Stimulation Of 5-HT2 Receptors Research Articles

Sexual functioning in depressed outpatients taking mirtazapine.

One-third of patients with untreated depression have sexual difficulties manifested by decreased libido, erectile dysfunction or delayed ejaculation. This dysfunction may be exacerbated by stimulation of post-synaptic serotonin 5HT2 receptors, a side-effect of most widely-used antidepressant medications, especially the selective serotonin reuptake inhibitors (SSRIs). Mirtazapine is an atypical antidepressant with alpha 2 adrenergic antagonist and serotonin 5-HT2 and 5-HT3 receptor-blocking activity. In theory, it should not worsen and perhaps may improve sexual function. This pilot study investigated sexual functioning and antidepressant activity in depressed patients taking mirtazapine. Twenty-five (F = 18, M = 7) sexually active adult outpatients with a DSM-IV-diagnosis of major depressive episode entered a 12-week, flexible-dosing, open-label pilot study. The Arizona Sexual Experiences Scale (ASEX) assessed sexual functioning and the Hamilton Depression Rating Scale (HAM-D) assessed depressive symptoms on a bimonthly basis. Desire, arousal/lubrication, and ease/satisfaction of orgasm improved (by 41%, 52%, and 48%, respectively) in the depressed women. In men, desire, arousal/erection, and ease/satisfaction of orgasm also improved (by 10%, 23% and 14%, respectively) but much more modestly. HAM-D, Clinical Global Impression (CGI) Sheehan Disability Scale (SDS), and Symptom Checklist-90 (SCL-90) scores improved in both groups. There was a 50% dropout rate among women before six weeks of treatment. However, the ASEX and HAM-D scores of the groups terminating before and after six weeks of treatment showed similar rates of improvement. Mirtazapine has a beneficial effect on sexual functioning in both depressed women and men. Longer-term double-blind research assessing sexual function during the administration of mirtazapine as well as other antidepressants is recommended.

Serotonin unmasks functional NK-2 receptors in vagal sensory neurones of the guinea-pig.

1. The regulation of substance P (SP) responsiveness in acutely isolated nodose neurones from adult guinea-pigs was investigated using standard intracellular recording techniques. 2. In control neurones, SP produced no measurable electrophysiological effects. However, following incubation with serotonin (5-HT, 10 microM), 64% of neurones were depolarized by 10 +/- 0.6 mV (n = 84 of 132 neurones) by SP (100 nM). 5-HT-induced SP responses were inhibited by SR48968 (100 nM, n = 6), a neurokinin 2 (NK-2) receptor antagonist, but were unaffected by CP99,994 and SR142801, NK-1 and NK-3 receptor antagonists (n = 3 each), respectively. 3. 5-HT-induced unmasking of SP responses was maximal within 5 min. Increasing the 5-HT incubation time up to 120 min did not increase the mean response amplitude or the percentage of SP responsive neurones (P = 0.611 and 0.867, respectively). 4. 5-HT-induced unmasking of SP responses was dose dependent (EC50 = 14 nM). A 5-HT3 receptor agonist CPBG (1 microM), mimicked the unmasking effects of 5-HT (n = 10 of 19 neurones), while 5-CT (10 microM), a non-selective 5-HT agonist devoid of action at 5-HT3 receptors, did not (n = 18). ICS205-930 (1 microM), a 5-HT3 receptor antagonist, completely blocked the 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 5. In 68% of the neurones tested, bath-applied 5-HT (10 microM) evoked a 178 +/- 29.5 nM increase in [Ca2+]i (n = 16), which was blocked by nominally zero [Ca2+]o (n = 4) or by ICS205-930 (1 microM, n = 4). Nodose neurones incubated with 5-HT in the presence of nominally zero [Ca2+]o did not respond to SP (n = 12 of 13 neurones) in Locke solution containing normal [Ca2+]o, indicating that the 5-HT-mediated elevation of [Ca2+]i is required for unmasking of SP responses. Calmidazolium (100 nM), a calmodulin inhibitor, inhibited the unmasking effects of 5-HT (n = 5 of 5 neurones). 6. Incubating neurones with the nitric oxide (NO) donors papaNONOate (1 mM, 15-30 min) or SNAP (50 microM, 30-60 min) unmasked depolarizing SP responses in 71% and 45% of the neurones studied, respectively. L-NMMA (30 microM), a NO synthase inhibitor, blocked 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 7. In sum, these results suggest that stimulation of 5-HT3 receptors activates an intracellular signalling cascade that couples calcium-calmodulin and NO activation to NK-2 receptor unmasking in sensory neurones.

The pharmacology of impulsive behaviour in rats IV: the effects of selective serotonergic agents on a paced fixed consecutive number schedule.

