Further evidence on the interaction of muramyl dipeptide with the serotonergic system

The mode of interaction between muramyl dipeptide (MDP), a compound with immunopharma-cological activities, and 5-hydroxytryptamine (5-HT, serotonin) was studied in isolated nerve-smooth muscle preparations of the carp stomach. Application of exogenous 5-HT evoked direct smooth muscle contractions; electric neurogenic stimulation evoked twitches due to the release of 5-HT from nerve endings. Contractions evoked by a high concentration of 5-HT (3–30 μM) were resistant to atropine and potentiated in the presence of MDP. Isamoltan (5-HT_ID antagonist) decreased the amplitude of contractions, whereas ketanserin (5-HT₂ antagonist) and MDL 72 222 (5-HT₃ antagonist) had no effect. The addition of low concentrations (0.1–1.5 μM) of 5-HT did not contract the preparation but caused a decrease in the amplitude of neurogenic twitches, which might be due to the presynaptic inhibition of serotonin release. This effect of 5-HT was not changed by isamoltan or ketanserin, but it was largely reduced in the presence of the 5-HT₃ antagonists tropisetron and MDL 72 222. This inhibitory effect of 5-HT on twitch amplitude was potentiated by MDP. The interaction of MDP with the serotonergic system thus involved not only potentiation of the postsynaptic effect of higher 5-HT concentrations, which might have been mediated via the 5-HT₁ subsystem, but also presynaptic inhibition. MDP enhancement of 5-HT's inhibitory effect, mediated via 5-HT₃ receptors, might represent a new feature in mutual 5-HT-MDP interactions.     

Presynaptic 5-HT1B receptor-mediated serotonergic inhibition of glutamate transmission in the bed nucleus of the stria terminalis

Activation of neurons in the bed nucleus of the stria terminalis (BNST) plays a critical role in stress and anxiety-related behaviors. Previously, we have shown that serotonin (5-HT) can directly modulate BNST neuronal excitability by an action at postsynaptic receptors. In this study we built upon that work to examine the effects of 5-HT on excitatory neurotransmission in an in vitro rat BNST slice preparation. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs). These effects were mimicked by the 5-HT_1B/D receptor agonist, sumatriptan, and by the 5-HT_1B receptor selective agonist, CP93129. Conversely, the effects of 5-HT and sumatriptan could be blocked by the 5-HT_1B receptor-selective antagonist, GR55562. In contrast, the 5-HT_1A receptor agonist 8-OH DPAT or antagonist WAY 100635 could not mimic or block the effect of 5-HT on eEPSCs. Together, these data suggest that the 5-HT-induced attenuation of eEPSCs was mediated by 5-HT_1B receptor activation. Moreover, sumatriptan had no effect on the amplitude of the postsynaptic current elicited by pressure applied AMPA, suggesting a possible presynaptic locus for the 5-HT_1B receptor. Furthermore, 5-HT, sumatriptan and CP93129 all increased the paired pulse ratio of eEPSCs while they concomitantly decreased the amplitude of eEPSCs, suggesting that these agonists act to reduce glutamate release probability at presynaptic locus. Consistent with this observation, sumatriptan decreased the frequency of miniature EPSCs, but had no effect on their amplitude. Taken together, these results suggest that 5-HT suppresses glutamatergic neurotransmission in the BNST by activating presynaptic 5-HT_1B receptors to decrease glutamate release from presynaptic terminals. This study illustrates a new pathway by which the activity of BNST neurons can be indirectly modulated by 5-HT, and suggests a potential new target for the development of novel treatments for depression and anxiety disorders.     

