Endurance performance in humans: the effect of a dopamine precursor or a specific serotonin (5-HT2A/2C) antagonist

In this study we examined the effect of a dopamine (DA) precursor (L-DOPA) or a serotonin (5-HT) antagonist (Ritanserin) on time to exhaustion. The study had a double-blind, randomised, placebo controlled and cross-over design. Seven moderately trained men performed three tests to exhaustion at 65% Wattmax. Each test was separated by two weeks to allow washout of the drugs (dose: 4 mg/kg Sinemet, and 0.3 mg/kg Ritanserin). Blood lactate, hematocrit, glucose, ammonia, free fatty acids (FFA), growth hormone (GH) and catecholamines were determined before and after exercise. Time to exhaustion did not differ between the three trials. Most of the parameters measured in this study responded as predicted during cycling to exhaustion in man. DA agonism significantly increased heart rate, lactate, and plasma DA values at rest, while other parameters such as FFA, lactate, plasma noradrenaline (NA) and adrenaline (A), and plasma GH showed the highest absolute levels at exhaustion. Ritanserin did not influence basal glucose and heart rate at rest, but this group showed a much lower increase in plasma catecholamine levels. We conclude that under the present conditions, neither a metabolic precursor of DA nor a specific centrally acting 5-HT2A/2C antagonist, when given in two single doses 24 h and immediately before the experiments, influences the time to exhaustion on a bicycle trial at 65% Wattmax.

Power spectral electroencephalogram analysis of the effects of ritanserin in freely moving rats during quiet waking

Changes in the spectral characteristics of the Fast Fourier Transformed (FFT) electroencephalogram (EEG) recorded from the frontal cortex of freely moving rats during quiet waking were studied after blockade of the 5-hydroxytryptamine-2 (5-HT2) receptors by the potent and long acting 5-HT2 antagonist ritanserin administered intraperitoneally (i.p.) at two doses (0.63 and 2.5 mg/kg). The rationale of this approach was to evaluate the subtle ritanserin-induced changes in the EEG in the frequency domain, and thus obtain more information about the neurophysiological effects mediated by brain 5-HT2 receptors and about the mechanisms underlying ritanserin's ability to increase sleep at the expense of wakefulness. Ritanserin induced significant changes in the power spectrum of EEG recorded from the frontal cortex. The activation index (the ratio of the dominant frequency in the power spectrum to the total power) significantly decreased after ritanserin, which was due to the increased EEG power in all frequency bands (except for beta 2) and to the decreased dominant frequency in the EEG spectrum, suggesting ritanserin-reduced EEG activation during quiet waking. The results favor the view of the excitatory modulating action of serotonin mediated via cortical 5-HT2 receptors and suggest that ritanserin could cause a faster transition from waking to sleep.

Serotonin directly stimulates luteinizing hormone-releasing hormone release from GT1 cells via 5-HT7 receptors

Luteinizing hormone-releasing hormone (LHRH release, which serves as the primary drive to the hypothalamic-pituitary gonadal axis, is controlled by many neuromediators. Serotonin has been implicated in this regulation. However, it is unclear whether the central effect of serotonin on LHRH secretion is exerted directly on LHRH neurosecretory neurons or indirectly via multisynaptic pathways. The present studies were undertaken in order to examine whether LHRH secretion from immortalized LHRH cell lines is directly regulated by serotonin and, if so, to identify the receptor subtype involved. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A/7 receptor agonist, stimulated LHRH release from GT1-1 cells. This effect was blocked by ritanserin, a 5-HT2/7 receptor antagonist, but not by SDZ-216-525, a 5-HT1A antagonist. Basal LHRH release was not affected by the 5-HT2 agonist DOI. Reverse transcription and polymerase chain reaction technique (RT-PCR) was used in order to identify 5-HT1A and 5-HT7 receptor mRNA in immortalized LHRH cell lines. GT1-1 cells express mRNA for the 5-HT7, but not the 5-HT1A receptor subtypes. These results demonstrate a direct stimulatory effect of serotonin on LHRH release via 5-HT7 receptor.

The apparent antipsychotic action of the 5-HT2a receptor antagonist M100907 in a mouse model of schizophrenia is counteracted by ritanserin. (Rapid communication)

The apparent antipsychotic action of the selective 5-HT2a receptor antagonist M100907 in MK-801-treated NMRI mice was shown to be markedly counteracted by the 5-HT2a/2c receptor antagonist ritanserin. The mechanism of action and potential implications are discussed.