An impulsive cognitive style may affect behaviour in several different ways, including rapid decision making, intolerance of the delay of reward and a tendency to terminate chains of responses prematurely. It has been proposed to measure the last of these in rats using fixed consecutive number (FCN) schedules. The present study uses a modified version of the FCN procedure in which responding was paced by retracting the response lever for short periods between presses. In this way, the experimenter can control the maximum rate of responding. The procedure was made up of two components. In both, the schedule requirement was FCN 8, but in the Fast component lever presses were spaced by a minimum of 2.5 s and in the Slow component by a minimum of 5 s. Alterations in impulsivity were inferred from changes in the mean chain length and the distribution of chain lengths. The 5-HT1A agonist, 8-OH-DPAT (0.03-0.3 mg/kg), increased chain lengths within a narrow dose range, whereas the 5-HT1A antagonist, WAY 100 635 (0.03-0.3 mg/kg), reduced chain lengths. The 5-HT2 agonist, DOI (0.1-1.0 mg/kg), markedly reduced chain lengths, whereas the 5-HT2 antagonist, ritanserin (0.03-0.3 mg/kg), had no effect. The 5-HT1A/1b agonist, RU 24969 (0.03-0.3 mg/kg), reduced chain lengths. The 5-HT releaser, p-chloramphetamine (0.1-1.0 mg/kg), had a weak, biphasic effect, slightly reducing the number of short chains at the lowest dose tested and slightly increasing this number at the highest dose. Other drugs tested, citalopram (1.0-10.0 mg/kg), metergoline (0.3-3.0 mg/kg) and MDL-72222 (0.1-3.0 mg/kg), had no significant effects. These results suggest that stimulation of 5-HT1A receptors reduces impulsivity, whereas stimulation of 5-HT2 receptors increases it. These data are in agreement with previous results using the DRL-72 schedule, and indicate that there is no simple role for serotonin in the control of impulsivity.

Effect of 5-HT4 receptor stimulation on the pacemaker current I(f) in human isolated atrial myocytes.

5-HT4 receptors are present in human atrial cells and their stimulation has been implicated in the genesis of atrial arrhythmias including atrial fibrillation. An I(f)-like current has been recorded in human atrial myocytes, where it is modulated by beta-adrenergic stimulation. In the present study, we investigated the effect of serotonin (5-hydroxytryptamine, 5-HT) on I(f) electrophysiological properties, in order to get an insight into the possible contribution of I(f) to the arrhythmogenic action of 5-HT in human atria. Human atrial myocytes were isolated by enzymatic digestion from samples of atrial appendage of patients undergoing coeffective cardiac surgery. Patch-clamped cells were superfused with a modified Tyrode's solution in order to amplify I(f) and reduce overlapping currents. A time-dependent, cesium-sensitive increasing inward current, that we had previously described having the electrophysiological properties of the pacemaker current I(f), was elicited by negative steps (-60 to -130 mV) from a holding potential of -40 mV. Boltzmann fit of control activation curves gave a midpoint (V1/2) of -88.9 +/- 2.6 mV (n = 14). 5-HT (1 microM) consistently caused a positive shift of V1/2 of 11.0 +/- 2.0 mV (n = 8, p < 0.001) of the activation curve toward less negative potentials, thus increasing the amount of current activated by clamp steps near the physiological maximum diastolic potential of these cells. The effect was dose-dependent, the EC50 being 0.14 microM. Maximum current amplitude was not changed by 5-HT. 5-HT did not increase I(f) amplitude when the current was maximally activated by cAMP perfused into the cell. The selective 5-HT4 antagonists, DAU 6285 (10 microM) and GR 125487 (1 microM), completely prevented the effect of 5-HT on I(f). The shift of V1/2 caused by 1 microM 5-HT in the presence of DAU 6285 or GR 125487 was 0.3 +/- 1 mV (n = 6) and 1.0 +/- 0.6 mV (n = 5), respectively (p < 0.01 versus 5-HT alone). The effect of 5-HT4 receptor blockade was specific, since neither DAU 6285 nor GR 125487 prevented the effect of 1 microM isoprenaline on I(f). Thus, 5-HT4 stimulation increases I(f) in human atrial myocytes; this effect may contribute to the arrhythmogenic action of 5-HT in human atrium.

Formula omitted] evidence of central 5-HT 1A agonist activity of the novel antipsychotic ziprasidone

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HTIC/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HTIA, 5-HTIC and 5-HT2, but not with 5-HTIB, 5-HTID or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HTIA, 5-HTIC and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HTIA, 5-HTIC and 5-HT2 receptors in depressed patients.

Role of 5-HT3 receptors in activation of abdominal sympathetic C fibre afferents during ischaemia in cats.

