Effect of 5HT2 receptor blockade on the startle reflex and its prepulse inhibition in mice and rats of various strains]

Effects of 5-HT2 receptor blockade on the amplitude of startle reflex, induced by an unexpected sound, and on its prepulse inhibition (PPI) were studied on mice of CBA strain and rats of Wistar and the genetically predisposed to catalepsy (GC) strains. The effect was dependent on type and dose of 5-HT2 antagonist used: 5-HT2A antagonist ketanserin increased startle amplitude at the dose of 0.5 mg/kg and decreased it at the dose of 2 mg/kg. Mixed 5-HT2A/2C antagonist ritanserin (0.1 and 0.2 mg/kg) markedly increased startle in mice. Ketanserin and cyproheptadine produced opposite effects on startle reflex in rats with inherited neuropathology and in rats with normal genotype: marked decrease in GC rats and increase in Wistar rats was shown. Ketanserin and cyproheptadine produced a pronounced potentiation of PPI in mice and rats of both strains, ritanserin was ineffective. Results suggest 5-HT2 receptors implication in both startle and PPI regulation with 5-HT2C receptors in startle response and 5-HT2A in PPI predominant involvement.     

Different receptor subtypes are involved in the serotonin-induced modulation of epileptiform activity in rat frontal cortex in vitro.

The frontal cortex is innervated by serotonergic terminals from the raphe nuclei and it expresses diverse 5-HT receptor subtypes. We investigated the effects of 5-HT and different 5-HT receptor subtype-selective agonists on spontaneous discharges which had developed in rat cortical slices perfused with a Mg2+-free medium and the GABA(A) receptor antagonist picrotoxin. The frequency of synchronous discharges, recorded extracellularly in superficial layers (II/III) of the frontal cortex, was dose-dependently enhanced by 5-HT (2.5-40 microM). That excitatory effect was blocked by the 5-HT2 receptor selective antagonist ketanserin. The 5-HT2A/2C receptor-selective agonist DOI and the 5-HT4 receptor agonist zacopride also increased the frequency of spontaneous discharges. In the presence of ketanserin, 5-HT decreased the discharge rate; a similar effect was observed when the 5-HT1A receptor agonist 8-OH-DPAT or the 5-HT1B receptor agonist CGS-12066B was applied. The 5-HT3 receptor agonist m-CPBG was ineffective. In conclusion, 5-HT produces multiple effects on epileptiform activity in the frontal cortex via activation of various 5-HT receptor subtypes. The excitatory action of 5-HT, which predominates, is mediated mainly by 5-HT2 receptors. The inhibitory effects can be attributed to activation of 5-HT1A and 5-HT1B receptors.     

Locomotor and peripheral effects of sibutramine modulated by 5-HT2 receptors

Sibutramine has been described as an anti-obesity drug with the ability to inhibit serotonin (5-HT), noradrenaline, and dopamine re-uptake, but without affinity to histamine and muscarinic receptors. On the other hand, cyproheptadine antagonizes serotonin 5-HT(2A), 5-HT(2B), and 5-HT(2C), histamine H1, and muscarinic (M) receptors. There are many reports concerning the influence of sibutramine on central serotoninergic pathways. In this study, we suggest that peripheral pathways may also be involved in the serotoninergic effects of sibutramine. In vivo experiments were undertaken to investigate the serotoninergic effects of sibutramine on body mass, the glycogen concentration in the diaphragm of rats, and locomotor behaviour. Rats were submitted to oral treatment with sibutramine, cyproheptadine, or sibutramine applied in combination with cyproheptadine, for a period of 2 months to investigate the 5-HT2 effects of sibutramine on these parameters. As the results demonstrated, the lower increase in body mass and the increased glycogen levels in the diaphragm muscle of rats treated with sibutramine seem to be modulated by 5-HT2 receptors, since these effects were completely antagonized by cyproheptadine in the group treated with the 2 drugs co-applied. Furthermore, the behavioural results also suggest that mechanisms modulated by 5-HT2 receptors are involved in the increase of locomotion in the rats treated with sibutramine, since the effect did not occur in the rats treated with sibutramine co-applied with the 5-HT2 receptor antagonist, cyproheptadine. The results suggest that sibutramine modifies energy-related parameters such as body mass, diaphragm glycogen, and locomotor behaviour in rats via 5-HT2 serotoninergic pathways.     