Cerebrovascular serotonergic receptors mediating vasoconstriction: further evidence for the existence of 5-HT2 receptors in rat and 5-HTVIike receptors in guinea-pig basilar arteries

Pharmacological experiments were carried out on isolated basilar arteries (BA) from the brain vasculature of guinea-pig and rat in order to characterize post-junctional serotonergic receptors mediating contraction by the use of selective agonists and antagonists. The sensitivity to 5-HT was higher, but the intrinsic activity lower, in guinea-pig compared to rat vessels. The contractile potency of the 5-HT₁ agonist, 5-carboxamidotryptamine (5-CT), was three times higher than 5-HT in guinea-pig but 16 times lower in rat BA. In arteries from both species the 5-HT_1A agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), only caused weak contraction. In rat BA, where the serotonergic contractile receptors are ketanserin-sensitive, mesulergine inhibited the contraction in doses high enough to block 5-HT₂ receptors, and also propranolol slightly inhibited the contraction, probably due to its binding to these receptors. Methiothepin, a potent antagonist of the 5-HT₁-like receptors, affected the contraction in a non-competitive manner. The antagonist profile was different in guinea-pig BA: propranolol was ineffective, mesulergine caused a slight, non-surmountable inhibition, whereas methiothepin acted as a true, competitive antagonist. The data support previous suggestions that the serotonergic contraction in rat BA is mediated by 5-HT₂ receptors, whereas the present data show that 5-HT₁-like receptors predominate in guinea-pig BA.     

Acute Ca-dependent desensitization of 5-HT1A receptors is mediated by activation of protein kinase A (PKA) in rat serotonergic neurons

This report investigates acute changes in the sensitivity of 5-HT_1A receptors in dorsal raphe (dr) neurons in response to elevated serotonin. DR neurons were isolated from adult rats and measurements of inhibition of Ca²⁺ current by 5-HT were obtained using the whole cell patch clamp technique. During a 10-min application of 5-HT (with normal [Ca²⁺]_i∼100 nM) a desensitization occurred. The response to 20 nM 5-HT decreased by 66% relative to control and remained depressed for about 30 min. When the internal [Ca²⁺] was buffered to <1 nM only a weak transient desensitization occurred that was surmountable with higher [5-HT]. Adenylyl cyclase activation with forskolin mimicked the desensitization and selective inhibition of protein kinase A (PKA), but not protein kinase C (PKC), partially antagonized the desensitization induced by 5-HT. To measure the activity of PKA and phosphatase enzymes, dr slices were incubated with the selective agonist dipropyl-5-carboxamidotryptamine (DP-5-CT, 1 μM) for 10 min and the phosphorylation of the PKA substrate Kemptide was followed using ATP-γ³²P. DP-5-CT inhibited the cAMP stimulated maximal activity of PKA but raised basal PKA activity, thus increasing the percentage of PKA in the active state (activity ratio), an effect that was prevented by the selective 5-HT_1A antagonist WAY100635. DP-5-CT also caused a significant inhibition of phosphatase activity. These data support a model in the dr where 5-HT_1A-receptor stimulation of PKA promotes phosphorylation of a target and phosphatase inhibition leading to heterologous desensitization. The effect would be expected to have physiological consequences for 5-HT-mediated inhibitory post synaptic potentials and the Ca²⁺ component of the action potentials of dr neurons.     

5-HT receptor binding in post-mortem brain from patients with affective disorder

Post-mortem brain tissue was obtained from a group of patients with well documented clinical histories of affective disorder. 5-Hydroxytryptamine-1 (5-HT₁) and 5-HT₂ receptor binding to homogenates of frontal cortex (Brodmann area 10) was measured using tritiated 5-HT and tritiated ketanserin respectively. 5-Hydroxyindoleacetic acid (5-HIAA) levels from the same brain samples were measured by reverse-phase high performance liquid chromatography with electrochemical detectionA tendency towards increased 5-HT receptor binding density in patients with major affective disorder was found compared to dysthymic disorder patients and normal controls. No relationship was found between receptor binding densities and metabolite values, nor were the differences in 5-HT binding correlated with time to autopsy, storage time prior to assay, or to clinical variables including DSM-III psychoticism/non-psychoticism and melancholia. Previous antidepressant drug histories were similar in the two patient groups and are unlikely to account for the findings.An increase in postsynaptic 5-HT₂ receptor binding in major affective disorder is a possible pathophysiological mechanism which is compatible with the observed down-regulatory effect of antidepressant drugs (although not electroconvulsive therapy) on 5-HT₂ sites. The methodological problems inherent in post-mortem studies in affective disorder are discussed.     

