

BACKGROUND

Inflammatory mediators participate in the pathogenesis and pathophysiology of skin disease (e.g. eczema). Although some investigators have suggested that serotonin may participate in the eczematous process, there has been little substantial evidence. Our objective was to study further the participation of serotonin in the pathogenesis of chronic eczema.

METHODS

Immunohistochemical techniques were used to study the expression of serotonin in the lesions of patients with chronic eczema.

RESULTS

There was a significantly stronger expression of serotonin in the prickle cells, sweat gland cells, sebaceous gland cells, and hair roots of the lesions in patients with chronic eczema. There was no positive expression of serotonin in normal skin.

CONCLUSIONS

Serotonin is probably important in the pathogenesis of chronic eczema. We demonstrated alterations in the expression of serotonin in chronic eczema lesions, which may in turn provide new insight into the pathogenesis of chronic eczema. Serotonin, along with cytokines, T lymphocytes, B lymphocytes, and macrophages, participates in the immune response and influences the inflammatory process by means of keratinocytes.

8-[3H]-hydroxy-2-(di-n-propylamino)tetralin binding sites in blood lymphocytes of rats and the modulation by mitogens and immobilization

Serotonin 5-HT(1A) receptors were characterized in rat resting lymphocytes obtained by cardiac puncture with the use of the ligand [3H]8-hydroxy-2-(di-n-propylamino)tetralin. Selectivity of the specific binding was demonstrated by inhibition experiments with various serotonergic and nonserotonergic drugs. The rank order of potency for inhibition was WAY-100478>pindobind>NAN-190>buspirone>imipramine>serotonin. While pimozide, desipramine, nomifensine, haloperidol and sulpiride did not inhibit the binding. Kinetic parameters calculated from saturation experiments indicated one site of interaction, with an equilibrium dissociation constant of 2.50 nM and maximum binding capacity of 487.21 nmol/10(6) cells. Complete dissociation was obtained with serotonin as the displacement agent, and equilibrium dissociation constant calculated by association and dissociation experiments was 2.03 nM. Thus, serotonin 5-HT(1A) receptors are present in resting lymphocytes. The in vivo administration of the mitogens lipopolysacharide (0.1 mg/kg, 18 h) or concanavalin A (0.2 mg/kg, 18 h) increased the number of sites. The elevation produced by the latter was of higher magnitude than that of lipopolysacharide, and two sites of the binding were determined by isotopic dilution. Immobilization stress (1 h daily for 7 days) also resulted in a significant increase of binding capacity, but was smaller than that produced by the mitogens. The affinity of binding was not affect by the treatments. The results indicate that serotonin 5-HT(1A) receptors are modulated by unspecific and specific immune system activation, as well as by a potent stress condition, which might result in relevant functional modifications in the response of rat lymphocytes.

Autologous stem cell transplantation as post-remission therapy in adult acute myelogenous leukemia: does platelet contamination of peripheral blood mobilized stem cell grafts influence the risk of leukemia relapse?

Conventional chemotherapy of acute myelogenous leukemia (AML) results in an overall long-term disease-free survival of less than 50%, but for selected subsets of younger patients the prognosis can be improved by allogeneic stem cell transplantation. The use of autologous stem cell transplantation is now investigated as an alternative to allotransplantation due to its lower risk of serious complications. However, autotransplantation is associated with a relatively high risk of post-transplant AML relapse that can be derived from contaminating leukemia cells in the autograft. Peripheral blood mobilized stem cell (PBSC) grafts usually contain a higher number of platelets. The degree of platelet contamination is determined by the peripheral blood platelet count at the time of harvesting, and the platelets become activated and release soluble mediators during the ex vivo handling of PBSC grafts. Many of these platelet-derived mediators can bind to specific receptors expressed by AML blasts, and the platelet contamination may then alter AML blast survival and thereby influence the risk of post-transplant leukemia relapse. Therefore, we conclude that the platelet contamination of autologous stem cell grafts is possibly of clinical importance, but the effect of this nonstandardized parameter is difficult to predict in individual patients because the number of graft-contaminating platelets, the degree of platelet activation, and the effects of platelet-derived mediators on AML blasts differ between patients.

