Potential Mechanisms Involved in the Negative Coupling Between Serotonin 5-HT1A Receptors and Carbachol-Stimulated Phosphoinositide Turnover in the Rat Hippocampus

Serotonin 5-HT1A receptors have been reported to be negatively coupled to muscarinic receptor-stimulated phosphoinositide turnover in the rat hippocampus. In the present study, we have investigated further the pharmacological specificity of this negative control and attempted to elucidate the mechanism whereby 5-HT1A receptor activation inhibits the carbachol-stimulated phosphoinositide response in immature or adult rat hippocampal slices. Various 5-HT1A receptor agonists were found to inhibit carbachol (10 microM)-stimulated formation of total inositol phosphates in immature rat hippocampal slices with the following rank order of potency (IC50 values in nM): 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (11) greater than ipsapirone (20) greater than gepirone (120) greater than RU 24969 (140) greater than buspirone (560) greater than 1-(m-trifluoromethylphenyl)piperazine (1,500) greater than methysergide (5,644); selective 5-HT1B, 5-HT2, and 5-HT3 receptor agonists were inactive. The potency of the 5-HT1A receptor agonists investigated as inhibitors of the carbachol response was well correlated (r = 0.92) with their potency as inhibitors of the forskolin-stimulated adenylate cyclase in guinea pig hippocampal membranes. 8-OH-DPAT (10 microM) fully inhibited the carbachol-stimulated formation of inositol di-, tris-, and tetrakisphosphate but only partially antagonized (-40%) inositol monophosphate production. The effect of 8-OH-DPAT on carbachol-stimulated phosphoinositide turnover was not prevented by addition of tetrodotoxin (1 microM), by prior destruction of serotonergic afferents, by experimental manipulations causing an increase in cyclic AMP levels (addition of 10 microM forskolin), or by changes in membrane potential (increase in K+ concentration or addition of tetraethylammonium). Prior intrahippocampal injection of pertussis toxin also failed to alter the ability of 8-OH-DPAT to inhibit the carbachol response. Carbachol-stimulated phosphoinositide turnover in immature rat hippocampal slices was inhibited by the protein kinase C activators phorbol 12-myristate 13-acetate (10 microM) and arachidonic acid (100 microM). Moreover, the inhibitory effect of 8-OH-DPAT on the carbachol response was blocked by 10 microM quinacrine (a phospholipase A2 inhibitor) but not by BW 755C (100 microM), a cyclooxygenase and lipoxygenase inhibitor. These results collectively suggest that 5-HT1A receptor activation inhibits carbachol-stimulated phosphoinositide turnover by stimulating a phospholipase A2 coupled to 5-HT1A receptors, leading to arachidonic acid release. Arachidonic acid could in turn activate a gamma-protein kinase C with as a consequence an inhibition of carbachol-stimulated phosphoinositide turnover. This inhibition may be the consequence of a phospholipase C phosphorylation and/or a direct effect on the muscarinic receptor.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of 5-HT2-selective agonists on cat platelet aggregation

The effects of the 5-HT2-sselective agonists 1-(4-bromo-2, 5-dimethoxyphenyl) -2- aminopropane (DOB) and 1- (2, 5-dimethenyl4-iodophenyl) -2- aminopropane (DOI) on cat platelet aggregation were investigated and compared with those produced by serotonin (5-HT) and a positional isomer of DOB (i.e., isoDOB). Serotonin, DOB, and DOI enhanced the aggregation of platelets induced by a suboptimal concentration of ADP. This effect was completely inhibited by pre-incubation of the platelet suspension with the 5-HT2-selective antagonist ketanserin. IsoDOB, an isomer of DOB with a very low affinity for central 5-HT2 binding sites, was inactive in the platelet aggregation assay. The present results are consistent with the proposed role of 5-HT2 receptors in serotonin-induced platelet aggregation     

Epinephrine stimulates inositol phospholipid metabolism by activating alpha-2 adrenergic receptors in human platelets