Cardiovascular and respiratory reflexes of the gulf toadfish (Opsanus beta) during acute hypoxia

The acute cardiovascular and respiratory responses of the gulf toadfish, Opsanus beta, to acute hypoxia or exposure to the O₂ chemoreceptor stimulant, sodium cyanide (NaCN) were characterized and the role of serotonin type 2 (5-HT₂) receptors in mediating these responses was investigated. Toadfish responded to hypoxia or NaCN exposure with a decrease in heart rate (fH) and an increase in breathing amplitude (V_AMP) but no change in breathing frequency (fR). The bradycardia appeared to be mediated to some extent by 5-HT₂ receptors, as methysergide, a non-selective 5-HT_1/2 receptor antagonist, and ketanserin, a 5-HT₂ receptor antagonist, attenuated the response. Injection of α-methyl-5-HT, a 5-HT₂ agonist, also resulted in bradycardia that was inhibited by ketanserin, lending further support for 5-HT₂ receptor involvement, possibly 5-HT_2A or 5-HT_2C, in the regulation of fH. External NaCN exposure resulted in a significant decrease in caudal arterial blood pressure (P_CA) that was attenuated by methysergide. In contrast, injection with α-methyl-5-HT resulted in a substantial increase in P_CA that was not affected by ketanserin, suggesting the possible involvement of 5-HT_2B or 5-HT_2C receptors. These data are the first to suggest a unique distribution of 5-HT_2B/2C receptors may be involved in mediating vasoconstriction of the systemic vasculature of toadfish. These data also provide mechanistic support for why pulsatile urea excretion, believed to be regulated by 5-HT via the toadfish 5-HT_2A receptor, is not triggered by hypoxia or external chemoreceptor activation.     

The Role of Serotonin Receptors in Delta-Opioid Immunosuppression

Studies in CBA mice and Wistar rats demonstrated the involvement of serotonin (5-HT) receptors in immunosuppression evoked using the selective δ2 opioid receptor (δ2-OR) agonist DSLET (100 μg/kg). This opioid effect was seen in conditions of selective activation of 5-HT_1A autoreceptors with 8-OH-DPAT (0.1 mg/kg) and in conditions of prior blockade of postsynaptic 5-HT_1A and 5-HT_2A receptors with WAY-100635 and ketanserin (1 and 3 mg/kg). The question of the different contributions of these types of 5-HT receptor to δ-induced suppression of the immune response is discussed.     

Platelet-vessel wall interactions: Implication of 5-hydroxytryptamine. A review

The evidence for an impact of platelet-derived 5-hydroxytryptamine (5-HT) on local tissue perfusion is reviewed. By interacting with 5-HT₂ serotonergic receptors, 5-HT, directly or through amplification, activates platelets, endothelial and vascular smooth muscle cells producing platelet aggregation, vascular permeability increase and large vessel constriction. Pharmacodissection in experimental animals with selective serotonergic 5-HT₂ receptor antagonists, e.g. ketanserin, shows that 5-HT largely contributes to the platelet-mediated increase in vascular permeability, to platelet-vessel wall interaction during hemostasis, to cardiopulmonary dysfunction provoked by thromboembolism and to the platelet-mediated inhibition of peripheral collateral circulation. Clinical results obtained with ketanserin further substantiate an involvement of platelet-derived 5-HT in the pathogenesis of impaired tissue perfusion in some cardiovascular conditions.     

