Comparisons of hallucinogenic phenylisopropylamine binding affinities at cloned human 5-HT2A, 5-HT2B and 5-HT2C receptors

Since the classical hallucinogens were initially reported to produce their behavioral effects via a 5-HT₂ agonist mechanism (i.e., the 5-HT₂ hypothesis of hallucinogen action), 5-HT₂ receptors have been demonstrated to represent a family of receptors that consists of three distinct subpopulations: 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors. Today, there is greater support for 5-HT_2A than for 5-HT_2C receptor involvement in the behavioral effects evoked by these agents. However, with the recent discovery of 5-HT_2B receptors, a new question arises: do classical hallucinogens bind at 5-HT_2B receptors? In the present study we examined and compared the binding of 17 phenylisopropylamines at human 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors. Although there was a notable positive correlation (r>0.9) between the affinities of the agents at all three populations of 5-HT₂ receptors, structural modification resulted only in small differences in 5-HT_2B receptor affinity such that the range of affinities was only about 50-fold. As with 5-HT_2A and 5-HT_2C receptor affinity, there is a significant correlation (r>0.9, n=8) between 5-HT_2B receptor affinity and human hallucinogenic potency. Nevertheless, given that 5-HT_2A and 5-HT_2A/2C antagonists – antagonists with low affinity for 5-HT_2B receptors – have been previously shown to block the stimulus effects of phenylisopropylamine hallucinogens, it is likely that 5-HT_2A receptors play a more prominent role than 5-HT_2B and 5-HT_2C receptors in mediating such effects despite the affinity of these agents for all three 5-HT₂ receptor subpopulations.     

5-HT_(2B)受体阻断剂对心肌肥厚大鼠心肌5-HT含量及5-HT_(2B)受体表达的影响

目的观察5-HT2B受体阻断剂对去甲肾上腺素(norepinephrine,NE)诱导的心肌肥厚大鼠心肌中5-羟色胺(5-hydrotriptamine,5-HT)含量及5-HT2B受体表达的影响。方法雄性SD大鼠24只,随机分为3组,8只/组,分别为对照组、肥厚组、实验组。采用腹腔注射NE(1.5 mg/kg,2次/d,28 d)的方法建立心肌肥厚模型,自第15天起实验组腹腔注射SB204741(5-HT2B受体阻断剂;2 mg/kg,2次/d),连续注射14 d。对照组腹腔注射相同体积的生理盐水(2次/d,28 d)。检测各组左心室重量与体重之比(LVW/BW)、心肌组织中5-HT的含量及5-HT2B受体的表达情况。结果在NE诱导心肌肥厚过程中,应用SB204741干预可显著减轻心肌肥厚的程度,降低心肌组织中5-HT的含量,并抑制5-HT2B受体表达的上调。结论应用5-HT2B受体阻断剂可减轻NE诱导心肌肥厚的程度,降低心肌组织中5-HT的含量,并抑制5-HT2B受体表达的上调。     

Both 5-hydroxytryptamine 5-HT2A and 5-HT1B receptors are involved in the vasoconstrictor response to 5-HT in the human isolated internal thoracic artery

1. The 5-hydroxytryptamine (5-HT, serotonin) receptor subtypes that mediate vasoconstriction in the human internal thoracic artery (ITA), which is frequently used as an arterial graft, remain unclear. The aim of the present study was to elucidate the 5-HT receptor subtypes responsible for 5-HT-induced contraction of the human ITA. 2. The contractile responses to 5-HT of endothelium-denuded human ITA obtained from patients undergoing coronary bypass surgery were examined. In addition, we investigated the effects of sarpogrelate and SB224289, antagonists of 5-HT(2A) and 5-HT(1B) receptors, respectively, on the 5-HT-induced vasoconstriction. Finally, 5-HT(2A) and 5-HT(1B) receptors in the human ITA were immunolabelled. 3. 5-Hydroxytryptamine (1 nmol/L-10 micromol/L) caused vasoconstriction in a concentration-dependent manner. Both sarpogrelate (1 micromol/L) and SB224289 (1 micromol/L) significantly, but not completely, inhibited 5-HT-induced vasoconstriction. 4. Conversely, simultaneous pretreatment with supramaximum concentrations (1 micromol/L for both) of sarpogrelate and SB224289 almost completely inhibited the 5-HT-induced vasoconstriction. 5. Immunopositive staining for 5-HT(2A) and 5-HT(1B) receptors was detected in smooth muscle cells of the human ITA. 6. These results demonstrate that, in human ITA, 5-HT-induced vasoconstriction is mediated by activation of both 5-HT(2A) and 5-HT(1B) receptors. Thus, when the human ITA is used as an arterial graft, a combination of 5-HT(2A) and 5-HT(1B) receptor antagonists would appear to be most useful to prevent 5-HT-induced vasospasm.     