The involvement of 5-HT1B receptors in the inhibitory effects of nitric oxide synthase inhibitor on 2-deoxy-D-glucose-induced hyperphagia in rats

We found previously that the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) significantly reduced 2-deoxy-D-glucose (2-DG)-induced hyperphagia in rats. To clarify the involvement of 5-HT, we investigated the effects of 5-HT receptor antagonists on inhibitory effects of L-NAME on 2-DG-induced hyperphagia. The effects of L-NAME on 2-DG-induced hyperphagia were inhibited by the 5-HT1B receptor antagonist metergoline. However, the 5-HT2 receptor antagonist ritanserin had no such effect. These results suggest that the anorectic effects of L-NAME may be related to serotonergic mechanisms.

Effects of ritanserin on haloperidol-induced dopamine (D2) receptor up-regulation in the rat

Rats were treated with vehicle, ritanserin (5 mg/kg, i.p.), haloperidol (1 mg/kg, i.p.) or both ritanserin and haloperidol for 19 days to determine whether chronic administration of the serotonin (5HT2) antagonist ritanserin affects D2 receptor up-regulation produced by haloperidol. Brain sections were prepared for D2 and 5HT2 receptor autoradiography with [3H]spiperone and [3H]ketanserin, respectively. Ritanserin significantly reduced 5HT2 receptors to 80% of vehicle in the sulcal area of the frontal cortex but had no effect on D2 receptors. Haloperidol significantly increased striatal and n. accumbens D2 receptors with no effect on 5HT2 receptors. In ritanserin/haloperidol-treated rats, D2 receptors were significantly increased along with significant decreases in 5HT2 receptors of the frontal cortex. These results suggest 5HT2 receptor antagonism by ritanserin does not significantly affect D2 receptor up-regulation produced by haloperidol.

Antagonism of meta-chlorophenylpiperazine-induced inhibition of exploratory activity in an emergence procedure, the open field test, in rats

The effects of meta-chlorophenylpiperazine (mCPP) were studied on exploratory behaviour in the open field test, using a procedure designed to evaluate the emergence of rats into a novel environment. mCPP reduced the exploratory activity in a dose-related manner after subcutaneous (s.c.), intraperitoneal (i.p.) and intravenous (i.v.) administration. The inhibition was manifest in all the parameters used to quantify the exploration of the open field area. Additional neuroendocrine experiments in a parallel group of rats revealed a dose-related increase in plasma prolactin and ACTH levels after i.v. mCPP, pointing to a general state of arousal in these mCPP-treated animals. A number of 5-HT antagonists were tested for their ability to prevent or reverse the behavioural inhibition induced by an i.v. injection of 1.0 g/kg mCPP given 15 min before testing in the open field. The antagonists were injected s.c. or given orally at various time intervals before mCPP, or they were injected i.v. 10 min after mCPP. The lowest active doses for the attentuation of the mCPP-induced behavioural inhibition after s.c., oral and i.v. administration, respectively, were 0.04, 40 and 10 mg/kg for pizotifen; 0.16, 0.16 and 0.16 mg/kg for mianserin; 0.63, 0.16 and 0.16 mg/kg for methysergide, and 0.16, 2.5 and 2.5 mg/kg for ritanserin. The lowest active doses of mirtazapine after s.c. and i.v. treatment were 0.01 and 0.16 mg/kg. These data indicate that mixed 5-HT1/5-HT2 receptor antagonists such as pizotifen and methysergide, and mixed 5-HT and catecholamine antagonists such as mianserin and mirtazapine are more potent antagonists of mCPP-induced behavioural inhibition in rats than the more selective 5-HT2A/5-HT2C antagonist ritanserin.

Some, but not all, antipsychotic drugs potentiate a low level of prepulse inhibition shown by rats of the Wistar strain