The metabolism of inositol phospholipids in response to epinephrine was investigated in intact human platelets. In platelets prelabelled with [3H]-myo-inositol in Ca2+-free HEPES buffer containing 10 mM LiCl, epinephrine caused an accumulation of inositol-1-phosphate in a concentration-dependent manner. The EC50 value for epinephrine was 5 microM. Yohimbine (1 microM), a selective alpha-2 adrenergic receptor antagonist, inhibited 88% of the epinephrine (10 microM) response, whereas prazosin (1 microM), a selective alpha-1 adrenergic receptor antagonist, failed to inhibit the response. Yohimbine inhibited the epinephrine (10 microM) response in a concentration-dependent manner. The inhibition constant (Ki) value for yohimbine was 60.3 nM. These data indicate that epinephrine stimulates phosphoinositide (PI) turnover by activating adrenergic receptors of the alpha-2 type in human platelets. In addition, this PI response elicited by epinephrine was found to be inhibited in a concentration-dependent manner by treatment of platelets with dibutyryl cyclic AMP and 8-bromo-cyclic GMP which are known as potent inhibitors for platelet activation, and may therefore be a useful biochemical index for the study of the function of human alpha-2 adrenergic receptors.     

Protein kinase C translocation in human blood platelets

Protein kinase C (PKC) activity and translocation in response to the phorbol ester, phorbol 12-myristate, 13-acetate (PMA), serotonin (5-HT) and thrombin was assessed in human platelets. Stimulation with PMA and 5-HT for 10 minutes or thrombin for 1 minute elicited platelet PKC translocation from cytosol to membrane. The catecholamines, norepinephrine or epinephrine at 10 microM concentrations did not induce redistribution of platelet PKC. Serotonin (0.5-100 microM) and the specific 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (10-100 microM) but not the 5-HT1A or 5-HT1B agonists, (+/-) 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT) or 5-methoxy-3-3-(1,2,3,6-tetrahydro-4-pyridin) 1H-indole succinate (RU 24969) induced dose-dependent PKC translocations. Serotonin-evoked PKC translocation was blocked by selective 5-HT2 receptor antagonists, ketanserin and spiroperidol. These results suggest that, in human platelets, PMA, thrombin and 5-HT can elicit PKC translocation from cytosol to membrane. Serotonin-induced PKC translocation in platelets is mediated via 5-HT2 receptors.     

Fluorescence digital image analysis of serotonin-induced calcium oscillations in single blood platelets

The intracellular concentration of Ca2+ [( Ca2+]i) was monitored continuously in single rabbit blood platelets by digital imaging microscopy in conjunction with Fura-2, a specific Ca(2+)-indicator dye. Ionomycin as well as aluminium fluoride caused sustained increase in [Ca2+]i in the platelet, but oscillations of [Ca2+]i were not observed. Serotonin (5-HT) induced oscillatory increases in [Ca2+]i in the presence of 1 mM CaCl2; these had not been detectable in cell populations because the oscillations were not in synchrony. This effect of 5-HT was diminished when CaCl2 was omitted from the medium, and was antagonized by 1 microM ketanserin, a specific 5-HT2 receptor antagonist. Furthermore, DOI, a specific 5-HT2 agonist, had the same effect as 5-HT at lower concentration. A specific effector mechanism, not fully understood at present, therefore appears to mediate 5-HT2 receptors thereby allowing rabbit platelets to generate [Ca2+]i oscillations. It is suggested that protein kinase C in platelets might play a key role in the regulation of [Ca2+]i, and possibly in [Ca2+]i oscillations.     

Thrombin-induced inositol trisphosphate production by rabbit platelets is inhibited by ethanol.

Ethanol has an inhibitory effect on some platelet functions, but the mechanisms by which it exerts this effect are not known. Using suspensions of washed platelets, we observed that ethanol (1-9 mg/ml) did not affect the aggregation of rabbit platelets stimulated with ADP (0.5-10 microM). When platelets were prelabelled with 5-hydroxy[14C]tryptamine, aggregation and secretion of granule contents in response to thrombin (0.01-0.10 unit/ml) were not inhibited by ethanol, but these responses to thrombin at lower concentrations (less than 0.01 unit/ml) were inhibited by ethanol (2-4 mg/ml). Platelets were prelabelled with [3H]inositol so that increases in inositol phosphates upon stimulation could be assessed by measuring the amount of label in these compounds. ADP-induced increases in IP (inositol phosphate) and IP2 (inositol bisphosphate) were not affected by ethanol. IP3 (inositol trisphosphate) was not changed by ADP or ethanol. Although ethanol did not affect the increases in IP, IP2 and IP3 caused by stimulation of platelets with thrombin at concentrations greater than 0.01 unit/ml, ethanol did inhibit the increases observed at 2 and 3 min in these inositol phosphates caused by lower concentrations of thrombin (less than 0.01 unit/ml). Since ADP did not cause formation of IP3 in rabbit platelets, and since no thromboxane B2 was detected in platelets stimulated with the lower concentrations of thrombin, it is unlikely that the inhibitory effect of ethanol in IP3 formation was due to effects on further stimulation of platelets by released ADP or by thromboxane A2. Ethanol may inhibit platelet responses to thrombin by inhibiting the production of the second messenger, IP3.     