Characterization of an endothelial 5-hydroxytryptamine (5-HT) receptor mediating relaxation of the porcine coronary artery

Summary The pharmacological properties of the endothelial 5-hydroxytryptamine (5-HT) receptors involved in relaxation of vascular smooth muscle were determined in rings of pig coronary artery contracted with 10 nmol/1 of the thromboxane A₂ receptor agonist 9,11-dideoxy-11,9-epoxy-methano-prostaglandin F₂ (U 46619).(1) In the presence of 10 mol/l ketanserin, relaxation was obtained with: 5-HT (apparent pD₂ value 7.00), 5-carboxamidotryptamine (5-CONH₂-T; 6.42), 5-aminotryptamine (5-NH₂-T; 5.96), 5-methoxytryptamine (5-OCH₃-T; 5.92), tryptamine, 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo(1,2-a)quinoxaline maleate (CGS 12066 A) and 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole succinate (RU 24969). The maximum relaxation obtainable with the agonists was about 40–60% of the U 46619-induced contraction and the concentration-response curves for 5-HT, 5-NH₂-T and 5-OCH₃-T were bell-shaped. The endothelium-dependence of this effect (i. e. the failure to relax the artery in endothelium-denuded preparations) was demonstrated for 5-HT, 5-CONH₂-T, RU 24969, CGS 12066A and tryptamine.(2) 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 4-hydroxytryptamine, quipazine and yohimbine were ineffective in decreasing the tension of arteries with or without endothelium. Ipsapirone elicited full relaxation of U 46619-induced contraction, but this effect was not endothelium-dependent.(3) Metitepine (0.03-1 mol/l), 6-chloro-2-(1-piperazinyl)pyrazine (MK 212; 10 mol/l), methysergide (1 gmol/l) and cyanopindolol (0.1 mol/l) antagonized the relaxing effect of 5-HT in a non-surmountable manner, whereas metergoline (0.1 mol/l), quipazine (10 mol/l), yohimbine (1 mol/l), propranolol (1 mol/l) and (3-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930; 0.1 mol/l) did not. However, spiroxatrine (0.1 mol/l) and mesulergine (10 mol/l) enhanced the 5-HT-induced relaxation. The endothelium-dependent relaxation induced by 5-CONH₂-T was also inhibited by metitepine 1 gmol/l.(4) The 5-HT-induced relaxation was probably mediated by release of an endothelium-derived relaxing factor (EDRF). Gossypol, an inhibitor of EDRF, virtually abolished the 5-HT-induced relaxation while indometacin, an inhibitor of cyclooxygenase and accordingly of PGI₂ formation, did not.In conclusion, the failure of ketanserin and ICS 205–930 to counteract the relaxant effect of 5-HT receptor agonists excludes the involvement of 5-HT₂ and 5-HT₃ receptors, respectively, in the endothelium-dependent relaxation of the porcine coronary artery. The rather high potency of 5-CONH₂-T and the ability of certain 5-HT receptor antagonists, such as metitepine, methysergide and cyanopindolol, to counteract the effect of 5-HT are compatible with a 5-HT₁ character of the endothelial receptor. However, on the basis of the present data, no final classification, in particular with respect to the known 5-HT₁ receptor subtypes, is possible. Classification is also hampered by the bell-shaped character of the concentration-response curves for 5-HT receptor agonists and by their property to produce only partial relaxation. Send offprint requests to M. Gothert at the above address     

