Antagonism of a peripheral vascular but not an apparently central serotonergic response by xylamidine and BW 501C67

Central 5-HT(2A) receptors modulate the vagal bradycardia in response to activation of the von Bezold-Jarisch reflex in anesthetized rats

Activation of 5-hydroxytryptamine (5-HT) 5-HT(1A), 5-HT(2C), 5-HT(3), and 5-HT(7) receptors modulates the excitability of cardiac vagal motoneurones, but the precise role of 5-HT(2A/2B) receptors in these phenomena is unclear. We report here the effects of intracisternal (ic) administration of selective 5-HT(2A/2B) antagonists on the vagal bradycardia elicited by activation of the von Bezold-Jarisch reflex with phenylbiguanide. The experiments were performed on urethane-anesthetized male Wistar rats (250-270 g, N = 7-9 per group). The animals were placed in a stereotaxic frame and their atlanto-occipital membrane was exposed to allow ic injections. The rats received atenolol (1 mg/kg, iv) to block the sympathetic component of the reflex bradycardia; 20-min later, the cardiopulmonary reflex was induced with phenylbiguanide (15 µg/kg, iv) injected at 15-min intervals until 3 similar bradycardias were obtained. Ten minutes after the last pre-drug bradycardia, R-96544 (a 5-HT(2A) antagonist; 0.1 µmol/kg), SB-204741 (a 5-HT(2B) antagonist; 0.1 µmol/kg) or vehicle was injected ic. The subsequent iv injections of phenylbiguanide were administered 5, 20, 35, and 50 min after the ic injection. The selective 5-HT(2A) receptor antagonism attenuated the vagal bradycardia and hypotension, with maximal effect at 35 min after the antagonist (pre-drug = -200 ± 11 bpm and -42 ± 3 mmHg; at 35 min = -84 ± 10 bpm and -33 ± 2 mmHg; P < 0.05). Neither the 5-HT(2B) receptor antagonists nor the vehicle changed the reflex. These data suggest that central 5-HT(2A) receptors modulate the central pathways of the parasympathetic component of the von Bezold-Jarisch reflex.

Investigation of the role of 5-HT2 receptor subtypes in the control of the bladder and the urethra in the anaesthetized female rat

BACKGROUND AND PURPOSE

Micturition is controlled by central 5-HT-containing pathways. 5-HT2 receptors have been implicated in this system especially in control of the urethra, which is a drug target for treating urinary incontinence. This study investigates the role of each of the three subtypes of this receptor with emphasis on sphincter regulation.

EXPERIMENTAL APPROACH

Recordings of urethral and bladder pressure, external urethral sphincter (EUS) EMG, as well as the micturition reflex induced by bladder distension along with blood pressure and heart rate were made in anaesthetized rats. The effects of agonists and antagonists for 5-HT2 receptor subtypes were studied on these variables.

KEY RESULTS

The 5-HT2C agonists Ro 60-0175, WAY 161503 and mCPP, i.v., activated the EUS, increased urethral pressure and inhibited the micturition reflex. The effects of Ro 60-0175 on the EUS were blocked by the 5-HT2C antagonist SB 242084 and the 5-HT2A antagonists, ketanserin and MDL 100907. SB 242084 also blocked the inhibitory action on the reflex, while the 5-HT2B antagonist RS 127445 only blocked the increase in urethral pressure. The 5-HT2A receptor agonist DOI given i.v. or i.t. but not i.c.v. activated the EUS.

CONCLUSIONS AND IMPLICATIONS

5-HT2A/2C receptors located in the sacral spinal cord activate the EUS, while central 5-HT2C receptors inhibit the micturition reflex and 5-HT2B receptors, probably at the level of the urethra, increase urethral smooth muscle tone. Furthermore, 5-HT2B and 5-HT2C receptors do not seem to play an important role in the physiological regulation of micturition.

Serotonin 2A receptors modulate tail-skin temperature in two rodent models of estrogen deficiency-related thermoregulatory dysfunction

Menopause-associated thermoregulatory dysfunction, including hot flushes and night sweats, is effectively treated by hormonal therapies that include estrogens. Evidence suggests that estrogen regulates serotonin 2A (5-HT(2A)) receptor expression and that 5-HT(2A) receptors are involved in thermoregulation. Therefore, the role of 5-HT(2A) receptors in thermoregulation was assessed in two rat models of ovariectomy-induced thermoregulatory dysfunction. The first model is based on measurement of the tail-skin temperature (TST) increase following naloxone-induced withdrawal in morphine-dependent ovariectomized (OVX) rats (MD model), while the second model relies on telemetric assessment of diurnal TST changes in ovariectomized rats (telemetry model). Treatment with a 5-HT(2A/2C) receptor agonist, (-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI), prevented the naloxone-induced TST increase in the MD model and restored normal active-phase TST in the telemetry model. The selective 5-HT(2A) receptor antagonist, MDL-100907, had no effect on the naloxone-induced flush when administered alone in the MD model, but it decreased DOI's ability to abate the flush. In the telemetry model, MDL-100907 attenuated the DOI-induced decrease in active-phase TST. Interestingly, MDL-100907 increased TST in both models when given alone, with the TST increase occurring prior to the naloxone-induced flush in the MD model. To evaluate the role of central nervous system (CNS) 5-HT(2A) receptors in TST regulation, DOI was administered in combination with a known peripheral 5-HT(2A/2C) receptor antagonist, xylamidine, in the MD model. Xylamidine had no effect on DOI's ability to abate the naloxone-induced flush. These results indicate that activation of central 5-HT(2A) receptors restores temperature regulation in two rodent models of ovariectomy-induced thermoregulatory dysfunction.

Role of central 5-HT2 receptors in mediating head bobs and body shakes in the rabbit

Systemic administration of the 5-HT(2A/2C) agonist DOI [(1(2,5-dimethoxy-4-iodophenyl)-2-aminopropane)hydrochloride] in rabbits elicits head bobs and body shakes, which are mediated by 5-HT(2A) and 5-HT(2C) receptors, respectively. This study was designed to determine whether the receptors mediating these behaviors are primarily located in the brain or in the periphery. Systemic administration of the peripheral 5-HT(2A/2C) antagonist xylamidine 30 min before systemic DOI challenge attenuated DOI-elicited body shakes by 50% without an effect on head bobs, suggesting a central origin for head bobs and a partial peripheral and a partial central origin for body shakes. Central administration of DOI into the lateral ventricle (ICV) elicited head bobs but not body shakes, demonstrating that the receptors mediating head bobs are centrally located. Pretreatment with ICV xylamidine blocked head bobs elicited by ICV DOI, indicating that the lack of inhibition, when systemically administered, is due to xylamidine's failure to reach central receptors. ICV pretreatment with the 5-HT(2A) receptor antagonist ketanserin inhibited ICV DOI-elicited head bobs establishing that 5-HT(2A) receptors activation elicits head bobs. In conclusion, 5-HT(2A) receptors mediating head movements are located in the brain whereas 5-HT(2C) receptors mediating the body movements appear to be located at different central sites as well as in the periphery.

