Comparison of the cardiovascular effects of the 5-HT1A receptor agonist flesinoxan with that of 8-OH-DPAT in the rat

Distinct methylome profile of cfDNA in AMI patients reveals significant alteration in cAMP signaling pathway genes regulating cardiac muscle contraction

BACKGROUND

The role of epigenetics in cardiovascular diseases has paved the way for innovative therapeutic approaches. Investigating epigenetic changes using cell-free DNA (cfDNA) holds substantial promise beyond mere diagnostics, especially for heart-related conditions like acute myocardial infarction (AMI), where obtaining tissue samples is a challenge. This study explores the methylation patterns of cfDNA in AMI patients and compares them with genomic DNA (gDNA) from the same individuals, aiming to evaluate the effectiveness of cfDNA as a valuable resource for studying heart-related diseases.

METHODOLOGY

We generated global methylome profiles of cfDNA and gDNA from 25 AMI patients using EM-Seq. Tissue deconvolution analysis was performed to estimate tissue specificity based on the methylation patterns. Differentially methylated loci were identified and explored to understand AMI pathophysiology.

RESULTS

Comparative analysis of cfDNA and gDNA methylation patterns in AMI patients reveals cfDNA holds more significance than gDNA. Principal component analysis revealed distinct clusters for cfDNA and gDNA, indicating distinct methylome profiles. cfDNA originated from multiple sources, predominantly from neutrophils (~ 75%) and about 10% from the left atrium, highlighting cardiac-specific changes. In contrast, immune cells are the major source of gDNA, indicative of inflammatory responses. Gene set enrichment analysis (GSEA) associates cfDNA methylation patterns with pathways related to cardiac muscle contraction, inflammation, hypoxia, and lipid metabolism. The affected genes include G protein-coupled receptors (GHSR, FFAR2, HTR1A, and VIPR2) that are part of the cAMP signaling pathway.

CONCLUSION

Epigenetic changes in cfDNA are more specific to cardiac tissue compared to those in gDNA, providing better insights into the molecular mechanisms involved in AMI. Genes that are differentially methylated in cfDNA and regulate core pathways, such as cAMP signaling, could be targeted for clinical applications, including the development of effective biomarkers and therapeutic targets.

Serotonin Receptors in Myocardial Infarction: Friend or Foe?

Acute myocardial infarction (AMI) is one of the leading causes of death worldwide and treatment costs pose a major burden on the global health care system. Despite the variety of treatment options, individual recovery can be still poor and the mortality rate, especially in the first few years after the event, remains high. Therefore, intense research is currently focused on identifying novel target molecules to improve the outcome following AMI. One of the potentially interesting targets is the serotonergic system (5-HT system), not at least because of its connection to mental disorders. It is known that patients suffering from AMI have an increased risk of developing depression and vice versa. This implicates that the 5-HT system can be affected in response to AMI and might thus represent a target structure for patients' treatment. This review aims to highlight the importance of the 5-HT system after AMI by describing the role of individual serotonin receptors (5-HTR) in the regulation of physiological and pathophysiological responses. It particularly focuses on the signaling pathways of the serotonin receptors 1, 2, 4, and 7, which are expressed in the cardiovascular system, during disease onset, and the following remodeling process. This overview also emphasizes the importance of the 5-HT system in AMI etiology and highlights 5-HTRs as potential treatment targets.

The piperazine analogue para-fluorophenylpiperazine alters timing of the physiological effects of the synthetic cannabinoid receptor agonist AMB-FUBINACA, without changing its discriminative stimulus, signalling effects, or metabolism

AMB-FUBINACA is a synthetic cannabinoid receptor agonist (SCRA), which has been associated with substantial abuse and health harm since 2016 in many countries including New Zealand. A characteristic of AMB-FUBINACA use in New Zealand has included the observation that forensic samples (from autopsies) and drugs seized by police have often been found to contain para-fluorophenylpiperazine (pFPP), a relatively little-characterised piperazine analogue that has been suggested to act through 5HT1a serotonin receptors. In the current study, we aimed to characterise the interactions of these two agents in rat physiological endpoints using plethysmography and telemetry, and to examine whether pFPP altered the subjective effects of AMB-FUBINACA in mice trained to differentiate a cannabinoid (THC) from vehicle. Though pFPP did not alter the ability of AMB-FUBINACA to substitute for THC, it did appear to abate some of the physiological effects of AMB-FUBINACA in rats by delaying the onset of AMB-FUBINACA-mediated hypothermia and shortening duration of bradycardia. In HEK cells stably expressing the CB1 cannabinoid receptor, 5HT1a, or both CB1 and 5HT1a, cAMP signalling was recorded using a BRET biosensor (CAMYEL) to assess possible direct receptor interactions. Although low potency pFPP agonism at 5HT1a was confirmed, little evidence for signalling interactions was detected in these assays: additive or synergistic effects on potency or efficacy were not detected between pFPP and AMB-FUBINACA-mediated cAMP inhibition. Experiments utilising higher potency, classical 5HT1a ligands (agonist 8OH-DPAT and antagonist WAY100635) also failed to reveal evidence for mutual CB1/5HT1a interactions or cross-antagonism. Finally, the ability of pFPP to alter the metabolism of AMB-FUBINACA in rat and human liver microsomes into its primary carboxylic acid metabolite via carboxylesterase-1 was assessed by HPLC; no inhibition was detected. Overall, the effects we have observed do not suggest that increased harm/toxicity would result from the combination of pFPP and AMB-FUBINACA.