The 5-hydroxytryptamine stimulated formation of inositol phosphate is inhibited in platelets from alcoholics

The accumulation of inositol monophosphate (IP1) was measured after stimulation of 5-hydroxytryptamine2 (5-HT2) receptors on platelets from alcoholics and healthy controls. In controls, 5-HT induced a dose-dependent response with an EC50 = 2 x 10(-6) M and a maximal response at 10(-5) M. Ritanserin, a selective 5-HT2 antagonist, markedly reduced the accumulation. The IP1 formation after stimulation by 10(-5) M 5-HT was significantly impaired in platelets from alcoholics as compared to controls. This study indicates that the 5-HT2 receptor function is inhibited in alcoholics. It also illustrates the possibility of using IP1 formation in peripheral cells as a mean of studying receptor function in disease.     

Involvement of phosphoinositide metabolism in potentiation by adrenaline of ADP-induced aggregation of rabbit platelets.

Changes in phosphoinositide metabolism were examined in washed rabbit platelets stimulated with 0.5 microM-ADP, 50 microM-adrenaline, or ADP and adrenaline in combination. Adrenaline does not stimulate platelet aggregation when used alone, but does potentiate aggregation stimulated by ADP. In platelets prelabelled with [32P]Pi and [3H]glycerol, adrenaline was found to potentiate the ADP-induced changes in platelet phospholipids, causing larger increases in the amount and labelling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid than was observed with ADP alone. The combination of ADP and adrenaline did not produce a greater decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) than was produced by ADP alone. In platelets prelabelled with [3H]inositol, adrenaline potentiated the increases in labelling of inositol phosphate and inositol bisphosphate stimulated by ADP; no increase in inositol trisphosphate labelling was detected with ADP alone or with the combination of ADP and adrenaline. Phentolamine, an alpha-adrenergic-receptor antagonist, blocked potentiation by adrenaline of ADP-induced changes in phosphoinositide metabolism. Propranolol and sotalol, beta-adrenergic-receptor antagonists, augmented the potentiation; this is consistent with the concept that the effect of adrenaline is mediated by beta-adrenergic receptors. The effect of adrenaline on phosphoinositide metabolism appears to be to potentiate the mechanisms by which ADP causes turnover of PIP and possibly degradation of PI, rather than the mechanism by which PIP2 is decreased.     

Inhibition of serotonin-amplified human platelet aggregation by ketanserin, ritanserin, and the ergoline 5HT2 receptor antagonists-LY53857, sergolexole, and LY237733

Human platelets are known to possess 5HT2 receptors which, when activated, amplify the aggregation response produced by other aggregating agents. Several 5HT2 receptor antagonists, including ketanserin and ritanserin, are known to antagonize serotonin-mediated aggregation of human platelets. In the present study, we document the ability of three ergoline 5HT2 receptor antagonists, LY53857, sergolexole, and LY237733, to antagonize the serotonergic component of the human platelet aggregation response. Potencies of the ergoline esters (LY53857 and sergolexole) and the ergoline amide (LY237733) to inhibit serotonin-amplified platelet aggregation responses were similar to the potencies of ketanserin and ritanserin under the conditions of our study. Furthermore, all five 5HT2 receptor antagonists were capable of fully inhibiting the serotonergic component of the platelet aggregation response. In contrast to these potent ergoline esters and amides, 1-isopropyl dihydrolysergic acid (up to 10(-5)M), a putative metabolite of the ergoline esters, was ineffective under these in vitro conditions. These data are consistent with the high potency of these ergolines as antagonists of 5HT2 receptors and further support the involvement of 5HT2 receptors on human platelets in the amplifying response to serotonin.     