1. Activation of abdominal sympathetic afferents during ischaemia reflexly excites the cardiovascular system. We have shown previously that exogenous 5-hydroxytryptamine (5-HT, i.e. serotonin) stimulates abdominal sympathetic afferent nerve endings, and recently have documented increased concentrations of 5-HT in intestinal lymph and portal venous plasma during brief abdominal ischaemia. The present investigation evaluated the role of endogenously produced 5-HT in activation of ischaemically sensitive abdominal sympathetic afferents. 2. Nerve activity of single-unit C fibre afferents innervating duodenum, mesentery, pancreas, portal hepatis, bile duct, gall bladder and jejunum was recorded from the right thoracic sympathetic chain of anaesthetized cats. Ischaemically sensitive C fibre afferents were identified according to their response to 5-10 min of abdominal ischaemia. 3. Intra-arterial injection of 5-HT (20 microg kg-1) increased discharge activity of twelve afferents from 0. 23 +/- 0.05 to 0.96 +/- 0.09 impulses s-1 after an onset latency of 5.7 +/- 1.4 s. Also, 2-methylserotonin (100 microg kg-1, i.a.), a 5-HT3 receptor agonist, stimulated eleven of twelve afferents to significantly increase their discharge activity from 0.25 +/- 0.05 to 0.90 +/- 0.10 impulses s-1 after a latency of 3.3 +/- 0.4 s. Furthermore, intravenous injection of tropisetron (200 microg kg-1), a 5-HT3 receptor antagonist, significantly attenuated the increase in activity of twelve other C fibre afferents during 10 min of abdominal ischaemia from 1.62 +/- 0.18 to 0.94 +/- 0.22 impulses s-1, and eliminated the response of eleven other afferents to 5-HT. 4. Both the 5-HT2 receptor agonist, alpha-methylserotonin (100 microg kg-1, i.a.), and the 5-HT1 receptor agonist, 5-carboxamidotryptamine (100 microg kg-1, i.a.), did not alter the impulse activity of these twelve afferents (0.29 +/- 0.05 to 0.31 +/- 0.06, and 0.26 +/- 0.06 to 0.29 +/- 0.06 impulses s-1, respectively). 5. Treatment with indomethacin (5 mg kg-1, i.v.) in eight different cats did not alter the response of nine C fibre afferents to exogenous 5-HT (0.91 +/- 0. 17 vs. 1.19 +/- 0.25 impulses s-1, P > 0.05). 6. The results suggest that, during mesenteric ischaemia, endogenous 5-HT contributes to the activation of abdominal sympathetic afferents, mainly through direct stimulation of 5-HT3 receptors and that the action of 5-HT on these afferents appears to be independent of the cyclo-oxygenase pathway.

Dorsal medullary 5-HT3 receptors and sympathetic premotor neurones in the rat.

1. Our aim was to determine whether the cardiovascular neurones in the rostro-ventrolateral medulla (CV-RVLM neurones) were involved in the sympathoexcitation induced by stimulation of 5-HT3 receptors in the region of the nucleus tractus solitarii (NTS). Experiments were performed in pentobarbitone-anaesthetized rats, artificially ventilated and paralysed with pancuronium bromide. 2. Using extracellular recordings, different types of RVLM neurones were characterized: cardiovascular (CV), ventilation-related and baroreflex-insensitive (unidentified) neurones. The CV-RVLM cells were further subdivided into three populations according to their axonal conduction velocities: A (1.2 +/- 0.1 m s-1), B (2.5 +/- 0.2 m s-1) and C (6.8 +/- 1.1 m s-1). 3. Only the CV-RVLM neurones of the A and B categories were partially inhibited (-30 %) by a hypotensive dose (2.5 microg kg-1 i.v.) of clonidine. 4. Microinjections into the region of the commissural NTS of 1-(m-chlorophenyl)-biguanide (CPBG, 2 nmol), a selective 5-HT3 receptor agonist, elicited an increase in both lumbar sympathetic nerve discharge (SND) and arterial pressure. In addition, this treatment produced a marked excitation of CV-RVLM neurones of the A and B categories, without affecting those of the C type, as well as ventilation-related and unidentified RVLM cells. 5. The activity of the CV neurones in the caudo-ventrolateral part of the medulla oblongata (CV-CVLM) was not modified by 5-HT3 receptor stimulation in the NTS. 6. Prior intra-NTS microinjections of ondansetron (300 pmol, a selective 5-HT3 receptor antagonist) into the region of the commissural NTS prevented the excitation of A and B CV-RVLM neurones induced by CPBG. 7. Intracarotid administration of saline saturated with CO2 (chemoreceptor activation) elicited both an increase in the SND and an excitation of the clonidine-insensitive CV-RVLM neurones of the C type, without affecting A and B neurones. 8. In conclusion, the sympathoexcitation elicited following 5-HT3 receptor stimulation in the region of the commissural NTS of pentobarbitone-anaesthetized rats seems to result from the excitation of two different pools of clonidine-sensitive CV-RVLM neurones. These neurones are apparently not involved in the sympathetic component of the chemoreceptor reflex.