Effects of imipramine and serotonin-2 agonists and antagonists on serotonin-2 and beta-adrenergic receptors following noradrenergic or serotonergic denervation

The effects of chronic (14 day) administration of the tricyclic antidepressant imipramine, the serotonin-2 (5-HT2) antagonist ketanserin, and the serotonin agonist quipazine on 5-HT2 receptor binding parameters and 5-HT2-mediated behavior were examined in rats with or without prior serotonergic denervation [via 5,7-dihydroxytryptamine (5,7-DHT)] or noradrenergic denervation [via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)]. Chronic administration of imipramine, ketanserin, or quipazine produced a marked reduction in the number of 5-HT2 binding sites which was accompanied by reductions in the 5-HT2-mediated quipazine-induced head shake response. In animals receiving DSP4 or 5,7-DHT lesions and continuous vehicle treatment, beta-adrenergic receptor binding sites were significantly up-regulated while 5-HT2 receptor binding sites did not change. Imipramine normalized the lesion-induced increases in beta-adrenergic binding observed in DSP4 and 5,7-DHT-lesioned rats but failed to down-regulate beta-adrenergic binding sites below non-lesioned control levels. Chronic imipramine, ketanserin, and quipazine reduced quipazine-induced head shakes and down-regulated 5-HT2 binding sites in rats with noradrenergic denervation. While imipramine, ketanserin, and quipazine all down-regulated 5-HT2 binding sites in animals with serotonergic denervation, only imipramine's ability to reduce quipazine-induced head shakes was attenuated in 5,7-DHT-lesioned rats. The present results suggest that imipramine-induced down-regulation of 5-HT2 receptors may not involve presynaptic 5-HT mechanisms, and imipramine-induced alterations in 5-HT2 sensitivity as reflected in the quipazine-induced head shake may, in part, be influenced by beta-adrenergic receptors.     

Serotonin-induced hypoglycemia and increased serum insulin levels in mice

The effects of serotonin (5-HT) on plasma glucose levels were studied. 5-HT above 20 mg/kg induced apparent hypoglycemia in mice. The hypoglycemic effects of 5-HT were strongly antagonized by methysergide but only partially inhibited by ketanserin. However, ICS 205-930 was without effect. This indicates that the hypoglycemia induced by 5-HT is mediated by both the 5-HT1 and 5-HT2 receptors. 5-HT also produced an increase in serum immunoreactive insulin (IRI) which was completely inhibited by methysergide and partially antagonized by ketanserin. It is suggested that the 5-HT-induced increase in IRI is elicited by the activation of the 5-HT1 and 5-HT2 receptors, which is similar to the results obtained with plasma glucose. These results indicate that the 5-HT receptors may regulate blood glucose levels by modifying the release of insulin.     

Divergent effects of pirenperone, a 5-HT2 antagonist, on the pressor and tachycardic responses to 5-HT in guinea-pigs

The effects of pirenperone and cyproheptadine on the pressor and tachycardic responses to 5-hydroxytryptamine (5-HT) and to dimethylphenylpiperazinium (DMPP) were compared. Both 5-HT antagonists suppressed in a dose-dependent manner the pressor effect of 5-HT, whilst did not noticeably affect the tachycardic effect of 5-HT and the cardiovascular effects of DMPP. On the molecular base, pirenperone was 15 times more potent antagonist of the pressor response to 5-HT than cyproheptadine. It is concluded that not only the 5-HT receptors in arterial smooth muscle but also the 5-HT receptors in sympathetic ganglia and the adrenal medulla responsible for the pressor response to 5-HT are sensitive to the 5-HT antagonists and probably analogous to the central 5-HT2 receptors. The 5-HT receptors in cardiac tissue mediating tachycardia differ in their pharmacological properties from those in arterial smooth muscle responsible for contraction. It is suggested that the ganglionic components of the pressor and tachycardic responses to 5-HT are mediated via different populations of 5-HT receptors in sympathetic ganglia.     