Evidence that central 5-HT2A and 5-HT2B/C receptors regulate 5-HT cell firing in the dorsal raphe nucleus of the anaesthetised rat

1. Systemic administration of phenethylamine-derived, 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists inhibits the firing of midbrain 5-HT neurones, but the 5-HT receptors involved are poorly defined, and the contribution of peripheral mechanisms is uncertain. This study addresses these issues using extracellular recordings of 5-HT neurones in the dorsal raphe nucleus of anaesthetised rats. 2. The 5-HT(2) receptor agonists DOI ((+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride) and DOB ((+/-)-2,5-dimethoxy-4-bromoamphetamine hydrobromide), caused a dose-related (10-100 micro g kg(-1) i.v.) inhibition of 5-HT neuronal activity, with the highest dose reducing firing rates by >80%. 3. Pretreatment with the 5-HT(2) receptor antagonist ritanserin (1 mg kg(-1) i.v.) completely blocked the action of DOI. The 5-HT(2A) receptor antagonist MDL 100,907 (0.2 mg kg(-1) i.v.) blocked the action of both DOI and DOB. In comparison, the 5-HT(2B/C) receptor antagonist SB 206553 (0.5 mg kg(-1) i.v.) caused a small, but statistically significant, shift to the right in the dose response to DOI and DOB. 4. Pretreatment with the peripherally acting 5-HT(2) receptor antagonist BW 501C67 (0.1 mg kg(-1) i.v.) had no effect on the DOI-induced inhibition of 5-HT cell firing, but completely blocked the DOI-induced rise in mean arterial blood pressure. 5. These data indicate that the inhibition of 5-HT cell firing induced by systemic administration of DOI and DOB is mediated predominantly by the 5-HT(2A) receptor-subtype, but that 5-HT(2B/C) receptors also play a minor role. Moreover, central and not peripheral mechanisms are involved. Given evidence that 5-HT(2) receptors are not located on 5-HT neurones, postsynaptic 5-HT feedback mechanisms are implicated.

Neurogenic dural protein extravasation induced by meta-chlorophenylpiperazine (mCPP) involves nitric oxide and 5-HT2B receptor activation

The compound m-chlorophenylpiperazine (mCPP), which is known to trigger migraine-like head pain in some subjects, was evaluated for its ability to induce dural plasma protein extravasation (PPE) in guinea pigs. Intravenous mCPP dose-dependently increased PPE. This effect was inhibited by non-selective 5-HT2 receptor antagonists (methysergide, LY53857, LY215840), by a peripherally restricted 5-HT2 receptor antagonist (xylamidine) and by a 5-HT2B selective receptor antagonist (LY202146). These data suggests that peripheral 5-HT2B receptors mediate mCPP-induced PPE. The nitric oxide synthase inhibitor L-NAME and 5-HT1 agonist sumatriptan also blocked mCPP-induced PPE, suggesting a role for nitric oxide (NO) and the trigeminal system, respectively. NO release has been linked to activation of the 5-HT2B receptor on the vascular endothelium. However, LY202146 did not block PPE induced by electrical stimulation of the trigeminal ganglion. These data are consistent with activation of peripheral 5-HT2B receptors initiating PPE and the theory that selective 5-HT2B antagonists might be effective prophylactic therapies for migraine.

Evidence that serotonin affects female sexual functioning via peripheral mechanisms

A review of the literature indicates that serotonin is active in several peripheral mechanisms that are likely to affect female sexual functioning. Serotonin has been found in several regions of the female genital tract in both animals and humans. In the central nervous system (CNS), serotonin acts primarily as a neurotransmitter, but in the periphery, serotonin acts primarily as a vasoconstrictor and vasodilator. Since, in the periphery, the principal component of sexual arousal is vasocongestion of the genital tissue, it is likely that serotonin participates in producing normal sexual arousal. In addition, serotonin administration produces contraction of the smooth muscles of the genito-urinary system and is found in nerves innervating the sexual organs. Taken together, this evidence suggests that peripheral serotonergic activity may be involved in the normal sexual response cycle. In addition, exogenous substances that alter serotonin activity, such as selective serotonin uptake inhibitors (SSRIs) and the atypical antipsychotics, can produce sexual dysfunction. It is possible that sexual side effects seen with these drugs may result, at least in part, from their action on peripheral mechanisms.

Evidence that activation of central 5-HT(2B) receptors causes renal sympathoexcitation in anaesthetized rats

The effects of injections i.c.v. of alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (BW723C86; 0.02 - 2 micromol kg(-1)), a 5-HT(2B) receptor agonist, on renal sympathetic and phrenic nerve activity, mean arterial blood pressure and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT(2) receptor antagonist. BW723C86 i.c.v. caused a dose-related increase in renal nerve activity reaching a maximum of 67+/-6%, which at the highest dose was associated with a small and maintained fall in mean arterial blood pressure of 7+/-3 mmHg. These changes were not associated with any significant changes in heart rate or phrenic nerve activity. BW723C86-evoked increases in renal nerve activity and hypotension were attenuated by pretreatment (i.c.v.) with SB204741 (300 nmol kg(-1); a 5-HT(2B) receptor antagonist) but not by the same dose (i.c.v.) of ketanserin (a 5-HT(2A) receptor antagonist) or RS102221 (a 5-HT(2C) receptor antagonist). None of these antagonists alone had any effect on the variables being measured. It is concluded that central 5-HT(2B) receptors may play a selective role in the control of sympathetic supply to the kidney, which could be important in the central mechanisms involved in blood volume regulation. British Journal of Pharmacology (2000) 129, 177 - 183

Evidence for a role for central 5-HT2B as well as 5-HT2A receptors in cardiovascular regulation in anaesthetized rats

1. The effects of injections i.c.v. of quipazine, (2 micromol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 2 micromol kg-1) on renal sympathetic and phrenic nerve activity, mean arterial blood pressure (MAP) and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT2 receptor antagonist. 2. Quipazine or DOI caused a rise in MAP which was associated with a tachycardia and renal sympathoinhibition in rats pretreated (i.c.v.) with the antagonist vehicle 10% PEG. These effects of quipazine were completely blocked by pretreatment with cinanserin (a 5-HT2 receptor antagonist) and attenuated by spiperone (a 5-HT2A receptor antagonist). However, pretreatment with SB200646A (a 5-HT2B/2C receptor antagonist) only blocked the sympathoinhibition, while pretreatment with SB204741 (a 5-HT2B receptor antagonist) reversed the sympathoinhibition to excitation as it also did for DOI. Quipazine also caused renal sympathoexcitation in the presence (i.v.) of a vasopressin V1 receptor antagonist. 3. Injection (i.v.) of the V1 receptor antagonist at the peak pressor response evoked by quipazine alone and in the presence of SB204741 caused an immediate fall in MAP. For quipazine alone the renal sympathoinhibition was slowly reversed to an excitation, while the renal sympathoexcitation observed in the presence of SB204741 was potentiated. In both, the quipazine-evoked tachycardia was unaffected. 4. The data indicate that cardiovascular responses caused by i.c.v. quipazine and DOI are primarily due to activation of central 5-HT2A receptors, which causes the release of vasopressin and a tachycardia. This released vasopressin appears to suppress a 5-HT2A receptor-evoked central increase in sympathetic outflow, which involves the activation of central 5-HT2B receptors indirectly by the released vasopressin.