Modulation by 5-HT1A receptors of the 5-HT2 receptor-mediated tachykinin-induced contraction of the rat trachea in vitro.

1. In the Fisher 344 rat, tachykinins have been shown to cause the release of 5-hydroxytryptamine (5-HT) from airway mast cells, which then causes direct smooth muscle activation as well as the release of acetylcholine from cholinergic nerves. The aim of the present study was to examine the modulatory effects of 5-HT receptors on the neurokinin A (NKA)-induced release of endogenous 5-HT and airway smooth muscle contraction in the isolated Fisher 344 rat trachea. 2. The selective 5-HT2 receptor antagonist ketanserin (0.1 microM) produced an almost complete inhibition of the contractions caused by NKA (n=4, P<0.0001, two-way ANOVA), and a significant rightward shift of the concentration-response curve to 5-HT (n=8, P<0.001, two-way ANOVA). 3. The partial agonist for 5-HT1A receptors, 8-OH-DPAT (1 microM), and the full agonist for 5-HT1 receptors, 5-CT (0.3 microM), potentiated the submaximal contractions induced by the 5-HT2 receptor agonist alpha-methyl-5-HT (0.1 microM) (n=4; P<0.005 and P<0.05, respectively). 8-OH-DPAT (1 microM), as well as the 5-HT1A receptor antagonists pMPPI, SDZ 216525 and NAN-190 (0.1 microM each), caused significant inhibition of the tracheal contractions induced both by NKA (10 nM-3 microM) and 5-HT (10 nM-10 microM) (n=4-10). This suggests that activation of 5-HT1A receptors potentiates the 5-HT2 receptor-mediated contractions. 4. SDZ 216525 (0.1 microM) significantly reduced the maximal contraction produced by 1 microM NKA (n=10, P< 0.001), without affecting the release of endogenous 5-HT. These data rule out the involvement of a 5-HT1A receptor-mediated positive feedback mechanism of the 5-HT release from mast cells. 5. Even in the presence of atropine (1 microM), 8-OH-DPAT (1 microM) further reduced the maximal NKA-induced contraction (n=4, P<0.0001), while the contractions of the rat isolated trachea induced by electrical field stimulation and the concentration-response curve to carbachol were unaffected by pMPPI (0.1 microM), SDZ 216525 (0.1 microM), NAN-190 (0.1 microM) and 8-OH-DPAT (1 microM) (n=4-6). These data demonstrate that the 5-HT1A receptor-mediated potentiation of contractile responses is not due to nonspecific inhibition of airway smooth muscle contraction or to modulation of postganglionic nerve activation. 6. The selective 5-HT1B/1D receptor antagonist GR 127935, the selective 5-HT3 receptor antagonist tropisetron and the selective 5-HT4 receptor antagonists SB 204070 and GR 113808 (0.1 microM each) had no effect on the concentration-response curve for NKA (n=6-10), ruling out the involvement of 5-HT1B/1D, 5-HT3 and 5-HT4 receptors. 7. The alpha-adrenoreceptor antagonist phentolamine (1 microM) had no effect on the 5-HT-induced contractions (n=4), ruling out the involvement of alpha-adrenoreceptors. 8. In conclusion, the tachykinin-induced contraction of the F334 rat isolated trachea is mediated by the stimulation of 5-HT2 receptors. Activation of 5-HT1A receptors located on airway smooth muscle potentiates the direct contractile effects of 5-HT2 receptor activation. The 5-HT1B/1D, 5-HT3 and 5-HT4 receptors are not involved in the NKA-induced contraction of rat airways.

5-HT1A receptor agonist (8-OH-DPAT) and 5-HT2 receptor agonist (DOI) disrupt the non-cognitive performance of rats in a working memory task.

The present study investigated the role of 5 -HT1A and 5 -HT2 receptors in the execution of a working memory task (delayed non-matching to position, DNMTP) by assessing the influence of 8-OH-DPAT (5-HT1A receptor agonist) and DOI (5-HT2 receptor agonist) on the performance of rats lesioned with 5,7-dihydroxytryptamine (5,7-DHT) and their controls. Post-mortem neurochemical analysis revealed that serotonin and 5-hydroxyindoleacetic acid levels were reduced in examined brain areas (especially in the hippocampus where there was a 90 percent reduction). Noradrenaline concentrations were also decreased (mostly on the same side of the injection) by about 20 percent. 5,7-DHT lesioned rats did not significantly differ from their controls in performance in the DNMTP task. At the 30 microg/kg dose, 8-OH-DPAT did not affect the DNMTP-performance of rats, but at the higher dose (100 microg/kg) it reduced the probability of responding to the sample lever. DOI (100 and 300 microg/kg) also interfered with the non-cognitive performance of rats. Since neither of these agonists affected significantly the choice accuracy, they do not appear to influence the working memory per se. The 5,7-DHT lesioned rats did not differ from their controls in response to these agonists. These results suggest that the combination of 5-HT1A receptor stimulation by 8-OH-DPAT and 5-HT2 receptor stimulation by DOI can interfere with the non-cognitive performance of rats in the DNMTP task. The results further indicate that the effect of 8-OH-DPAT may be mediated through post-synaptic rather than pre-synaptic 5-HT1A receptors.