Stimulation of the immune response during blockade of serotonin receptors by cyproheptadine

The role of serotonin receptors in the inhibitory effect of serotoninergic system on immunogenesis was studied using cyproheptadine, a specific blocker of 5-HT2 receptors. It was shown that cyproheptadine administration to CBA mice stimulated the immune response, which was dopamine-dependent and was realized via thymus. With the pituitary stalk destruction, the stimulatory effect of cyproheptadine was not observed, which suggests the participation of 5-HT2 brain receptors in immunogenesis.     

In vivo regulation of the serotonin-2 receptor in rat brain

Serotonin-2 (5-HT-2) receptors in brain were measured using [3H]ketanserin. We examined the effects of amitriptyline, an antidepressant drug, of electroconvulsive shock (ECS) and of drug-induced alterations in presynaptic 5-HT function on [3H]ketanserin binding to 5-HT-2 receptors in rat brain. The importance of intact 5-HT axons to the up-regulation of 5-HT-2 receptors by ECS was also investigated, and an attempt was made to relate the ECS-induced increase in this receptor to changes in 5-HT presynaptic mechanisms. Twelve days of ECS increased the number of 5-HT-2 receptors in frontal cortex. Neither the IC50 nor the Hill coefficient of 5-HT in competing for [3H]ketanserin binding sites was altered by ECS. Repeated injections of amitriptyline reduced the number of 5-HT-2 receptors in frontal cortex. Reserpine, administered daily for 12 days, caused a significant increase in 5-HT-2 receptors, but neither daily injections of p-chlorophenylalanine (PCPA) nor lesions of 5-HT axons with 5,7-dihydroxytryptamine (5,7-DHT) affected 5-HT-2 receptors. However, regulation of 5-HT-2 receptors by ECS was dependent on intact 5-HT axons since ECS could not increase the number of 5-HT-2 receptors in rats previously lesioned with 5,7-DHT. Repeated ECS, however, does not appear to affect either the high-affinity uptake of [3H]5-HT or [3H]imipramine binding, two presynaptic markers of 5-HT neuronal function. 5-HT-2 receptors appear to be under complex control. ECS or drug treatments such as reserpine or amitriptyline, which affect several monoamine neurotransmission systems including 5-HT, can alter 5-HT-2 receptors. While depleting 5-HT alone (5,7-DHT or PCPA) does not alter [3H]ketanserin binding to 5-HT-2 receptors, intact 5-HT axons are necessary for the adaptive up-regulation of the receptor following ECS.     

Pharmacological characterization of a 5-hydroxytryptamine-sensitive receptor/adenylate cyclase complex in the mandibular closer muscles of the cricket, Gryllus domestica.

The mandibular closer muscles of the cricket, Gryllus domestica, contain a 5-hydroxytryptamine (5-HT)-sensitive receptor that is coupled to adenylate cyclase. A structure-activity study of the 5-HT molecule indicates that the integrity of the ethylamine sidegroup and the presence of a negatively charged moiety at the 5 position (-OH, -OCH3) are essential for activity. A pharmacological profile is presented for this receptor. The receptor differs from any reported mammalian 5-HT receptor in that none of the mammalian agonists tested were effective. However, the mammalian antagonists for 5-HT receptors, spiperone, mianserin, and ketanserin as well as the anti-histaminic cyproheptadine were all effective antagonists in this preparation. Preliminary analysis of antagonism, particularly by spiperone, shows that these antagonists are probably acting non-competitively. On the basis of the pharmacological data, and comparisons with other insect systems, the 5-HT receptor present in the cricket mandibular muscles has been tentatively classified as 5-HT2-like.     