Peripheral 5-HT1A and 5-HT7 serotonergic receptors modulate parasympathetic neurotransmission in long-term diabetic rats

We analyzed the modulation of serotonin on the bradycardia induced in vivo by vagal electrical stimulation in alloxan-induced long-term diabetic rats. Bolus intravenous administration of serotonin had a dual effect on the bradycardia induced either by vagal stimulation or exogenous Ach, increasing it at low doses and decreasing it at high doses of 5-hydroxytryptamine (5-HT), effect reproduced by 5-carboxamidotryptamine maleate (5-CT), a 5-HT_1/7 agonist. The enhancement of the bradycardia at low doses of 5-CT was reproduced by 5-HT_1A agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) and abolished by WAY-100,635, 5-HT_1A antagonist. Pretreatment with 5-HT₁ antagonist methiothepin blocked the stimulatory and inhibitory effect of 5-CT, whereas pimozide, 5-HT₇ antagonist, only abolished 5-CT inhibitory action. In conclusion, long-term diabetes elicits changes in the subtype of the 5-HT receptor involved in modulation of vagally induced bradycardia. Activation of the 5-HT_1A receptors induces enhancement, whereas attenuation is due to 5-HT₇ receptor activation. This 5-HT dual effect occurs at pre- and postjunctional levels.

Inhibition of the cardiovascular response to stress by systemic 5-HT1A activation: sympathoinhibition or anxiolysis?

5-HT1A agonists given systemically are known to produce anxiolytic effects. In addition, a growing body of research is showing that those compounds also have central sympathoinhibitory properties. Since emotional arousal gives rise to sympathetic activation, it is not clear whether systemic treatment with a 5-HT1A agonist reduces the sympathetic response to emotional stress primarily by a direct action on sympathetic-related sites in the brain or indirectly through reducing anxiety. To test this, we compared the effect of intraperitoneal injections of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.05 and 0.25 mg/kg), a preferential 5-HT1A agonist, or vehicle on the cardiovascular responses to four stressors known to produce sympathetic activation, three being emotional stressors, and one physiological. In conscious rats, 30-min exposure to either a neutral context, a fear-conditioned context, or to restraint stress led to increases in heart rate and blood pressure, which were attenuated by 8-OH-DPAT. In contrast, the same treatment did not reduce the cardiovascular response to 30-min cold exposure (4°C). The results suggest that 8-OH-DPAT acts preferentially on limbic, rather than central, autonomic sites. Hence, doses of 5-HT1A agonists, which are just sufficient to produce anxiolysis, are not enough to cause true sympathoinhibition.

Diabetes-induced changes in 5-hydroxytryptamine modulation of vagally-induced bradycardia in rat heart

1. In the present study, we investigated how alloxan-induced diabetes affects the ability of 5-hydroxytryptamine (5-HT) to modulate bradycardia induced in vivo by electrical stimulation of the vagus nerve in pithed rats. We also analysed the type and/or subtype of 5-HT receptors involved. 2. Diabetes was induced in male Wistar rats with a single injection of alloxan (150 mg/kg, s.c.). Four weeks later, rats were anaesthetized, pretreated with atenolol and pithed. Electrical stimulation (3, 6 and 9 Hz) of the vagus nerve resulted in frequency dependent decreases in heart rate (HR). 3. In diabetic rats, intravenous bolus administration of high doses of 5-HT (100 and 200 microg/kg) increased the bradycardia induced by vagal electrical stimulation. Similarly, low doses (10 microg/kg) of the 5-HT(1/7) receptor agonist 5-carboxamidotryptamine (5-CT), increased vagally induced bradycardia. However, at high doses (50, 100 and 150 microg/kg), 5-CT reduced the bradycardia. Attenuation of the vagally induced bradycardia evoked by the higher doses of 5-CT was reproduced by L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors. Enhancement of the vagally induced bradycardia elicited by low doses of 5-CT was reproduced by the selective 5-HT(1A) receptor agonist 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT; 50 microg/kg). These stimulatory and inhibitory actions on vagal stimulation-induced bradycardia in diabetic rats were also observed after administration of exogenous acetylcholine. 4. Vagally induced bradycardia in diabetic rats was not affected by administration of the selective 5-HT(2) receptor agonist alpha-methyl-5-HT (150 microg/kg), the selective 5-HT(3) receptor agonist 1-phenylbiguanide (150 microg/kg) or the selective 5-HT(1B) receptor agonist CGS-12066B (50 microg/kg). 5. Enhancement of the electrical stimulation-induced bradycardia in diabetic rats caused by 5-CT (10 microg/kg) or 8-OH-DPAT (50 microg/kg) was abolished by the selective 5-HT(2/7) receptor antagonist mesulergine (1 mg/kg) and the selective 5-HT(1A) receptor antagonist WAY-100,635 (100 microg/kg), respectively. Similarly, pretreatment with the non-selective 5-HT(1) receptor antagonist methiothepin (0.1 mg/kg) blocked the inhibitory effect of 5-CT (50 microg/kg) on the bradycardia induced by vagal electrical stimulation in diabetic rats. BRL-15572 (2 microg/kg), a selective 5-HT(1D) receptor antagonist, inhibited the action of L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors, on the vagally induced bradycardia. 6. In conclusion, in the present study, experimental diabetes evoked changes in both the nature and 5-HT receptor types/subtypes involved in vagally induced bradycardia.

Functions of 5-HT2A receptor and its antagonists in the cardiovascular system

The serotonin (5-hydroxytryptamine, 5-HT) receptors have conventionally been divided into seven subfamilies, most of which have several subtypes. Among them, 5-HT(2A) receptor is associated with the contraction of vascular smooth muscle, platelet aggregation and thrombus formation and coronary artery spasms. Accordingly, selective 5-HT(2A) antagonists may have potential in the treatment of cardiovascular diseases. Sarpogrelate, a selective 5-HT(2A) antagonist, has been introduced clinically as a therapeutic agent for the treatment of ischemic diseases associated with thrombosis. Molecular modeling studies also suggest that sarpogrelate is a 5-HT(2A) selective antagonist and is likely to have pharmacological effects beneficial in the treatment of cardiovascular diseases. This review describes the above findings as well as the signaling linkages of the 5-HT(2A) receptors and the mode of agonist binding to 5-HT(2A) receptor using data derived from molecular modeling and site-directed mutagenesis.