Effect of acute and chronic exercise on plasma amino acids and prolactin concentrations and on [3H]ketanserin binding to serotonin2A receptors on human platelets

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) has been shown to modulate various physiological and psychological functions such as fatigue. Altered regulation of the serotonergic system has been suggested to play a role in response to exercise stress. In the present study, the influence was investigated of acute endurance exercise and short-term increase in the amount of training on the concentrations of the 5-HT precursor tryptophan (TRP), of prolactin (PRL) and of branched-chain amino acids (BCAA) in the blood, as well as on the binding of [3H]ketanserin to the serotonin-2A (5-HT2A) receptors on platelets. Nine healthy endurance-trained men were tested the day before (I) and after (II) a 9-day training programme. Samples of venous blood were drawn after an overnight fast and following 5 h of cycling. Fasted and post-exercise plasma concentrations of free TRP, BCAA and free TRP:BCAA ratio did not differ between I and II. A significant decrease of plasma BCAA (P < 0.01) and significant augmentations of plasma free TRP, free TRP:BCAA ratio and PRL (P < 0.01) were found post-exercise. The increase in plasma PRL was smaller in II compared with I. Acute endurance exercise reduced the density of platelet 5-HT2A receptor [3H]ketanserin binding sites at I and II (P < 0.05). The basal density of the binding sites and the affinity of [3H]ketanserin for these binding sites were unaffected by an increase in the amount of training. The present results support the hypothesis that acute endurance exercise may increase 5-HT availability. This was reflected in the periphery by increased concentration of the 5-HT precursor free TRP, by increased plasma PRL concentration, and by a reduction of 5-HT2A receptors on platelets. It remains to be resolved whether these alterations in the periphery occur in parallel with an increase in the availability of 5-HT in the brain.     

The decrease in phosphatidylinositol 4,5-bisphosphate in ADP-stimulated washed rabbit platelets is not primarily due to phospholipase C activation.

Addition of 10 micron-ADP to washed rabbit platelets caused platelet shape change and aggregation without release of the contents of the amine-storage granules, and caused a transient decrease (8.8% at 10 s) in the amount of phosphatidylinositol 4,5-bisphosphate (PIP2). By 20 s the decrease in PIP2 was no longer apparent, but by 60 s the amount of PIP2 was again decreased. Addition of thrombin (1 unit/ml), which causes platelet shape change, aggregation and the release of the contents of the amine-storage granules, caused a decrease in the amount of PIP2 (8.0% at 10 s); at 60 s the amount of PIP2 was not significantly different from that in controls. In platelets prelabelled with [3H]glycerol, the specific radioactivity of PIP2 was increased at 10 s in ADP-stimulated platelets, and unchanged in thrombin-stimulated platelets. In platelets prelabelled with [3H]inositol and incubated with 20 mM-Li+ to inhibit the degradation of the inositol phosphates to inositol, there was no increase in the labelling of inositol trisphosphate (IP3) upon stimulation with ADP. In contrast, stimulation with thrombin caused a significant increase in the labelling of IP3 at 10 s. These differences in the changes in polyphosphoinositide metabolism in ADP- and thrombin-stimulated platelets are consistent with the hypothesis that the decrease in PIP2 in ADP-stimulated platelets may be due not to degradation of PIP2 by phospholipase C, but rather to a shift in the equilibrium between PIP2 and phosphatidylinositol 4-phosphate (PIP). Increases in the labelling of phosphatidic acid at 10 s and of inositol bisphosphate and inositol phosphate after 20 s are consistent with phospholipase C being stimulated through some other mechanism that leads to the degradation of PIP and phosphatidylinositol; one possibility is that ADP causes an increase in cytoplasmic Ca2+.     