Endogenous 5-HT2B receptor activation regulates neonatal respiratory activity in vitro

An implication of 5-HT(2B) receptors in central nervous system has not yet been clearly elucidated. We studied the role of different 5-HT(2) receptor subtypes in the medullary breathing center, the pre-B?tzinger complex, and on hypoglossal motoneurons in rhythmically active transversal slice preparations of neonatal rats and mice. Local microinjection of 5-HT(2) receptor agonists revealed tonic excitation of hypoglossal motoneurons. Excitatory effects of the 5-HT(2B) receptor agonist BW723C86 could be blocked by bath application of LY272015, a highly selective 5-HT(2B) receptor antagonist. Excitatory effects of the 5-HT(2A/B/C) receptor agonist alpha-methyl 5-HT could be blocked by the preferential 5-HT(2A) receptor antagonist ketanserin. Therefore, 5-HT-induced excitation of hypoglossal motoneurons is mediated by convergent activation of 5-HT(2A) and 5-HT(2B) receptors. Local microinjection of BW723C86 in the pre-B?tzinger complex increased respiratory frequency. Bath application of LY272015 blocked respiratory activity, whereas ketanserin had no effect. Therefore, endogenous 5-HT appears to support tonic action on respiratory rhythm generation via 5-HT(2B) receptors. In preparations of 5-HT(2B) receptor-deficient mice, respiratory activity appeared unaltered. Whereas BW723C86 and LY272015 had no effects, bath application of ketanserin disturbed and blocked rhythmic activity. This demonstrates a stimulatory role of endogenous 5-HT(2B) receptor activation at the pre-B?tzinger complex and hypoglossal motoneurons that can be taken up by 5-HT(2A) receptors in the absence of 5-HT(2B) receptors. The presence of functional 5-HT(2B) receptors in the neonatal medullary breathing center indicates a potential convergent regulatory role of 5-HT(2B) and -(2A) receptors on the central respiratory network.     

Serotonin (5-HT2) receptor mediated enhancement of cortical unit activity.

Simultaneous single-unit and intracortical activity were recorded from neocortical neurons in urethane-anaesthetized rats to investigate the role of serotonin (5-HT) in modifying cortical excitability. Units, at a depth of 775-1100 microns from the pial surface, discharged in a burst-pause pattern that was correlated with slow wave activity. Application of noxious somatic stimulation resulted in cortical desynchronization and altered the pattern of unit activity such that firing was continuous, i.e., the pauses were eliminated. Intravenous administration of the mixed 5-HT1C/5-HT2 antagonists (cinanserin, cyproheptadine, ketanserin, and ritanserin) prevented both desynchronization and the change in unit activity induced by noxious stimulation within 2.5-15 min of the injection. The basic pattern of burst-pause activity remained intact, but the number of spikes per burst was typically reduced, whereas interburst intervals were increased. Iontophoretic application of these antagonists onto cortical neurons resulted in actions similar to those observed following systemic administration. Intravenous and iontophoretic application of m-trifluomethylphenylpiperazine (5-HT1C agonist, 5-HT2 antagonist) resulted in actions indistinguishable from those observed with the above antagonists, from which we conclude 5-HT2 and not 5-HT1C receptors mediate the alteration in unit activity observed with noxious stimulation. The results are discussed with respect to an interaction between N-methyl-D-aspartate and 5-HT2 receptors leading to the enhanced unit activity observed with noxious stimulation.     

The effects of serotonin agonists on the hypothalamic regulation of sexual receptivity in Syrian hamsters

One brain region that has been implicated in the regulation of lordosis is the medial preoptic-anterior hypothalamic continuum (MPOA-AH). Previous studies have suggested that this zone may be part of the circuit mediating the effects of serotonin (5-HT) on sexual receptivity. In the present experiments, we investigated the role of 5-HT(1a/7) and 5-HT(2) receptor subtypes in the MPOA-AH in the control of lordosis. In two experiments, either 5-HT(1a/7) or 5-HT(2) agonists were injected unilaterally into the MPOA-AH of ovariectomized, hormonally primed female hamsters. In the first experiment, microinjections of the 5-HT(1a/7) agonist 8-hydroxy-2,9-(di-n-propylamino)tetralin resulted in an attenuation of the lordosis posture by causing a decrease in the mean lordosis duration and an increase in the number of lordosis episodes over the entire testing period. In the second experiment, microinjections of the 5-HT(2b/2c) agonist m-chlorophenylpiperazine did not result in any changes in sexual receptivity. However, microinjections of the 5-HT(2) agonist (2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl facilitated lordosis by increasing the mean lordosis duration and decreasing the number of lordosis episodes in the first 5 min of the testing period. These data indicate that serotonin may act in the MPOA-AH via 5-HT(1a/7) receptors to attenuate and 5-HT(2) receptors to facilitate sexual receptivity.     

