Comparison of effects of some 5-HT1 agonists on blood pressure and heart rate of normotensive anaesthetized rats

Pharmaco-toxicological effects of the novel tryptamine hallucinogen 5-MeO-MiPT on motor, sensorimotor, physiological, and cardiorespiratory parameters in mice-from a human poisoning case to the preclinical evidence

RATIONALE

The 5-methoxy-N-methyl-N-isopropyltryptamine (5-MeO-MiPT, known online as "Moxy") is a new psychedelic tryptamine first identified on Italian national territory in 2014. Its hallucinogen effects are broadly well-known; however, only few information is available regarding its pharmaco-toxicological effects.

OBJECTIVES

Following the seizure of this new psychoactive substances by the Arm of Carabinieri and the occurrence of a human intoxication case, in the current study we had the aim to characterize the in vivo acute effects of systemic administration of 5-MeO-MiPT (0.01-30 mg/kg i.p.) on sensorimotor (visual, acoustic, and overall tactile) responses, thermoregulation, and stimulated motor activity (drag and accelerod test) in CD-1 male mice. We also evaluated variation on sensory gating (PPI, prepulse inhibition; 0.01-10 mg/kg i.p.) and on cardiorespiratory parameters (MouseOx and BP-2000; 30 mg/kg i.p.). Lastly, we investigated the in silico ADMET (absorption, distribution, metabolism, excretion, toxicity) profile of 5-MeO-MiPT compared to 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) and N,N-dimethyltryptamine (DMT).

RESULTS

This study demonstrates that 5-MeO-MiPT dose-dependently inhibits sensorimotor and PPI responses and, at high doses, induces impairment of the stimulated motor activity and cardiorespiratory changes in mice. In silico prediction shows that the 5-MeO-MiPT toxicokinetic profile shares similarities with 5-MeO-DIPT and DMT and highlights a cytochrome risk associated with this compound.

CONCLUSIONS

Consumption of 5-MeO-MiPT can affect the ability to perform activities and pose a risk to human health status, as the correspondence between the effects induced in mice and the symptoms occurred in the intoxication case suggests. However, our findings suggest that 5-MeO-MiPT should not be excluded from research in the psychiatric therapy field.

Effects of hallucinogenic drugs on the human heart

Hallucinogenic drugs are used because they have effects on the central nervous system. Their hallucinogenic effects probably occur via stimulation of serotonin receptors, namely, 5-HT2A-serotonin receptors in the brain. However, a close study reveals that they also act on the heart, possibly increasing the force of contraction and beating rate and may lead to arrhythmias. Here, we will review the inotropic and chronotropic actions of bufotenin, psilocin, psilocybin, lysergic acid diethylamide (LSD), ergotamine, ergometrine, N,N-dimethyltryptamine, and 5-methoxy-N,N-dimethyltryptamine in the human heart.

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring, short-acting psychedelic tryptamine, produced by a variety of plant and animal species. Plants containing 5-MeO-DMT have been used throughout history for ritual and spiritual purposes. The aim of this article is to review the available literature about 5-MeO-DMT and inform subsequent clinical development.

METHODS

We searched PubMed database for articles about 5-MeO-DMT. Search results were cross-checked against earlier reviews and reference lists were hand searched. Findings were synthesised using a narrative synthesis approach. This review covers the pharmacology, chemistry and metabolism of 5-MeO-DMT, as well epidemiological studies, and reported adverse and beneficial effects.

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

5-MeO-DMT is a potentially useful addition to the psychedelic pharmacopoeia because of its short duration of action, relative lack of visual effects and putatively higher rates of ego-dissolution and mystical experiences. We conclude that further clinical exploration is warranted, using similar precautions as with other classic psychedelics.

Psychedelic Treatment for Trauma-Related Psychological and Cognitive Impairment Among US Special Operations Forces Veterans

BACKGROUND

U.S. Special Operations Forces Veterans are at increased risk for a variety of mental health problems and cognitive impairment associated with military service. Current treatments are lacking in effectiveness and adherence. Therefore, this study examined psychedelic treatment with ibogaine and 5-methoxy-N,N-dimethyltryptamine for trauma-related psychological and cognitive impairment among U.S. Special Operations Forces Veterans.

METHOD

We conducted a survey of Veterans who completed a specific psychedelic clinical program in Mexico between 2017 and 2019. Questions probed retrospective reports of mental health and cognitive functioning during the 30 days before and 30 days after treatment. A total of 65 people completed treatment during this time frame and were eligible for contact. Of these, 51 (78%) completed the survey and were included in data analyses (mean age = 40; male = 96%; married = 55%; Caucasian/White = 92%; Operation Enduring Freedom/Operation Iraqi Freedom Service = 96%).

RESULTS

Results indicated significant and very large reductions in retrospective report of suicidal ideation (p < .001; d = -1.9), cognitive impairment (p < .001; d = -2.8), and symptoms of posttraumatic stress disorder (p < .001; d = -3.6), depression (p < .001; d = -3.7), and anxiety (p < .001; d = -3.1). Results also showed a significant and large increase in retrospective report of psychological flexibility (p < .001; d = 2.9) from before-to-after the psychedelic treatment. Increases in the retrospective report of psychological flexibility were strongly associated with retrospective report of reductions in cognitive impairment, and symptoms of posttraumatic stress disorder, depression, and anxiety (rs range -0.61 to -0.75; p < .001). Additionally, most participants rated the psychedelic experiences as one of the top five personally meaningful (84%), spiritually significant (88%), and psychologically insightful (86%) experiences of their lives.Limitations: Several limitations should be considered including the retrospective, self-report, survey design of the study, and the lack of randomization and blinding, thus making these finding preliminary.