5-HT(1B) receptor knockout mice show no adaptive changes in 5-HT(1A) receptor function as measured telemetrically on body temperature and heart rate responses

Two presynaptic receptors play an important role in the regulation of serotonergic neurotransmission, i.e., the 5-HT(1A) and 5-HT(1B) receptor. The present study focuses on putative adaptive changes in the 5-HT(1A) receptor system in mice that lack 5-HT(1B) receptors (5-HT(1B) KO). 5-HT(1A) receptor sensitivity was assessed in vivo in two models of presynaptic 5-HT(1A) receptor activity: agonist-induced hypothermia and prevention of stress-induced hyperthermia. The effects of 5-HT(1A) receptor activation by flesinoxan (0.1-3.0 mg/kg s.c.) were determined telemetrically on body temperature and heart rate in 5-HT(1B) KO and wild-type (WT) mice. Flesinoxan induced hypothermia dose-dependently without affecting heart rate and prevented stress-induced hyperthermia and tachycardia equipotently in both genotypes. Specificity of these responses was confirmed by blockade with the selective 5-HT(1A) receptor antagonist WAY100635 (1.0 mg/kg s.c.). The importance of continuous sampling in freely moving subjects to improve appropriate characterization of mutants is discussed. 5-HT(1B) KO mice showed no shift in 5-HT(1A) receptor sensitivity compared to WT mice. This study found no indications for adaptive changes in presynaptic 5-HT(1A) receptor function in 5-HT(1B) KO mice as measured telemetrically on body temperature and heart rate responses.

Triptans in migraine: a comparative review of pharmacology, pharmacokinetics and efficacy

Triptans are a new class of compounds developed for the treatment of migraine attacks. The first of the class, sumatriptan, and the newer triptans (zolmitriptan, naratriptan, rizatriptan, eletriptan, almotriptan and frovatriptan) display high agonist activity at mainly the serotonin 5-HT1B and 5-HT1D receptor subtypes. As expected for a class of compounds developed for affinity at a specific receptor, there are minor pharmacodynamic differences between the triptans. Sumatriptan has a low oral bioavailability (14%) and all the newer triptans have an improved oral bioavailability and for one, risatriptan, the rate of absorption is faster. The half-lives of naratriptan, eletriptan and, in particular, frovatriptan (26 to 30h) are longer than that of sumatriptan (2h). These pharmacokinetic improvements of the newer triptans so far seem to have only resulted in minor differences in their efficacy in migraine. Double-blind, randomised clinical trials (RCTs) comparing the different triptans and triptans with other medication should ideally be the basis for judging their place in migraine therapy. In only 15 of the 83 reported RCTs were 2 triptans compared, and in 11 trials triptans were compared with other drugs. Therefore, in all placebo-controlled randomised clinical trials, the relative efficacy of the triptans was also judged by calculating the therapeutic gain (i.e. percentage response for active minus percentage response for placebo). The mean therapeutic gain with subcutaneous sumatriptan 6mg (51%) was more than that for all other dosage forms of triptans (oral sumatriptan 100mg 32%; oral sumatriptan 50mg 29%: intranasal sumatriptan 20mg 30%; rectal sumatriptan 25mg 31%; oral zolmitriptan 2.5mg 32%; oral rizatriptan 10mg 37%; oral eletriptan 40mg 37%; oral almotriptan 12.5mg 26%). Compared with oral sumatriptan 100mg (32%), the mean therapeutic gain was higher with oral eletriptan 80mg (42%) but lower with oral naratriptan 2.5mg (22%) or oral frovatriptan 2.5mg (16%). The few direct comparative randomised clinical trials with oral triptans reveal the same picture. Recurrence of headache within 24 hours after an initial successful response occurs in 30 to 40% of sumatriptan-treated patients. Apart from naratriptan, which has a tendency towards less recurrence, there appears to be no consistent difference in recurrence rates between the newer triptans and sumatriptan. Rizatriptan with its shorter time to maximum concentration (tmax) tended to produce a quicker onset of headache relief than sumatriptan and zolmitriptan. The place of triptans compared with non-triptan drugs in migraine therapy remains to be established and further RCTs are required.

Stress-induced hyperthermia in mice: effects of flesinoxan on heart rate and body temperature

Stress-induced hyperthermia in mice has predictive validity for anxiolytic properties of drugs. In this paradigm, 60 min after drug administration rectal temperature is measured, which causes hyperthermia of 1-1.5 degrees C (DeltaT) in about 10 min. Flesinoxan, a selective 5-HT(1A) receptor agonist with anxiolytic-like properties, causes hypothermia, which complicates interpretation of stress-induced hyperthermia. Therefore, we combined flesinoxan treatment and the stress paradigm with radiotelemetric measurement of body temperature and heart rate, which is also related to anxiety. Subjects were either undisturbed or injected with flesinoxan (0-0.1-0.3-1.0 and 3.0 mg/kg), with or without the stress paradigm. Flesinoxan (1.0 and 3.0 mg/kg) caused a relatively long-lasting hypothermia, but did not lower heart rate. The rectal temperature procedure caused hyperthermia and tachycardia. Flesinoxan reduced the stress-induced hyperthermia and the tachycardia evoked by the stress procedure. Continuous radiotelemetric measurement of heart rate, apart from body temperature, revealed that flesinoxan has anxiolytic-like properties in mice.