5-Hydroxytryptamine3 Receptors Sited on Cholinergic Axon Terminals of Human Cerebral Cortex Mediate Inhibition of Acetylcholine Release

Synaptosomes prepared from freshly obtained human cerebral cortex and labeled with [3H]choline have been used to investigate the modulation of [3H]acetylcholine ([3H]ACh) release by 5-hydroxytryptamine (5-HT). The Ca(2+)-dependent release of [3H]-ACh occurring when synaptosomes were exposed in superfusion to 15 mM KCl was inhibited by 5-HT (0.01-1 microM) in a concentration-dependent manner. The effect of 5-HT was mimicked by 1-phenylbiguanide, a 5-HT3 receptor agonist, but not by 8-hydroxy-2-(di-n-propylamino)tetralin, a 5-HT1A receptor agonist. The 5-HT3 receptor antagonists tropisetron and ondansetron blocked the effect of 5-HT, whereas spiperone and ketanserin were ineffective. It is suggested that cholinergic axon terminals in the human cerebral cortex possess 5-HT receptors that mediate inhibition of ACh release and appear to belong to the 5-HT3 type.     

Attenuating effects of prior oestradiol benzoate priming on 5-HT-mediated lordosis behavior in rats are dose-dependent

The present study was designed to investigate the role of the 5-HT7 receptors in lordosis and compare the lordotic responses with 5-HT1A agent under the influence of different steroid-priming regimens in ovariectomized, non-receptive and receptive rats. 8-OH DPAT, a 5-HT1A agonist and 5-CT, a 5-HT7 agonist inhibited the lordosis differently in non-receptive and receptive rats, however, the response was attenuated in a dose-dependent manner following 5-CT treatment in the first two tests. Treatment with 5-HT1A antagonist, WAY 100 135 caused a protective effect which was evident in the second test only. Priming with 25 microg OB attenuated in the first test in non-receptive rats whereas the same dose repeated a similar pattern in receptive rats. The attenuation of LQ was evident in rats treated with 5-HT7 antagonist, SB 269970-A. This finding shows that WAY 100 135, a 5-HT1A antagonist has potency to attenuate inhibitory influence of 8-OH DPAT by enhancing lordosis behavior acutely in female rats with a low estrous state. Treatment with 5-CT and SB 269970-A as 5-HT7, agonist and antagonist, respectively, have mimicked 5-HT-mediated lordotic response as moderate affinity towards 5-HT1A receptors has been reported. This offers a comparable effect on lordosis as a result of the two 5-HT agents used.     

Activation of peripheral serotonin2 receptors induces hypothermia in mice

The effects of peripherally administered serotonin (5-HT) on the rectal temperature were investigated. 5-HT i.p. induced a dose-dependent hypothermia in mice. The hypothermic effects of 5-HT were strongly antagonized by the 5-HT1 and 5-HT2 receptor antagonist methysergide and the 5-HT2 receptor antagonist ketanserin. However, the 5-HT1 receptor antagonist pindolol and the 5-HT3 receptor antagonist ICS 205-930 were without effect. In addition, the peripheral 5-HT2 receptor antagonist xylamidine strongly reduced 5-HT-induced hypothermia. These results indicate that the activation of the peripheral 5-HT2 receptors induces hypothermia, although the central 5-HT2 receptors have been suggested to relate to hyperthermia.     