CONCLUSION

U.S. Special Operations Forces Veterans may have unique treatment needs because of the sequela of problems associated with repeated trauma exposure and the nature of the exposure. Psychedelic-assisted therapy with these under-researched psychedelics may hold unique promise for this population. However, controlled studies are needed to determine whether this treatment is efficacious in relieving mental health and cognitive impairment among U.S. Special Operations Forces Veterans.

Plasma serotonin levels and the platelet serotonin transporter

Serotonin (5HT) is a platelet-stored vasoconstrictor. Altered concentrations of circulating 5HT are implicated in several pathologic conditions, including hypertension. The actions of 5HT are mediated by different types of receptors and terminated by a single 5HT transporter (SERT). Therefore, SERT is a major mechanism that regulates plasma 5HT levels to prevent vasoconstriction and thereby secure a stable blood flow. In this study, the response of platelet SERT to the plasma 5HT levels was examined within two models: (i) in subjects with chronic hypertension or normotension; (ii) on platelets isolated from normotensive subjects and pretreated with 5HT at various concentrations. The platelet 5HT uptake rates were lower during hypertension due to a decrease in Vmax with a similar Km; also, the decrease in Vmax was primarily due to a decrease in the density of SERT on the platelet membrane, with no change in whole cell expression. Additionally, while the platelet 5HT content decreased 33%, the plasma 5HT content increased 33%. Furthermore, exogenous 5HT altered the 5HT uptake rates by changing the density of SERT molecules on the plasma membrane in a biphasic manner. Therefore, we hypothesize that in a hypertensive state, the elevated plasma 5HT levels induces a loss in 5HT uptake function in platelets via a decrease in the density of SERT molecules on the plasma membrane. Through the feedback effect of this proposed mechanism, plasma 5HT controls its own concentration levels by modulating the uptake properties of platelet SERT.

WAY 405, a new silent 5-HT1A receptor antagonist with low affinity for vascular alpha1-adrenoceptors

1 The effect of WAY 405 ((R)-N-(2-methyl-(4-indolyl-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide), a putative 5-HT(1A) receptor antagonist, on cardiovascular function was studied. 2 In anaesthetized rats, the i.v. injection of WAY 405 did not significantly modify basal heart rate nor blood pressure at doses of 1, 3, 10 and 30 microg kg(-1); while the antagonist dose dependently antagonized the 5-HT(1A) receptor agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin)-induced hypotension and bradycardia. 3 WAY 405 antagonized noradrenaline-induced contraction in isolated arteries, with pK(B) values of 6.6+/-0.1, 6.5+/-0.1 and 6.5+/-0.1, for rat tail artery (alpha(1A)-adrenoceptors), rabbit aorta (alpha(1B)-adrenoceptors), and rat aorta (alpha(1D)-adrenoceptors) respectively. 4 The results show that in the control of blood pressure the new compound, WAY 405, behaves as a silent 5-HT(1A) receptor antagonist in the anaesthetized rat, also having low affinity for vascular alpha(1)-adrenoceptors.

The hypotensive effect of BMY 7378 involves central 5-HT1A receptor stimulation in the adult but not in the young rat

BACKGROUND

Stimulation of central 5-hydroxytryptamine-1A (5-HT(1A)) receptors produces hypotension and bradycardia. We describe BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9 dione) effects in cardiovascular function and [(3)H] 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino) tetralin) binding sites in rat brain of different ages.

METHODS

BMY 7378 was administered to anesthetized male Wistar rats (1, 3 and 6 months old) and blood pressure and heart rate were continuously recorded. Saturation of [(3)H] 8-OH-DPAT binding to 5-HT(1A) sites in brain membranes was determined.

RESULTS

Basal diastolic blood pressure increased with age, 85 +/- 2, 106 +/- 3, and 113 +/- 2 mmHg for 1-, 3- and 6-month-old rats, respectively (p <0.05 among groups). BMY 7378 induced significant dose- and age-dependent hypotension. The selective 5-HT(1A) receptor antagonist, WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-(2-pyridinyl) cyclohexanecarboxamide), antagonized BMY 7378 effects in 6 month-old but not in younger rats. [(3)H] 8-OH-DPAT binding sites decreased in hippocampi and brainstem with maturation.

CONCLUSIONS

Data suggest that BMY 7378 is a hypotensive agent in the rat, but that its actions are mediated, in part, by central 5-HT(1A) receptor stimulation in the adult and by a nonserotonergic mechanism in the young rat.

Evidence that NAN-190-induced hypotension involves vascular alpha1-adrenoceptor antagonism in the rat

The effect of NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido]-butyl] piperazine), described as a mixed 5-HT(1A) receptor agonist/antagonist, on cardiovascular function was studied. The i.v. injection of NAN-190 (1-300 micro/kg) dose-dependently decreased blood pressure (p<0.001), while heart rate was not significantly modified compared to saline-treated, anaesthetized adult rats. WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide), a highly selective 5-HT(1A) receptor antagonist, increased NAN-190-induced hypotension (p<0.05). In the pithed rat NAN-190 displaced the phenylephrine dose-pressor response curve to the right; ED(50) values were: approximately 14, 20, 40 and 270 microg/kg for saline and NAN-190 (1, 10 and 100 microg/kg, respectively); similar ED(50) values were obtained with prazosin ( approximately 20, 69 and 358 microg/kg for 1, 10 and 100 microg/kg of prazosin, respectively). NAN-190 shifted to the right the concentration-response curves to phenylephrine in rat tail artery (alpha(1A)-adrenoceptors), in rabbit aorta (alpha(1B)-adrenoceptors) and in rat aorta (alpha(1D)-adrenoceptors), with pA(2) values of 9.47, 9.02 and 9.99; while Schild slopes were -0.78, -1.13 and -0.90, respectively (not significantly different from unity). The results show that NAN-190 induced hypotension in the anaesthetized, adult rat and suggest that this effect could be explained by antagonism of vascular alpha(1)-adrenoceptors.