Interaction of the beta adrenergic receptor antagonist bucindolol with serotonergic receptors

Bucindolol is a nonselective beta-adrenergic receptor antagonist that has additional vasodilating properties. Because some beta-adrenergic receptor antagonists such as cyanopindolol are used as 5-HT1A/5-HT1B receptor antagonists, we tested the hypothesis that bucindolol can interact with 5-HT receptors. Both in vitro and in vivo methods were used to examine the interaction of bucindolol with 5-HT receptors relevant to the cardiovascular system-the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptors-and with alpha1-adrenergic receptors. In binding studies, bucindolol displayed high affinity for the 5-HT1A receptor (Ki, 11 nM), modest affinity for the 5-HT2A receptor (Ki, 382 nM), and no measurable affinity for the 5-HT1D receptor; binding affinity for the 5-HT2B receptor was not studied. Bucindolol also displayed significant binding affinity (Ki, 69 nM) for the alpha1-adrenergic receptors. Alpha1-Adrenergic receptor antagonist activity was confirmed by the ability of bucindolol (1 mg/kg) to act as a competitive antagonist against 0.01-30 microg/kg phenylephrine-induced pressor responses in conscious rats. In conscious permanently instrumented rats, bucindolol (0.1-3.0 mg/kg, i.v.) did not cause bradycardia similar to that elicited by the 5-HT1A-receptor agonist 8-OH-DPAT (3-300 microg/kg, i.v.), nor did bucindolol (1 mg/kg) block the 8-OH-DPAT-induced bradycardia. Bucindolol (10(-9)-10(-5) M) did not cause relaxation in the PGF2alpha-contracted, endothelium-intact porcine coronary artery, nor did bucindolol (10(-5) M) block 5-HT-induced coronary artery relaxation, indicating that bucindolol does not have significant interactions at the 5-HT1D receptor. Bucindolol also displayed no agonist activity at the 5-HT2A and 5-HT2B receptor (endothelium-denuded rat thoracic aorta and rat stomach fundus, respectively), but did act as a weak 5-HT2A-receptor antagonist (-log K(B) [M] = 5.4+/-0.1) and 5-HT2B-receptor antagonist (-log K(B) [M] = 7.8+/-0.1). Thus, these data suggest that bucindolol lacks the ability to activate the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptor, but can block alpha1-adrenergic receptors and act as a weak 5-HT2A- and 5-HT2B-receptor antagonist. The relevance of these serotoninergic effects as it pertains to the mechanism of bucindolol-induced vasodilation is unknown.

Effect of 5-methylurapidil, an alpha 1a-adrenergic antagonist and 5-hydroxytryptamine1a agonist, on aqueous humor dynamics in monkeys and rabbits

PURPOSE

To evaluate the effects of 5-methylurapidil (5-MU) on intraocular pressure (IOP) and aqueous humor dynamics in female cynomolgus monkeys and albino rabbits.

METHODS

IOP was measured by pneumatonometer prior to and up to 6 hours after AM administration of 5-MU to one eye of each of 8 normal monkeys and to the laser-induced glaucomatous eye of 8 monkeys. During single-dose and 5-day multiple-dose testing, pupillary diameter (PD) was measured at the same time and same intervals as IOP measurements in the normal monkeys. Outflow facility and aqueous humor flow rates were measured in 8 normal monkeys before and after treatment. Uveoscleral outflow was measured in 8 rabbits before and after treatment.

RESULTS

In normal monkeys, unilateral topical application of 2 x 25 microliters of 1% or 2% 5-MU significantly (p < 0.05) reduced pupil size and IOP bilaterally as compared to baseline measurements. The reduction in IOP (mean +/- SEM, mmHg) was up to 2.8 +/- 0.7 (1% 5-MU) and 4.4 +/- 0.5 (2% 5-MU) in the treated eyes, and 2.3 +/- 0.8 (1%) and 3.0 +/- 0.7 (2%) in the contralateral eyes. In glaucomatous monkeys, the maximum reduction in IOP was 6.5 +/- 1.0 mmHg (1%) and 7.5 +/- 0.8 mmHg (2%). The ocular hypotensive effect increased over time with twice-daily administration for 5 days. Compared with baseline values, outflow facility and aqueous flow rates in the treated eyes of normal monkeys were increased (p < 0.01) by 51% and by 11%, respectively. Uveoscleral outflow was unaltered (p > 0.3) in rabbits compared with baseline values. Mild corneal edema, corneal punctate erosions, and conjunctival discharge occurred in some eyes treated with either 1% or 2% 5-MU.

CONCLUSIONS

5-Methylurapidil, an antagonist at the alpha 1A-adrenergic receptor subtype and an agonist at the 5-HT1A receptor subtype, lowers IOP predominantly by increasing outflow facility and may have potential for the therapy of glaucoma.

The cardiovascular and renal functional responses to the 5-HT1A receptor agonist flesinoxan in two rat models of hypertension

1. This study investigated the importance of renal sympathetic nerves in regulating sodium and water excretion from the kidneys of stroke prone spontaneously hypertensive and 2K1C Goldblatt hypertensive rats anaesthetized with chloralose/urethane (17.5/300 mg initially and supplemented at regular intervals), and prepared for measurement of renal function. 2. In stroke prone spontaneously hypertensive rats, flesinoxan, 30-1000 micrograms kg-1, i.v., caused graded reductions in blood pressure and heart rate of 74 +/- 5 mmHg and 63 +/- 9 beats min-1, respectively at the highest dose (P < 0.001). Renal blood flow did not change at any dose of drug while glomerular filtration rate fell by some 20% (P < 0.001) at the highest dose of drug, absolute and fractional sodium excretions, approximately doubled at 100 micrograms kg-1, and thereafter fell to below the baseline level at 1000 micrograms kg-1. 3. This pattern of excretory response was abolished following acute renal denervation when flesinoxan caused dose-related reductions in urine flow and sodium excretion, similar to that obtained by a mechanical reduction of renal perfusion pressure. 4. Flesinoxan administration (30-1000 micrograms kg-1, i.v.) into 2K1C Goldblatt hypertensive rats caused a maximum decrease in blood pressure and heart rate (both P < 0.001) of 34 +/- 3 mmHg and 20 +/- 6 beats min-1 and while renal blood flow and glomerular filtration rate were autoregulated, from 160 to 125 mmHg, there were dose-related decreases in urine volume and sodium excretion from the clipped and non-clipped kidneys of approximately 50-60% at the highest dose. 5. These findings suggest that in the stroke prone spontaneously hypertensive rat the renal nerves importantly control sodium and water reabsorption at the level of the tubules, whereas in 2K1C Goldblatt hypertensive rats, they play a minor role.