Neurochemical profile in vitro of irindalone: A5-HT2-receptor antagonist

A new potential antihypertensive drug, irindalone (Lu 21-098;(+)-(1R,3S)-1-[2-[4-[3-(p-fluorophenyl)-1-indanyl]-1-piperazinyl] -ethyl]-2-imidazolidinone), has been characterized by a series of in vitro methods. Irindalone competitively inhibits peripheral and central serotonin₂ (5-HT₂) receptors as measured by 5-HT-induced contractions of rabbit pulmonary artery and receptor binding methodology with ³H-ketanserin as a ligand. The inhibitory effect on 5-HT receptors is confirmed using other organs in vitro (rat stomach fundus, rat jugular vein, guinea pig trachea). Irindalone has lower affinity for α₁-adrenoceptors and histamine-H₁ receptors and only negligible affinity for α₂-adrenoceptors and muscarine cholinergic receptors. No inhibition of the uptake of noradrenaline (NA), dopamine (DA), or 5-HT is seen. The neurochemical profile resembles that of ketanserin, which has slightly higher affinity for 5-HT₂ receptors. In the rat fundus, irindalone inhibits 5-HT-induced contractions, differing in this respect from ketanserin and cinanserin. Irindalone clearly deviates from the neuroleptics haloperidol and tefludazine, and from prazosin with which irindalone has been compared. The (-)enantiomer of irindalone, Lu 21-099, is considerably weaker than irindalone in the different test systems. Due to the strong blockade of 5-HT₂ receptors and the somewhat weaker blockade of α-adrenoceptors, irindalone is expected to lower blood pressure potently and thus be a candidate for the treatment of hypertension.     

Interactions of GR127935, a 5-HT1B/D receptor ligand, with functional 5-HT receptors

GR127935 (N-[methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2’-methyl-4’-(5-methyl-1,2,4-oxadiazol-3-yl) [1, 1-biphenyl]-4-carboxamide hydrochloride) has been recently introduced as an experimental tool to antagonize 5-HT_1B/D receptor-mediated functional responses. The compound indeed exhibits a very high affinity and selectivity for 5-HT_1B/D binding sites and it antagonizes a number of 5-HT_1B/D receptor-mediated responses. The present experiments were performed to investigate the selectivity of GR127935 against functional responses mediated by 5-HT₁-like, ‘orphan’ 5-HT₁-like (5-ht₇?), 5-HT₂, 5-HT₃ or 5-HT₄ receptors in several invivo preparations. Intravenous (i.v.) treatment with GR127935 (300μg?kg^-1) potently antagonized decreases in total carotid blood flow as well as hypotensive responses induced by the 5-HT₁-like receptor agonist sumatriptan in rabbits. I.v. bolus injections of GR127935 (up to 500 and/or 1500μg?kg^-1) did not significantly modify 5-HT-induced: (i) tachycardia in the pig (5-HT₄ receptor-mediated) and cat (‘orphan’ 5-HT₁-like or, perhaps, 5-ht₇ receptor-mediated); (ii) depressor effects in the rat and cat (‘orphan’ 5-HT₁-like or 5-ht₇ receptor-mediated); (iii) vonBezold-Jarisch reflex in the rat or the early phase of the urinary bladder contraction in the cat (both 5-HT₃ receptor-mediated). In contrast, high doses (500-1500μg?kg^-1) of GR127935 suppressed 5-HT-induced pressor responses in the rat and cat and urinary bladder contractions (secondary phase) in the cat as well as the DOI ((±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride)-induced pressor responses in the rat, which are all mediated by 5-HT_2A receptors. In conclusion, the present study demonstrates that GR127935 is a selective 5-HT_1B/D receptor antagonist devoid of interactions at ‘orphan’ 5-HT₁-like (5-ht₇?), 5-HT₃ and 5-HT₄ receptors. However, GR127935 possesses a moderate 5-HT_2A receptor blocking property, which is consistent with its binding profile (pK_i: 7.4). Lastly, in view of the potent antagonist action of GR127935, the sumatriptan-induced hypotension in rabbits seems to be mediated by 5-HT_1B/D receptors.     