Evidence that the hypotensive effect of WAY 100635, a 5-HT1A receptor antagonist, is related to vascular alpha 1-adrenoceptor blockade in the adult rat

1. The effect of WAY100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride), a 5-HT1A receptor antagonist, on cardiovascular function was studied. 2. The i.v. injection of WAY100635 dose-dependently decreased blood pressure in anaesthetized rats; while in pithed rats WAY100635 (1 mg kg(-1)) displaced the phenylephrine pressor effect. 3. WAY100635 antagonized phenylephrine-induced contraction in rabbit and rat aorta (pA2 of 6.88 and 7.93 and Schild slopes of -0.83 and -1.21, respectively); while in rat caudal artery pK(B) was 7.45 and the Schild slope of -0.56, suggesting a complex interaction in this vessel. 4. The results show that WAY100635 induced hypotension in the anaesthetized rat and suggest that this effect could be partially explained by antagonism of vascular alpha1-adrenoceptors.

The hypotensive effect of BMY 7378 is antagonized by a silent 5-HT(1A) receptor antagonist: comparison with 8-hydroxy-dipropylamino tetralin

BACKGROUND

Stimulation of central 5-HT(1A) receptors produces bradycardia and diminishes blood pressure in conscious or anesthetized rats. Our objective was to investigate the effects on blood pressure and heart rate of the partial 5-HT(1A) receptor agonist and selective alpha1D-adrenoceptor antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-8-azaspiro [4.5] decane-7,9 dione hydrochloride) compared to the full 5-HT(1A) receptor agonist 8-OH-DPAT (8-hydroxy-dipropylamino tetralin) in adult anesthetized rats.

METHODS

Male Wistar rats of 6 months of age were exposed intravenously (i.v.) to increasing doses of BMY 7378 or 8-OH-DPAT in the absence and presence of WAY 100635. Blood pressure and heart rate were continuously recorded.

RESULTS

BMY 7378 induced a decrease in blood pressure with no apparent change in heart rate compared to basal values, while 8-OH-DPAT decreased both hemodynamic parameters. BMY 7378 hypotensive effect was antagonized by the selective, silent 5-HT(1A) receptor antagonist WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). However, a remnant yet significant hypotensive effect was not blocked by the antagonist. In contrast, 8-OH-DPAT actions were completely blocked by WAY 100635.

CONCLUSIONS

Data suggest that BMY 7378 cardiovascular effects are related to activation, as a full agonist, of central 5-HT(1A) receptors in adult rats; however, participation of other systems such as vascular alpha1-adrenoceptors in cardiovascular function is suggested.

Bopindolol: pharmacological basis and clinical implications

Bopindolol, a non-selective antagonist of beta 1- and beta 2-adrenoceptors (ARs), has been found by pharmacological, molecular biological techniques and molecular modeling to have several unique properties. Bopindolol produces sustained blockade of beta 1- and beta 2-ARs, has intrinsic sympathomimetic as well as membrane stabilizing actions, inhibits renin secretion, and interacts with 5-HT receptors. Also, our recent molecular modeling studies identified possible interaction sites between bopindolol and beta-AR subtypes. The reviewed studies support our findings that bopindolol is non-selective for beta 1- and beta 2-ARs, has low affinity for beta 3-AR subtype and has pharmacological properties that are likely to be beneficial in the treatment of cardiovascular diseases.

Effect of 5-HT(1A) receptor ligands on Fos-like immunoreactivity in rat brain: evidence for activation of noradrenergic transmission

This study investigated the effects of 8-OH-DPAT and various other 5-HT(1A) receptor agonists on brain noradrenergic transmission using Fos-like immunoreactivity (Fos-LI) as a marker of neural activation. Administration of 8-OH-DPAT (0.1 and 1 mg/kg) induced a marked and dose-related increase in the number of cells positive for Fos-LI in the locus coeruleus (LC), the main source of noradrenergic projections to the forebrain. This effect was also induced by the non-selective, partial 5-HT(1A) receptor agonist buspirone (10 mg/kg). The effect of both 8-OH-DPAT (0.1 mg/kg) and buspirone (10 mg/kg) on Fos-LI in the LC was blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). The active S(-)-enantiomer of the partial 5-HT(1A) receptor agonist (+/-)-MDL 75005EF (1 mg/kg) also induced the expression of Fos-LI in the LC, whereas the inactive R(+)-enantiomer of (+/-)-MDL 73005EF at the same dose did not. In addition to the LC, 8-OH-DPAT (0.1 mg/kg) also induced a marked increase in Fos-LI in various forebrain areas including the medial prefrontal cortex (infralimbic and cingulate cortical areas). More detailed analysis of the Fos response to 8-OH-DPAT in the medial prefrontal cortex revealed that the effect was attenuated by pretreatment with a combination of the beta(1)- and beta(2)-adrenoceptor antagonists ICI 118551 (4 mg/kg) and metoprolol (4 mg/kg), but not the alpha(1)-adrenoceptor antagonist prazosin (5 mg/kg). Taken together, the present findings provide immunocytochemical evidence that 5-HT(1A) receptor agonists activate noradrenergic neurones in the LC and that this leads to increased noradrenergic transmission at postsynaptic sites in the forebrain (specifically medial prefrontal cortex).

Central serotonergic responsiveness in neurocardiogenic syncope: a clomipramine test challenge

BACKGROUND

Central serotonergic mechanisms appear to participate in the pathogenesis of recurrent neurally mediated syncope. The aim of the study was to investigate the responsiveness of the central serotonergic system by measuring the prolactin and cortisol responses to intravenous administration of the serotonin reuptake inhibitor clomipramine.

