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

2B Determined: The Future of the Serotonin Receptor 2B in Drug Discovery

The cardiotoxicity associated with des-ethyl-dexfenfluramine (norDF) and related agonists of the serotonin receptor 2B (5-HT2B) has solidified the receptor's place as an "antitarget" in drug discovery. Conversely, a growing body of evidence has highlighted the utility of 5-HT2B antagonists for the treatment of pulmonary arterial hypertension (PAH), valvular heart disease (VHD), and related cardiopathies. In this Perspective, we summarize the link between the clinical failure of fenfluramine-phentermine (fen-phen) and the subsequent research on the role of 5-HT2B in disease progression, as well as the development of drug-like and receptor subtype-selective 5-HT2B antagonists. Such agents represent a promising class for the treatment of PAH and VHD, but their utility has been historically understudied due to the clinical disasters associated with 5-HT2B. Herein, it is our aim to examine the current state of 5-HT2B drug discovery, with an emphasis on the receptor's role in the central nervous system (CNS) versus the periphery.

Blocking properties of terguride at the 5-HT2 receptor subtypes mediating cardiovascular responses in the rat

In vitro studies have suggested that terguride blocks the contractile and relaxant responses produced by 5-hydroxytryptamine (5-HT) via 5-HT_2A/2B receptors. This study has now investigated terguride's blocking properties on central/peripheral 5-HT₂ receptors in anaesthetized or pithed rats. Male Wistar anaesthetized/pithed rats were cannulated for recording blood pressure and heart rate and for i.v. administration of several compounds. In both groups of rats, i.v. bolus injections of 5-HT or (±)-DOI (a 5-HT₂ receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and heart rate. These responses were dose-dependently antagonized by terguride (10-3000 μg/kg). In anaesthetized rats, i.v. bolus injections of BW723C86 (a 5-HT_2B receptor agonist; 1-1000 μg/kg) produced dose-dependent increases in diastolic blood pressure and not dose-dependent increases in heart rate, while in pithed rats, these responses were attenuated. The vasopressor responses elicited by BW723C86 in anaesthetized rats were dose-dependently blocked by terguride (10-300 μg/kg), whereas its the tachycardic responses were dose-independently blocked. These results, taken together, suggest that terguride behaved as an antagonist at the 5-HT₂ receptors located in the central nervous system and (or) the systemic vasculature. This is the first evidence demonstrating that terguride can block central/peripheral 5-HT₂ receptors mediating cardiovascular responses in anaesthetized or pithed rats.

Immunomodulatory capacity of the serotonin receptor 5-HT2B in a subset of human dendritic cells

Serotonin is a monoamine neurotransmitter that signals through a wide array of receptors (5-HT_1–7) many of which are also involved in immune processes. Dendritic cells (DCs) are crucial players in immune defense by bridging innate and adaptive immune responses via their vast repertoire of pattern recognition receptors and antigen-presenting capability. Although serotonin is known to influence immunity at many levels, cell type-specific expression and function of its receptors remains poorly understood. Here we aimed to study 5-HT_1–7 expression and function in CD1a⁻ and CD1a⁺ human monocyte-derived DCs (moDCs). We found that the 5-HT_2B receptor-subtype is solely expressed by the inflammatory CD1a⁺ moDC subset. Specific 5-HT_2B activation potently inhibited TLR2, TLR3, and TLR7/8-induced proinflammatory cytokine and chemokine (TNF-α, IL-6, IL-8, IP-10, IL-12) but not type I interferon-β responses. 5-HT_2B agonism also interfered with the polarization of CD1a⁺ moDC-primed CD4⁺ T cells towards inflammatory Th1 and Th17 effector lymphocytes. Here we report the subset-specific expression and immunomodulatory function of 5-HT_2B in human moDCs. Our results expand the biological role of 5-HT_2B which may act not only as a neurotransmitter receptor, but also as an important modulator of both innate and adaptive immune responses.