Potent 5-hydroxytryptamine3 receptor antagonist activity of RG 12915

RG 12915, N-(1-azabicyclo[2.2.2.]octan-3(S)-yl)-2-chloro-(5aS, 9aS)-5a,6,7,8,9,9a-hexahydrodibenzofuran-4-carboxamide, a potent antiemetic agent active in reducing emesis produced by antieoplastic agents, was examined in tests of 5-hydroxytryptamine₃ (5-HT₃) receptor blockade. RG 12915 was found to be a potent antagonist of 5-HT₃ receptor activation both in blocking 5-HT-induced contractions of guinea-pig ileum and in blocking the Bezold-Jarisch effect in the rat. Compared to several other 5-HT₃ antagonists, RG 12915 had a greater pA2 value (11.2) for blocking 5-HT-induced contractions of guinea pig ileum than zacopride (8.3); BRL 43694 (granisetron) (9.1); and GR 38032F (ondansetron) (7.4). Falls in heart rate due to 5-HT₃ receptor activation following intravenous (i.v.) administration of 5-HT (the Bezold-Jarisch effect) were also potently reduced by RG 12915. Minimum effective dose (MED) levels (in parentheses), defined as the lowest dose at which each compound produced a significant reduction in the Bezold-Jarisch effect, were determined for RG 12915 (1.0μg/kg, i.v.); zacopride (3.0); granisetron (9.0); and ondansetron (27.0). The order of potency in blocking 5-HT₃ receptor activation was generally the same as the order of potency in 5-HT₃ receptor binding. RG 12915 had a lower K_i value (0.17±0.02 nM, mean ± SEM) in binding studies using ³H-GR-65630 as the ligand in rat entorhinal cortex tissue than either zacopride (1.5±0.4); granisetron (1.7±0.3); or ondansetron (6.2±2.1). RG 12915 was also found active in blocking contractions of guinea pig ileum and the Bezold-Jarisch effect induced by the somewhat selective 5-HT₃ receptor agonist 2-methyl-secrotonin. The results support the idea that this orally active antiemetic agent is a potent antagonist of 5HT₃ receptor activation. © 1993 Wiley-Liss, Inc.     

Inhibition of the 5-HT-induced cardiogenic hypertensive chemoreflex by the selective 5-HT3 receptor antagonist ICS 205-930

Summary We investigated the effect of ICS 205-930 [(3-tropanyl)-1H-indole-3-carboxylic acid ester], a selective antagonist at 5-HT₃ receptors, on the cardiogenic hypertensive chemoreflex in the anaesthetized dog. The reflex was elicited by injection of 5-HT (12.5–1600 g) into the left cardiac ventricle and consisted of a dose-dependent systemic hypertension associated with a decrease in heart rate. ICS 205-930 (10, 30, and 100 g/kg i.v.) caused a displacement to the right of both the dose-response curves of 5-HT-induced blood pressure increase and heart rate reduction. Its blocking effects upon the action of 5-HT could be surmounted by increasing the dose of the agonist. The selective 5-HT₂ receptor antagonist, ketanserin (0.1 mg/kg i.v.) and the combined 5-HT₁ and 5-HT₂ receptor antagonist, methiothepin (0.1 mg/kg i.v.) had no influence on the hypertensive reflex. When the reflex was elicited by the ganglionic stimulant, 1,1-dimethyl-4-phenyl-piperazinium (DMPP; 100–1600 g), ICS 205-930 had no blocking effect. The results suggest that the 5-HT-induced cardiogenic hypertensive chemoreflex is mediated by 5-HT₃ receptors. Send offprint requests to H. Berthold at the above address     

5-HT_(2B)受体阻断剂在去甲肾上腺素诱导大鼠心肌肥厚中的作用

目的观察应用5-HT2B受体阻断剂对去甲肾上腺素(NE)诱导的心肌肥厚的影响,并初步分析5-HT2B受体在NE诱导心肌肥厚中的作用及机制。方法雄性SD大鼠46只随机分为试验组(n=32)及对照组(n=14),试验组采用腹腔注射NE(1.5 mg/kg,2次/d,28 d)的方法建立心肌肥厚模型,自第15天起按分组分别注射SB204741(5-HT2B受体阻断剂;0.5 mg/kg,n=8;2 mg/kg,n=8)或SDZ SER 082(另一种5-HT2B受体阻断剂;1 mg/kg,n=8)14 d,对照组分别注射生理盐水(n=8)或SB204741(2 mg/kg,n=8)。检测心肌肥厚指数包括左心室重量与体重之比(LVW/BW)和左心室游离壁厚度与体重之比(LVWT/BW),以及心肌组织中5-HT含量。结果 NE诱导心肌肥厚过程中注射SB204741可显著减轻心肌肥厚的程度,并剂量依赖地降低肥厚心肌组织中5-HT含量;而单独给予SB204741处理对心肌肥厚指数和心肌组织中5-HT含量没有影响。SDZ SER 082可产生与SB204741相同的作用效果。结论 5-HT2B受体参与NE诱导的心肌肥厚的进展。     