METHODS AND RESULTS

Twenty subjects free of any medical treatment were tested. Twelve had a history of recurrent syncopal attacks and positive tilt test (patient group, mean age 47+/-18 years, 8 men); 8 subjects without syncope and a negative tilt test result served as control subjects (mean age 49+/-10 years, 5 men). Twenty-five milligrams of clomipramine was administered intravenously within 15 minutes, and blood samples were taken at 0, 15, 30, 45, and 60 minutes. Two days later, a tilt test was performed at 60 degrees for 30 minutes and blood samples were taken at 0, 10, 20, and 30 minutes. During the clomipramine challenge, plasma prolactin levels increased in both groups. The levels at 30 minutes were higher in the patient group compared with the control group (17.3+/-7.2 vs 9.3+/-7.6 ng/mL, P=0.05). Similar results were observed for cortisol at 30 minutes (172+/-15 vs 118+/-21 ng/mL P=0. 04) and at 45 minutes (189+/-20 vs 116+/-23 ng/mL, P=0.03). The tilt test was positive in 8 (67%) out of 12 of the patient group and negative in all control subjects. In the samples taken during the tilt test, significant increases in prolactin and cortisol were observed only in the subjects with positive tilt test results.

CONCLUSIONS

Patients with a history of neurocardiogenic syncope show a higher responsiveness of the central serotonergic system to clomipramine challenge. The results support the view that central serotonergic mechanisms are involved in the pathophysiology of the syndrome.

8-OH-DPAT-induced hypotensive action and sympathoexcitatory neurons in the rostral ventrolateral medulla of the rat

We examined whether the selective 5-hydroxytryptamine 1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) injected systemically can act directly on sympathoexcitatory neurons located in the rostral ventrolateral medulla (RVLM) to cause the hypotensive effect of this agent in rats. Microinjections of 8-OH-DPAT and buspirone into the RVLM produced a dose-dependent decrease in blood pressure. Microinjections of spiperone and pindolol, 5-HT1A antagonists, into the RVLM inhibited the depressor response to 8-OH-DPAT intravenously injected or injected into the RVLM. Microiontophoretic application of 8-OH-DPAT onto RVLM sympathoexcitatory neurons inhibited the firing of RVLM sympathoexcitatory neurons and the inhibition of unit activity by 8-OH-DPAT was blocked by microiontophoretic spiperone. Intravenous administration of 8-OH-DPAT also inhibited the firing of these neurons. Microiontophoretic application of spiperone onto the RVLM sympathoexcitatory neurons reversed the inhibitory response to intravenous 8-OH-DPAT. These results are consistent with the hypothesis that 8-OH-DPAT may exert a portion of its hypotensive effect through a direct inhibition of RVLM sympathoexcitatory neurons in rats. The receptor involved is probably the 5-HT1A type.

Further evidence that the tachycardiac response to 5-hydroxytryptamine in rats is not mediated via typical 5-HT2 receptors

1. In pithed rats, the receptor type of cardiac tissue mediating the tachycardiac response to 5-hydroxytryptamine (5-HT) was analysed. 2. Ketanserin, mesulergine and methiothepin, administered in doses which antagonized the pressor response to 5-HT, slightly reduced, considerably reduced and antagonized, respectively, its tachycardiac response. 3. 8-OH DPAT and RU 24969 produced no significant tachycardia, while DOI neither mimicked nor blocked the 5-HT-induced tachycardia. 4. The tachycardia was not affected by MDL 72222, metoclopramide and cocaine. 5. The hypothesis is proposed that in rats the tachycardiac response to 5-HT is mediated via 5-HT receptors belonging to an yet unclassified subtype of 5-HT1 or 5-HT2 receptors.

Vascular receptors for 5-hydroxytryptamine: distribution, function and classification

The complex actions of 5-hydroxytryptamine (5-HT) on blood vessels result from interactions with a multiplicity of specific 5-HT receptors. Dramatic advances recently have been made in defining the receptor subtypes involved in terms of their pharmacology and biochemistry, as well as their molecular biology, provoking an evolution of the criteria used for classifying and naming them. This article reviews the distribution and function of different 5-HT receptor types present on vascular smooth muscle, endothelium and perivascular neurones, and considers ways in which they can be positively defined and differentiated using traditional pharmacological approaches. The characteristics of each receptor type are also considered in terms of current biochemical and molecular perspectives on 5-HT receptor classification.

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.

Pharmacological analysis of the cardiac effects of 5-HT and some 5-HT receptor agonists in the pithed rat

A pharmacological analysis of the effects of 5-HT on heart rate has been performed in the pithed rat. 5-HT induced a dose-dependent increase in heart rate whereas 5-HT1 receptor agonists--8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 5-methoxy-N,N-dimethyl-tryptamine (5-MeODMT), 5-methoxy 3-(1,2,3,6-tetrahydro-4-piridinyl) 1H indole (RU 24969) and 1-(m-trifluoromethylphenyl)-piperazine (TFMPP)--failed to increase heart rate. The increase in heart rate induced by the selective 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) was not significant. The dose-response curve to 5-HT for its tachycardic effects was shifted two-fold to the right by ketanserin and LY 53857 and nine-fold to the right by methiothepin. The effects of high doses of 5-HT (higher than 100 micrograms/kg iv) were antagonized by methiothepin, (-)propranolol, 2-(2-[4(O-methoxyphenyl)-piperazine-1-yl]-ethyl)4,4-dimethyl-1,3 (2H-4H) isoquinoline-dione (AR-C 239) and by pretreatment with reserpine. The 5-HT1 receptor antagonists, pindolol and spiroxatrine, the 5-HT3 receptor antagonist MDL 72222 and the alpha 2-adrenoceptor blocking agent idazoxan failed to antagonize the tachycardia induced by 5-HT. It is concluded that in the pithed rat, the tachycardia induced by 5-HT remained unexplained (implication of 5-HT2 receptors probably different from the classical vascular 5-HT2 receptor, or implication of 5-HT1C receptors?). Moreover, at high doses (higher than 100 micrograms/kg iv), 5-HT may increase heart rate by releasing catecholamines.