Interactions between the serotoninergic and noradrenergic systems of the brain and their role in the regulation of animal behavior

Studies were carried out using DBA/2 mice on the relationship between the behavioral effects of antagonists of different types of 5-HT receptors and the level of activity of α₂-adrenoceptors. The 5-HT_1c receptor antagonists mianserin (2 mg/kg) and cyproheptadine (2 mg/kg) and the 5-HT₂ receptor antagonist ketanserin (3 mg/kg) had no effect on movement activity, while the 5-HT₃ and 5-HT₄ receptor antagonists zacopride (1 mg/kg) and ICS 205-930 (1 mg/kg) reduced movement behavior in intact animals. On a background of treatment with the α₂-adrenoceptors blocker yohimbine (0.5 mg/kg), mianserin, cyproheptadine, and ketanserin inhibited movement activity and significantly reduced the sensitivity of animals to audiogenic convulsions. These data indicate that administration of α₂-adrenoceptors antagonists increases the efficiency with which serotonin receptors regulate behavior. Institute of Cell Biophysics, Russian Academy of Sciences, Pushchino, Russia; Department of Pharmacology, Scientific Health Center, University of Texas, San Antonio, USA. Translated from Fiziologicheskii Zhurnal im.I. M. Sechenova, Vol. 81, No. 8, pp. 101–103, August, 1995.     

Excitation and inhibition of rat medial vestibular nucleus neurones by 5-hydroxytryptamine

The effects of 5-hydroxytryptamine (5-HT) and related compounds on the discharge rate of tonically active medial vestibular nucleus (MVN) neurones were studied in an in vitro slice preparation of the dorsal brainstem of the rat. The majority (87 of 107, 82%) of MVN neurones were excited by 5-HT. Nine cells (8%) showed a biphasic response to 5-HT, which consisted of a brief inhibition followed by excitation. Eleven cells (10%) were inhibited by 5-HT. The excitatory effects of 5-HT were mimicked by alpha-methyl-5-HT and antagonised by ketanserin and ritanserin, indicating the involvement of the 5-HT₂ subtype of 5-HT receptor. In biphasic cells, blockade of 5-HT₂ receptors by ketanserin reduced the excitatory component of the response and revealed an enhanced initial inhibition. The inhibitory effects in biphasic cells, and in cells that showed a pure inhibition in response to 5-HT, were blocked by pindobind-5-HT and mimicked by 8-hydroxy-2-(di-n-propylamino)-tetralin indicating the involvement of 5-HT1A receptors. The significance of these findings in relation to the effects of 5-HT on vestibular reflex function is discussed.     

Actions of serotonin antagonists on cholera-toxininduced intestinal fluid secretion

The effects of several 5-hydroxytryptamine (5-HT) receptor antagonists were tested in rats in vivo on the intestinal fluid secretion evoked by cholera toxin. Five receptor antagonists were used, namely 2-bromolysergic acid diethylamine (2-bromo-LSD), granisetron, ketanserin, methysergide and ondansetron. The drugs were used in doses that inhibited the arterial hypertension and/or bradycardia evoked by 5-HT given i.v. Granisetron and ondansetron markedly diminished cholera-toxin-evoked secretion, whereas ketanserin was without any effect. Methysergide also diminished cholera-toxininduced fluid secretion particularly when the drug was given as an i.v. infusion. The results are considered in relation to the pathophysiology of cholera secretion and to the current views of receptor subtypes for 5-HT. It is proposed that the receptor involved is a 5-HT₃ receptor, possibly also a receptor of the 5-HT₁ type. Results from experiments in which 5-HT (20 mM) was placed in the intestinal lumen to evoke an intestinal secretion suggest that the 5-HT₃ receptor is located in the villus tissue. It was also demonstrated that zimeldine, an inhibitor of presynaptic 5-HT reuptake, diminished choleraic secretion, an effect that may be ascribed to a 5-HT tachyphylaxis caused by an accumulation of 5-HT in a synaptic cleft.     