Effects of 5-HT and 5-HT1A receptor agonists and antagonists on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study

1. Effects of ionophoretic administration of 5-hydroxytryptamine (5-HT) and selective 5-HT1A receptor agonists and antagonists on identified dorsal vagal preganglionic and dorsal raphe neurones were studied in pentobarbitone sodium or chloral hydrate-anaesthetized rats, respectively. 2. Extracellular recordings were made from 176 preganglionic neurones in the dorsal vagal nucleus (DVN). Application of 5-HT at low currents (< or = 10 nA) increased the activity of these neurones. However, at increased currents (10-60 nA), it had a predominantly depressant effect. Application of selective 5-HT1A receptor antagonists, (+/-)-pindolol or WAY-100635, attenuated the excitatory responses evoked by 5-HT. 3. Ionophoresis of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-30 nA) increased the firing rate of 19 and decreased that of 67 of the 104 vagal neurones tested. Other 5-HT1A receptor agonists, flesinoxan and N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) also had predominantly depressant effects. 4. (+/-)-Pindolol attenuated excitations but not inhibitions evoked by 8-OH-DPAT. Surprisingly, WAY-100635 and 8-OH-DPAT produced the same effect on these neurones and when applied together, WAY-100635 failed to attenuate the 8-OH-DPAT responses. 5. Dorsal raphe neurones were identified by their low, regular firing rate and their subsequent histological localization. 8-OH-DPAT reversibly reduced the activity in all 7 neurones tested and this was antagonized by WAY-100635 in all 3 neurones tested. 6. In conclusion, 5-HT applied to vagal preganglionic neurones evokes excitatory and inhibitory responses. The excitatory, but not the inhibitory responses may be mediated, at least in part, by activation of 5-HT1A receptors.

5-Carboxamidotryptamine attenuates the development of deoxycorticosterone acetate-salt hypertension in rats

The effect of chronic i.v. infusion of the 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), was evaluated during the development of deoxycorticosterone acetate-salt (DOCA-salt) hypertension in rats over 4 weeks. Vehicle-treated (n = 10) Sprague-Dawley rats given DOCA (100 mg/kg, s.c.) and 1% saline as drinking fluid developed hypertension with systolic blood pressure reaching 194.6 +/- 8.99 mm Hg at 27 days. In DOCA-salt rats treated with 5-CT infusions (15.0 micrograms/kg per day, n = 10) for 4 weeks via osmotic minimpumps, systolic blood pressure was significantly lower by 41.7 mm Hg at day 27 when compared to vehicle-treated DOCA-salt rats. Systolic blood pressure values on day 27 in 5-CT-treated DOCA-salt rats were however greater than those in vehicle-treated control rats which were not given DOCA. Systolic blood pressure in 5-CT-treated DOCA-salt rats was significantly lower by day 7 compared to vehicle-treated DOCA-salt rats and remained lowered for the rest of the observation period. Heart rate was significantly greater in 5-CT-treated DOCA-salt rats on day 7 when compared to vehicle-treated DOCA-salt rats. Baroreflex sensitivity on day 28 was significantly greater in 5-CT-treated DOCA-salt rats as compared to vehicle-treated DOCA-salt rats. On day 28, hypotensive responses to hexamethonium (20 mg/kg) in 5-CT-treated DOCA-salt rats were markedly reduced compared to those in vehicle-treated DOCA-salt rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Different types of centrally acting antihypertensives and their targets in the central nervous system

The central regulation of blood pressure and other cardiovascular parameters may involve the baroreceptor reflex are, including both adrenergic and serotonergic pathways, as well as amino acids, as neurotransmitters. Both adrenergic and serotonergic pathways have been recognized as targets for clinically relevant, centrally acting antihypertensives, such as clonidine, guanfacine, and alpha-methyl-DOPA. The central components of the hybrid drugs urapidil and ketanserin also involve serotonergic pathways and receptors. For urapidil the stimulation of 5-HT1A-receptors is assumed to induce peripheral sympathoinhibition, whereas for ketanserin the central mechanism is unknown in detail. More recently central imidazoline (I1) receptors have been proposed as the major target for the newer antihypertensives rilmenidine and moxonidine. Clonidine, however, is assumed to be mixed I1- and alpha2-receptor agonist. The distinction between central I1- and alpha2-receptors may potentially offer the design of new antihypertensives, acting like clonidine but with fewer side effects. Finally, the amino acid pathways should be considered as potential targets for centrally acting antihypertensives. Experimental compounds on this basis are available but clinical implications appear to be very remote. In the present survey an outline is given of the various pathways, neurotransmitters, and receptors involved in the central regulation of blood pressure. The different types of centrally acting antihypertensives are subsequently discussed on this basis.

Are the cardiovascular effects and '5-HT syndrome' induced by MDL 73,975 and flesinoxan in the dog mediated by 5-HT1A receptors?

The cardiovascular effects of the 5-HT1A receptor agonists MDL 73,975 (8-[2-(2,3-dihydro-8-methoxy-1,4-benzodoxin-2-yl)methylaminol++ +]-ethyl]-8- azaspiro[4,5]decane-7,9-dione hydrochloride) and flesinoxan (10-300 micrograms/kg subcutaneously, s.c.), the 5-HT1A receptor antagonist NAN 190 (2-[4-[4-(2-methoxyphenyl)-1-piperazinyl]butyl]-1H-isoindole-1,3(2H)- dione,1,2-ethanedioate), and the alpha 1-adrenoceptor antagonist prazosin have been investigated in conscious normotensive and renal hypertensive dogs. In normotensive dogs the increases in heart rate and respiratory rate induced by both agonists were dose-related, as were the decreases in systolic and diastolic blood pressure induced by MDL 73,975. Both compounds caused a dose-related increase in the intensity of the '5-HT syndrome'. After pretreatment with NAN 190 (100 micrograms/kg s.c.) the increases in heart rate, respiratory rate and symptoms of the '5-HT syndrome' were significantly reduced but the decreases in systolic and diastolic pressure were additive. Pretreatment with prazosin (100 micrograms/kg s.c.) antagonized the '5-HT syndrome' and the increase in respiratory rate. Similar responses were evident in renal hypertensive dogs. Tolerance did not develop to the increases in heart rate, respiratory rate and manifestations of the '5-HT syndrome' in normotensive dogs during 5 days of treatment with MDL 73,975 or flesinoxan. In conclusion, MDL 73,975 and flesinoxan induced a 5-HT1A receptor-mediated fall in blood pressure but the changes in heart rate, respiratory rate and the '5-HT syndrome' are probably mediated by alpha 1-adrenoceptors.