Autoradiographic localization of 5HT1 binding sites in the medulla oblongata of the rat

Serotonin (5HT) binding sites in the medulla oblongata of the rat were localized using autoradiographic techniques with radioactive ligands that express high affinity for the 5HT1 (3H-5HT), 5HT1A (3H-80H-DPAT), or 5HT1B (125I-CYP with isoproterenol) receptor subtypes. 5HT1A sites were concentrated primarily in certain sensory regions of the medulla and in regions that contain serotonergic neurons. 5HT1B sites were diffusely distributed throughout the reticular formation and motor regions as well as being localized to certain sensory regions. A surprising finding was an association of 5HT1B binding sites with the corticospinal tract. 3H-5HT binding generally reflected the combined pattern of 5HT1A and 5HT1B sites but was also extremely dense in the choroid plexus, which exhibited virtually no 5HT1A or 5HT1B ligand binding. Presumably this binding, which was blocked by ketanserin, corresponds to 5HT1C sites. Administration of the serotonergic neurotoxin 5,7-dihydroxytryptamine reduced 5HT1A binding sites in regions of the medulla that contain serotonergic neuronal cell bodies. 5HT1B binding was not significantly altered in any area of the medulla. These studies indicate an important role for 5HT1A ligands in the processing of visceral and somatic sensory information, in regulation of certain cerebellar afferent projections, and in the regulation of serotonergic neuronal activity. 5HT1B ligands would be expected to regulate visceral and somatic efferent activity, as well as sensory information and reticular efferent activity, and might presynaptically regulate cortical inputs to the brain stem and spinal cord.

[3H]5-methyl-urapidil labels 5-HT1A receptors and alpha 1-adrenoceptors in the rat CNS. In vitro binding and autoradiographic studies

The tritiated derivative of the potent antihypertensive agent, 5-methyl-urapidil, was used as a radioligand in binding studies with rat brain membranes and tissue sections. Serotonin and prazosin inhibited [3H]5-methyl-urapidil binding to membranes from the rat hippocampus, cerebral cortex and brainstem biphasically, leading to the definition of serotonin high-affinity and prazosin high-affinity [3H]5-methyl-urapidil binding sites. Comparison of these serotonin-sensitive [3H]5-methyl-urapidil binding sites with the 5-HT1A sites labelled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) revealed striking similarities regarding pharmacological properties, respective densities and regional distribution. On the other hand, the prazosin-sensitive [3H]5-methyl-urapidil binding sites should correspond to the alpha 1A-subtype of adrenoceptors which has recently been defined. Detailed autoradiographic investigations allowed the detection of 5-HT1A sites labelled by both [3H]5-methyl-urapidil and [3H]8-OH-DPAT in the posterior raphe nuclei (pallidus and obscurus) which are possibly involved in the hypotensive action of 5-methyl-urapidil. These data demonstrate that [3H]5-methyl-urapidil is a useful radioligand for the visualization and quantification of both 5-HT1A serotonin receptors and alpha 1A-adrenoceptors in the central nervous system.

Molecular design of novel ligands for 5-HT1A receptors

Serotonin (5-HT) is a potent bioactive substance known to function through a number of different receptor types and subtypes. In our attempt to develop new agents that would interact selectively at certain 5-HT receptors, especially the 5-HT1A subtype, 8-hydroxy-2-di-n-propylamino tetralin (8-OH-DPAT) served as a template for the design of novel agents sharing aspects of the pharmacophore of 8-OH-DPAT and 5-HT. 5-HT contains no center of asymmetry, and 8-OH-DPAT shows only very modest stereospecificity for 5-HT1A receptors. To develop agents having enhanced potency and selectivity for the 5-HT1A site, several ring systems offering enhanced conformational rigidity which approximate the oxygen to nitrogen interatomic distances of 8-OH-DPAT and (to a lesser extent) 5-HT were synthesized. Exemplary ring systems include the 8-alkoxy-hexahydroindeno[1,2-c]pyrrole, 5-alkoxy-hexahydro-1H-indeno-[2,1-c]pyridine, and 9-alkoxy-hexahydro-1H-benz[e]isoindole systems. These conformationally restricted molecules demonstrated moderate stereospecificity in their interaction with the 5-HT1A binding site, which was enhanced in compounds with larger nitrogen substituents. Appropriate choice of such derivatives led to highly potent compounds selective for 5-HT1A sites compared with their activity at other 5-HT and/or adrenergic receptors. The pharmacological profile of compounds which appear to act as agonists at 5-HT1A receptors in the central nervous system to lower blood pressure in animal models of hypertension is presented.

Unmasking of a neonatal somatovesical reflex in adult cats by the serotonin autoreceptor agonist 5-methoxy-N,N-dimethyltryptamine