Role of various types of serotonin receptors in regulation of drinking behavior and salt appetite in vasopressin-deficient brattleboro rats

Serotonin 5-HT_1A receptor agonist 8-OH DPAT suppressed drinking behavior in Brattleboro and Wistar rats. 5-HT_1B agonist CGS-12066A and 5-HT_2A antagonist ketanserin did not affect drinking behavior in Brattleboro rats; 5-HT₃ antagonist ondansetron suppressed water consumption and 5-HT_1A agonist stimulated salt appetite in Brattleboro, but not in Wistar rats. Presumably, vasopressin regulates thirst and salt appetite by modulating sensitivity/density of various types of 5-HT receptors.     

Indoleaminergic mechanisms in brain vessels; localization, concentration, uptake and in vitro responses of 5-hydroxytryptamine

Immunocytochemical studies have revealed the presence of 5-HT-containing nerve fibres in all parts of the cerebrovascular bed (arteries, arterioles and veins) of mouse, rat, guinea-pig, rabbit and cat. Biochemical measurements (using HPLC) revealed substantial concentrations of 5-HT and 5-HIAA in the pial vessels of the rat, rabbit, cat and man, the amounts corresponding well with the density of the perivascular nerve supply. The uptake of 3H-5-HT was studied in arteries removed from the circle of Willis in rats. Maximum uptake was reached after 15 min of incubation at 37 degrees C and plateaued at 30 min. The reaction was temperature-dependent and found to be absent if performed at 0 degrees C. Pharmacological experiments on isolated middle cerebral and basilar arteries showed that vessels from rat and dog were contracted by approximately 90% upon administration of 5-HT, whereas vessels from guinea-pig, rabbit, cat and man were contracted by 40 to 60% relative to 124 mM K+. The EC50 values in the different species varied by between 1.5 X 10(-7) M (rat) and 3 X 10(-9) M (dog). The 5-HT-induced contractions were blocked by the 5-HT antagonists, methysergide, methergoline and ketanserin. Transmural nerve stimulation (TNS) of the rabbit basilar artery revealed a tetrodotoxin sensitive constriction whereas TNS of cat and dog middle cerebral arteries caused a tetrodotoxin-sensitive relaxation. The relaxation was not significantly attenuated until high doses of methergoline (3 X 10(-6) M) or ketanserin (3 X 10(-5) M) had been given.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective serotonin receptor stimulation of the medial nucleus accumbens differentially affects appetitive motivation for food on a progressive ratio schedule of reinforcement

Previously, we reported that stimulation of selective serotonin (5-HT) receptor subtypes in the nucleus accumbens shell differentially affected consumption of freely available food. Specifically, activation of 5-HT₆ receptors caused a dose-dependent increase in food intake, while the stimulation of 5-HT_1/7 receptor subtypes decreased feeding [34]. The current experiments tested whether similar pharmacological activation of nucleus accumbens serotonin receptors would also affect appetitive motivation, as measured by the amount of effort non-deprived rats exerted to earn sugar reinforcement. Rats were trained to lever press for sugar pellets on a progressive ratio 2 schedule of reinforcement. Across multiple treatment days, three separate groups (N = 8–10) received bilateral infusions of the 5-HT₆ agonist EMD 386088 (at 0.0, 1.0 and 4.0 μg/0.5 μl/side), the 5-HT_1/7 agonist 5-CT (at 0, 0.5, 1.0, or 4.0 μg/0.5 μl/side), or the 5-HT_2C agonist RO 60-0175 fumarate (at 0, 2.0, or 5.0 μg/0.5 μl/side) into the anterior medial nucleus accumbens prior to a 1-h progressive ratio session. Stimulation of 5-HT₆ receptors caused a dose-dependent increase in motivation as assessed by break point, reinforcers earned, and total active lever presses. Stimulation of 5-HT_1/7 receptors increased lever pressing at the 0.5 μg dose of 5-CT, but inhibited lever presses and break point at 4.0 μg/side. Injection of the 5-HT_2C agonist had no effect on motivation within the task. Collectively, these experiments suggest that, in addition to their role in modulating food consumption, nucleus accumbens 5-HT₆ and 5-HT_1/7 receptors also differentially regulate the appetitive components of food-directed motivation.     