The renal functional responses to 5-HT1A receptor agonist, flesinoxan, in anaesthetized, normotensive rat

1. The present study was designed to examine the effects of a centrally acting 5-HT1A receptor agonist, flesinoxan, on the cardiovascular system and renal haemodynamics and excretory function. 2. In chloralose-urethane anaesthetized Wistar rats, i.v. administration of bolus doses of flesinoxan, at 30, 100, 300 and 1000 micrograms kg-1, caused significant, dose-dependent decreases in mean arterial pressure, of 33 +/- 2 mmHg (P < 0.001) and heart rate of 57 +/- 9 beats min-1 (P < 0.001) at the highest dose used. Despite this substantial fall in perfusion pressure there were no meaningful changes in the renal excretion of water and sodium. In a second group of rats, reduction of renal perfusion pressure mechanically to the same values as observed in rats given flesinoxan (i.e. 100, 92, 84 and 76 mmHg) produced reductions in urine flow, absolute and fractional sodium excretions reaching a maximum of 74, 86 and 84% respectively (all P < 0.001) at the lowest pressure. These reductions were significantly larger than those seen in the previous group of animals. 3. In the group of rats subjected to renal denervation, flesinoxan produced changes in blood pressure and heart rate which were not different from those observed in intact animals. However, the reduction in pressure was accompanied by significant decreases in urine flow of 71%, absolute sodium excretion of 68% and fractional sodium excretion of 67% (all P < 0.001) at the highest dose, which were all significantly greater than the changes seen in the innervated animals but were not different from those observed when renal perfusion pressure was reduced mechanically. 4. The findings of this investigation showed that flesinoxan was effective in lowering blood pressure and heart rate in the anaesthetized rat, which was probably due to decreased sympathetic nerve activity.Renal excretion of water and sodium was well preserved in the face of the flesinoxan-induced hypotension.The maintenance of fluid excretion with flesinoxan appeared to be mediated via changes in renal nerve activity, since it did not occur when the kidney was denervated.

Effects of ipsapirone in healthy subjects: a dose-response study

A dose-response study of ipsapirone (IPS), a 5HT1a partial agonist, was conducted in healthy male subjects. IPS was administered in doses of 5, 10 and 20 mg PO in a placebo-controlled, double-blind design to 15 subjects on 4 test days separated by at least 3 days. Oral temperature, ACTH, cortisol, prolactin, blood pressure, pulse rate and behavioral variables were assessed every 30 min for 3 h after administration of tablets (at 10:00 A.M.). IPS at 20 mg significantly decreased temperature and increased cortisol levels. Although IPS increased ACTH levels at 20 mg, this effect was variable and not significant. IPS did not affect prolactin levels nor did it have any behavioral effects. Although 20 mg IPS decreased blood pressure and pulse rate in one subject, overall it had no significant effect on these parameters. IPS at 20 mg PO appears a useful probe to test 5HT1a function when temperature and cortisol are used as response variables. These results replicate earlier studies on the effect of IPS in healthy human subjects.

5-Hydroxytryptamine (5-HT) contracts the guinea-pig isolated iliac artery via 5-HT1-like and 5-HT2 receptors

The characterization of 5-hydroxytryptamine (5-HT) receptors mediating contractions of the guinea-pig isolated iliac artery was studied when the basal tone was slightly increased by prostaglandin F2 alpha (PGF2 alpha). In the presence of ketanserin (1 mumol/l), 5-HT and several 5-HT receptor agonists induced contractile responses with the rank order of agonist potency: 5-HT = 5-carboxyamidotryptamine (5-CT) = lysergol > ergometrine = methylergometrine > RU 24969 approximately 5-methoxytryptamine (5-MeOT) > methysergide > sumatriptan > tryptamine. Concentration-effect curves to the ergot alkaloids, lysergol, ergometrine, methylergometrine and methylsergide, were biphasic. In the presence of ketanserin (1 mumol/l), contractile responses to 5-HT, 5-CT, RU 24969, 5-MeOT, sumatriptan and tryptamine were antagonized by methiothepin (30 nmol/l) and flesinoxan (3 mumol/l) with approximate pKB values of 8.5-9.0 and 6.0-6.3, respectively. The first phase of contraction produced by the ergot alkaloids, lysergol, ergometrine, methylergometrine and methysergide, were blocked by methiothepin (30 nmol/l) and flesinoxan (3 mumol/l), respectively, with approximate pKB values about 8.4-8.7 and 6.2-6.4, respectively. The mechanism underlying the second phase of contraction remains to be established. Maximum responses of the concentration-effect curves to 5-HT (1 nmol/l-1 mumol/l) were concentration-dependently depressed by ketanserin (1 nmol/l-1 mumol) and spiperone (30 nmol/l-0.3 mumol/l) and reached approximately 60% of the 5-HT maximum response in the presence of ketanserin (1 mumol/l) and spiperone (0.1 mumol/l), respectively. Agonist potency of 5-HT was not affected by the antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

The effects of intravenous (i.v.) administration of four agonists at central 5-HT1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The i.v. injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The i.v. administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 micrograms of 8-OH-DPAT or 100 micrograms of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 micrograms of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily i.v. injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of alpha 1-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 microgram/i.v.) at 5-min intervals before and after the i.v. injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35-40 mm Hg after saline pre-treatment, and these responses were not affected by the i.v. injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT1A receptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral alpha 1-adrenoceptors.