Role of peripheral and spinal 5-HT2B receptors in formalin-induced nociception

In this study we assessed the role of local peripheral and spinal serotonin 2B (5-HT(2B)) receptors in rats submitted to the formalin test. For this, local peripheral ipsilateral, but not contralateral, administration of the highly selective 5-HT(2B) receptor antagonist 2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyridine (RS-127445, 0.01-1 nmol/paw) significantly prevented 1% formalin-induced flinching behavior. Moreover, local peripheral ipsilateral, but not contralateral, of the selective 5-HT(2) receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI, 1-10 nmol/paw) augmented 0.5% formalin-induced nociceptive behavior. The local pronociceptive effect of the 5-HT(2) receptor agonist DOI (10 nmol/paw) was significantly prevented by the local injection of RS-127445 (0.01 nmol/paw). Moreover, intrathecal injection of the selective 5-HT(2B) receptor antagonist RS-127445 (0.1-10 nmol/rat) also prevented 1% formalin-induced nociceptive behavior. In contrast, spinal injection of the 5-HT(2) receptor agonist DOI (1-10 nmol/rat) significantly increased flinching behavior induced by 0.5% formalin. The spinal pronociceptive effect of the 5-HT(2) receptor agonist DOI (10 nmol/rat) was prevented by the intrathecal injection of the 5-HT(2B) receptor antagonist RS-127445 (0.1 nmol/rat). Our results suggest that the 5-HT(2B) receptors play a pronociceptive role in peripheral as well as spinal sites in the rat formalin test. 5-HT(2B) receptors could be a target to develop analgesic drugs.

Elevated cyclic stretch and serotonin result in altered aortic valve remodeling via a mechanosensitive 5-HT(2A) receptor-dependent pathway

INTRODUCTION

Serotonin/5-hydroxytryptamine (5-HT) has been implicated in valve disease and in the modulation of valve mechanical properties. Several 5-HT receptor subtypes are also known to be mechanosensitive in other cell types, but this has not been studied in the context of the valve. In this study, we sought to understand the effects of elevated 5-HT levels and stretch overload on aortic valve remodeling and the dominant 5-HT receptor subtype that regulates these processes.

METHODS AND RESULTS

Collagen biosynthesis and tissue mechanical properties of porcine aortic valve cusps were evaluated after 10% (physiologic) and 15% (pathologic) dynamic stretch. These studies were performed in normal medium or medium supplemented with 5-HT (1, 10, 100 μM) in the absence and presence of 5-HT(2A) or 5-HT(2B) receptor antagonists. Fresh valves served as controls. Valve collagen content was maximal at the 10-μM 5-HT concentration for both 10% and 15% stretch. The 5-HT(2A) receptor antagonist reduced collagen synthesis, cell proliferation, and hsp47 expression under elevated and normal stretch, whereas the 5-HT(2B) receptor antagonist was effective only at normal stretch. The pretransition stiffness of the valve cusps was also increased in response to 5-HT via a stretch-sensitive 5-HT(2A) mechanism, with the post-transition stiffness unaltered.

CONCLUSIONS

Combined elevated stretch and 5-HT resulted in increased valve collagen biosynthesis, cell proliferation, and tissue stiffness. These responses were inhibited by a 5-HT(2A) antagonist. This strongly suggests that the 5-HT(2A) receptor subtype is sensitive to elevated stretch.

5-HT(2B) receptors modulate visceral hypersensitivity in a stress-sensitive animal model of brain-gut axis dysfunction

BACKGROUND

Irritable bowel syndrome (IBS) is associated with an enhanced perception to visceral stimuli and exaggerated stress response. The serotonergic neurotransmitter system has been strongly implicated as a key player in the manifestation of IBS symptomatology including visceral hypersensitivity. However the role of 5-HT(2B) receptors in visceral pain, although speculated, is currently unclear. Thus we assessed the impact of a selective 5-HT(2B) receptor antagonist, RS-127445, on visceral hypersensitivity in a model of brain gut axis dysfunction the Wistar Kyoto (WKY) rat.

METHODS

Colorectal distension (CRD) was used to assess the visceral sensitivity of the WKY rat compared to normosensitive Sprague Dawley (SD) rats. Once we verified the visceral sensitivity of the WKY rat we assessed the efficacy of RS-127445 in pain signalling from the colorectum. We administered the compound peripherally (i.p.) and centrally (i.c.v.) in order to ascertain the site of action of RS 127445. Behavioural responses to colorectal distention were then monitored.