5-HT3 receptor-channels coupled with Na+ influx in human T cells: role in T cell activation

The study was conducted on a human (Jurkat) T cell line, loaded with a Na+ fluorescent probe, SBFI/AM. Serotonin and an agonist of 5-HT3 receptor-channels, 2-methyl-5HT, evoked Na+ influx, whereas the agonists of other serotonergic receptor subtypes, i.e., 5-HT1A and 5-HT1B receptors, failed to induce Na+ influx in these cells. By using 3H-BRL43694, an agonist of 5-HT3 receptor-channels, we characterized 5-HT3 lymphocyte receptors which exhibited a density (Bmax) of 300 +/- 20 fmol/10(6) cells and a Kd of 30 nM in Jurkat T cells. The T-cell 5-HT3 receptor-channel is not regulated either by the protein kinase C or by the free intracellular calcium concentrations as the agents known to activate the PKC and to induce increases in intracellular free calcium concentrations failed to influence the free intracellular Na+ concentrations, [Na+]i, in these cells. Furthermore, an increase in [Na+]i, induced by 2-methyl-5HT, via 5-HT3 receptor-channels seems to stimulate T-cell activation by facilitating the progression of T cells from S to G2/M phase of the cell cycle.

Characterisation of serotonin transport mechanisms in rainbow trout peripheral blood lymphocytes: role in PHA-induced lymphoproliferation

In this study, we investigated the serotonin transport mechanisms in rainbow trout (Oncorhynchus mykiss) peripheral blood Lymphocytes. We have observed that the transport of serotonin is a membrane transport process that have the properties of a secondary active transport system. The binding isotherm of [3H]-paroxetine, a serotonin transport blocker, demonstrated a high-affinity binding site with a positive type of cooperativity, Hill coefficient being higher than unity. Known specific inhibitors of the mammalian serotonin transporter significantly inhibited the uptake process in fish lymphocytes. In order to demonstrate the physiological relevance of the serotonin transporter in T-cell activation, we conducted experiments on lymphocytes activated or not by phytohemagglutinin (PHA), a T-cell mitogen. We have observed that addition of PHA for 24hrs, increased the Vmax but not the Km of this transporter. Serotonin uptake inhibitors diminished the PHA-activated proliferation of fish lymphocytes. The intracellular concentrations of cAMP were found to regulate the serotonin uptake and the PHA-stimulated proliferation as the agents known to augment cAMP stimulated serotonin uptake, and inhibited the lymphoproliferation. Inhibitory effects of increased cAMP on the proliferation were reversed by the addition of the nanomolar concentrations of 8-OH-DPAT, a 5-HT1A receptor agonist which is known to diminish the intracellular cAMP concentrations, suggesting that serotonin also regulates PHA-induced proliferation via 5-HT1A membrane receptors in an autocrine manner. These results all together demonstrate that fish lymphocytes possess an active serotonin transporter that is implicated in the proliferation of these immunocompetent cells.

A putative alpha-helical G beta gamma-coupling domain in the second intracellular loop of the 5-HT1A receptor

We have identified a conserved threonine residue in the second intracellular (i2) loop of the 5-HT1A receptor that when mutated to alanine prevents coupling to G beta gamma-mediated signaling, while preserving G alpha i-induced actions. In this review, we investigate the characteristics and potential role of the i2 domain in the coupling of the 5-HT1A receptor and other receptors to G proteins. The i2 domain, as well as portions of the i3 domain, is predicted to form an amphipathic alpha-helix with a positively charged face and a hydrophobic face. Mutagenesis experiments support a model in which the hydrophobic faces of these alpha-helical domains form an intracellular binding "pocket" for interaction with G proteins. Embedded in the hydrophobic face, Thr 149 is crucial for signaling through G beta gamma subunits, perhaps via interaction with its hydroxyl side-chain. Mutation of other residues of the i2 domain of Gi-coupled receptors is required to substantiate the importance of the alpha-helical i2 domain in receptor-G beta gamma signaling. If confirmed in other receptors, these results support a general model in which activated receptor and G beta gamma subunits remain associated to interact with effectors in a receptor-specific manner.

5-Hydroxytryptamine-induced calcium-channel gating in rainbow trout (Oncorhynchus mykiss) peripheral blood lymphocytes

The present study was conducted on peripheral blood lympho-cytes of rainbow trout (Oncorhynchus mykiss) to assess the role of 5-hydroxytryptamine (5-HT; 'serotonin') in calcium signalling. 5-HT-induced increases in intracellular free calcium concentrations, [Ca2+]i, and its action was mediated by 5-HT receptor subtype 3 (5-HT3), but not by 5-HT receptor subtype 1A (5-HT1A) or subtype 2 (5-HT2) in these cells. In Ca2+-containing medium (1 mM CaCl2), 5-HT and 2-methyl-5-HT (5-HT3 receptor agonist) induced increases in [Ca2+]i, whereas in Ca2+-free medium (0 Ca2+, 1 mM EGTA), these two agents failed to evoke increases in [Ca2+]i in these cells, demonstrating that 5-HT mobilizes Ca2+ from the extracellular environment. Furthermore, 5-HT-induced increases in [Ca2+]i are not contributed to by the intracellular endoplasmic reticulum (ER) pool, as thapsigargin, an agent that recruits Ca2+ from ER stores, had additive effects on 5-HT-induced [Ca2+]i responses in fish peripheral lymphocytes. 5-HT-induced increases in [Ca2+]i were mediated by 5-HT3 receptors via gating the calcium through L-type, but not N-type, calcium channels in trout lymphocytes.