In a previous study using Sprague-Dawley rats, we showed that in a prepulse inhibition (PPI) procedure with levels of PPI ranging from approximately 10 to 40% (for prepulse intensities 2, 9 and 15 dB above background noise), the antipsychotics clozapine and haloperidol, but also the alpha 1 adrenoceptor antagonist prazosin, robustly and dose-dependently potentiated PPI. In contrast, the antipsychotics risperidone, amisulpride, raclopride and remoxipride did not potentiate PPI. The false positive (prazosin) and the four false negatives led us to conclude that this PPI-enhancing procedure had poor predictive validity as a screening tool for potential antipsychotics. In the present study, we used Wistar rats, which under the same protocol as that used for Sprague-Dawley rats show a very low level of PPI. We examined the ability of six antipsychotics, given intraperitoneally (i.p.), to reverse this PPI deficit. It was found that clozapine (5-20 mg/kg), olanzapine (5-20 mg/kg) and sertindole (1-10 mg/kg) reversed this deficiency of PPI (i.e. potentiated the low level of PPI). In contrast, risperidone (0.1-1 mg/kg), remoxipride (1-10 mg/kg) and haloperidol (0.1-1 mg/kg) were inactive. The negative results with three clinically active antipsychotics (risperidone, remoxipride and haloperidol) indicate that reversal of this PPI deficit in Wistar rats has poor predictive validity to screen for potential antipsychotic activity. In an attempt to investigate the mechanism that might underlie the reversing effect of clozapine, olanzapine and sertindole, we tested the ability of the alpha 1 adrenoceptor antagonist prazosin (3-20 mg/kg), the dopamine D1 receptor antagonist SCH 23390 (0.01-0.1 mg/kg) and the 5-HT2 antagonist ritanserin (0.3-3 mg/kg) to reverse the PPI deficit. Negative results with these three drugs did not allow us to characterize the receptor(s) that might be implicated in the reversal of this type of PPI deficit.

Pharmacological characterization of in vivo properties of putative mixed 5-HT1A agonist/5-HT(2A/2C) antagonist anxiolytics. II. Drug discrimination and behavioral observation studies in rats

To characterize their in vivo 5-hydroxytryptamine (5-HT)2A antagonist properties, the ability of the putative mixed 5-HT1A agonists/5-HT(2A,2C) antagonists (N-(29(4-(2-pyrimidinyl)-1-piperazinyl)ethyl)tricyclo(3.3.1.1(3,7) ) decane-1-carboxamide (WY-50,324), (2-(4-(4,4-bis(4-fluorophenyl)butyl)-1-piperazinyl)-3-pyridinecarboxy lic acid hydrochloride (FG5974), 9,10-didehydro-N-(2-propynyl)-6-methylergoline-8b-carboxamid e (LEK-8804) and trans-1,3,4,a5,10b-hexahydro10-methoxy-4-propyl-2H-(1)benzopyra nol[3,4-b]pyridine (CGS 18102A) to antagonize both head twitches and discriminative stimulus (DS) effects produced by (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) in rats were compared with those of the 5-HT2 antagonists ketanserin and ritanserin, and the 5-HT1A agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone. All of these compounds produced dose-related decreases in DOI-induced head twitches; however pretreatment with the 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide (WAY-100635) failed to alter the ability of ritanserin, ketanserin or CGS 18102A to attenuate DOI-induced head twitches. In contrast, WAY-100635 completely blocked the effects of 8-OH-DPAT, buspirone and WY-50,324, and partially blocked the effects of LEK-8804, demonstrating that 5-HT1A agonist properties are involved in the effects of all of the mixed compounds except CGS 18102A. In rats trained to discriminate DOI (0.63 mg/kg) from saline in a two-lever, FR10 drug discrimination paradigm, ketanserin, ritanserin and CGS 18102A blocked the DS effects of the training dose by more than 50%. In contrast, WY-50,324, FG5974, LEK-8804, buspirone and 8-OH-DPAT, up to doses that completely suppressed responding, failed to produce more than a 33% blockade of the DS effects of DOI. In vivo 5-HT1A agonist effects were demonstrated by the finding that relatively selective- and mixed-5-HT1A agonists produced one or more elements of the "serotonin syndrome," i.e., flat-body posture, forepaw treading, or lower-lip retraction, and produced high levels of drug-lever selection in rats trained to discriminate 8-OH-DPAT (0.16 mg/kg) from saline. Because DOI-induced head twitches and DS effects are thought to be mediated by 5-HT2A receptors, the results demonstrate that the putative mixed compound, CGS 18102A has prominent 5-HT2A antagonist properties in vivo, whereas 5-HT2A antagonist effects of WY-50,324, FG5974 and LEK-8804 could not be clearly identified.

How to express an effect mean as percentage of a control mean?

The expression of effects as % of corresponding controls requires to consider the variance of these controls. Fieller's Theorem allows the appropriate calculation of the deviation of % values. The Zero-Method, which wrongly assumes that the variance of controls is zero, may lead to doubtful interpretations, as shown with experimental examples on the inhibition of depolarization-evoked transmitter release.

Ritanserin attenuates the in vitro effects of the 5-HT2 receptor agonist DOI on skeletal muscles from malignant hyperthermia-susceptible patients

STUDY OBJECTIVES

To study the in vitro effects of the serotonin2 (5-HT2) receptor agonist 1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in skeletal muscle specimens from malignant hyperthermia-susceptible (MHS) and normal (MHN) patients following pretreatment with the 5-HT2 receptor antagonist ritanserin.