Serotonin in migraine: theories, animal models and emerging therapies

A role for serotonin in migraine has been supported by changes in circulating levels of serotonin and its metabolites during the phases of a migraine attack, along with the ability of serotonin-releasing agents to induce migraine-like symptoms. The development of serotonin receptor agonists with efficacy in the clinic for the alleviation of migraine pain further implicates serotonin as a key molecule in migraine. Several theories regarding the etiology of migraine have been proposed. The vasodilatory theory of migraine suggested that extracranial arterial dilation during an attack was related to migraine pain; a theory supported when vasoconstrictors such as sumatriptan alleviated migraine pain. The neurological theory of migraine proposed that migraine resulted from abnormal firing in brain neurons. Cortical spreading depression, one facet of the neurological theory, could explain the prodrome of migraine. The neurogenic dural inflammation theory of migraine supposed that the dural membrane surrounding the brain became inflamed and hypersensitive due to release of neuropeptides from primary sensory nerve terminals. Substance P, calcitonin gene related peptide and nitric oxide are all though to play a role in the dural inflammatory cascade. Animal models of migraine have been utilized to study the physiology of migraine and develop new pharmaceutical therapies. One model measures the shunting of blood to arteriovenous anastomoses based on a proposal that migraine primarily involves cranial arteriovenous vasodilation. Another model utilizes electrical stimulation of the trigeminal ganglion to induce neurogenic dural inflammation quantified by the resulting extravasation of proteins. Pharmacological agents such as meta-chlorophenylpiperazine (mCPP) and nitroglycerin have also been used to induce dural extravasation in animals. Both compounds also induce migraine attacks in individuals with a history of migraine. In addition, Fos, a protein produced by activation of the c-fos gene, has been measured as an index of migraine-like pain transmission to the CNS following chemical or electrical stimulation of the trigeminal nerve. A role for serotonin in migraine is further supported by the efficacy of serotonin receptor ligands. Sumatriptan is an agonist at 5-HT1D and 5-HT1B receptor subtypes, and effective in treating migraine pain and associated symptoms. Recently, selective 5-HT1F agonists have been proposed for the treatment of migraine, without the side effects associated with the present 5-HT1D and 5-HT1B receptor agonists. A role for 5-HT2B receptors has also been suggested the initiation of migraine, supporting use of selective 5-HT2B receptor antagonists in migraine. Thus, agents that modulate 5-HT1B, 5-HT1D, 5-HT1F and 5-HT2B receptors either have or may have clinical utility in the therapy of migraine headache.

Modulation of 5-HT receptor subtype-mediated behaviours by corticosterone

Malfunction of the serotonergic system and dysregulation of the hypothalamo-pituitary-adrenocortical axis have been implicated in the pathophysiology of depression. Several studies provide evidence for reciprocal influences between glucocorticoids and 5-HT receptors. The effect of repeated treatment with a high dose of corticosterone (50 mg/kg s.c. twice daily for 4 days) on 5-HT receptor subtype-mediated behaviours was studied. It was found that in rats that were repeatedly treated with corticosterone the number of 2-chloro-6-(1-piperazinyl)pyrazine HCl (MK 212)-induced, 5-HT2C receptor-mediated penile erections were reduced, whereas both MK 212 and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced 5-HT2A receptor-mediated head shakes were increased. The (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-induced lower lip retraction mediated by presynaptic 5-HT1A receptors was unchanged, whereas the open field activity induced by 8-OH-DPAT was enhanced in corticosterone pretreated rats. These changes in 5-HT receptor subtype-mediated behaviours were not seen after a single injection with corticosterone given 24 h or 5 days before. The results suggest that 5-HT2A, 5-HT2C and postsynaptic 5-HT1A receptor-mediated behaviour can be modulated by repeated treatment with a high dose of corticosterone.

Spinal 5-HT2 receptor-mediated facilitation of pudendal nerve reflexes in the anaesthetized cat

1. 5-Hydroxytryptamine (5-HT) is intimately associated with central sympathetic and somatic control of the lower urinary tract. The sympathetic and somatic innervation of the lower urinary tract is conveyed through efferent axons of the hypogastric and pudendal nerves, respectively. 2. The present study examined the effects of 2,5-dimethoxy-4-iodophenylisopropylamine (DOI), a 5-HT2 receptor subtype-selective agonist, on evoked potentials recorded from the central ends of the hypogastric and pudendal nerves in response to electrical stimulation of afferent fibres in the pelvic and pudendal nerves, respectively. Various spinalization paradigms were employed to localize the site of action. All cats were pretreated with xylamidine (1 mg kg-1), a peripherally-restricted 5-HT2 receptor antagonist. 3. In acute spinal cats, DOI (0.01-3 mg kg-1, i.v.) reliably produced dose-dependent increases in the pudendal nerve reflex (to 228 +/- 31% of control). These increases were reversed by the 5-HT2 receptor-selective antagonist, LY53857 (0.3-3 mg kg-1, i.v.). On the other hand, in spinally-intact cats, DOI produced no significant changes in the pudendal reflex. However, within minutes of spinalization of DOI-pretreated cats, a marked increase (to 221 +/- 16% of control) in the pudendal reflex was observed which could be reversed by LY53857. No significant effects were observed on hypogastric reflexes in either acute spinal or spinally-intact cats following DOI administration. No effects were seen in either spinally-intact or acute spinal animals when LY53857 was administered as the initial drug. 4. These results indicate that activation of spinal 5-HT2 receptors facilitates pudendal reflexes. In spinally-intact cats, it is hypothesized that DOI activates supraspinal pathways that mediate inhibition of the pudendal reflexes and counteracts the facilitatory effects of spinal 5-HT2 receptor activation.

Anteroventral third ventricle lesions attenuate pressor responses to serotonin in anesthetized rats

When administered intravenously, serotonin (5-hydroxytryptamine; 5-HT) evokes a triphasic blood pressure response, consisting of the Bezold-Jarisch-associated depressor response, a pressor action, and long-lasting depressor response. Because the pressor response may, in part, be caused by central nervous system (CNS) activation by 5-HT, we predicted that destruction of the anteroventral third ventricle (AV3V) region, an area rich in 5-HT receptors, would attenuate increases in blood pressure to intravenous 5-HT. In anesthetized sham-lesioned and AV3V-lesioned Sprague-Dawley rats, we measured mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (SNA) to increasing bolus doses of intravenous 5-HT (1, 2.5, 5, 10, 25 mu g/kg), before and after blockade of bradycardia using methylatropine (200 mu g/kg). In all rats, bolus injections of 5-HT elicited bradycardia accompanied by a fall in lumbar SNA and an initial hypotension followed by a pressor response and a longer lasting hypotensive response. The bradycardia, reduction in lumbar SNA, and both depressor responses were equivalent in sham-lesioned and AV3V-lesioned groups. Importantly, AV3V lesions attenuated pressor responses to increasing doses of 5-HT (3 +/- 1, 6 +/- 4, 6 +/- 4, 17 -/+ 4 35 +/- 3 mmHg) compared to sham-lesioned controls (6 +/- 3, 16 +/- 7, 33 +/- 5, 54 +/- 4, 51 +/- 6 mmHg; P < 0.0001). This attenuation was conserved following blockade of bradycardia with methylatropine (P < 0.01). In summary, pressor responses to intravenous 5-HT are diminished by AV3V lesions. These data indicate that the pressor component of the blood pressure response to intravenous 5-HT is partly dependent upon interaction with the CNS.