Identification of lymphocyte 5-HT3 receptor subtype and its implication in fish T-cell proliferation

In the present study, we identified the serotonergic receptor of type 3 (5-HT3) on the lymphocytes of a teleost fish, Oncorhynchus mykiss. In the pharmacological studies on the binding of [3H]serotonin to membrane receptor sites, 2-methyl-5-HT, an agonist of 5-HT3 receptors, displaced the binding of [3H]serotonin to fish lymphocytes, indicating the presence of 5-HT3 receptors on these cells. The known antagonists of the mammalian 5-HT3 receptor, ICS-205-930 and metoclopramide, failed to displace [3H]serotonin binding to lymphocytes during the period of association equilibrium (8 min); however, these antagonists progressively displaced [3H]serotonin binding from 10 to 40 min of incubation. These results suggest that fish 5-HT3 lymphocyte receptors may differ pharmacologically from mammalian receptors. As mammalian 5-HT3 receptors are coupled with Na+ inward movements, we undertook a study on Na+ influx by using SBFI/AM, a fluorescent probe. In SBFI/AM loaded fish lymphocytes, 2-methyl-5-HT leaked Na+ inward movements. Prior incubation of lymphocytes for 30 min in the presence of 5-HT3 antagonists, ICS-205-930, metoclopramide and MDL-72222, curtailed significantly the Na+ influx evoked by 2-methyl-5-HT, demonstrating that Na+ is leaked into fish lymphocytes via the 5-HT3 receptor-channel whose functioning is blocked by these antagonists. Furthermore, 2-methyl-5-HT exerted immunosuppressive effects in a dose dependent manner on fish T-lymphocytes stimulated by phytohaemagglutinin (PHA). Serotonin and 2-methyl-5-HT blocked the cell cycle progression of PHA-stimulated T-cells from G0/G1 to S phase. The immunosuppressive effects of 2-methyl-5-HT on T-cells were partially reversed by the antagonists, metoclopramide and ICS-205-930; however, the latter antagonist at high concentrations synergized with the immunosuppressive effects of 2-methyl-5-HT. These results demonstrate that the fish lymphocyte 5-HT3 receptor, which may be pharmacologically different from mammalian receptor subtype, is functionally implicated in fish T-cell proliferation.

Serotonin modulation of lymphocyte proliferation via 5-HT1A receptors in rainbow trout (Oncorhynchus mykiss)

In this study, the effects of serotonin (5-HT) on in vitro lymphoproliferation in rainbow trout (Oncorhynchus mykiss) are investigated. Serotonin exerted immunosuppressive effects on lipopolysaccharide (LPS) and phytohemagglutinin (PHA)-stimulated proliferation of fish peripheral blood lymphocytes (PBL). 8-OH-DPAT (an agonist of 5-HT1A receptors) mimicked the inhibitory effects of serotonin on lymphocyte proliferation, whereas addition of spiperone (an antagonist of 5-HT1A and 5-HT1B receptors) reversed these inhibitory effects, indicating that 5-HT1A receptors may be implicated in serotonin-induced immunosuppression. Furthermore, in this study the serotonergic receptors present on fish peripheral lymphocytes were characterized. A Scatchard plot of serotonin binding to fish lymphocytes followed the 'bell' shape curve with a Bmax of 0.63 microM and a Kd of 1.54 x 10(-8) M/10(6) cells. These results demonstrate the presence of positive-type co-operation among receptor populations. In a displacement study, serotonin inhibited the binding of 3H-5HT to the receptor sites both in resting and LPS/PHA-stimulated trout lymphocytes. Interestingly, the agonists (8-OH-DPAT and buspirone) and antagonist (NAN-190) of the 5-HT1A receptor subtype failed to displace 3H-5HT binding to receptor sites in resting cells, whereas these agents inhibited 3H-5HT binding in LPS- and PHA-stimulated lymphocytes significantly, suggesting that after mitogenic stimulation, 5-HT1A receptors are expressed on lymphocytes. CGS-12066B (an agonist of 5-HT1B receptors) failed to influence significantly 3H-5HT binding to receptor sites both in resting and mitogen-stimulated lymphocytes, indicating that the 5-HT receptor subpopulation is not expressed either on resting or on LPS- or PHA-stimulated lymphocytes. Taken together, these results suggest that trout peripheral blood lymphocytes express functional serotonergic receptors, and 5-HT1A receptors, which are not expressed by resting lymphocytes, are expressed after mitogenic stimulation and implicated in the inhibition of mitogenic (LPS and PHA) responses.