DESIGN

Prospective study.

SETTING

Malignant hyperthermia (MH) laboratory at a university hospital.

PATIENTS

41 patients undergoing in vitro contracture test for diagnosis of MH susceptibility.

INTERVENTIONS

Skeletal muscle biopsies in adult patients were performed with a 3-in-1 nerve block with 40 ml prilocaine 1%. In children, general anesthesia was induced with 50 micrograms/kg alfentanil intravenously (i.v.) and 2 to 2.5 micrograms/kg propofol i.v. and maintained with a continuous infusion of propofol (< or = 150 micrograms/kg/min) and nitrous oxide (66%) in oxygen.

MEASUREMENTS AND MAIN RESULTS

Patients were first classified as MHS or MHN by the in vitro contracture test according to the European MH protocol. Surplus muscle specimens of 21 MHS and 20 MHN patients were used in this study. At first, DOI was added to the organ bath at a concentration of 0.02 mM. In the second part of the study, muscle specimens were preincubated with ritanserin 0.01 mM for 10 minutes before DOI 0.02 mM was added to the bath. Muscle specimens from all patients developed contractures after administration of DOI. The onset of contractures was significantly faster in MHS muscles, and the magnitude of contracture was significantly greater than in MHN. The muscle twitch decreased significantly in both groups after DOI. After pretreatment with ritanserin, start of contracture was significantly delayed in MHS muscles. MHN muscles failed to develop contractures. The maximum level of contracture was significantly reduced in MHS. Muscle twitch decreased also in both MHS and MHN groups.

CONCLUSIONS

The findings may indicate that stimulation of 5-HT2 receptors is involved in MH induction. Furthermore, 5-HT2 receptor antagonists could possibly be effective in preventing MH. Additional studies are required to determine if administration of 5-HT2 receptor antagonists could be of additional value in the treatment or prevention of anesthetic-induced MH.

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D1 receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K(+)-evoked [3H]ACh release while SCH 23390, a dopamine D1 receptor antagonist, had no effect. [3H]ACh release was decreased by the dopamine D2 receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D2 receptor antagonist haloperidol. The selective neurokinin NK1 receptor agonist [Sar9, met(O2)11]SP also potentiated K(+)-evoked release of [3H]ACh. GR 82334, a NK1 receptor antagonist, blocked not only the effect of [Sar9, met(O2)11]SP but also the release of ACh induced by the D1 receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D1 receptor stimulation. Mesulergine, a more selective 5-HT2C antagonist, showed an intrinsic releasing effect but did not affect K(+)-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT1/2 antagonists, as well as ondansetron, a 5-HT3 receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT2 receptor agonists were ineffective. However, the 5-HT2 agonist DOI was able to prevent the antagonism by ketanserin of the increased [3H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK1 agonist. The results suggest that 5-HT2 receptors, probably of the 5-HT2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D1 receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK1 receptors located on cholinergic interneurons.

8-OH-DPAT, a 5-HT1A agonist and ritanserin, a 5-HT2A/C antagonist, reverse haloperidol-induced catalepsy in rats independently of striatal dopamine release

In this study, both catalepsy and changes in extracellular levels of striatal dopamine (DA) and dihydroxyphenyl acetic acid (DOPAC) induced by the typical neuroleptic haloperidol (HAL) were simultaneously assessed, using intracerebral microdialysis in freely moving rats, in the presence of either the 5-HT1A agonist 8-OH-DPAT or the 5-HT2A/C antagonist ritanserin. HAL (1 mg/kg, SC) elicited a strong cataleptic state, reaching its maximal intensity (about 240 s) 2 h after the drug administration. This effect was paralleled by a long-lasting enhancement of striatal DA and DOPAC extracellular levels, reaching 230 and 350% of basal values, respectively. 8-OH-DPAT (0.1 mg/kg, SC) given 2.5 h after, and ritanserin (0.63 and 1.25 mg/kg, IP), given 15 min prior to HAL, significantly reduced the neuroleptic-induced catalepsy. However, both 5-HT agents failed to modify basal DA and DOPAC striatal outflow as well as the stimulatory effect of HAL on these parameters. It can thus be concluded that the anticataleptic effect of these compounds is not related to an alteration of DA release within the striatum.