In neonatal kittens, micturition is induced by a spinal somatovesical reflex pathway that is activated by the mother cat licking the perigenital region of the kitten. The somatovesical reflex pathway disappears about the time of weaning and is replaced by a vesicovesical reflex pathway that produces micturition via a supraspinal reflex pathway that is activated by distension of the urinary bladder. Furthermore, stimulation of the perigenital region in adult cats actually inhibits the supraspinal vesicovesical micturition reflex. Spinalization prompts the return of the somatovesical reflex, immediately in weaned kittens but over a course of days to weeks in adult cats. The purpose of the present experiments was to determine if the somatovesical reflex could be demonstrated acutely, and reversibly, in adult cats with an intact spinal cord via pharmacological suppression of the serotonergic system. The serotonergic system was suppressed by the intravenous administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), a serotonin agonist that inhibits the firing of serotonergic neurons via activation of inhibitory somatodendritic autoreceptors. 5-MeODMT in low doses (20-50 micrograms/kg) abolished inhibition of the bladder produced by either light tactile stimulation of the perigenital region or by electrical stimulation of the pudendal nerve, which carries the afferent fibers from the perigenital region, in 9 of 10 adult cats. Furthermore, in 8 of the 10 cats, the bladder inhibition was reversed to an excitation of variable amplitudes in each cat. Higher doses of 5-MeODMT (100-1000 micrograms/kg) abolished spontaneous bladder activity but did not inhibit perigenital-induced bladder contractions in those 8 animals in which the drug unmasked the excitatory somatovesical reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The cardiovascular response to flesinoxan and 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetralin), 5-HT1A receptor agonists, has been investigated in anaesthetized Wistar rats and spontaneously hypertensive rats (SHR) and in conscious SHR. Flesinoxan and 8-OH-DPAT potently lowered blood pressure and heart rate in these models. In conscious SHR, atropine reversed the bradycardia induced by flesinoxan partially and that induced by 8-OH-DPAT completely. In pithed rats with vasopressin-raised blood pressure, neither flesinoxan nor 8-OH-DPAT lowered blood pressure or heart rate. Intracisternal administration of either flesinoxan or 8-OH-DPAT was less efficacious than intravenous administration. The cardiovascular responses to flesinoxan and 8-OH-DPAT in the anaesthetized Wistar were inhibited by the putative 5-HT1A antagonists methiothepin, buspirone, spiroxatrine and 8-MeO-C1EPAT (8-methoxy-2-(N-2-cholroethyl-N-n-propylamino)tetralin). 8-MeO-C1EPAT appeared to be the most suitable antagonist in this model. The 5-HT1C, antagonist ritanserin or the 5-HT3 antagonist GR 38032F had no effect on the responses to flesinoxan or 8-OH-DPAT. In conscious SHR however, 8-MeO-C1EPAT did not antagonize these cardiovascular responses. This study confirms the involvement of central 5-HT1A receptors in the cardiovascular effects of flesinoxan and 8-OH-DPAT.

Anxiolytics and stress-induced behavioural and cardiac responses: a study of diazepam and ipsapirone (TVX Q 7821)

The present study has been designed to investigate the effects of the 5-HT1A receptor agonist, ipsapirone (TVX Q 7821), a representative of a novel class of anxiolytics, and the classical benzodiazepine anxiolytic, diazepam, on cardiac and behavioural responses in an emotional stress situation. The emotional stress of fear of punishment, induced by training male Wistar rats in an inhibitory avoidance situation, was followed by a bradycardiac response relative to similarly trained, but non-punished, freely moving rats. The behavioural response of stressed rats was immobility in the dark compartment in which an electric footshock (0.6 mA a.c. for 3 s) had been administered as punishment a day earlier. Diazepam administered i.p. in doses of 2.5 mg/kg and 7.5 mg/kg caused a decrease in the interbeat interval (IBI) in shocked and non-shocked rats whereas ipsapirone administered i.p. in doses of 2.5 and 12.5 mg/kg decreased the IBI in shocked rats only. Ipsapirone diminished the duration of immobility in both shocked and non-shocked animals whereas diazepam decreased immobility in shocked rats only. These results suggest a differential effect of the two anxiolytics on the behavioural and cardiac responses to an emotional stress situation. It is suggested that ipsapirone has an anxiolytic-like effect and 'anti-stress' action that is clearly reflected in the cardiac physiology in an anxiety-inducing situation.

5-HT1C receptors in the serotonergic control of periaqueductal gray induced aversion in rats

The functional role of brain 5-HT and 5-HT receptor subtypes in periaqueductal gray (PAG) induced aversion has been investigated in rats. Antiaversive effects were found with the serotonin agonists TFMPP, mCPP and DOI but not with RU 24969 which was found to facilitate PAG aversion. The first three serotonin agonists share potent 5-HT1C activity while RU 24969 differs with a high 5-HT1A activity. Proaversive effects were found with the mixed 5-HT1C/5-HT2 antagonists cyproheptadine and ritanserin; this effect was already reported for the mixed 5-HT1C/5-HT2 antagonists metergoline and mianserin and is opposite to the effects of the selective 5-HT2 antagonists ketanserin, pirenperone, trazodone and spiperone. The antiaversive effects of mCPP (1 mg/kg) could be prevented by pretreatment of the animals with mianserin (1 and 10 mg/kg). These results suggest that 5-HT1C receptors play an important role in the serotonergic control of PAG aversion. 5-HT1C receptor activation seems to mediate antiaversive effects whereas acute 5-HT1C receptor blockade appears to facilitate PAG aversion. Functional interactions take place between several receptor types in the in vivo control of PAG aversion, where 5-HT1C receptors appear to play a predominant function.

Involvement of brain 5-HT1A receptors in the hypotensive response to urapidil

Stimulation of serotonin-1A (5-hydroxytryptamine) (5-HT1A) receptors in the brain stem has been suggested to contribute to the antihypertensive action of the alpha 1-adrenoceptor antagonist urapidil. This hypothesis was tested by analyzing the influence of the 5-HT1A receptor antagonist spiroxatrine on the hypotensive responses to urapidil and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). Chloralose/urethane-anesthetized cats underwent thoracotomy and were artificially ventilated. Blood pressure was monitored in the femoral artery. Urapidil (0.01 to 10 mumol/kg) or 8-OH-DPAT (3 to 30 nmol/kg) was injected into a femoral vein and the maximal hypotensive response recorded. A dose-response test with both drugs was performed before and after administration of spiroxatrine (3 and 10 nmol/kg); the latter was given through the vertebral artery, thus delivering the antagonist to the brain stem. Blood pressure was dose-dependently reduced by urapidil and 8-OH-DPAT after intravenous injection. Central administration of spiroxatrine through the vertebral artery shifted the dose-response curves of both drugs markedly and in a dose-dependent manner to the right, while the hypotensive response to the peripheral vasodilator nitroglycerin remained unchanged. The results suggest that the hypotensive response after peripheral administration of urapidil is mediated in part by stimulation of brain 5-HT1A receptors and this effect on central cardiovascular regulation is additive to the blood pressure reduction resulting from peripheral alpha-adrenoceptor blockade.