Platelet aggregation may not be a prerequisite for collagen-stimulated platelet generation of nitric oxide

By determining the sum of the supernatant concentrations of nitrite and nitrate the stimulated generation of nitric oxide (NO) by human washed platelets induced by a range of fibrillar collagen concentrations (0.0156-25 microg ml(-1)) was investigated. Platelet serotonin (5-hydroxytryptamine, 5-HT) efflux and platelet aggregation were also measured. Under resting conditions (0 microg ml(-1) collagen) platelet NO release was equivalent to 1.06+/-0.17 nmol per 10(8) platelets. Maximal NO release, equivalent to 2.1+/-0. 37 nmol per 10(8) platelets, was observed with only 0.0625 microg ml(-1) collagen (P<0.02, stimulated vs. resting release), higher collagen concentrations producing no further increases in platelet NO output. By contrast, maximal platelet aggregation and 5-HT efflux did not occur until collagen concentrations of 2.5 microg ml(-1) and 10-25 microg ml-1), respectively, had been achieved. L-NAME (1 mmol l(-1)) and L-NMMA (1 mmol l(-1)) inhibited stimulated platelet NO generation by 78+/-6% and 72%, respectively. Contrasting with fibrillar collagen, fibrillar beta-amyloid protein had no effect on platelet NO generation, or on 5-HT efflux or aggregation. These data perhaps indicate that NO generation by human platelets is stimulated by concentrations of fibrillar collagen insufficient to elicit an aggregatory response. Such a mechanism could operate in vivo to inhibit platelet aggregation which might otherwise be induced by low concentrations of circulating agonists.     

Serotonin agonist and antagonist actions in hippocampal CA1 neurons

The actions of serotonin (5-HT) and its putative agonists and antagonists were examined in vitro on hippocampal CA1 neurons using intracellular recordings, demonstrating that the cellular pharmacological effects can not necessarily be predicted from binding characteristics alone. The first response following 5-HT application was often a long-lasting (several minutes) hyperpolarization associated with decreased input resistance. Subsequent 5-HT applications caused only brief hyperpolarizations (30-120 s) and associated decreased input resistance, often followed by membrane depolarization. The post-spike train afterhyperpolarization (AHP) was prolonged for several minutes following the 5-HT induced hyperpolarization. 5-HT1 agonists (8-hydroxy-2-(di-n-propylamino)tetralin, 5-methoxytryptamine, MK-212) caused a prolonged hyperpolarization, decreased input resistance, and enhancement of the AHP. 5-HT applied following agonist application elicited only short-lasting hyperpolarizations. The 5-HT2 antagonists, cyproheptadine and mianserin, and a nonspecific 5-HT antagonist, methysergide, also caused a prolonged hyperpolarization with decreased input resistance. Spiperone, a nonspecific 5-HT antagonist, and ritanserin, a putative specific 5-HT2 receptor antagonist, depolarized CA1 neurons with little or no change in input resistance. The 5-HT-induced short-lasting hyperpolarization was not affected by drop application of 5-HT antagonists, except for methysergide, but perfusion of methysergide, ritanserin, and spiperone attenuated this response. The long-lasting 5-HT hyperpolarization might be mediated by 5-HT1A receptor activation, and the short-lasting hyperpolarization by another serotonergic receptor subtype.     

Translocation-independent activation of protein kinase C by platelet-activating factor, thrombin and prostacyclin. Lack of correlation with polyphosphoinositide hydrolysis in rabbit platelets.

The relationship between polyphosphoinositide hydrolysis and protein kinase C (PKC) activation was explored in rabbit platelets treated with the agonists platelet-activating factor (PAF), thrombin and 12-O-tetradecanoylphorbol 13-acetate (TPA), and with the anti-aggregant prostacyclin (PGI2). Measurement of the hydrolysis of radiolabelled inositol-containing phospholipids relied upon the separation of the products [3H]inositol mono-, bis- and tris-phosphates by Dowex-1 chromatography. PKC activity, measured in platelet cytosolic and Nonidet-P40-solubilized particulate extracts that were fractionated by MonoQ chromatography, was based upon the ability of the enzyme to phosphorylate either histone H1 in the presence of the activators Ca2+, diacylglycerol and phosphatidylserine, or protamine in the absence of Ca2+ and lipid. Treatment of platelets for 1 min with PAF (2 nM) or thrombin (2 units/ml) led to the rapid hydrolysis of inositol-containing phospholipids, a 2-3-fold stimulation of both cytosolic and particulate-derived PKC activity, and platelet aggregation. Exposure to TPA (200 nM) for 5 min did not stimulate formation of phosphoinositides, but translocated more than 95% of cytosolic PKC into the particulate fraction, and induced a slower rate of aggregation. PGI2 (1 microgram/ml) did not enhance phosphoinositide production, and at higher concentrations (50 micrograms/ml) it antagonized the ability of PAF, but not that of thrombin, to induce inositol phospholipid turnover, even though platelet aggregation in response to both agonists was blocked by PGI2. On the other hand, PGI2 alone also appeared to activate (by 3-5-fold) cytosolic and particulate PKC by a translocation-independent mechanism. The activation of PKC by PGI2 was probably mediated via cyclic AMP (cAMP), as this effect was mimicked by the cAMP analogue 8-chlorophenylthio-cAMP. It is concluded that this novel mechanism of PKC regulation by platelet agonists may operate independently of polyphosphoinositide turnover, and that activation of cAMP-dependent protein kinase represents another route leading to PKC activation.     