Ritanserin administration potentiates amphetamine-stimulated dopamine release in the rat prefrontal cortex

1. Administration of serotonin 5-HT2 receptor antagonists increases the basal release of dopamine in the mesocorticolimbic pathway. 2. Treatment with dopamine D2 receptor antagonists increases impulse-dependent basal dopamine release in the nigrostriatal pathway. D2 antagonists also potentiate carrier-mediated increases in DA efflux from this pathway. 3. The present study compared the effects of a 5-HT2A/C antagonist (ritanserin) and a D2 antagonist (haloperidol) on carrier-mediated (amphetamine-induced) DA release in the mesocortical system. 4. In vivo microdialysis was used to recover extracellular fluid from the medial prefrontal cortex of conscious rats. Samples were then assayed for dopamine content by HPLC with electrochemical detection. Haloperidol or ritanserin were administered systemically (i.p.) 30 min before d-amphetamine (5.0 mg/kg i.p.). 5. Results demonstrated that 5.0 mg/kg ritanserin, but not 1.0 mg/kg, potentiated amphetamine-induced DA release in the prefrontal cortex. Similar to previous findings in the striatum, haloperidol (1.0 mg/kg) also augmented amphetamine-stimulated DA efflux in the cortex. 6. These results suggest that 5-HT2 and D2 receptor antagonists increase impulse-mediated dopamine release in the rat prefrontal cortex which in turn potentiates carrier-mediated release.

Partial role of 5-HT2 and 5-HT3 receptors in the activity of antidepressants in the mouse forced swimming test

The present study was designed to evaluate the roles of 5-HT2 and 5-HT3 receptors in the mouse forced swimming test, by using selective agonists and antagonists of 5-HT(2A/C) and 5-HT3 receptor sites. Agonists/antagonists and antidepressants were administered 45 min and 30 min, respectively, prior to testing. Pretreatment with (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) (4 mg/kg, i.p.) or 2-methyl-5-HT (4 mg/kg, i.p.) had no effect on the anti-immobility effects of any antidepressant tested. Prior administration of ritanserin (4 mg/kg, i.p.) or ketanserin (8 mg/kg, i.p.), on the other hand, potentiated the effects of sub-active doses of imipramine (8 mg/kg, i.p.) and desipramine (16 mg/kg, i.p.) but not of maprotiline (8 mg/kg, i.p.), fluoxetine (16 mg/kg, i.p.), citalopram (16 mg/kg, i.p.) or fluvoxamine (8 mg/kg, i.p.). Pretreatment with ondansetron (1 X 10(-5) mg/kg, i.p.) enhanced the antidepressant-like effects of sub-active doses of the selective serotonin reuptake inhibitors. The results of the present study suggested that, in the forced swimming test, the selective serotonin reuptake inhibitors act partially through 5-HT3 receptor sites, whereas the tricyclic antidepressants exert effects at 5-HT(2A/C) receptor sites. Anti-immobility effects of the selective noradrenaline reuptake inhibitor, maprotiline, do not seem to be mediated by 5-HT(2A/C) or 5-HT3 receptor function.

5-HT2B receptor-mediated serotonin morphogenetic functions in mouse cranial neural crest and myocardiac cells

During embryogenesis, serotonin has been reported to be involved in craniofacial and cardiovascular morphogenesis. The detailed molecular mechanisms underlying these functions, however remain unknown. From mouse and human species, we have recently reported the cloning of 5-HT2B receptors which share signal transduction pathways with other 5-HT2 receptor subtypes (5-HT2A and 5-HT2C). In addition to phospholipase C stimulation, it appears that these three subtypes of receptor transduce a common serotonin-induced mitogenic activity, which could be important for cell differentiation and proliferation. We have first investigated the expression of 5-HT2 receptor mRNAs in the mouse embryo. Interestingly, a peak of 5-HT2B receptor mRNA expression was detected 8–9 days postcoitum, whereas there was only low level 5-HT2A and no 5-HT2C receptor mRNA expression at this stage. Expression of this receptor was confirmed by binding assays using a 5-HT2-specific ligand which revealed a peak of binding to membrane preparations from 9 days postcoitum embryos. In addition, whole mount in situ hybridisation and immunohistochemistry on similar stage embryos detected 5-HT2B expression in neural crest cells, heart myocardium and somites. The requirement for functional 5-HT2B receptors between 8 and 9 days postcoitum is supported by culture of embryos exposed to 5-HT2-specific ligands; 5-HT2B high-affinity antagonist such as ritanserin, induced morphological defects in the cephalic region, heart and neural tube. These antagonistic treatments interfere with cranial neural crest cell migration, induce their apoptosis, and are responsible for abnormal sarcomeric organisation of the subepicardial layer and for the absence of the trabecular cell layer in the ventricular myocardium. This report indicates for the first time that 5-HT2B receptors are actively mediating the action of serotonin on embryonic morphogenesis, probably by preventing the differentiation of cranial neural crest cells and myocardial precursor cells.