KEY RESULTS

The WKY rats were more viscerally hypersensitive than the SD as previously shown. RS-127445 (5 mg kg(-1), i.p.) significantly reversed visceral hypersensitivity in WKY animals. Moreover, when administered intracerebroventricularly RS-127445 (100 nM) also decreased the number of pain behaviours during noxious CRD in the WKY animals.

CONCLUSIONS & INFERENCES

Taken together, blockade of 5-HT(2B) receptors offers an exciting novel therapeutic target for pain relief in stress-related gastrointestinal disorders such as IBS.

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.

Cardiovascular responses produced by 5-hydroxytriptamine:a pharmacological update on the receptors/mechanisms involved and therapeutic implications

The complexity of cardiovascular responses produced by 5-hydroxytryptamine (5-HT, serotonin), including bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, has been explained by the capability of this monoamine to interact with different receptors in the central nervous system (CNS), on the autonomic ganglia and postganglionic nerve endings, on vascular smooth muscle and endothelium, and on the cardiac tissue. Depending, among other factors, on the species, the vascular bed under study, and the experimental conditions, these responses are mainly mediated by 5-HT(1), 5-HT(2), 5-HT(3), 5-HT(4), 5-ht(5A/5B), and 5-HT(7) receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT(6) receptors do not seem to be involved in the cardiovascular responses to 5-HT. Regarding heart rate, intravenous (i.v.) administration of 5-HT usually lowers this variable by eliciting a von Bezold-Jarisch-like reflex via 5-HT(3) receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT(1B/1D) receptors and, in the case of the rat, an additional 5-ht(5A/5B) receptor component. Moreover, i.v. 5-HT can increase heart rate in different species (after vagotomy) by a variety of mechanisms/receptors including activation of: (1) myocardial 5-HT(2A) (rat), 5-HT(3) (dog), 5-HT(4) (pig, human), and 5-HT(7) (cat) receptors; (2) adrenomedullary 5-HT(2) (dog) and prejunctional sympatho-excitatory 5-HT(3) (rabbit) receptors associated with a release of catecholamines; (3) a tyramine-like action mechanism (guinea pig); and (4) unidentified mechanisms (certain lamellibranch and gastropod species). Furthermore, central administration of 5-HT can cause, in general, bradycardia and/or tachycardia mediated by activation of, respectively, 5-HT(1A) and 5-HT(2) receptors. On the other hand, the blood pressure response to i.v. administration of 5-HT is usually triphasic and consists of an initial short-lasting vasodepressor response due to a reflex bradycardia (mediated by 5-HT(3) receptors located on vagal afferents, via the von Bezold-Jarisch-like reflex), a middle vasopressor phase, and a late, longer-lasting, vasodepressor response. The vasopressor response is a consequence of vasoconstriction mainly mediated by 5-HT(2A) receptors; however, vasoconstriction in the canine saphenous vein and external carotid bed as well as in the porcine cephalic arteries and arteriovenous anastomoses is due to activation of 5-HT(1B) receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT(7) receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT(1A/1B/1D) receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT(2B) and/or 5-HT(1B/1D) receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT(1A) receptors) and hypertension (mainly mediated by 5-HT(2) receptors). The increasing availability of new compounds with high affinity and selectivity for the different 5-HT receptor subtypes makes it possible to develop drugs with potential therapeutic usefulness in the treatment of some cardiovascular illnesses including hypertension, migraine, some peripheral vascular diseases, and heart failure.