Antihypertensive effect of chronic i.v. administration of 5-carboxamidotryptamine in spontaneously hypertensive rats

The effects of chronic i.v. administration of the serotonin 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), on blood pressure (BP), heart rate (HR) and baroreflex sensitivity were studied in Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Acute i.v. injection of increasing doses of 5-CT resulted in a dose-dependent reduction in mean arterial pressure (MAP) in SHR and WKY with concomitant tachycardia. In chronic experiments, 5-CT (15.0 micrograms/kg per day) or vehicle (24.0 microliters/day) was infused i.v. for 7 days, using osmotic minipumps. Systolic blood pressure (SBP) and HR were monitored daily before and during infusions. In SHR (n = 8) and WKY rats (n = 9) receiving 5-CT, a significant reduction in SBP was observed during the infusion period. HR was slightly increased in WKY rats on days 1 and 2. There were no HR changes in the SHR group. The fall in SBP was significantly larger in the SHR than in the WKY rats. Baroreflex sensitivity on day 7 was significantly greater in 5-CT-treated SHR than in control rats. There was no change in baroreflex sensitivity in WKY rats. Administration of a single dose of 5-CT (0.5 microgram/kg i.v.) on day 7 of infusion resulted in attenuated responses in WKY rats while SHR responded as their respective controls. Our data suggest that chronic administration of 5-CT results in a sustained antihypertensive effect. This is associated with an improved BRS in the SHR either as a consequence of a resetting of the baroreflex due to sustained lowering of BP or a direct action of 5-CT on baroreflex sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypotensive action of 5-HT receptor agonists in the vitamin B6-deficient hypertensive rat

Feeding a vitamin B6-deficient diet to rats causes a moderate hypertension. The blood pressure responses to 5-HT1A receptor agonists were studied in conscious vitamin B6-deficient hypertensive rats. They were all effective in lowering blood pressure with the following rank order of potency: 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) > flesinoxan > 5-methylurapidil > urapidil. The putative 5-HT1A receptor antagonist spiroxatrine by itself, did not have any effect on the blood pressure at the doses used (0.01-1 mumol/kg). However, dose dependently, it antagonized the hypotensive effect of flesinoxan and urapidil. The alpha 1-adrenoreceptor antagonist prazosin, on prior treatment, did not change the hypotensive effect of either flesinoxan or urapidil. The alpha 2-adrenoreceptor agonist clonidine dose dependently (0.01-0.1 mumol/kg) reduced blood pressure. This effect of clonidine was unaffected by spiroxatrine, but was dose dependently antagonized by the alpha 2-adrenoreceptor antagonist yohimbine. Binding studies with [3H]8-OH-DPAT indicated that the affinity and Bmax of 5-HT1A receptors was increased in vitamin B6-deficient hypertensive rats. The results suggest that decreased synthesis of 5-HT in brain regions and the consequent alterations in 5-HT receptors in the vitamin B6-deficient rats may be the underlying cause of the hypertension seen in these animals.

Cardiovascular effects of the 5-HT1A receptor ligand, MDL 73005EF, in conscious spontaneously hypertensive rats

The effects of pretreatment with the potent and selective 5-HT1A receptor ligand, MDL 73005EF, on the cardiovascular responses to administration of the 5-HT1A receptor agonists, 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT), flesinoxan and 5-methylurapidil were studied in conscious spontaneously hypertensive rats (SHR) and compared with those of putative 5-HT1A receptor antagonists. MDL 7300EF (0.1-3 mg/kg) induced a dose-dependent but transient decrease in mean arterial pressure (MAP). Pretreatment with doses of 1 or 3 mg/kg MDL 73005EF significantly inhibited the hypotensive and bradycardiac effects of 8-OH-DPAT (0.03-1 mg/kg). Pretreatment with 1 mg/kg MDL 73005EF similarly reduced the hypotensive actions of flesinoxan (0.3-1 mg/kg) and 5-methylurapidil (0.1 mg/kg). In contrast, MDL 73005EF did not significantly affect the decrease in blood pressure induced by administration of 0.01 mg/kg clonidine, 0.3 mg/kg hydralazine or 0.2 mg/kg nifedipine. The effect of 8-OH-DPAT (0.1 mg/kg) on MAP was also reduced by pretreatment with 1 mg/kg BMY 7378, buspirone or pindolol, but not NAN 190 or spiperone. BMY 7378, NAN 190, pindolol and spiperone induced a significant decrease in blood pressure. To rule out the possibility that the reduced baseline may have influenced responses to 8-OH-DPAT, we showed that pretreatment with the vasodilator, hydralazine (0.3 mg/kg), had no effect on the MAP response to 8-OH-DPAT although it significantly reduced MAP. We conclude that MDL 73005EF acts as a mixed agonist/antagonist at 5-HT1A receptors since it caused a decrease in blood pressure, but also reduced the cardiovascular responses to the 5-HT1A receptor agonists, 8-OH-DPAT, flesinoxan and 5-methylurapidil.

Serotonergic receptors and drugs in hypertension

The possible role of 5-hydroxytryptamine (5HT) and 5HT-receptors in hypertension, already suggested by Page in 1954, has been subject to a renaissance of interest owing to the development of antihypertensive drugs which interact with 5HT-receptors. These drugs, like ketanserin, urapidil and flesinoxan are used as tools to study the role of 5HT and its receptors in hypertension. Some arguments would plead in favour of a certain role of 5HT and 5HT-receptors in the pathogenesis and maintenance of hypertension: hyperresponsiveness of blood vessels from hypertensive patients and animals to 5HT-induced constriction; the antihypertensive/vasodilator activity of the 5HT2-receptor antagonist ketanserin; enhanced sensitivity of platelets from hypertensives to 5HT. However, there are also several arguments which do not support a causal role of 5HT in hypertensive disease: 5HT is not a generally accepted pressor agent, whereas its concentration in the circulating blood is subthreshold; the 5HT2-receptor antagonist ketanserin is the only agent of this type which lowers blood pressure, other 5HT2-receptor blockers (ritanserin; LY 53587) being inactive. The various data and arguments available do not unequivocally support a relevant role of peripheral 5HT and its receptors in hypertensive disease. 5HT2-receptor blockade may, however, have a favourable effect on the microcirculation under pathological conditions. The stimulation of central 5HT1A-receptors by drugs like urapidil, 8-OH-DPAT or flesinoxan, has been demonstrated to induce peripheral sympathoinhibition and a fall in blood pressure. This mechanism appears to be a novel target for centrally acting antihypertensives, clearly different from that of clonidine and related drugs, which are centrally acting alpha 2-adrenoceptor agonists.