Stimulation of adenylate cyclase in the heart of Aplysia californica by biogenic amines

The effects of serotonin (5-HT), dopamine (DA), several peptides including FMRFamide and arginine vasotocin, the diterpene forskolin and Ca2+ were examined on adenylate cyclase in a particulate fraction from hearts of Aplysia californica. Enzyme activity was stimulated 6-7-fold by 5-HT (EC50, 1 microM) in the presence of GTP. Several 5-HT analogs particularly 5-methoxytryptamine and 5-methoxy-N-N-dimethyltryptamine were also active. The stimulatory action of 5-HT was antagonized by the 5-HT receptor blockers methergoline and metitepine and by the DA receptor blocker chlorpromazine. Dopamine had weak stimulatory action (EC50, 10 microM) and an efficacy relative to that of 5-HT of 0.3. The action of DA was antagonized by chloropromazine and metitepine. Several peptides including FMRFamide and arginine vasotocin had no effect on adenylate cyclase when tested over the concentration range 0.1-100 microM. The enzyme was stimulated 6-fold by the diterpene forskolin (EC50, 2 microM). 5-HT-stimulated activity was strongly inhibited by Ca2+. Calmodulin had no action on the enzyme in the presence of Ca2+.     

Antihypertensive properties of ketanserin (R 41 468)

The serotonergic receptor antagonist 3-(2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl)-2,4-[1H, 3H] quinazolinedione (ketanserin) causes dose-dependent inhibition of the effects of 5-hydroxytryptamine (5-HT) on 5-HT2-serotonergic receptors. These receptors mediate facilitation of platelet aggregation, direct vasoconstriction in several arteries and veins, and direct amplification of vasoconstrictor responses to other neurohumoral mediators. Ketanserin does not inhibit vasodilator effects of 5-HT. At higher concentrations, ketanserin has alpha 1-adrenergic blocking properties. The compound causes dose-related reductions in arterial blood pressure in hypertensive animals and humans that are larger and occur at lower doses than in normotensive controls. In humans, the antihypertensive properties of ketanserin do not appear to involve alpha 1-adrenergic inhibition, because the compounds given i.v. (10 mg) do not affect the pressor dose-response curve to phenylephrine.     

5-HT2A serotoninergic receptor in the locus coeruleus participates in the first phase of lipopolysaccharide-induced fever

This study was aimed at testing the hypothesis that serotoninergic receptors in the locus coeruleus (LC) play a role in bacterial lipopolysaccharide-induced fever. To this end, 5-HT1A (WAY-100635; 3 microg/100 nL) and 5-HT2A (ketanserin; 2 microg/100 nL) antagonists were microinjected into the LC and body temperature was monitored by biotelemetry. Intra-LC microinjections of ketanserin or WAY-100635 caused no change in body temperature of euthermic animals. 5-HT2A antagonism abolished the first phase of the lipopolysaccharide-induced fever. Taken together, these results indicate that serotonin acting on 5-HT2A receptors in the LC mediates the first phase of the febrile response, whereas 5-HT1A receptors are not involved in the lipopolysaccharide-induced fever.     

Serotonin-induced enhancement of vasopressin and oxytocin secretion in rat neurohypophyseal tissue culture

The effects of serotonin (5-hydroxytryptamine; 5-HT) on vasopressin (VP) and oxytocin (OT) secretion were studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP and OT contents of the supernatant were determined by radioimmunoassay after a 1 or 2 h incubation. Significantly increased levels of VP and OT production were detected in the tissue culture media following 5-HT administration, depending on the 5-HT dose. The elevation of NH hormone secretion could be partially blocked by previous administration of the 5-HT antagonist ketanserin or metergoline. WAY-100635 did not influence the increased VP secretion induced by 5-HT, but the elevated OT production was prevented by WAY-100635 before 5-HT administration. The application of WAY-100635, ketanserin or metergoline, after 5-HT administration proved ineffective. The results indicate that NH hormone release is influenced directly by the serotonergic system. The serotonergic control of VP and OT secretion from the NH tissue in rats can occur at the level of the posterior pituitary.     

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.