Serotonin receptors involved in regulation of pituitary-adrenocortical function in rats

Drugs that enhance brain serotonin function (direct agonists, serotonin uptake inhibitors and serotonin releasers) increase serum corticosterone concentration in rats, and in many cases ACTH has been shown to be similarly increased. At least two distinct serotonin receptor subtypes can mediate this effect. One is the 5-HT1A receptor, and the other seems to be a 5-HT2A receptor. Possibly, the 5-HT2C receptor or other receptors can also mediate these effects. The increase in serum corticosterone seems to be one useful marker in characterizing drug effects on serotonergic function. Much needs to be learned about the physiological importance and roles of this serotonergic influence on pituitary-adrenocortical function. Some studies have suggested it may be important in the diurnal rhythmicity of glucocorticoid secretion and perhaps in some stress effects. Serotonergic activation can also increase plasma levels of ACTH and cortisol in humans. Although the effects in humans are less well characterized than in rats, they seem to be useful as a way of probing brain serotonergic function in disease or after drug treatment. As more drugs that act selectively on a single receptor subtype become available for use in humans, more precise information about particular receptor subtypes should be attainable.

Evidence that activation of 5-HT2 receptors in the forebrain of anaesthetized cats causes sympathoexcitation

1. The aim of the present experiments was to determine whether the effects of lateral ventricular application of 5-HT on cardiovascular and respiratory variables in anaesthetized cats are mediated by forebrain 5-HT2 receptors. This was carried out by determining whether the effects of 5-HT are blocked by the 5-HT2 antagonist, cinanserin and if they are mimicked by the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). 2. Cats were anaesthetized with a mixture of alpha-chloralose and pentobarbitone sodium, neuromuscularly blocked and artifically ventilated. The following cardiovascular and respiratory variables were recorded: renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, heart rate, arterial blood pressure, femoral arterial conductance and tracheal pressure. All drugs were administered via the lateral ventricle and the action of these agonists was restricted to forebrain sites by a cannula placed in the Aqueduct of Sylvius. 3. Cumulative doses of 5-HT (10-160 nmol kg-1) and DOI (80-320 nmol kg-1) injected into the lateral ventricle caused significant increases in blood pressure, heart rate, cardiac and splanchnic sympathetic nerve activity and a decrease in femoral arterial conductance. DOI and 5-HT caused a greater increase in cardiac compared with splanchnic nerve activity and failed to change renal nerve activity. 5-HT but not DOI significantly increased the magnitude and the number of phrenic bursts as well as significantly increasing tracheal pressure. The effects of 5-HT also differed from DOI in that 5-HT evoked maximal pressor and near maximal sympathoexcitatory effects after the first dose, whereas the pressor and sympathoexcitatory effects of DOI were graded over the complete dose-range.4 The 5-HT2 antagonist, cinanserin (265 nmol kg-1, i.c.v.) caused significant falls in blood pressure,heart rate and cardiac nerve activity and an increase in femoral arterial conductance. Splanchnic andrenal sympathetic nerve activity, phrenic nerve activity and tracheal pressure were unaffected by cinanserin. After pretreatment with cinanserin all cardiovascular and respiratory effects of 5-HT were significantly attenuated.5 It is concluded that in the cat, as DOI and 5-HT have similar effects on the cardiovascular variables recorded and as the effects of 5-HT are blocked by cinanserin, 5-HT can act on 5-HT2 receptors located in the forebrain to cause differential sympathoexcitation and a rise in arterial blood pressure. Further,the sympathoexcitatory effects mediated by 5-HT2 receptors located in the forebrain differ from those located in the hindbrain in that they mediate increases in cardiac nerve activity and heart rate and also have no effect on renal nerve activity.

Further studies to examine the nature of dexfenfluramine-induced suppression of heroin self-administration

The present series of experiments sought to investigate further the mechanism by which dexfenfluramine, a selective 5-HT releaser/reuptake inhibitor, reduces heroin self-administration by male Wistar rats. In experiment 1, the effect of combined intravenous heroin and intraperitoneal dexfenfluramine injections on operant responding for food was examined. In experiment 2, the maintenance of dexfenfluramine suppression of heroin self-administration following chronic (7 day) treatment was evaluated. Finally, in experiment 3, the ability of various 5-HT antagonists to block the dexfenfluramine suppression was examined. The results from experiment 1 suggest that sensorimotor deficits/malaise potentially associated with heroin/dexfenfluramine combinations are unlikely to account for the reductions in heroin self-administration. Experiment 2 suggested that the suppressant effect of dexfenfluramine on heroin responding may diminish rapidly following chronic treatment. Finally, central 5-HT1 and/or 5-HT2, but not 5-HT3, receptors may underlie the suppressant effects of dexfenfluramine on heroin self-administration.

Systemic and regional haemodynamic effects of 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) and alpha-methyl-5-HT, in the anaesthetised rat

The present experiment was performed to investigate the haemodynamic effects of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT) in the anaesthetised normotensive rat. DOI (1-300 micrograms/kg i.v.) increased mean arterial pressure (MAP), total peripheral resistance (TPR) and decreased cardiac output (CO) and heart rate (HR). DOI increased all vascular resistances investigated (hindquarters, mesenteric and renal). Alpha-methyl-5-HT (10-300 micrograms/kg i.v.) dose-dependently increased MAP, TPR, all regional vascular resistances and decreased CO and HR. The bradycardia induced by alpha-methyl-5-HT was suppressed by bivagotomy. Both DOI and alpha-methyl-5-HT were more effective on renal vascular bed than hindquarters and mesenteric vascular beds. The effects of DOI and alpha-methyl-5-HT were antagonised by spiperone (10 or 100 micrograms/kg i.v.) and LY 53857 (10 micrograms/kg i.v.). Intracerebroventricular administration of DOI (100 micrograms/kg) increased MAP, TPR, regional vascular resistances and did not change HR and CO. Pretreatment with xylamidine (10 micrograms/kg i.v.), a selective peripheral 5-HT2 receptor antagonist, blocked i.v. and i.c.v. effects of DOI. These results suggest that: 1) the increase in MAP induced by DOI and alpha-methyl-5-HT is due to an increase in TPR. All regional vascular beds and in particular the renal vascular bed participate in the increase of TPR. 2) Peripheral--and may be--central 5-HT2 receptors seem to be implicated in the control of regional vascular resistances. 3) Cardiac effects of alpha-methyl-5-HT are baroreflexly-mediated whereas those of DOI are--at least in part--centrally mediated.

Effect of dexfenfluramine on saccharin drinking: behavioural and pharmacological studies

We have previously reported that the 5-hydroxytryptamine (5-HT) releaser/reuptake blocker dexfenfluramine suppresses voluntary ethanol intake. To further analyse the generality of these findings, in the present study we examined the effect of equivalent doses of dexfenfluramine (0.5-2.5 mg/kg) on the intake of another preferred fluid, saccharin. Saccharin was made available for 2 h daily across a wide concentration range chosen to promote varying degrees of intake. Following stable levels of intake, the behaviour of vehicle-pretreated rats was assessed immediately prior to (anticipatory/preparatory phase) and during (consumatory phase) saccharin access. These behaviours were compared and contrasted with those produced following dexfenfluramine pretreatment at the optimally preferred saccharin concentration (0.2%). In a preliminary study the effects of various 5-HT antagonists were also examined against the dexfenfluramine response. The present results suggest that dexfenfluramine produced a dose-related suppression of saccharin intake at doses similar to those which reduced ethanol intake. However, the magnitude of this suppression was similar across each saccharin concentration. Behavioural analysis indicated that the profile of the dexfenfluramine (0.5- and 1-mg/kg doses only) suppression of the 0.2% solution was similar to that observed in vehicle-pretreated rats presented with saccharin solutions of lesser palatability to this concentration. Pharmacological studies indicated a 5-HT1 (non-5-HT1C) receptor involvement in the dexfenfluramine response. These studies imply that at certain doses dexfenfluramine may produce a subtle alteration in the motivation to consume a preferred fluid.