The role of calcium channels and serotonin (5-HT2) receptors for tumour cell lodgement in the liver

The effect of blockade of calcium channels and/or 5-HT receptors on the hepatic lodgement of intraportally injected fibrosarcoma cells was studied in rats. The calcium channel blocker verapamil and the 5-HT₂ antagonist ketanserin each had a significant lodgement reducing effect. The pure 5-HT₂ antagonist R 56413 was most effective and dose-dependent in its tumour cell lodgement reducing effect. At the highest dose tested (2·5 mg/kg body weight) R 56413 was as effective as thrombocytopenia, indicating that 5-HT is the most important platelet-released product that influences tumour cell lodgement.     

Neuronal responses to 5-hydroxytryptamine in the red nucleus of rats

The effects of microiontophoretic 5-hydroxytryptamine (5-HT) on the firing rate of red nucleus (RN) neurons were studied in urethane-anesthetized rats. The background discharge rate of almost all the neurons tested (97%) was modified by 5-HT, and generally increased (89%). Responses were dose dependent. Twenty-three percent of the excitatory responses were preceded by a short inhibitory phase. No significant difference in the effect of 5-HT was found between those RN neurons that project to the spinal cord and those that do not. The excitatory responses to 5-HT were blocked or greatly reduced by the 5-HT antagonists methysergide and ketanserin, and were even reversed in some cases. The 5-HT₂/5-HT_1A antagonist spiperone, in small doses, also blocked the transient inhibitory phases in addition to the excitatory effects. In RN neurons exhibiting a short-lasting inhibition in the response to 5-HT, the 5-HT_1A agonist 8-hydroxy-2(di-n-propyl-amino)tetralin (8-OH-DPAT) induced inhibitory effects. These results support the hypothesis that 5-HT exerts control throughout the RN, mostly by acting on 5-HT₂ receptors. Furthermore, an influence of this amine on the electrical activity of small groups of RN neurons by 5-HT_1A receptors, and eventually by different mechanisms, appears probable. The functional significance of serotoninergic control of RN neuronal activity is discussed.     

Serotonin 5-HT2 and 5-HT1A-like receptors differentially modulate aggressive behaviors in Drosophila melanogaster

Aggressive behavior is widespread throughout the animal kingdom, and is a complex social behavior influenced by both genetics and environment. Animals typically fight over resources that include food, territory, and sexual partners. Of all the neurotransmitters, serotonin (5-HT) has been the most implicated in modulating aggressive behaviors in mammalian systems. In the fruit fly, Drosophila melanogaster, the involvement of 5-HT itself in aggressive behaviors has been recently established, however, the underlying mechanisms have largely remained elusive. Here we describe the influence of different 5-HT receptor subtypes on aggressive behaviors in Drosophila. Drosophila express homologs of three mammalian 5-HT receptors: the 5-HT_1A, 5-HT₂, and 5-HT₇ receptors. Significantly, these receptors mediate important behaviors in mammalian systems ranging from feeding, aggression, and sleep, to cognition. To examine the role of the 5-HT₂Dro receptor, we utilized the selective 5-HT₂ receptor agonist (R)-1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI), and the 5-HT₂ receptor antagonist, ketanserin. To examine the role of 5-HT_1A-like receptors we used the 5-HT_1A receptor agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT), and the 5-HT_1A receptor antagonist WAY100635. We find that activation of 5-HT₂ receptors with (R)-DOI appears to decrease overall aggression, whereas activation of 5-HT_1A-like receptors with 8-OH-DPAT increases overall aggression. Furthermore, the different 5-HT receptor circuitries appear to mediate different aspects of aggression: 5-HT₂ receptor manipulation primarily alters lunging and boxing, whereas 5-HT_1A-like receptor manipulation primarily affects wing threats and fencing. Elucidating the effects of serotonergic systems on aggression in the fly is a significant advancement not only in establishing the fly as a system to study aggression, but as a system relevant to elucidating molecular mechanisms underlying aggression in mammals, including humans.     