Chronic treatment with mianserin prevents DOCA–salt hypertension in rats—evidence for the involvement of central 5-HT2 receptors

Central 5-HT2A receptors have been implicated in central volume control by activating a central angiotensinergic pathway to cause the release of vasopressin. Interestingly, to induce DOCA-salt hypertension in rats vasopressin release is required. Thus the present experiments were carried out to determine whether continuous blockade of these receptors over 20 days, with the non-selective 5-HT2 receptor antagonist mianserin would prevent the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Mianserin, given i.c.v. 90 or 60 microg twice daily for 20 days prevented the development of hypertension in conscious rats receiving DOCA-salt but did not affect blood pressure in rats on salt alone. Further, the dose of 30 microg given i.c.v. twice daily had no effect nor did the vehicle, polyethylene glycol (PEG), on the development of the hypertension. Mianserin 90 microg twice daily i.c.v. was also shown to prevent the increase in fluid intake, urinary flow and sodium excretion caused by DOCA-salt treatment. These data indicate that this action of mianserin is not due to an intrinsic hypotensive action but an action which involves interference with the mechanism by which DOCA-salt treatment causes hypertension. Thus the data overall support the view that to induce hypertension with DOCA-salt a central 5-HT-containing pathway needs to be activated, which then activates 5-HT2 receptors to cause the release of vasopressin which has previously been shown to be responsible for the initiation of DOCA-salt treatment hypertension.

Effect of the activation of central 5-HT2C receptors by the 5-HT2C agonist mCPP on blood pressure and heart rate in rats

In the present study we investigated the role of central 5-HT2C receptors in the control of blood pressure and heart rate in non-stressed and stressed, adult, male, Wistar rats. Third ventricle injections of the 5-HT2C agonist mCPP elicited a significant increase in blood pressure in non-stressed animals. The initial period of this hypertensive response (10-30 min after mCPP administration) was accompanied by baroreflex-mediated bradycardia, while after this period the coexistence of hypertension and tachycardia was observed. These cardiovascular effects promoted by the central administration of mCPP were blocked by pretreatment with the 5-HT2C antagonist, SDZ SER 082. The administration of SDZ SER 082 alone induced no significant changes in blood pressure or heart rate. The pharmacological stimulation of central 5-HT2C receptors by mCPP did not change the hypertensive or tachycardic responses induced by restraint stress. Conversely, the blockade of central 5-HT2C receptors by SDZ SER 082 blunted stress-induced hypertension without modifying stress-induced tachycardia. It is concluded that the activation of central 5-HT2C receptors induces hypertension in non-stressed rats and that the normal function of these receptors is essential for the rise in blood pressure that occurs in the course of restraint stress.

5-HT7, but not 5-HT2B, receptors mediate hypotension in vagosympathectomized rats

This study evaluated the possible involvement of 5-HT(2B) receptors in long-lasting hypotension to 5-hydroxytryptamine (5-HT), which is predominantly mediated by 5-HT7 receptors, in anaesthetised vagosympathectomized rats. Intravenous injections of 5-HT and 5-carboxamidotryptamine (5-CT) elicited a dose-dependent hypotension that was dose-dependently antagonised by (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl) ethyl] pyrrolidine (SB-269970; a selective 5-HT7 receptor antagonist), but not by saline. Interestingly, alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (BW723C86; a 5-HT(2B) receptor agonist) produced vasopressor responses without affecting hypotension to 5-HT. These results suggest that hypotension to 5-HT and 5-CT is mainly mediated by 5-HT7 receptors, whilst the role of 5-HT(2B) receptors seems unlikely.

Metachlorophenylpiperazine (m-CPP) induced intracavernous pressure responses in anaesthetized rats

Here we have recorded the effects of metachlorophenylpiperazine (m-CPP) on intracavernous pressure (ICP) in anesthetized rats pretreated with various pharmacological agents in an attempt to determine the mechanism and relevance of the m-CPP induced ICP response to other models of erection. m-CPP elicited consistent and significantly greater increases in ICP (71.5+/-6.6 mmHg) compared with the mixed 5-HT(2a/2c) agonists trifluoromethylphenylpiperazine (3.4+/-1.3 mmHg) and quipazine (10.9+/-1.8 mmHg). Blockade of 5-HT(2a) receptors with ketanserin failed to unmask any stimulatory effect of quipazine (7.2+/-1.0 mmHg). m-CPP induced ICP responses (71+/-7.0 mmHg) were unaffected in the presence of mianserin (63+/-5 mmHg) and ketanserin (51+/-12 mmHg). Spiperone significantly reduced the m-CPP induced increase in ICP (8.0+/-1.0 mmHg). Naloxone, yohimbine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) failed to elicit increases in ICP on their own. All three drugs significantly reduced the latency to the first m-CPP induced ICP response compared to saline. Yohimbine increased the duration of m-CPP induced ICP responses whereas 8-OHDPAT increased the mean number of m-CPP induced ICP responses compared to saline. The effects of m-CPP on ICP in anesthetized rats may not be mediated by 5-HT(2c) receptors and appears to be similar to erection in copula, but not erection elicited by other drugs or penile sheath retraction.