5-HT and carbohydrate suppression: effects of 5-HT antagonists on the action of d-fenfluramine and DOI

The effects of several 5-hydroxytryptamine (5-HT) receptor antagonists on the anorectic effect of d-fenfluramine and the 5-HT2/5-HT1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were examined in a dietary paradigm that appears to be sensitive to 5-HT-induced carbohydrate suppression. In this paradigm, deprived rats are provided with a nutritionally complete hydrated chow mash diet together with an optional carbohydrate supplement of powdered Polycose. Both d-fenfluramine and DOI produced a clear suppression of total energy intake and carbohydrate (Polycose) intake. However, the mechanisms underlying these effects are different. The effect of d-fenfluramine in this paradigm was attenuated by the 5-HT1/5-HT2 receptor antagonist metergoline and partially attenuated by the 5-HT1A/5-HT1B receptor antagonist (+/-)cyanopindolol. In contrast, d-fenfluramine's effect was not antagonised by the 5-HT2 receptor antagonist ketanserin, the 5-HT3 receptor antagonist (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS-205,930), the 5-HT2/5-HT1C receptor antagonist ritanserin, or the peripheral 5-HT receptor antagonist xylamidine. However, the effect of DOI in this paradigm was significantly attenuated by ketanserin but was not antagonised by either ritanserin or (+/-)cyanopindolol. Therefore, the suppressive effect of these two 5-HT drugs on total and Polycose intake appears to be mediated, respectively, by 5-HT1B/5-HT1C receptors (d-fenfluramine) and 5-HT2 receptors (DOI).

5-HT1C/5-HT2 receptor blockade prevents 1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane-, but not stress-induced increases in brain tryptophan

We have previously shown that acute administration of the 5-HT1C/5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane (DOI), elevates brain tryptophan levels. The present work aimed to investigate the mechanisms responsible for this elevation. Acute s.c. administration of a 2-mg/kg dose of DOI increased brain tryptophan levels but did not affect either plasma free tryptophan, plasma total tryptophan, brain 5-HT, or brain 5-hydroxyindoleacetic acid. Pretreatment with the 5-HT1C/5-HT2 receptor antagonist, LY 53857, prevented the DOI-induced increase in brain tryptophan levels, whilst the increase was reduced by the 5-HT2 receptor/alpha 1-adrenoceptor antagonist, ketanserin, and to a lesser extent, by the ganglionic blocker, chlorisondamine. On the other hand, pretreatment with either the peripherally acting 5-HT1C/5-HT2 receptor blocker, BW 501C67, the 5-HT uptake enhancer, tianeptine, the 5-HT uptake blocker, paroxetine, or the beta 2-adrenoceptor antagonist, ICI 118.551, proved ineffective. Lastly, pretreatment with LY 53857 did not affect the immobilization-induced elevation in brain tryptophan levels. It is concluded that the elevation in brain tryptophan levels induced by DOI but not that induced by stress is due to central 5-HT1C and 5-HT2 receptor stimulation.

Can the 5-HT2/1c agonist DOI cause differential sympatho-excitation in nerves supplying the heart in anaesthetized cats?

A comparison of the effects on sympathetic nerve activity to the heart of intravenous administration of the selective 5-HT2/1c agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) alone and in the presence of the peripherally acting 5-HT2/1c antagonist BW501C67 were made in alpha-chloralose anaesthetized cats. Activity in both cardiac sympathetic nerves running in the vagus and in both inferior cardiac nerves was simultaneously recorded. In addition renal and phrenic nerve activity, heart rate, arterial blood pressure, femoral arterial flow and tracheal pressure were also recorded. DOI evoked a rise in blood pressure and increased femoral arterial resistance in both groups of animals. In the BW501C67 pretreated animals, DOI had no effect on heart rate but caused a significant increase in all sympathetic nerve activities. In non-pretreated animals, however, the rise in blood pressure was associated with variable effects on sympathetic nerve activity, a significant rise only occurring in renal nerve activity. In these experiments DOI evoked a bradycardia. The variability in sympathetic nerve activity in the non-pretreated animals may have resulted from the rise in blood pressure evoking a baroreceptor-mediated central sympathoinhibition which would interfere with the central sympathoexcitatory effects of DOI. It is concluded that centrally, DOI will cause sympathoexcitation. In addition, intravenous DOI acting on 5-HT2 receptors on bronchial smooth muscle evokes bronchoconstriction as indicated by the very large rise in tracheal pressure in non-BW501C67-pretreated animals. If not controlled this severely compromises respiration leading to a large overestimate of the sympathoexcitatory effects of stimulation of central 5-HT2/1c receptors.

Central administration of 5-HT activates 5-HT1A receptors to cause sympathoexcitation and 5-HT2/5-HT1C receptors to release vasopressin in anaesthetized rats

1. The effects of intracerebroventricular injections to the right lateral ventricle (i.c.v.) of 5-hydroxytryptamine (5-HT, 40 and 120 nmol kg-1), N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT; 3 nmol kg-1), 5-carboxamidotryptamine (5-CT; 3 nmol kg-1), 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT; 3, 40 and 120 nmol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 40 and 120 nmol kg-1) on renal sympathetic nerve activity, blood pressure, heart rate and phrenic nerve activity were investigated in normotensive rats anaesthetized with alpha-chloralose. 2. 5-HT caused a long lasting pressor response which was associated with an initial bradycardia and renal sympathoinhibition followed by a tachycardia and renal sympathoexcitation. Pretreatment with the 5-HT2/5-HT1C receptor antagonists, cinanserin (300 nmol kg-1, i.c.v.) or LY 53857 (300 nmol kg-1, i.c.v.) reversed the initial bradycardia and sympathoinhibition to tachycardia and sympathoexcitation. Combined pretreatment with LY 53857 (300 nmol kg-1, i.c.v.) and the 5-HT1A antagonist, spiroxatrine (300 nmol kg-1, i.c.v.), blocked the effects of 5-HT on all the above variables. 3. Pretreatment with the vasopressin V1-receptor antagonist, beta-mercapto-beta,beta-cyclopentamethylene-propionyl1, O-Me-Tyr2, Arg8-vasopressin [(d(CH2)5Tyr(Me)AVP, 10 micrograms kg-1, i.v.] did not affect the magnitude but reduced the duration of the pressor response produced by i.c.v. 5-HT and reversed the initial bradycardia and renal sympathoinhibition to tachycardia and sympathoexcitation. 4. 1-(2,5-Di-methoxy-4-iodophenyl)-2-aminopropane (DOI) caused a pressor effect which was associated with a bradycardia and sympathoinhibition. These effects were blocked by pretreatment with BWSOlC67 (0.1 mg kg-', i.v.), a peripherally acting 5-HT2/5-HTc receptor antagonist. However,BWSOlC67 (0.1 mg kg-', i.v.) failed to block the effects of i.c.v. 5-HT.5. DP-5-CT, 5-CT and 8-OH-DPAT (3 nmol kg-', i.c.v.) caused sympathoexcitation, tachycardia and a rise in blood pressure. Pretreatment with methiothepin (1 mg kg-', i.v.) or spiroxatrine (300 nmol kg-',i.c.v.) attenuated the response to i.c.v. DP-5-CT.6. It is concluded that i.c.v. administration of 5-HT activates 5-HTlA receptors to cause sympathoexcitation and 5-HT2 or 5-HT1c receptors to cause the release of vasopressin.