5-hydroxytryptamine and precapillary vessels

The complexity of the vascular effects of 5-hydroxytryptamine (5-HT) is illustrated by differences in sensitivity to the amine among arterial tissues of different origin. The interaction of 5-HT with 5-HT2 receptors is inhibited by specific antagonists such as ketanserin and methysergide. Such compounds also inhibit the contractile responses to endogenous 5-HT released from aggregating platelets. The vasodilator component of the response to 5-HT can be unmasked in the presence of serotonergic blockade, provided the antagonist used has no partial agonistic properties. 5-HT augments (amplifies) the vasoconstrictor responses to adrenergic and nonadrenergic neurohumoral mediators. The amplifying effect of the monoamine is prevented by 5-HT2-serotonergic antagonists such as ketanserin.     

Neomycin inhibits inositol phosphate formation in human platelets stimulated by thrombin but not other agonists

Neomycin (0.1-1 mM) added to human platelet-rich plasma or washed platelets prelabeled with [3H]inositol inhibits aggregation, ATP secretion (ID50 0.2 mM) and formation of [3H]inositol mono-, bis- and trisphosphate (ID50 0.6-0.8 mM) in response to thrombin (0.25 U/ml). The production of inositol phosphates in response to other platelet agonists (vasopressin, platelet activating factor, prostaglandin endoperoxide analogs and collagen) is not inhibited by neomycin, even at a concentration of 2 mM. At this concentration neomycin reduces the secretion of ATP stimulated by these agents (by up to 50%). The results indicate that neomycin has multiple effects on platelets that are unrelated to a specific inhibition of inositol phospholipid degradation by phospholipase C. Low concentrations (0.1-1 mM) of neomycin might selectively inhibit the interaction of thrombin with the platelet surface, and high concentrations (greater than 2 mM) might unspecifically reduce platelet secretion in response to various platelet agonists.     

Influence of serotonergic transmission anD postsynaptic 5-HT2C action on the feeding behavior of Coturnix japonica (Galliformes: Aves).

We investigated the role of 5-HT2C receptors and serotonergic transmission in the feeding behavior control of quails. Administration of serotonin releaser, fenfluramine (FEN) and 5-HT2C agonists, mCPP and MK212, 1.0 and 3.3 mg/Kg induced significant inhibition of food intake in previously fasted fowls (0.71 +/- 0.18 g and 0.47 +/- 0.2 g; 0.49 +/- 0.22 g and 0.48 +/- 0.29 g; 0.82 +/- 0.13 g and 0.71 +/- 0.16 g, respectively). Control groups ranged from 2.89 +/- 0.21 g to 2.97 +/- 0.22 g, 60 min after reintroduction of food, P < 0.0001). Similar results were obtained with normally fed quails. Both serotonin releaser and 5-HT2C agonists, in a 3.3 mg/Kg dose, induced hypophagy (FEN, 0.78 +/- 0.08 g; mCPP, 0.89 +/- 0.07 g; MK212, 1.25 +/- 0.17 g vs. controls, 2.05 +/- 0.12 g, 120 min after food was presented, P < 0.0001 to P < 0.01). Previous administration of 5-HT2C antagonist, LY53857 (5.0 mg/Kg) blocked the hypophagic response induced by 5-HT2C agonists 60 min after food was reintroduced. Current data show a modulatory role of serotonin release and postsynaptic 5-HT2C receptors in the feeding behavior of quails.     