C-Fos induction in the rostroventrolateral medulla by 5-HT3 receptor activation in the nucleus tractus solitarius.

The nucleus tractus solitarius (NTS) plays a crucial role in the reflex control of sympathetic tone through direct projections to the caudal and rostral regions of the ventrolateral medulla (CVLM and RVLM respectively). 5-HT3 receptor stimulation in the NTS increases both blood pressure and sympathetic tone. In order to investigate whether the ventrolateral medulla participates in these effects, the expression of c-Fos protein, a marker of neuronal activation, was examined at this level after intra-NTS microinjections of 1-(m-chlorophenyl)-biguanide, a selective 5-HT3 receptor agonist. A marked increase (93%) in the density of c-Fos-immunoreactive neurons was restricted to the most rostral part of the RVLM after this treatment. This effect could be prevented by prior intra-NTS microinjection of ondansetron, a selective 5-HT3 antagonist. These results suggest the participation of sympathoexcitatory neurons in the rostral RVLM in the pressor response to 5-HT3 receptor stimulation in the NTS.

Direct and indirect actions of 5-hydroxytryptamine on the discharge of mesenteric afferent fibres innervating the rat jejunum.

1. This study was performed to elucidate the actions of 5-hydroxytryptamine (5-HT) on mesenteric afferent discharge and to determine the receptor-mechanisms responsible for these effects. The activity of mesenteric afferents innervating the mid-jejunum of urethane-anaesthetized rats was recorded with extracellular microelectrodes. The discharge of single nerves within the whole nerve recording was monitored using waveform discriminator software. 2. The intravenous injection of 5-HT produced a complex pattern of afferent activation with two distinct components which could be distinguished both in terms of the response characteristics and the receptors involved. Initially, in 64% of nerve bundles, there was a brief (2.0 +/- 0.1 s) but intense activation of afferent discharge with peak afferent firing increasing with incremental doses of 5-HT. The discharge frequency in seventeen single units from these bundles during the initial response to 10 micrograms 5-HT was 13.0 +/- 1.8 impulses s-1 from a baseline discharge of 1.0 +/- 0.1 impulses s-1. 3. This initial response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, whereas 5-methoxytryptamine (5-MEOT, 10-100 micrograms) had no comparable effect. Similarly, the initial 4. 5-HT response was completely abolished by the 5-HT3 receptor antagonist, granisetron (0.5 mg kg-1). 5-HT also evoked, in approximately 35% of nerve bundles, a delayed response that single unit analysis showed to be mediated by an entirely different population of afferents from those activated during the initial response. This secondary response to 5-HT was characterized by a more prolonged (> 30 s) but less intense period of afferent activity which was coincident with an increase in intrajejunal pressure, and was mimicked by 5-MEOT (10-100 micrograms). The secondary response to 5-HT and the response to 5-MEOT were significantly attenuated by the 5-HT2A receptor antagonist, ketanserin (0.5 mg kg-1), which had no effect on the initial response. The initial response to 5-HT was unaffected by the L-type calcium channel inhibitor nifedipine (1 mg kg-1) or the N-type calcium channel inhibitor omega-conotoxin GVIA (25 micrograms kg-1). However, the secondary response to 5-HT was significantly reduced after treatment with nifedipine. 5. These results demonstrate that 5-HT activates different populations of afferent fibres innervating the rat jejunum. One population of afferents is activated directly via stimulation of 5-HT3 receptors, while another population responds to 5-HT with a time course consistent with secondary activation of mechanosensitive afferents following 5-HT2A-mediated contractile activity.

Effect of prolonged administration of the serotonin4 (5-HT4) receptor agonist cisapride on aldosterone secretion in healthy volunteers

In man, serotonin (5-HT) has been shown to stimulate aldosterone secretion through activation of 5-HT4 receptors, In particular, we have observed that oral administration of a single dose of the 5-HT4 receptor agonist cisapride (10 mg) induces a 5-fold increase in plasma aldosterone levels in healthy volunteers. Surprisingly, the usual disorders associated with hyperaldosteronism, i.e. hypertension and hypokalemia, have never been reported during chronic treatment with cisapride. In the present study, we have investigated the effect of prolonged oral administration of cisapride (10 mg, 3 times/day during 7 days) on aldosterone secretion in 12 healthy volunteers, in a simple blind fashion versus placebo. On day I of the treatment, cisapride induced a significant increase in plasma aldosterone levels (PAL) which returned to the values observed after placebo treatment within 10 hrs. On days 2 and 3, PAL were similar in cisapride- and placebo-treated subjects. Urinary aldosterone, kalemia and reninemia were not influenced by cisapride during the 7 days of the treatment. The present study shows that cisapride only exerts a transient stimulatory effect on aldosterone secretion in healthy volunteers. These data explain why long-term administration of 5-HT4 agonists does not affect blood pressure in man. They also indicate that prolonged stimulation of adrenal 5-HT4 receptors in vivo yields to a rapid desensitization phenomenon, as previously observed in vitro.