3H]A-804598 ([3H]2-cyano-1-[(1S)-1-phenylethyl]-3-quinolin-5-ylguanidine) is a novel, potent, and selective antagonist radioligand for P2X7 receptors

ATP-sensitive P2X7 receptors are localized on cells of immunological origin including peripheral macrophages and glial cells in the CNS. Activation of P2X7 receptors leads to rapid changes in intracellular calcium concentrations, release of the pro-inflammatory cytokine IL-1β, and following prolonged agonist exposure, the formation of cytolytic pores in plasma membranes. Data from gene knockout studies and recently described selective antagonists indicate a role for P2X7 receptor activation in inflammation and pain. While several species selective P2X7 antagonists exist, A-804598 represents a structurally novel, competitive, and selective antagonist that has equivalent high affinity at rat (IC₅₀ = 10 nM), mouse (IC₅₀ = 9 nM) and human (IC₅₀ = 11 nM) P2X7 receptors. A-804598 also potently blocked agonist stimulated release of IL-1β and Yo-Pro uptake from differentiated THP-1 cells that natively express human P2X7 receptors. A-804598 was tritiated ([³H]A-804598; 8.1 Ci/mmol) and utilized to study recombinant rat P2X7 receptors expressed in 1321N1 cells. [³H]A-804598 labeled a single class of high affinity binding sites (K_d = 2.4 nM and apparent B_max = 0.56 pmol/mg). No specific binding was observed in untransfected 1321N1 cells. The pharmacological profile for P2X antagonists to inhibit [³H]A-804598 binding correlated with their ability to block functional activation of P2X7 receptors (r = 0.95, P < 0.05). These data demonstrate that A-804598 is one of the most potent and selective antagonists for mammalian P2X7 receptors described to date and [³H]A-804598 is a high affinity antagonist radioligand that specifically labels rat P2X7 receptors.     

5-HT_(2B)受体阻断剂对去甲肾上腺素诱导的肥厚心肌组织中心肌细胞凋亡的影响

目的观察5-HT2B受体阻断剂对去甲肾上腺素(NE)诱导的肥厚心肌组织中心肌细胞凋亡的影响。方法雄性SD大鼠24只,随机分为3组,每组8只,分别为对照组,肥厚组、实验组。采用腹腔注射NE(1.5 mg/kg,2次/d,28 d)的方法建立心肌肥厚动物模型,自第15天起实验组腹腔注射SB204741(5-HT2B受体阻断剂)2 mg/kg,2次/d,连续注射14 d。对照组腹腔注射相同体积的生理盐水2次/d,28 d。采用TUNEL法检测各组心肌组织中心肌细胞的凋亡情况,Western blot方法分析Bcl-2、Bax、Caspase-3的表达情况。结果对照组的心肌组织中几乎看不到凋亡的心肌细胞,肥厚组心肌组织中凋亡心肌细胞数量显著增多,同时Bcl-2/Bax表达比例显著降低,Caspase-3的表达显著升高;与肥厚组相比,实验组心肌组织中凋亡心肌细胞的数量显著减少,Bcl-2/Bax表达比例显著升高,Caspase-3的表达显著降低。结论应用5-HT2B受体阻断剂不仅可减轻NE诱导的心肌肥厚的程度,还可通过上调Bcl-2/Bax表达比率、降低Caspase-3的活性抑制心肌细胞的凋亡。     

Synthesis and 5-HT2A radioligand receptor binding assays of DOMCl and DOMOM,two novel 5-HT2A receptor ligands

A synthesis of two new active substances, DOMCl (1-(4-chloromethyl-2, 5-dimethoxyphenyl)-2-propanamine; 2) and DOMOM (1-(2, 5-dimethoxy-4-methoxymethylphenyl)-2-propanamine; 3), was developed. Unexpectedly, the Blanc reaction permitted successful synthesis of 2, 5-dimethoxyphenylpropylamine derivatives having a substituted methyl group in position 4 since solvation of the reactant occurs during the reaction. Afterwards, their affinities towards the 5-HT(2A) receptor were examined in 5-HT(2A) radioligand receptor binding assays. The study of these substances is of considerable interest because they were predicted, by preliminary molecular modeling studies based on mescalin units, to be potential new hallucinogens that should be added to the list of substances prohibited by law. It was assumed that DOMCl would be 82 times more potent as a hallucinogen than mescalin, and DOMOM would be 94 times more potent. The 5-HT(2A) radioligand receptor binding studies showed that the affinities of DOMCl and DOMOM for the 5-HT(2A) receptor are less than expected but are nevertheless 1.6 and 8.7 times higher, respectively, than that of mescalin. Therefore, scheduling these substances as potential drugs of abuse might be considered.     