Cardiovascular effects from stimulation of 5-hydroxytryptamine receptors

The cardiovascular effects of 5-hydroxytryptamine (5-HT), consisting of bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, are mediated by three main types of receptors called 5-HT1-like, 5-HT2, and 5-HT3. In intact animals 5-HT elicits a short-lasting bradycardia, accompanied by hypotension, via stimulation of 5-HT3 receptors located on sensory vagal nerve endings in the heart (Bezold-Jarisch reflex). The nature of 5-HT receptors mediating tachycardiac responses is species-dependent. Myocardial 5-HT1-like and 5-HT2 receptors subserve tachycardia in the cat and rat, respectively. Tachycardia in the dog and rabbit is due to a release of catecholamines effected via the 5-HT2 receptors on the adrenal medulla and the 5-HT3 receptors on postganglionic cardiac sympathetic nerve fibres, respectively. The receptors mediating tachycardia in the pig are unique as they do not resemble any of the three 5-HT receptors characterized so far. The blood pressure response to 5-HT is usually triphasic: initial short-lasting hypotension due to reflex bradycardia (via 5-HT3 receptors), a middle pressor phase (via 5-HT2 receptors), and a longer-lasting hypotension (via 5-HT1-like receptors). Vascular contraction by 5-HT is generally mediated by 5-HT2 receptors (located primarily on the large conducting vessels), though in some instances (e.g., dog saphenous vein, dog and human basilar artery, and porcine arteriovenous anastomoses) the contractile response is (also) mediated via 5-HT1-like receptors. Venous dilatation and arteriolar dilatation (leading to increased capillary ['nutrient'] blood flow) occur via 5-HT1-like receptors located mainly on the vascular smooth muscles but also on the endothelium; the smooth muscle and endothelial 5-HT1-like receptors seem to be heterogeneous. In addition, 5-HT can elicit vasodilatation and hypotension as a result of decreased sympathetic nervous tone by acting within the central nervous system and by inhibiting noradrenaline release by a presynaptic action. Both these effects also involve 5-HT1-like receptors that do not appear to be identical. Last, knowledge of the cardiovascular effects of 5-HT and the nature of the receptors involved should be helpful in developing 5-HT-related compounds that may be useful in the treatment of hypertension, migraine, and peripheral vascular diseases.

5-HT(1)-like receptor agonists and the pathophysiology of migraine

Recently, AG-25086 and GR-43175 have been reported to be highly effective in aborting acute migraine attacks. These compounds seem selectively to stimulate a sub-population of 5-HT(1)-like receptors to inhibit noradrenaline release from certain sympathetic neurovascular terminals, to contract dog saphenous vein and dog, monkey and human basilar arteries, and to decrease the arteriovenous anastomotic component of carotid blood flow in the cat. GR-43175 neither has any antinociceptive effect nor crosses the blood-brain barrier. Pramod Saxena and Michel Ferrari review the clinical effectiveness of these 5-HT(1)-like receptor agonists and their selective pharmacology, both of which strongly suggest that excessive dilatation in the extracerebral cranial (scalp and/or dural) vasculature is an integral part of the pathophysiology of migraine.

8-OH-DPAT, flesinoxan and guanfacine: systemic and regional haemodynamic effects of centrally acting antihypertensive agents in anaesthetized rabbits

1. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and flesinoxan, agents which show high affinity and selectivity for 5-HT1A receptors, were administered intravenously in doses of 0.003 to 0.1 and 0.01 to 0.3 mg kg-1 respectively to 5 rabbits each. Their effects were compared with those of the centrally acting agent and alpha 2-adrenoceptor agonist, guanfacine, 0.01-0.3 mg kg-1, administered to a group of 5 rabbits. Five further rabbits were used as controls and treated with the vehicle of the active agents. 2. Both flesinoxan and 8-OH-DPAT induced similar systemic and regional haemodynamic changes. Both lowered mean arterial pressure and heart rate. The principal blood pressure lowering mechanism was vasodilatation; cardiac output changed minimally despite the falls in heart rate and myocardial contractile force. 3. With guanfacine the maximal fall of blood pressure was comparable to that obtained with the 5-HT1A receptor ligands; however, in contrast to the latter, the dose-response curve was U-shaped, the highest dose eliciting a pressor effect with reversal of the vasodilatation. 4. Widespread peripheral vasodilatation was found with all the agents in the splanchnic circulation and also in the brain and skeletal muscle. A weak tendency towards vasodilatation was found in the kidneys where the dose-response curve was bell-shaped for guanfacine. 5. This spectrum of activity is different from that of peripheral vasodilators, such as calcium antagonists, potassium channel activating agents or hydralazine; it is, however, consistent with the putative mechanism of action of these compounds to reduce peripheral sympathetic tone by a central mechanism of action.