Blockade of cocaine sensitization and tolerance by the co-administration of ondansetron, a 5-HT3 receptor antagonist, and cocaine.

The present experiment evaluated the ability of the 5-HT3 antagonist, ondansetron, administered during chronic cocaine administration, to block the development of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg per day cocaine for 14 days by either SC injections or osmotic minipumps, or 0.9% saline, administered by SC injection. During this chronic (cocaine) treatment, all rats received a daily SC injection of 0-1.0 mg/kg ondansetron. The rats were then withdrawn from the pretreatment regimen for 7 days. On day 7 of withdrawal from the cocaine pretreatment all subjects received a 15.0 mg/kg IP cocaine challenge, and their behavior was then rated according to the modified Ellinwood and Balster scale for 60 min. The results indicated that daily injections of ondansetron had no consistent or significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron with cocaine significantly blocked the development of sensitization with an inverted U-shape dose-response curve. In the continuous cocaine group ondansetron injections also attenuated the development of behavioral tolerance. The results therefore indicate that 5-HT3 receptor stimulation during continuous and intermittent cocaine administration is an important link in the development of behavioral tolerance and sensitization.

Role of serotonin3 receptors in the nucleus tractus solitarii on the carotid chemoreflex.

The effects of serotonin3 (5-HT3)-receptor stimulation in the nucleus tractus solitarii (NTS) on the cardiovagal, sympathetic, and respiratory responses to activation of carotid body chemoreceptors were investigated in anesthetized rats. The chemoreflex responses were triggered by an intravenous administration of KCN (40 microg/kg) in spontaneously breathing urethan-chloralose-anesthetized rats or by an intracarotid administration of saline saturated with 100% CO2 in pancuronium bromide-paralyzed and artificially ventilated urethan-anesthetized rats. Microinjections of 5-HT (2.5-5 nmol) or the 5-HT3 agonist 1-(m-chlorophenyl)-biguanide (CPBG, 300-1,200 pmol) into the commissural NTS blocked in a dose-dependent manner the atropine-sensitive chemoreflex bradycardia elicited by KCN. However, neither 5-HT nor CPBG affected the KCN-induced increase in respiratory volume and the CO2-induced increases in blood pressure and lumbar sympathetic nerve discharge. The inhibitory effect of 5-HT or CPBG on KCN-induced bradycardia was blocked by prior intra-NTS microinjection of a 5-HT3 antagonist, such as zacopride (100 pmol) or ondansetron (100 pmol), or the A-type gamma-aminobutyric acid (GABA(A)) antagonist bicuculline (10 pmol). In contrast, local microinjections of antagonists acting at 5-HT1 and 5-HT2 receptors, such as methysergide (100 pmol) and ketanserin (10 pmol), respectively, did not prevent the actions of 5-HT or CPBG. These data show that the stimulation of 5-HT3 receptors in the NTS exerted an inhibitory influence, probably through the activation of a local GABAergic system, on the cardiovagal component of the chemoreflex. Because similar effects of 5-HT3-receptor stimulation in the NTS were previously found on the baroreflex and Bezold-Jarisch reflex responses, it can be inferred that NTS 5-HT3 receptors play a key modulatory role in the reflex control of the heart rate.

Pulmonary serotonin 5-HT3-sensitive afferent fibers modulate renal sympathetic nerve activity in rats.

Cardiopulmonary reflexes with vagal afferents may control volume homeostasis by influencing renal nerve activity. Such reflexes can be stimulated mechanically and chemically, e.g., by serotonin 5-HT). We have demonstrated that stimulation of epicardial 5-HT3 receptors inhibits renal sympathetic nerve activity (RSNA) by a cardiorenal reflex. We now tested the hypothesis that pulmonary 5-HT3-sensitive vagal afferent fibers participate in the control of renal nerve activity. Two sets of experiments were performed. First, the responses of multifiber RSNA, heart rate (HR), and blood pressure (BP) to the 5-HT3-receptor agonist phenylbiguanide (PBG; 10 microg iv) were recorded in the presence of intact pulmonary afferents. Abdominal afferents were removed by subdiaphragmatic vagotomy. Cardiac afferents were blocked by intrapericardial injection of 10% procaine. Second, the responses of 25 single vagal pulmonary afferent C fibers to PBG were assessed. PBG decreased BP, HR, and RSNA (-90 +/- 8%). When cardiac afferents were blocked by procaine, BP and HR failed to decrease in response to PBG; however, the RSNA decrease was still -48 +/- 8%. Single fibers generally responded to PBG by a slight increase in firing rate. A distinct subset of fibers (5 of 25) showed an activity increase of >15 Hz that preceded changes in BP and HR. The decreased RSNA in the absence of cardiac and abdominal vagal afferents and the strong response of 20% of pulmonary single fibers to intravenous PBG suggest that pulmonary fibers play a role in a 5-HT3 serotenergic reflex. Thus pulmonary serotonin could influence the neural control of renal function.