Vagal afferent-mediated inhibition of a nociceptive reflex by i.v. serotonin in the rat. II. Role of 5-HT receptor subtypes

In the rat, intravenous (i.v.) serotonin (5-HT) is a noxious stimulus which produces distinct vagal afferent-mediated pseudoaffective responses, a passive avoidance behavior, a vagal afferent-mediated inhibition of the nociceptive tail-flick (TF) reflex and a complex triad of cardiovascular responses. In the present study, we have used a variety of 5-HT receptor antagonists to characterize the receptor subtype(s) in the rat that mediate (1) inhibition of the TF reflex and (2) the cardiovascular responses produced by i.v. 5-HT. 5-HT produced a dose-dependent (3-72 micrograms/kg, i.v.) inhibition of the TF reflex (ED50 = 15.3 +/- 0.7 micrograms/kg). Following administration of the 5-HT2 receptor-selective antagonists ketanserin (50-250 micrograms/kg, i.v.) or xylamidine (10-100 micrograms/kg, i.v.), or the 5-HT3 receptor-selective antagonists ICS 205-930 (50-250 micrograms/kg, i.v.) or MDL 72222 (25-250 micrograms/kg, i.v.), there appeared to be a parallel shift of the 5-HT dose-response curve to the right. Following co-administration of xylamidine (50 micrograms/kg, i.v.) with ICS 205-930 (100 micrograms/kg, i.v.), the 5-HT-induced inhibition of the TF reflex was completely abolished at all doses of 5-HT tested (3-288 micrograms/kg, i.v.). In contrast, administration of the centrally acting 5-HT2 receptor-selective antagonist LY 53857 (10-100 micrograms/kg, i.v.) or the non-specific receptor antagonist methysergide (25-500 micrograms/kg, i.v.) resulted in a dose-dependent, but not parallel shift of the 5-HT dose-response curve to the right. The maximal doses of LY 53857 and methysergide tested (250 micrograms/kg and 500 micrograms/kg, respectively) completely abolished the effects of 5-HT (3-288 micrograms/kg, i.v.). Administration of the alpha 1-adrenoceptor antagonist prazosin (25-100 micrograms/kg, i.v.) failed to alter the 5-HT dose-response curve, indicating that the effects of ketanserin were due to blockage of 5-HT2 receptors rather than alpha 1 receptors. Administration of each of the antagonists also produced marked, but selective effects on components of the complex cardiovascular response to i.v. 5-HT. Each of the 5-HT3 receptor selective antagonists (ICS 205-930 or MDL 72222) produced a dose-dependent attenuation of the Bezold-Jarisch reflex-mediated hypotension and bradycardia, and each of the 5-HT2 receptor selective antagonists (xylamidine, ketanserin or LY 53857) produced a dose-dependent attenuation of the pressor response. The non-specific 5-HT receptor antagonist methysergide produced a dose-dependent attenuation of the 5-HT-induced pressor response.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperglucagonemia induced in mice by tryptamine: involvement of the peripheral 5-HT2 receptors

The effects of an indoleamine, tryptamine, on plasma glucagon levels were investigated in mice. Tryptamine induced dose-related increases in plasma glucagon levels. The hyperglucagonemia effects of tryptamine were completely antagonized by methysergide and ketanserin which have a high affinity to 5-HT2 receptors. In addition, the peripheral 5-HT2 receptor antagonist, xylamidine, also strongly inhibited tryptamine-induced hyperglucagonemia. Our results indicate that the peripheral 5-HT2 receptors mediate the increase in plasma glucagon levels induced by tryptamine and that these receptors may have a role in the control of glucagon secretion.

Mechanisms involved in the hyperglycemic effect of the 5-HT1C/5-HT2 receptor agonist, DOI

Previous experiments have indicated that 5-HT2 receptors and catecholaminergic systems mediate the rise in plasma glucose levels elicited by acute administration of the 5-HT1c/5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). On this basis, we investigated the location of these serotonin receptors and the nature of this catecholaminergic involvement. Administration of DOI (0.4 mg/kg i.v.) to conscious rats (bearing jugular catheters) elicited a rapid rise in plasma glucose which was associated with a decreased insulin response to a glucose bolus (300 mg/kg i.v.). Pretreatment with the peripherally acting 5-HT1c/5-HT2 receptor antagonist, BW 501C67 (0.5 mg/kg i.v. 10 min beforehand) prevented the rise in plasma glucose triggered by the peripherally acting 5-HT1c/5-HT2 receptor agonist, alpha-methyl-5-HT (0.75 mg/kg i.v.), but amplified the rise elicited by DOI. Pretreatment with chlorisondamine (1 mg/kg i.v. 10 min beforehand) or adrenalectomy 20 h beforehand prevented the DOI-induced hyperglycemia. On the other hand, pretreatment with dexamethasone (0.35 mg/kg s.c. 2 h and 20 min beforehand) did not affect the DOI-induced hyperglycemia. It is concluded that the hyperglycemic effect of DOI administration is mediated by centrally located 5-HT2 receptors and, in turn, adrenal epinephrine release.

Effect of drugs influencing 5-HT function on ethanol drinking and feeding behaviour in rats: studies using a drinkometer system

In the present study, we have investigated how various 5-HT agonists (m-chlorophenylpiperazine (mCPP) (0.1-1 mg/kg), 8-hydroxy 2-(di-N-propylamino) tetralin (8-OH DPAT) (0.125-2 mg/kg) and 5-HT (0.5-2 mg/kg)), the 5-HT uptake blocker sertraline (1-10 mg/kg), and the 5-HT uptake blocker and releaser dexfenfluramine (0.5-2.5 mg/kg), affect ethanol intake in a continual access paradigm using Wistar rats. By means of a drinkometer system the effect of each drug on microdrinking parameters (e.g., drink latency, number, and duration of drinking bouts) was assessed. The effect of various 5-HT antagonists (metergoline, ritanserin, ondansetron, and xylamidine) against the dexfenfluramine-induced suppression was studied. Furthermore, threshold doses for the anorectic and the suppressant effects of mCPP, sertraline and dexfenfluramine on ethanol intake were identified. From these studies, it seemed that similar mechanisms may be responsible for the suppressant effects of the various 5-HT agonists studied (direct and indirect) on ethanol and food intake. The 5-HT3 receptor antagonist, ondansetron, also reduced ethanol (but not food) intake. However, the profile of this effect may suggest an alternative means by which 5-HT3 receptors regulate ethanol intake in the rat by comparison to the various 5-HT agonists studied.

Peripherally administered 5-hydroxytryptamine elicits the full behavioural sequence of satiety

If 5-HT acts as a peripheral satiety signal, then it should not only produce anorectic effects when administered systemically but should allow the normal behavioural sequence of satiety to develop. Food-deprived rats treated with 5 mg/kg 5-HT were found to show, when given access to food, the full normal behavioural sequence of satiety, although the sequence occurred earlier than in rats treated with saline. It is therefore argued that 5-HT must be considered as a likely candidate for a peripheral satiety signal.