Ventrolateral medullary pressor area: site of hypotensive and sympatho-inhibitory effects of (+/-)8-OH-DPAT in anaesthetized dogs

Injections of 8-OH-DPAT (0.1-6 micrograms/kg) into the vertebral artery or into the cisterna magna (5 micrograms/kg) produced a dose-dependent decrease in blood pressure, heart rate and renal sympathetic nerve activity in intact anaesthetized dogs and baroreceptor-denervated dogs. 8-OH-DPAT reduced the renal sympathetic nerve activity without changing the blood pressure or heart rate in catecholamine-depleted animals. Methiothepin (0.2 mg/kg) injected into the vertebral artery reduced the blood pressure without changing the heart rate or renal sympathetic nerve activity in baroreceptor-denervated dogs. The pressor response to i.v. phenylephrine was largely attenuated. Subsequent administration of 8-OH-DPAT (3 micrograms/kg) into the vertebral artery failed to alter the sympathetic discharge. Methiothepin (0.2 mg/kg) injected into the vertebral artery reversed the sympatho-inhibitory effect of 8-OH-DPAT (3 micrograms/kg) injected by the same route without changing the blood pressure. (+/-)Pindolol (0.2 mg/kg) injected into the vertebral artery of baroreceptor-denervated dogs reduced the blood pressure and heart rate without changing renal sympathetic activity. Subsequent administration of 8-OH-DPAT (3 micrograms/kg) failed to alter the sympathetic discharge. Bilateral microinjection of 8-OH-DPAT (1 microgram) into the nucleus tractus solitarii or into the medullary raphe nuclei failed to alter the blood pressure, heart rate or renal sympathetic activity. In contrast, bilateral microinjections of 8-OH-DPAT into the ventrolateral pressor area (VLPA) (0.2 microgram) produced a marked decrease in blood pressure, heart rate and renal sympathetic nerve activity. These effects were prevented or reversed by microinjections of methiothepin (10 micrograms) at the same sites. These results indicate that the central sympatho-inhibitory effects of 8-OH-DPAT were due to the stimulation of 5-HT1A receptors located in the ventrolateral pressor area. 5-HT autoreceptors did not seem to be involved.

Evidence for the interaction of urapidil with 5-HT1A receptors in the brain leading to a decrease in blood pressure

Current knowledge about the role of serotonin (5-HT) in central cardiovascular regulation is reviewed. Results from experiments with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) suggest that activation of somatodendritic 5-HT1A receptors in the medulla oblongata decreases the firing of serotoninergic neurons and thus reduces their excitatory input to the sympathetic neurons in the intermediolateral cell column. As a consequence, blood pressure is reduced by 5-HT1A receptor agonists. Urapidil is an antihypertensive drug that has a dual mode of action: peripheral alpha-adrenoceptor antagonism and interaction with 5-HT1A receptors in the brain. This profile can adequately explain the vasodilation and lack of significant sympathetic activation observed during urapidil treatment.

Effects of serotonin receptor antagonists on PAG stimulation induced aversion: different contributions of 5HT1, 5HT2 and 5HT3 receptors

The effects of serotonin receptor antagonists with differential selectivity for the various classes of 5HT receptors (5HT1, 5HT2 and 5HT3) were tested for their effects on the response to aversive brain stimulation. Electrical stimulation was administered to the dorsal part of the periaqueductal gray matter (PAG), one of the main cerebral structures subserving negative reinforcement. Stimulation frequency thresholds for escape responses were recorded before and following administration of the compounds. Ketanserin (0.32-32 mg/kg IP), trazodone (1.0-22 mg/kg), pirenperone (0.032-1.0 mg/kg) and spiperone (0.1-0.2 mg/kg) dose-dependently increased stimulation frequency thresholds necessary to induce escape responses. Opposite effects were observed with mianserin (0.01-32 mg/kg) and metergoline (0.032-10 mg/kg) which decreased threshold for escape. ICS 205-930 (0.01-10 mg/kg), did not affect the stimulation frequency threshold for escape. Prazosin (0.1-22 mg/kg) did not specifically affect aversive brain stimulation. Haloperidol (0.02-1.0 mg/kg) increased the frequency threshold for escape responses but with some motoric side effects. These data show that the various types of 5HT receptors differentially contribute to the control of central aversive systems in rats. It is suggested that blockade of 5HT2 receptors suppresses the central aversive system, whereas blockade of some 5HT1 receptors enhances aversion and overcomes the 5HT2-mediated suppression. Blockade of 5HT3 receptors has no effects. Dopamine receptor blockade further contributes to the suppression of the central aversive system. The relevance of these findings to some pathophysiological mechanisms of anxiety and depressive disorders is discussed.

Vascular postsynaptic effects of some 5-HT1-like receptor agonists in the pithed rat

5-HT induced an increase in blood pressure in the pithed rat which was antagonized by LY 53857 a selective 5-HT2 receptor antagonist. It was not antagonized by spiroxatrine, MDL 72222, idazoxan or AR-C 239, respectively 5-HT1-like and 5-HT3 receptor antagonists, alpha 2- and alpha 1-adrenoceptor antagonists. 5-MeODMT also induced an increase in blood pressure which was antagonized by LY 53857 but not by the other 5-HT receptor antagonists and alpha-adrenoceptor antagonists used, suggesting a 5-HT2 component in the pressor effect of 5-MeODMT. The maximal effect of 5-MeODMT was less marked than that of 5-HT. 8-OH-DPAT, RU 24969 and TFMPP were far less effective than 5-HT and 5-MeODMT to increase blood pressure. In contrast, 5-CT induced a vasodepressor effect. It is therefore suggested that the vasoconstriction induced by 5-HT and by 5-MeODMT in pithed rats could be due mainly to the selective stimulation of postjunctional 5-HT2 receptors because selective alpha 1- and alpha 2-adrenoceptor antagonists were ineffective against the vasoconstrictor effects of 5-HT and 5-MeODMT. The relative lack of effect of 8-OH-DPAT, RU 24969 and TFMPP to increase blood pressure suggested that postjunctional 5-HT1-like receptors play only a minor role - if any - in 5-HT induced vasoconstriction in the pithed rat.