Subthreshold stimulation of a serotonin 5-HT3 reflex attenuates cardiovascular reflexes.

Volume-sensitive and chemosensitive cardiopulmonary reflexes modulate volume homeostasis via renal sympathetic nerve activity (RSNA). Blunting of volume-sensitive cardiopulmonary reflexes is associated with volume retention, e.g., in hypertension, whereas the role of chemosensitive cardiopulmonary reflexes is largely unknown. To elucidate the possible role of chemosensitive cardiopulmonary reflexes in control of volume homeostasis, we investigated whether subthreshold stimulation of 5-HT3 receptors modulates the control of RSNA by volume-sensitive cardiopulmonary reflexes or the arterial baroreceptor reflex in rats. Phenyl biguanide (PBG) was infused intravenously to stimulate 5-HT3 receptors. Higher doses of PBG lowered RSNA, but a dose of 6 micrograms/min, given as a background infusion throughout the experiment, did not change arterial pressure, heart rate (HR), or RSNA. Ten minutes after beginning the 6 micrograms/min PBG infusion, a 15-min volume expansion (0.9% saline, 5 or 10% body weight) was started to stimulate volume-sensitive cardiopulmonary reflexes. In separate experiments, 5-min ramp infusions of methoxamine and nitroglycerin to stimulate the arterial baroreceptor reflex (evaluated by a 4-parameter logistic regression) were performed 15 min after beginning the PBG background infusion (6 micrograms/min). During PBG infusion, the RSNA responses to volume expansions were significantly impaired (5% body weight: PBG -6 +/- 6%, n = 7 vs. control -39 +/- 9%, n = 6, P < 0.001; 10% body weight: PBG -33 +/- 6%, n = 8 vs. control -52 +/- 5%, n = 7, P < 0.05). The 5-HT3 receptor antagonist odansetron (GR-38032F) abolished these effects of PBG. The maximum HR gain of the arterial baroreceptor reflex was impaired but the arterial baroreceptor control of RSNA was unaffected by PBG background infusion. We conclude that 5-HT3-serotonergic cardiopulmonary chemoreceptors blunt the RSNA decrease to volume loading. This mechanism may facilitate volume retention when cardiac serotonin is increased.

Peripheral 5-HT4 receptors.

The 5-HT4 receptor is a member of the seven transmembrane spanning G-protein-coupled family of receptors. The receptor is positively coupled to adenylate cyclase and exists in two isoforms (5-HT4S and 5-HT4L) that differ in the length and sequence of their carboxy termini. The 5-HT4 receptor is pharmacologically defined by selective agonists such as SC 53116 and RS 67506, and selective antagonists such as GR 113808, SB 204070, and RS 39604. The receptor is widely distributed in the central nervous system and peripheral tissues. In the periphery, the receptor plays an important role in the function of several organ responses including the alimentary tract, urinary bladder, heart and adrenal gland. In the alimentary tract, stimulation of 5-HT4 receptors has a pronounced effect on smooth muscle tone, mucosal electrolyte secretion, and the peristaltic reflex. In the urinary bladder, activation of 5-HT4 receptors modulates cholinergic/purinergic transmission. In the heart, stimulation of atrial 5-HT4 receptors produces positive inotropy and tachycardia that can precipitate arrhythmias. In the adrenal gland, agonism of 5-HT4 receptors stimulates release of cortisol, corticosterone, and aldosterone. Since its discovery in 1988, significant advances have been made in our understanding of the physiology and pharmacology of the 5-HT4 receptor. These advances have led to the development of several selective 5-HT4 receptor agonists and antagonists that may have therapeutic utility in the treatment of peripheral disorders such as irritable bowel syndrome, gastroparesis, urinary incontinence and cardiac arrhythmias.

Medullary pathways of cardiovascular responses to 5-HT2 and 5-HT3 receptor stimulation in the rat nucleus tractus solitarius.