alpha2-Adrenoceptor antagonists reverse the 5-HT2 receptor antagonist suppression of head-twitch behavior in mice

The alpha2-adrenoceptor agonist clonidine, as well as 5-HT2 receptor antagonists, reportedly suppress 5-HT2 receptor-mediated head-twitch behavior. We investigated the effect of alpha2-adrenoceptor antagonists on the suppressive action of 5-HT2 receptor antagonists in mice pretreated with the noradrenaline toxin 6-hydroxydopamine (6-OHDA) or the 5-HT synthesis inhibitor p-chlorophenylalanine (p-CPA). In normal mice, idazoxan (0.08-0.2 mg/kg, IP) or yohimbine (0.2-2.0 mg/kg, IP), both alpha2-adrenoceptor antagonists, had no effect on the head-twitch response caused by 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 16 mg/kg, IP), but idazoxan significantly enhanced the response at 0.5 mg/kg. On the other hand, these alpha2-adrenoceptor antagonists, at doses that had no effect on the basal number of head-twitches (idazoxan 0.2 mg/kg and yohimbine 0.5 mg/kg), significantly attenuated not only the suppressive effect of clonidine (0.01 mg/kg, IP) on head-twitch response but also that of the 5-HT2 receptor antagonist ritanserin (0.03 mg/kg, IP). Moreover, idazoxan (0.2 mg/kg) also significantly reversed the inhibition by 0.01 mg/kg (IP) ketanserin, a selective 5-HT2 receptor antagonist. Pretreatment with 6-OHDA plus nomifensine but not with p-CPA significantly attenuated the effect of idazoxan (0.2-0.5 mg/kg) on the ritanserin inhibition of the head-twitch response. Prazosin, an alpha1-adrenoceptor antagonist, dose-dependently suppressed the response, and the effect of prazosin (1.25 mg/kg) was significantly attenuated by 0.5 mg/kg idazoxan. These results indicate that endogenous noradrenaline is involved in the apparent antagonistic interaction between selective alpha2-adrenoceptor antagonists and 5-HT2 receptor antagonists in the head-twitch response, and suggest that noradrenaline stimulation of alpha1-adrenoceptors may be involved in this apparent antagonism.

[Ritanserin prolongs the analgesic effect of morphine and slows the development of tolerance]

The effect of 5-HT2 receptor blocker ritanserine on the analgesic effect of morphine was studied in experiments on mice. A single simultaneous injection of ritanserine (10 mg/kg) and morphine (1, 10, and 20 mg/kg) prolonged the duration of analgesia in the tail clip test. Combined subchronic (6 days) injection of morphine and ritanserine (1, 5, and 10 mg/kg) twice a day delayed the development of tolerance to the opiate analgesic effect in the hot plate and tail clip tests.

Differential involvement of serotonin 2A/C and thromboxane A2/prostanoid receptors in high- vs. low-shear rate arterial thrombosis in rabbits

Experiments performed in 226 pentobarbitone-anesthetized rabbits were designed to investigate the involvement of thromboxane/prostanoid and 5-hydroxytryptamine (5-HT)2A/C receptors during arterial thrombus formation in distinct low- and high-shear rate thrombosis models. Antithrombotic activities of the thromboxane/prostanoid receptor antagonist SQ 29,548 and two chemically distinct 5-HT2A/C receptor antagonists, ritanserin and ketanserin, were assessed first in low-shear rate (approximately 600 sec(-1)) arterial thrombosis, produced by insertion of a silk thread as thrombogenic substrate into the central section of an extracorporeal arteriovenous shunt established between the left carotid artery and the right jugular vein (n = 77), and second in high-shear rate (approximately 40,000 sec(-1)) arterial thrombosis, produced by critical stenosis and local endothelial injury of a carotid artery, characterized by cyclic flow reductions (CFRs) due to recurrent platelet aggregation and subsequent dislodgement of the thrombus (n = 149). Under low shear rate, SQ 29,548 (10-2500 microg/kg plus 10-2500 microg/kg/hr i.v.), but not ritanserin or ketanserin (both at 2500 microg/kg i.v.), dose-dependently inhibited thrombus formation. In contrast, under high shear rate, SQ 29,548 (10-160 microg/kg plus 10-160 microg/kg/hr i.v.) and both ritanserin and ketanserin (both at 10-2500 microg/kg i.v.) dose-dependently reduced CFR frequency, with ID50 values of 35 microg/kg (95% confidence limits, 24-58 microg/kg), 77 microg/kg (95% confidence limits, 40-132 microg/kg) and 89 microg/kg (95% confidence limits, 36-285 microg/kg) i.v., respectively. Furthermore, local infusion of the stable thromboxane A2 analog U-46619 (0.63 microg/kg/min) or 5-HT (20.8 microg/kg/min) proximal to the site of injury and stenosis in rabbits pretreated with either SQ 29,548 (40 microg/kg plus 40 microg/kg/hr i.v.) or ritanserin (160 microg/kg i.v.), respectively, restored CFR frequency to vehicle group levels in animals whose CFR frequency was previously reduced. The inhibitory activity of ketanserin and ritanserin on CFRs could not be attributed to 5-HT1B/D or alpha-1 adrenoceptor antagonist properties or to any hypotensive activity. These results provide firm evidence that thromboxane/prostanoid receptors are involved in arterial thrombosis in rabbits independently of the shear rate, whereas 5-HT2A/C receptors play a major role only in high-shear rate thrombus formation.