Effects of central serotonin on autonomic control of heart rate in intact and baroreceptor deficient rats

The intracerebroventricular (i.c.v.) injection of serotonin (5-HT) increases blood pressure and decreases heart rate (HR) in conscious rats by activation of 5-HT2/1C receptors. Since the bradycardia is eliminated by pretreatment with a ganglionic or V1-vasopressin antagonist, we proposed that the decrease in HR results from an effect on cardiac autonomic activity which is potentiated by vasopressin. The present study aimed first, to further characterize mechanisms by which the i.c.v. injection of 5-HT (2.5 micrograms) decreases HR in conscious rats, and second to determine the cardiovascular responses to 5-HT (2.5 micrograms, i.c.v.) in rats with chronic sinoaortic deafferentation (SAD). In intact rats, the bradycardia elicited by 5-HT was eliminated by a combination of the muscarinic antagonist atropine and the beta-adrenoceptor antagonist sotalol; neither antagonist was effective alone. In rats with SAD, 5-HT produced a larger increase in blood pressure and a marked tachycardia, both of which were eliminated by the 5-HT2/1C antagonist LY 53857. Furthermore, in rats with SAD the 5-HT-induced increase in HR was blocked by sotalol alone. In conclusion, 5-HT (2.5 micrograms, i.c.v.) acts on central 5-HT2/1C receptors to increase arterial pressure. In intact rats this decreases HR by vasopressin-potentiated activation of baroreceptor reflexes and subsequent increase in vagal tone and decrease in cardiac sympathetic tone. In the absence of baroreflexes, a direct central effect of 5-HT to produce a beta-adrenoceptor-mediated cardioacceleration is unmasked.

Cardiovascular and side effects of flesinoxan in conscious hypertensive dogs. Modulation by prazosin

Previous studies on anaesthetized animals indicate that flesinoxan exerts hypotensive effects via stimulation of central 5-HT1A receptors. The purpose of the present study was to investigate the cardiovascular and side effects of flesinoxan in conscious, renal hypertensive dogs at rest and during exercise. Animals were pretreated with prazosin (2.5 or 7.5 nmol/kg) to verify a reduction of dose-dependent side effects, as occurred in normotensive dogs. A decrease in blood pressure without reflex tachycardia was observed only with the lower dose of flesinoxan (0.1 mumol/kg). The higher dose (0.2 mumol/kg) led to an increase in blood pressure and heart rate. The increase in heart rate during exercise was diminished by 0.2 mumol/kg flesinoxan. Pretreatment with prazosin resulted in an additive hypotensive effect at rest. Side effects, occurring primarily after the higher dose of flesinoxan, were not influenced by prazosin. It is concluded that flesinoxan is not likely to be efficacious in antihypertensive therapy.

Behavioral and biochemical effects of the 5-HT1A receptor agonists flesinoxan and 8-OH-DPAT in the rat

The 5-HT1A receptor agonists flesinoxan (0.2-3.2 mg kg-1 s.c.) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.025-0.4 mg kg-1 s.c.) produced (1) a dose-dependent facilitation of male rat ejaculatory behavior and (2) characteristic, dose-dependent effects on spontaneous motor activity. Thus, total locomotor activity and rearing activity were decreased. However, forward locomotion and peripheral locomotion were increased relative to the total horizontal activity. Furthermore, (3) 5-HTP accumulation, after inhibition of cerebral decarboxylase, was dose dependently decreased by both compounds in the ventral striatum and in the prefrontal cortex. There was a statistically significant decrease in DOPA accumulation in the ventral striatum after administration of a high dose of flesinoxan (3.2 mg kg-1), and a tendency for 8-OH-DPAT to produce the same effect. The efficacy of the compounds to affect male rat sexual behavior, spontaneous motor activity in the open-field and forebrain 5-HT synthesis was approximately the same, whereas flesinoxan was about an order of magnitude less potent than 8-OH-DPAT.

Pressor effects following microinjection of 5-HT1A receptor agonists into the raphe obscurus of the anaesthetized rat

1. The effects of electrical stimulation and microinjections (90 nl) of the 5-HT1A receptor agonists, flesinoxan and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), and glutamate into the raphe obscurus on blood pressure, heart rate and phrenic nerve activity (central inspiratory drive) were investigated in rats anaesthetized with alpha-chloralose. 2. Electrical stimulation of the raphe obscurus caused a rise in blood pressure which was associated with bradycardia, while glutamate (2.7 nmol) caused only a rise in blood pressure. 3. Flesinoxan (1.3 nmol) and 8-OH-DPAT (0.7 nmol) increased blood pressure by 9 +/- 1 and 14 +/- 2 mmHg, respectively and did not affect heart rate. For both agonists the effect on blood pressure was shown to be dose-dependent; again no effect on the heart rate was observed over the dose-ranges chosen. 4. Microinjections of the non-selective 5-HT1A receptor antagonists, (+/-)-pindolol (2.7 nmol) or methiothepin (5.2 nmol), into the raphe obscurus prevented the increase in blood pressure caused by microinjection of flesinoxan. However, (+/-)-pindolol caused a sustained rise in blood pressure of 15 +/- 1 mmHg while methiothepin caused a transient rise in blood pressure. Neither drugs affected heart rate. The ability of methiothepin to attenuate the pressor effect of flesinoxan was found to be partially reversed after 30 min. 5. It is suggested that activation of 5-HT1A receptors within the raphe obscurus can cause sympatho-excitation.