A Search for New 5-HT1A/5-HT2A Receptor Ligands. In Vitro and in vivo Studies of 1-[ω-(4-Aryl-1-piperazinyl)alkyl]indolin-2(1H)-ones

A series of 1-[ω-(4-aryl-1-piperazinyl)alkyl]indolin-2(1H)-one derivatives 2–14 was synthesized in order to obtain ligands with a dual 5-HT_1A/5-HT_2A activity. The majority of those compounds ( 2–5, 7, 10–13 ) exhibited a high 5-HT_1A (K_i = 2 – 44 nM) and/or 5-HT_2A affinity (K_i = 51 and 39 for 5 and 7 , respectively). Induction of lower lip retraction (LLR) and behavioral syndrome and inhibition of these efects evoked by 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) were used for determination the agonistic and antagonistic activity, respectively, at 5-HT_1A receptors. The 5-HT_2A antagonistic activity was assessed by the blocking effect on the head twitches induced by (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in mice. Two of the tested compounds, 1-{3-[4-(3-chlorophenyl)-1-piperazinyl]propyl}-6-fluoroindolin-2(1H)-one ( 5 ) and 1-{3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl}indolin-2(1H)-one ( 7 ), demonstrated a high 5-HT_1A/5-HT_2A affinity and an in vivo antagonistic activity towards both receptor subtypes.     

Synthesis of [3H]fenobam,a radioligand for the mGlu5 receptor

Fenobam is a clinically efficacious anxiolytic that acts as metabotropic glutamate receptor 5 (mGlu5) antagonist by binding to an allosteric site. Other mGlu5 receptor antagonists such as MPEP and MTEP bind to the same allosteric site and are efficacious in preclinical models of anxiety and depression. Consequently, the allosteric‐binding site of the mGlu5 receptor is an attractive target for the discovery of novel psychiatric therapies. Radioligands of this binding site can be used for in vitro and in vivo pharmacodynamic studies. We report here a short synthesis of such a radioligand for the allosteric mGlu5 receptor‐binding site, [³H]fenobam. Copyright © 2008 John Wiley & Sons, Ltd.     

3H-TVX Q 7821: identification of 5-HT1 binding sites as target for a novel putative anxiolytic

Summary In order to clarify the mechanism of action of the putative nonbenzodiazepine anxiolytic TVX Q 7821 [2-(4-(4-(2-pyrimidinyl)-1-piperazinyl)butyl)-1,2-benzisothiazol-3-(2H)one-1,1-dioxidehydrochloride], binding studies with the radio labelled compound were performed.³H-TVX Q 7821 bound apidly, reversibly and in a saturable manner with high affinity to calf brain structures with preference for the hippocampus (K _D 1.62 nmol/l;B _max 320 fmol/mg protein).³H-TVX Q 7821 binding was displaced only by 5-hydroxytryptamine and its agonists and antagonists including spiperone, but was not displaced by a variety of other neurotransmitters and drugs. The 5-HT₂ receptor antagonist ketanserin was a weak displacer. The hippocampal binding sites for³H-TVX Q 7821 were pharmacologically very similar to the 5-HT₁-binding sites in this region. TVX Q 7821 is likely to be an important tool in research on functional aspects of 5-HT₁ binding sites.     