5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption in rabbit jejunum

The aim of the present study was to determine the 5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption across rabbit jejunum in vitro. A number of agonists and antagonists were used to characterize the receptors through which serotonin inhibits this absorption. The results show that 2.5x10(-6) M 5-HT inhibits the amino acid absorption by about 20%. The 5-HT receptor agonists, alpha-methyl-5-HT (5-HT2), 2-methyl-5-HT (5-HT3) and zacopride (5-HT4) at concentrations 2.5x10(-6) and 2.5x10(-5) M produced 10-30% inhibition on L-leucine intestinal absorption. 5-carboxyamidotryptamine (5-HT1) did not produce any inhibition. The 5-HT antagonists, GR 113808A (5-HT4) at 2.5x10(-6) M and ritanserin (5-HT2) and ondansetron (5-HT3) at 2.5x10(-5) M completely blocked the effect of 5-HT. However, methiothepin (5-HT1) did not produce any effect on serotonin action in the intestinal absorption of amino acid. It can be concluded that 5-HT2, 5-HT3 and 5-HT4 receptors could mediate inhibition of L-leucine absorption across rabbit jejunum.

Effects of 5HT2 receptor antagonists on responses to potassium depolarization in rat isolated aorta

1. In order to determine whether 5HT2 receptor antagonists can modify Ca2+ uptake via voltage-operated Ca2+ channel (VOC) in arterial smooth muscle, a comparative study of the effects of selected Ca2+ uptake blockers and 5HT2 receptor antagonists on K(+)-induced contractions of rat aortic strip was undertaken. 2. The antagonist drugs studied included the Ca2+ uptake blockers verapamil, nifedipine, felodipine, diltiazem and cinnarizine, the 5HT2 receptor antagonists cyproheptadine, ritanserin, mianserin, and ketanserin and the alpha 1-adrenoceptor antagonist prazosin. 3. With the notable exception of prazosin, each of these compounds diminished K(+)-induced aortic responses. 4. The following order of potencies (mean IC50 values in mol/L) was established: felodipine (7.0 x 10(-11)) > nifedipine (4.8 x 10(-9) > verapamil (5.5 x 10(-8)) > cyproheptadine (6.2 x 10(-8)) > diltiazem (4.1 x 10(-7)) > cinnarizine (1.3 x 10(-6)) > ritanserin (1.8 x 10(-6)) > ketanserin (9.0 x 10(-6)) > mianserin (2.0 x 10(-5)). 5. The results suggest that antagonists of 5HT2 receptors can modulate Ca2+ uptake via VOC in rat aorta.

A quantitative evaluation of the relationships between growth hormone secretion and delta wave electroencephalographic activity during normal sleep and after enrichment in delta waves