Differences in potency and efficacy of a series of phenylisopropylamine/phenylethylamine pairs at 5-HT(2A) and 5-HT(2C) receptors

The pharmacological profile of a series of (+/-)-2,5-dimethoxy-4-(X)-phenylisopropylamines (X=I, Br, NO(2), CH(3), or H) and corresponding phenylethylamines, was determined in Xenopus laevis oocytes injected with cRNA coding for rat 5-HT(2A) or 5-HT(2C) receptors. The efficacy and relative potency of these drugs were determined and compared to classical 5-HT(2) receptor agonists and antagonists. The rank order of agonist potency at the 5-HT(2A) receptor was: alpha-methyl-5-HT=5-HT>m-CPP>MK-212; at the 5-HT(2C) receptor the order was: 5-HT>alpha-methyl-5-HT>MK-212>m-CPP. All these compounds were full agonists at the 5-HT(2C) receptor, but alpha-methyl-5-HT and m-CPP showed lower efficacy at the 5-HT(2A) receptor. 4-(4-Fluorobenzoyl)-1-(4-phenylbutyl)piperidine (4F 4PP) was 200 times more potent as a 5-HT(2A) antagonist than at 5-HT(2C) receptors. Conversely, RS 102221 was 100 times more potent as a 5-HT(2C) antagonist, confirming their relative receptor selectivities. The phenylisopropylamines were partial agonists at the 5-HT(2A) receptor, with I(max) relative to 5-HT in the 22+/-7 to 58+/-15% range; the corresponding phenylethylamines had lower or undetectable efficacies. All these drugs had higher efficacies at 5-HT(2C) receptors; DOI was a full 5-HT(2C) agonist. 2C-I and the other phenylethylamines examined showed relative efficacies at the 5-HT(2C) receptor ranging from 44+/-10% to 76+/-16%. 2C-N was a 5-HT(2) receptor antagonist; the mechanism was competitive at the 5-HT(2A), but non-competitive at the 5-HT(2C) receptor. The antagonism was time-dependent at the 5-HT(2C) receptor but independent of pre-incubation time at the 5-HT(2A) receptor subtype. The alpha-methyl group determines the efficacy of these phenylalkylamines at the 5-HT(2A) and 5-HT(2C) receptors.

Central cardiovascular regulation and 5-hydroxytryptamine receptors

Evidence is provided to support the view that central 5-HT(1A) and 5-HT(2) receptors are the major receptor subtypes important in cardiovascular regulation. Data are also provided to implicate 5-HT(1B/1D/1F) receptors in central cardiovascular regulation. Activation of 5-HT(2) receptors generally causes sympathoexcitation and a rise in blood pressure and this is mainly mediated by 5-HT(2A) receptors. However, presympathetic vasomotor neurones located in the hindbrain (RVLM), controlling sympathetic outflow to the heart, are not activated in the same way as other presympathetic vasomotor neurones, although activation of 5-HT(2) receptors located in the midbrain can activate sympathetic outflow to the heart. Furthermore, at least in the rat, these midbrain 5-HT(2A) receptors are also responsible for the release of vasopressin by activation of a central angiotensinergic pathway. The ability of vasopressin directly and/or indirectly to modify renal sympathetic outflow involves the activation of central 5-HT(2B) receptors, which in turn, when activated via the i.c.v. route, can cause selective renal sympathoexcitation. Evidence is also provided which indicates that the reflex control of parasympathetic outflow to the heart and to other organs involves central 5-HT(1A) receptors located in the vicinity of these preganglionic vagal neurones. Finally, 5-HT(3) receptors are implicated in the afferent regulation of central sympathetic and parasympathetic tone.

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.