The peripheral nociceptive actions of intravenously administered 5-HT in the rat requires dual activation of both 5-HT2 and 5-HT3 receptor subtypes

In 16-week-old Sprague-Dawley rats lightly anesthetized with pentobarbital, 5-HT (3-96 micrograms/kg, i.v.; n = 6) produced distinct pseudaffective responses and a dose-dependent (slope = 17.2 +/- 6.8 s/log10 dose) inhibition of the tail-flick (TF) reflex (ED50 = 32.6 +/- 9.2 micrograms/kg). In the same rats, a 1:1 combination of alpha-methyl 5-HT (a 5-HT2, receptor selective agonist) and 2-methyl 5-HT (a 5-HT3 receptor selective agonist) (3-192 micrograms/kg, i.v.), produced the same profile of pseudaffective responses and also resulted in a dose-dependent (slope = 34.0 +/- 7.0 s/log10 dose) inhibition of the TF reflex (ED50 = 88.4 +/- 20.5 micrograms/kg). In contrast, administration of alpha-methyl 5-HT (3-192 micrograms/kg, i.v.) or 2-methyl 5-HT (3-192 micrograms/kg, i.v.) alone did not produce any pseudaffective responses or any change in TF latency from baseline. In conscious 16-week-old male Sprague-Dawley rats, administration of 5-HT (48 micrograms/kg, i.v.; n = 5), or a 1:1 combination of alpha-methyl 5-HT and 2-methyl 5-HT (total dose = 120 micrograms/kg, i.v.; n = 5), resulted in a passive avoidance behavior assessed in a step-down paradigm (slopes = 139.7 +/- 58.2 and 154.9 +/- 63.9 s/trial, respectively), and the same profile of distinct pseudaffective responses exhibited by the lightly pentobarbital-anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological evidence that circumventricular organs may participate in the pressor effect of central serotonin

The present study was to investigate the role of '5-HT2-like' receptors in the circumventricular organs in the pressor response elicited by the central injection of serotonin (5-HT) to conscious rats. The increase in blood pressure induced by intracerebroventricular (i.c.v.) 5-HT (2.5 micrograms) was attenuated by intravenous pretreatment will, the peripheral 5-HT2 antagonist xylamidine (100 or 300 micrograms/kg). A combination of xylamidine with prazosin or a V1-vasopressin antagonist did not produce a further attenuation in the pressor response. Our data suggest that when administered i.c.v., 5-HT increases blood pressure by acting on 5-HT2-like receptors located in circumventricular organs as well as areas of the brain protected by the blood-brain barrier.

Investigation of the effects of IVth ventricular administration of the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), on autonomic outflow in the anaesthetized cat

1. The effects of IVth ventricular injections of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, arterial blood pressure, heart rate, femoral arterial conductance, tracheal and intragastric pressures were investigated in alpha-chloralose anaesthetized, neuromuscular blocked and artificially ventilated cats. 2. Cumulative doses of DOI (80, 160 and 320 nmol kg-1) injected into the IVth ventricle caused an increase in mean arterial blood pressure, a fall in femoral arterial conductance, an increase in tracheal pressure and a decrease in the rate of phrenic nerve bursts but did not affect any of the other variables recorded. 3. Even after i.v. administration of the peripheral 5-HT2 antagonist BW501C67 (2 mg kg-1) following the highest dose of DOI there was still a significant pressor response, a fall in femoral arterial conductance and small increase in tracheal pressure. 4. In control experiments, intravenous infusion of noradrenaline to raise blood pressure to the levels obtained during the cumulative doses of DOI caused large falls in renal, splanchnic and cardiac nerve activities which were all significantly lower than those recorded during the cumulative doses of DOI. 5. The results of this study provide evidence for a brainstem site of action of DOI in producing hypertension and further support the hypothesis that central 5-HT2 receptors are involved in the control of skeletal muscle and skin vascular beds.

A peripheral 5-HT1D-like receptor involved in serotonergic induced hindlimb scratching in rats

The pharmacological characteristics of hindlimb scratching induced by serotonergic compounds were studied. We conclude that hindlimb scratching induced by serotonergic compounds is mediated by a serotonin1D (5-HT1D) or 5-HT1D-like receptor outside the blood-brain barrier because hindlimb scratching could be induced by s.c. injection of 5-methoxytryptamine (5-MeOT), 5-carboxamidotryptamine (5-CT), bufotenine, 5-hydroxytryptamine (5-HT) and tryptamine. These compounds have high affinity for 5-HT1A and 5-HT1D receptors. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5-HT1C receptor agonist MK 212, and the mixed 5-HT1C/5-HT2 receptor agonists (dl)-1-(2,5 dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and quipazine did not induce hindlimb scratching. Rather, the latter compounds attenuated 5-MeOT-induced hindlimb scratching. The 5-HT releasing compounds fenfluramine and p-chloroamphetamine (PCA) inhibited whereas the 5-HT re-uptake inhibitors fluvoxamine and indalpine potentiated 5-MeOT-induced hindlimb scratching. 5-MeOT-induced hindlimb scratching could be inhibited dose dependently by the alpha 2-adrenoceptor blockers yohimbine and rauwolsince, which also have high affinity for 5-HT1D receptors, whereas the alpha 2-adrenoceptor blocker piperoxan only weakly counteracted hindlimb scratching. Haloperidol, apomorphine, morphine, clonidine and methiothepin strongly attenuated hindlimb scratching, atropine, naloxone and ICS 205930 attenuated it weakly whereas domperidone, methylatropine and mepyramine were inactive in doses up to 10 mg/kg. Hindlimb scratching induced by 5-MeOT was potentiated by the 5-HT receptor antagonists metergoline, methysergide, mesulergine, mianserin, ritanserin and xylamidine. Hindlimb scratching was not induced by i.c.v. injection of 5-MeOT.(ABSTRACT TRUNCATED AT 250 WORDS)

Quipazine-induced hypertension in anaesthetized cats is mediated by central and peripheral 5-HT2 receptors: role of the ventrolateral pressor area

Quipazine (0.5 mg/kg i.v.) produced a sustained pressor response and an increase in splanchnic nerve activity in intact as well as in baroreceptor-denervated cats without causing a significant change in heart rate. These effects were prevented by the 5-HT2 receptor antagonists, ritanserin (0.5 mg/kg i.v.) or BW 501 C (0.5 mg/kg i.v.). Quipazine induced an hypertensive response and an increase in splanchnic discharge in cats pretreated with prazosin (0.1 mg/kg) or hexamethonium (10 mg/kg i.v.). Bilateral application of quipazine (25 micrograms/side) to the ventrolateral pressor area produced a rapid increase in mean blood pressure and in splanchnic discharge. Pretreatment with prazosin (0.1 mg/kg i.v.) abolished the hypertension but not the sympatho-excitatory effects of quipazine. Local application of the 5-HT2 receptor antagonists, LY53857 (10 micrograms/side) or cyproheptadine (10 micrograms/side), had no effects on blood pressure and splanchnic nerve activity but prevented or reversed the actions of locally applied quipazine. LY 53857 (10 micrograms/side) antagonized the sympatho-excitatory effects of systemically administered quipazine. These results indicate that the cardiovascular changes induced by quipazine in anaesthetized cats are mediated by central 5-HT2 receptors located in the ventrolateral pressor area and by peripheral vascular 5-HT2 receptors.