A study of 5-HT-receptors associated with afferent nerves located in normal and inflamed rat ankle joints

Neural recordings were made from sensory fibres in a nerve supplying the ankle joint in normal rats and in rats with a novel monoarticular arthritis. The responses of mechanically and chemically sensitive units to intra-arterial injections of 5-HT were measured. In most cases the mechanosensitivity of sensory receptors in the ankle joint was not altered by 5-HT. However, 5-HT produced an increase in afferent activity in units which were identified as C-fibres on the basis of action potential amplitude and duration. The receptive fields of these chemosensitive units were not located. The responses of these units to 5-HT were dose dependent and were abolished by the 5-HT₂-antogonist, ketanserin, but not by the 5-HT₃-receptor antagonist, MDL 72222. The responses of chemosensitive units to injections of 5-HT were similar in normal and arthritic rats although the response was slightly prolonged in arthritic animals.     

The effect of ketanserin,a 5-HT2-receptor antagonist,on 5-hydroxytryptamine-induced irreversible platelet aggregation in patients with cardiovascular diseases

Platelet aggregation in response to 5-hydroxytryptamine was investigated in 40 normal subjects, in 45 patients with acute myocardial infarction, and in 65 patients with peripheral arterial obstructive disease. It was found that of the 110 patients with cardiovascular disease, 40% had a biphasic irreversible platelet aggregation, whereas this phenomenon occurred in only 7.5% of the normal population. A double-blind placebo-controlled study further showed that a subacute treatment with ketanserin, a selective 5-HT₂-receptor antagonist both on platelets and on vascular tissue, efficiently abolished the irreversible platelet aggregation in patients hyperreactive to 5-hydroxytryptamine. In an additional open study, including 10 patients with peripheral arterial obstructive disease, a chronic treatment with ketanserin 40 mg t.i.d. for a period of 3 months significantly suppressed the primary platelet aggregation to 5-HT at 2×10^–5 M and at 2×10^–6 M and significantly lowered the plasma betathromboglobulin levels. Since 5-HT is a potent mediator of vasospam, treatment with ketanserin might be of therapeutic value in atherosclerotic diseases, where platelet activation is thought to be involved.     

Platelet-vessel wall interactions in hemostasis: Implication of 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is implicated in the platelet-vessel wall interactions during hemostasis. Indeed, the tall bleeding time and the initial blood loss in rats are significantly increased by platelet amine depletion with reserpine/parachlorophenylalanine, by the specific 5-HT₂ receptor antagonists ketanserin, R 55 667 and R 56 433 and by the ergot derivatives metergoline and methysergide. In the dose range affecting hemostasis, the serotonergic receptor antagonists also inhibit the 5-HT-amplified aggregation of ADP-sensitized platelets in rat whole blood. Bleeding times are also prolonged by -adrenergic receptor antagonism with prazosin or phentolamine, by specific thromboxane A₂-synthetase inhibition with R 59 655 or dazoxiben, and by anticoagulant treatment.The latter three mechanisms of action are not explanatory for the effect of the tested serotonergic antagonists, since bleeding times are prolonged by specific 5-HT₂ receptor antagonists without an effect on -adrenergic receptors, on platelet prostaglandin biosynthesis or on coagulation and fibrinolysis. The present study shows the interplay between the platelet-derived mediators 5-HT and TXA₂ at the level of the platelet or the blood vessel to be of major importance in the platelet-vessel wall interactions during hemostasis.