Pharmacological studies in vivo with ICI 169,369, a chemically novel 5-HT2/5-HT1C receptor antagonist

ICI 169,369 (2-(2-dimethylaminoethylthio-3-phenylquinoline hydrochloride) has been tested in vivo for its potency and selectivity as an antagonist at 5-HT₂ and 5-HT_1C receptors. It caused a 50% inhibition of 5-HTP-induced head twitches in mice and fenfluramine-induced hyperthermia in the rat at approximately 1 mg/kg following parenteral administration. Results showed that ICI 169,369 had good oral bioavailability, since in the fenfluramine test the oral and s.c. ID₅₀ values were similar. ICI 169,369 was a selective antagonist of 5-HT-induced bronhoconstriction in the guinea-pig and 5-HT-induced pressor effects in the anaesthetised dog. In a series of other test in vivo the compound was shown to be devoid of significant activity at ₂- and ₂-adrenoceptors, dopamine (D₂), muscarinic (M₁) and histamine (H₁) receptors at 30–100 times its ID₅₀ values used in the 5-HT tests. Thus, ICI 169, 369 is a selective, orally active 5-HT₂/5-HT_1C antagonist that should prove useful in the analysis of the role of 5-HT in physiological and pathological states.     

SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'methyl-4'-(5-methyl-1,2,3-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): a novel, potent and selective 5-HT1B receptor antagonist

SB-616234-A possesses high affinity for human 5-HT_1B receptors stably expressed in Chinese hamster ovary (CHO) cells (pK_i 8.3 ± 0.2), and is over 100-fold selective for a range of molecular targets except h5-HT_1D receptors (pK_i 6.6 ± 0.1). Similarly, affinity (pK_i) for rat and guinea pig striatal 5-HT_1B receptors is 9.2 ± 0.1. In [³⁵S]-GTPγS binding studies in the human recombinant cell line, SB-616234-A acted as a high affinity antagonist with a pA₂ value of 8.6 ± 0.2 whilst providing no evidence of agonist activity in this system. In [³⁵S]-GTPγS binding studies in rat striatal membranes, SB-616234-A acted as a high affinity antagonist with an apparent pK_B of 8.4 ± 0.5, again whilst providing no evidence of agonist activity in this system. SB-616234-A (1 μM) potentiated electrically stimulated [³H]-5-HT release from guinea pig and rat cortical slices (S₂/S₁ ratios of 1.8 and 1.6, respectively). SB-616234-A (0.3–30 mg kg⁻¹ p.o.) caused a dose-dependent inhibition of ex vivo [³H]-GR125743 binding to rat striatal 5-HT_1B receptors with an ED₅₀ of 2.83 ± 0.39 mg kg⁻¹ p.o. Taken together these data suggest that SB-616234-A is a potent and selective 5-HT_1B autoreceptor antagonist that occupies central 5-HT_1B receptors in vivo following oral administration.     

5-HT1B receptors modulate release of [3H]dopamine from rat striatal synaptosomes

The effect of the selective r5-HT_1B agonist 3-(1,2,5,6-tetrahydro)-4-pyridil-5-pyrrolo [3,2-b] pyril-5-one (CP93,129) on the K⁺-evoked overflow of [³H]dopamine was studied in rat striatal synaptosomes loaded with [³H]dopamine. The aim of the study was to investigate the participation of 5-HT_1B receptors in the serotonergic modulation of striatal dopaminergic transmission. The Ca²⁺-dependent, tetrodotoxin-resistant K⁺-evoked overflow of [³H]dopamine was inhibited by CP93,129 (0.01–100 μM) in a concentration-dependent manner (IC₅₀=1.8 μM; maximal inhibition by 35.5% of control). [±]8-OH-DPAT, a 5-HT_1A receptor agonist, [+/–]DOI, a 5-HT₂ receptor agonist, and 2-methyl-5-hydroxytryptamine, a 5-HT₃ receptor agonist, at concentrations ranging from 0.01 μM to 100 μM did not show any significant effect. Neither ketanserin (1 μM and 5 μM), a selective 5-HT₂/5-HT_1D receptor antagonist, nor ondansetron (1 μM), a 5-HT₃ receptor antagonist, changed the inhibitory effect of CP93,129. SB224289, GR55562, GR127935, isamoltane and metergoline, selective and non-selective 5-HT_1B receptor antagonists, in contrast, used at a concentration of 1 μM, antagonized the inhibitory effect of CP93,129 (3 μM and 10 μM). SB224289, a selective 5-HT_1B receptor antagonist, inhibited the effect of CP93,129 in a concentration-dependent manner; the calculated K _i value was 1.8 nM. Our results indicate that in rat striatal axon terminals the K⁺-evoked release of dopamine is regulated by the presynaptic 5-HT_1B heteroreceptors. Received: 7 September 1998 / Accepted: 2 November 1998