Evidence for a spinal serotonergic control of the peripheral inflammation in the rat

We investigated the effect of serotonergic agonists and antagonists injected intrathecally by direct punction of the spinal cord at the lumbar level (between L5-L6) on peripheral inflammatory edema. Edema was induced by carrageenan injected subcutaneously in one hindpaw 30 min after spinal treatments. Serotonin (0.1, 1, 10 pmol) caused a graded-inhibition of the inflammatory paw edema. The corticosteroid inhibitor aminoglutethimide (100 mg/kg, p.o. 1.5 h before spinal treatment) did not modify this effect. The 5-HT1A agonist buspirone and the 5-HT1B/1D agonist sumatriptan (0.1, 1.0 and 10 nmol) also inhibited paw edema. The 5-HT1,2 antagonist methysergide (10 and 100 pmol) enhanced edema, but higher doses ( 4 and 8 nmol) diminished edema. NAN-190 (5-HT1 antagonist; 1 and 10 nmol) increased paw edema, while ritanserin (5-HT2 antagonist; 1 nmol) inhibited paw edema. Ondansetron (5-HT3 antagonist; up to 10 nmol) did not affect edema, but metoclopramide (5-HT3 antagonist / 5-HT4 agonist; 5, 10 and 30 pmol) inhibited edema. These data suggest that a tonic release of serotonin in the spinal cord may occurs during ongoing peripheral inflammation, modulating the neurogenic component of edema either by an inhibitory action on 5-HT1 receptors or by a stimulatory action on 5-HT2 receptors. A disfunction in such mechanism may be involved in the pathophysiology of certain types of headaches or migraine, which seem to depend on neurogenic vasodilation, and may also help to explain the therapeuthic effectiveness of some serotonergic agents in these conditions.     

Platelet activating factor-stimulated formation of inositol triphosphate in platelets and its regulation by various agents including Ca2+, indomethacin, CV-3988, and forskolin

When myo-2-[3H]inositol-labeled rabbit platelets were stimulated with 1 X 10(-9)M sn-3-AGEPC (platelet activating factor) for 5 s, the levels of [3H]inositol monophosphate (IP), [3H]inositol diphosphate (IP2), and [3H]inositol triphosphate (IP3) increased about 1.5-, 3-, and 5-fold, respectively. Formation of these inositol polyphosphates was strikingly independent of extracellular Ca2+. Inactive analogs of sn-3-AGEPC, i.e., lysoGEPC and stereoisomer sn-1-AGEPC, did not cause production of any inositol polyphosphate. Pretreatment of platelets with indomethacin (5 microM) had little effect on this phenomenon. On the other hand, a platelet activating factor antagonist, CV-3988, blocked the AGEPC-stimulated production of radioactive IP, IP2, and IP3. Similarly forskolin, an activator of adenylate cyclase, at 5 microM or above completely abolished AGEPC-induced aggregation, [3H]serotonin secretion, and formation of [3H]inositol polyphosphates. In the light of the emerging role of AGEPC in inflammation, hypotension, and other cardiovascular processes, studies with platelets reported here indicate that forskolin could be a useful tool for manipulating AGEPC responses. It is further concluded that AGEPC-induced formation of inositol polyphosphate is an early response "specific" to AGEPC, mediated via extracellular Ca2+-independent phosphoinositide phosphodiesterase, and could play a role in intracellular Ca2+ mobilization and platelet shape change.     

Multiple populations of serotonin receptors may modulate the behavioral effects of serotonergic agents

In order to establish a functional role for the various populations of serotonin (5-HT) receptors, behavioral studies have been conducted over the past decade with serotonergic agonists and antagonists. And, although there is reason to believe that certain behavioral effects may be mediated via particular populations of 5-HT receptors, evidence now suggests that some serotonin-mediated behaviors may be modulated by the interaction of serotonergic agents at multiple subtypes of 5-HT receptors. The generality of these effects, and the exact mechanism(s) by which they occur, have yet to be elucidated. Nevertheless, over the past year, results from several different laboratories provide a growing recognition of this novel phenomenon.