The existence of a relationship between growth hormone (GH) release and slow-wave sleep (SWS), often studied in the past using conventional scoring of sleep stages, remains controversial. In the present study, this relationship was reevaluated by spectral analysis of the sleep electroencephalogram (EEG) and deconvolution analysis of the plasma GH concentrations during normal nocturnal sleep and after enrichment in SWS by means of ritanserin, a selective 5-HT2 receptor antagonist. Eight healthy male subjects each participated in two randomized night studies after having received either a placebo or a 5-mg dose of ritanserin. They were subjected to 8 hours of polysomnography, including spectral analysis of the sleep EEG. Plasma GH levels were measured at 10-minute intervals. The mean delta absolute power and the mean GH secretory rates were significantly higher under ritanserin than under placebo for the first 3 hours after sleep onset (+24% and +29%, respectively). Their nocturnal profiles were significantly and positively correlated in all subjects (average r = 0.710 under placebo, 0.567 under ritanserin; p < 0.0001 in both cases). GH secretory pulses were found to be coincident with delta activity peaks in both directions. The amount of GH secreted during significant GH pulses was correlated with the amount of concomitant delta wave activity (r = 0.803 under placebo, r = 0.764 under ritanserin, p < 0.0001). Similarly, the amount of delta wave activity found during delta wave peaks was correlated with the amount of GH secreted concomitantly (r = 0.715 under placebo, r = 0.723 under ritanserin: p < 0.0001). These results demonstrate a close temporal and quantitative relationship between GH secretion and delta wave activity, which may be evidence of common stimulatory mechanisms.

Effects of dopamine antagonists in a two-way active avoidance procedure in rats: interactions with 8-OH-DPAT, ritanserin, and prazosin

Using a conditioned avoidance procedure in rats, the present study examined the ability of 8-OH-DPAT, ritanserin, and prazosin to alter the effects of the dopamine antagonists, raclopride and haloperidol, on avoidance- and on escape responding. The 5-HT1A agonist 8-OH-DPAT (0.16 mg/kg) significantly enhanced the inhibitory effects of both raclopride and haloperidol on the conditioned avoidance response and produced a small enhancement of the effects of haloperidol on escape failures. the alpha 1-adrenoceptor antagonist prazosin (0.63 mg/kg) significantly enhanced the effects of raclopride on the conditioned avoidance response, but enhanced the effects of only a single dose of haloperidol; prazosin did not alter the effects of either dopamine antagonist on escape failures. The 5-HT2 antagonist ritanserin (0.16 mg/kg) failed significantly to alter the effects of the dopamine antagonists examined here. These findings suggest that blockade of 5-HT2 receptors may not enhance the antipsychotic efficacy of D2-like antagonists. Further, they confirm previous findings with respect to interactions between 5-HT1A agonists and neuroleptics, and support the hypothesis that combined 5-HT1A agonist/D2-like antagonist properties may be of clinical importance.

Enhancement of antipsychotic-like properties of the dopamine D2 receptor antagonist, raclopride, by the additional treatment with the 5-HT2 receptor blocking agent, ritanserin, in the rat

The effects of 5-HT2 receptor blockade on the ability of a dopamine (DA) D2 receptor antagonist to produce suppression of conditioned avoidance response (CAR) and to produce catalepsy in rats were examined. It was found that ritanserin (2 mg kg-1 s.c.) enhanced the raclopride (0.1 mg kg-1 s.c.)-induced suppression of CAR without affecting raclopride-induced catalepsy at either maximal (4 mg kg-1 s.c.) or submaximal (0.2 mg kg-1 s.c.) doses. Considering the CAR performance as an index of mesocorticolimbic dopaminergic functions, it is concluded that 5-HT2 receptor blockade confers a limbic profile on the DA D2 receptor antagonist.

Ritanserin blocks DOI-altered embryonic motility and posthatch learning in the developing chicken

Developing chicken embryos exposed to cocaine show altered motility, hatchability, and posthatch detour learning. Pretreating such subjects with the serotonin2 (5-HT2) antagonist ritanserin (RIT) can block the motility suppression and reduced hatchability, indicating 5-HT2 receptor involvement in these cocaine effects. To study behavioral consequences of more selective 5-HT2 receptor stimulation and its blockade during development and to compare such exposure with that of cocaine, we injected eggs with 15-day-old chicken embryos with the 5-HT2 agonist dimethoxyiodophenylaminopropane (DOI, 1.0 mg/kg egg) and 1 h later, with RIT (0.3 and 0.9 mg/kg egg). Motility was recorded 2.5 or 24 h after DOI. This DOI dose suppressed motility 2.5 h but not 24 h after administration. Both RIT doses blocked DOI's motility suppression. No treatment affected hatchability. Subjects were tested on posthatch days 6-9 for detour learning acquisition. DOI "enhanced" learning (i.e., reduced latency), a cocaine-like effect observed in prior work, which was also blocked by both RIT doses. Thus, some consequences of DOI exposure late during embryonic development resemble cocaine's and are blocked by RIT, suggesting a therapeutic role for RIT-like drugs against cocaine's potential developmental toxicity.