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.

Peripheral serotonergic inhibition of suckling

Systemically administered serotonin, which does not pass the blood-brain barrier, inhibited nipple attachment behavior in 20- and 30-day-old rat pups. Xylamidine, a peripheral serotonin antagonist, attenuated the effects of serotonin, quipazine, and fenfluramine on nipple attachment behavior. Thus, serotonin receptors in the periphery may play an important role in the serotonergic inhibitory mechanism that has been hypothesized as the developing system leading to weaning. However, unlike more general 5-HT antagonists, xylamidine given alone failed to facilitate suckling, suggesting different sites of action for facilitation and inhibition of this infantile behavior.

Hemodynamic and renin responses to (+-)-DOI, a selective 5-HT2 receptor agonist, in conscious rats

The effect of the selective 5-HT2 agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) on arterial pressure (AP), heart rate (HR), renal blood flow (RBF) and plasma renin activity (PRA) was determined in conscious rats. DOI increased AP and PRA, but decreased HR and RBF. All responses to DOI were abolished by central (LY 53857) or peripheral (xylamidine) 5-HT2 antagonists. Prazosin did not alter the AP or HR response to DOI. Chlorisondamine abolished the bradycardia but slightly increased the hypertension produced by DOI, while enalapril attenuated the pressor response. No further reduction was produced by the combination of enalapril and prazosin. Propranolol attenuated but did not eliminate the renin response, and blocked the bradycardia elicited by DOI. The data suggest that DOI activates 5-HT2 receptors located on vascular smooth muscle and/or the circumventricular organs of the brain to: (1) increase AP and reflexly decrease HR, (2) decrease RBF and (3) increase PRA. The hypertension is mediated by angiotensin II and direct vascular effects whereas the increase in PRA is mediated by an interaction of increased sympathetic nerve activity and decreased renal perfusion pressure.

Effects of xylamidine on peripheral 5-hydroxytryptamine-induced anorexia

Rats injected peripherally with 5-hydroxytryptamine (5-HT) showed a dose-dependent decrease in food intake following overnight fasting. The peripheral 5-HT-2 antagonist xylamidine had no effect on food intake when administered alone, but antagonised 5-HT-induced anorexia. However, at the highest dose of 5-HT (5 mg/kg), both doses of xylamidine (1.0 and 2.0 mg/kg) displayed the same degree of antagonism to the anorectic effect, but failed to block it completely. The results are discussed in terms of 5-HT receptor subtypes, and it is suggested that non-5-HT-2 receptors may be partially responsible for the mediation of peripheral 5-HT-induced anorexia.

Lack of a difference between ketanserin and ritanserin in central vs. peripheral serotonin receptor antagonism

Ketanserin and ritanserin antagonized with similar potency the pressor response to serotonin in pithed rats, a measure of antagonism of vascular 5HT2 receptors. Both compounds also antagonized the elevation of serum corticosterone concentration by quipazine, a centrally acting serotonin agonist; higher doses of both antagonists were needed, but ketanserin was not less potent than ritanserin. Earlier suggestions that ketanserin mainly blocks peripheral serotonin receptors and that ritanserin mainly blocks central serotonin receptors seem unfounded.

Evidence that d-fenfluramine anorexia is mediated by 5-HT1 receptors

The effects of eight serotonin (5-HT) receptor antagonists on the anorectic effect of d-fenfluramine (3.0 mg/kg, IP) were examined in a test of sweet mash consumption, using non-deprived male rats. d-Fenfluramine's effect was attenuated by the mixed 5-HT1/5-HT2 receptor antagonists, methiothepin and metergoline; by the 5-HT2 receptor antagonist ritanserin; and by (+/-)cyanopindolol, a mixed 5-HT1A/5-HT1B receptor antagonist. In contrast, d-fenfluramine's effect was not antagonised by the 5-HT2 receptor antagonists ketanserin and ICI 169 369; the 5-HT3 receptor antagonist ICS 205 930; or by xylamidine, a peripheral 5-HT receptor antagonist. In this feeding model, none of the 5-HT antagonists, when tested alone, had any effect to increase palatable food consumption. The pattern of results obtained strongly suggest that central 5-HT1 receptors play an important role in the mediation of d-fenfluramine-induced anorexia.

Suppression of food intake in rats by fluoxetine: comparison of enantiomers and effects of serotonin antagonists

R- and S-enantiomers of fluoxetine lowered food intake in meal-fed rats and in 2-deoxyglucose-induced hyperphagic rats. In both feeding paradigms, the S-enantiomer was slightly more potent. The potency of the two enantiomers of fluoxetine in producing anorectic effects paralleled their potency as inhibitors of 5-hydroxytryptamine (5HT) uptake in vivo. Both enantiomers were selective inhibitors of 5HT uptake in vitro and showed only weak affinity for 5HT-1, 5HT-1A and 5HT-2 receptors or for other receptors in rat brain. The anorectic effect of fluoxetine in meal-fed rats was not reversed by either centrally or peripherally acting 5HT-2 receptor antagonists (ritanserin, LY53857, xylamidine, BW 501C67) or a nonspecific 5HT receptor antagonist, metergoline. However, the serotonergic mechanism involved in the anorexic effect of fluoxetine is discussed.

Role of central serotonergic (5-HT2) receptor in blood pressure regulation in rats

To investigate the role of the serotonergic nervous system in blood pressure regulation, 5 micrograms of 5-hydroxytryptamine (5-HT) was given i.c.v. before and after 1 microgram of i.c.v. xylamidine or 200 micrograms of i.c.v. ketanserin or 200 micrograms of i.v. ketanserin in conscious Wistar Kyoto rats. Also i.v. (0.5, 1, 2 micrograms) or i.c.v. (1 microgram) phenylephrine (PHE) were given before and after 1 microgram of i.c.v. xylamidine. I.c.v. 5-HT elicited a consistent pressor response of approximately 27mmHg and slight decrease in heart rate. MAP and heart rate did not change after xylamidine or ketanserin. Whereas pressor response to i.c.v. 5-HT after i.c.v. ketanserin or i.c.v. xylamidine was suppressed, it did not change after i.v. ketanserin. Neither i.c.v. nor i.v. PHE-induced pressor response was influenced by i.c.v. xylamidine pretreatment. These data suggest that the central 5-HT2 receptor may subserve pressor function in rats.

Effects of fenfluramine, m-chlorophenylpiperazine, and other serotonin-related agonists and antagonists on penile erections in nonhuman primates

Fenfluramine, m-chlorophenylpiperazine (m-CPP), 1-phenylpiperazine, and the buspirone metabolite, 1-(2-pyrimidyl)piperazine given intravenously to adult rhesus monkeys regularly elicited penile erections. In contrast, serotonin (5-HT) agonists with 5-HT1A site specificity (8-OH-DPAT, buspirone) as well as trazodone, ritanserin, and metergoline were no different from saline in producing penile erections. Fenfluramine's effects were blocked by the 5-HT2 antagonists, ritanserin and metergoline, while m-CPP's effects were not blocked by the peripheral 5-HT antagonist, xylamidine, indicating that tumescence can be elicited by serotonergic agents which act at non-5-HT1A sites in the central nervous system.