As previously found for the baroreceptor reflex, microinjection of kynurenic acid (KYN, 2 nmol), a glutamate receptor antagonist, into the caudal ventrolateral medulla (CVL) blocked the hypotension and bradycardia elicited by microinjections of a 5-HT2 receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI, 0.5 pmol) into the nucleus tractus solitarius (NTS). In addition, as previously observed with the sympathetic chemoreflex response, microinjections of KYN into the RVI, blocked the increase in blood pressure elicited by microinjections of 1-m-(chlorophenyl)-biguanide (1200 pmol), a 5-HT1 receptor agonist, into the NTS. These results suggest that medullary pathways involved in the responses to 5-HT2 and 5-HT3 receptor stimulation in the NTS are similar to those that mediate the baroceptor and chemoreceptor reflex responses, respectively.

Stimulation of 5-HT3 receptors in the NTS inhibits the cardiac Bezold-Jarisch reflex response

Intra-atrial administration of phenylbiguanide has been shown to trigger, through the stimulation of vagal afferent C-fibers, reflex bradycardia, hypotension, and sympathoinhibition classically known as the Bezold-Jarisch (B-J) reflex (O. Krayer. Naunyn-Schmiedeberg's Arch. Exp. Pathol. Pharmacol. 240: 361-368, 1961). The effects of microinjections, into the nucleus tractus solitarius (NTS), of serotonin (5-HT) and 1-(m-chlorophenyl)-biguanide (CPBG), a potent 5-HT3 receptor agonist, on these reflex responses were studied in urethananesthetized rats. 5-HT (600 and 900 pmol) and CPBG (10-150 pmol) produced a dose-dependent inhibition of the atropine-sensitive bradycardiac component of the B-J reflex. The effect of both agonists was reversed by prior local microinjection of the 5-HT3 receptor antagonists zacopride (100 pmol) and ondansetron (100 pmol), but not by that of the 5-HT2 receptor antagonist ketanserin (10 pmol) or the mixed 5-HT1/5-HT2 receptor antagonist methysergide (100 pmol). In contrast, CPBG (150 pmol) did not affect the B-J reflex inhibition of lumbar sympathetic nerve discharge. These results show that stimulation of NTS 5-HT3 receptors produced an inhibition of the cardiovagal component of the B-J reflex without affecting its sympathetic component. Because the stimulation of these receptors also inhibits the cardiac component of the baroreflex, the present data suggest the participation of NTS 5-HT3 receptors in the mechanisms that modulate cardiac reflex responses elicited by messages from different vagal afferents.

5-HT3 and 5-HT4 receptor-mediated facilitation of the emptying phase of the peristaltic reflex in the marmoset isolated ileum.

1. The patterns of intestinal motility and the effect of an increase in intraluminal pressure were studied in vitro on segments obtained from the marmoset small intestine. 2. Segments obtained from the distal half of the marmoset small intestine exhibited segmentation, consisting of narrow annular contractions (lasting for 2-3 s) interposed between the relaxed segments of varying length. The subsequent contractions occurred slightly distal to the previous contraction, with ring-like contractions appearing to move in the aboral direction. Such segmentation was infrequent or absent in the segments obtained from the proximal half of the small intestine. An increase in intraluminal pressure inhibited segmentation and finally produced peristalsis in most of the tissues. 3. The influence of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on the threshold of the peristaltic reflex was investigated in the segments obtained from the distal half of the intestine after segmentation had subsided. The effect of drug application to the serosal surface was measured as a change in threshold pressure required to trigger the peristaltic reflex. A facilitation was defined in two ways (a) as a reduction in the threshold pressure required to trigger the reflex and (b) in those tissues that failed to respond with peristalsis on raising intraluminal pressure to the maximum attainable (1 kPa), as a reduction in threshold pressure compared to this value. 4. 5-HT (7.85 +/- 0.19), 5-methoxytryptamine (7.79 +/- 0.24), 5-carboxamidotryptamine (6.66 +/- 0.13) and 2-methyl-5-HT (6.24 +/- 0.16) caused a concentration related facilitation of the peristaltic reflex, the pD2 values (mean +/- s.e.mean) being shown in parentheses. 5. The concentration-response curves to both 5-HT and 5-methoxytryptamine were dextrally shifted in a surmountable manner in the presence of GR 113808 (30 nM). pD2 values for 5-HT and 5-methoxytryptamine were significantly decreased to 6.98 +/- 0.24 and 6.83 +/- 0.36 respectively in the presence of GR 113808 (30 nM). 6. In the presence of a high concentration of (10 microM) 5-methoxytryptamine the subsequent addition of 2-methyl-5-HT (3-10 microM) but not 5-methoxytryptamine (10 microM) facilitated peristalsis; the effect of 3 microM 2-methyl-5-HT was significantly decreased by 2 microM ondansetron. 7. It is concluded that the facilitation of the peristaltic reflex in the marmoset intestine induced by 5-HT at submicromolar concentrations involves a 5-HT4 receptor stimulation with an additional 5-HT3 receptor activation at higher concentrations.