Receptor Antagonist SB Research Articles

ROLE OF NEUROKININ RECEPTORS IN THE BEHAVIORAL EFFECT OF INTRAVESICAL ANTIGEN INFUSION IN GUINEA PIG BLADDER

To characterize a guinea pig behavior model of bladder pain due to intravesical antigen infusion and to determine the role of neurokinin receptor subtypes in mediating this behavior. The influence of subtype-selective neurokinin receptor antagonists on increased abdominal licking behavior in response to intravesical antigen infusion in guinea pigs immunized with ovalbumin (OA) was determined. Intravesical OA infusion for 30 minutes induced a significantly greater frequency (about 3-fold) of abdominal licking behavior than during either the 30 minutes pre-challenge or post challenge saline infusions. Treatment with IP capsaicin 7 to 10 days before OA challenge abolished the intravesical antigen-induced behavior. IP injection of the NK1 receptor antagonist CP 99994 (10 mg./kg. or 30 mg./kg.), 30 minutes pretreatment, inhibited the increase in the average number of abdominal licks during antigen infusion. The 30 mg./kg., but not the 10 mg./kg. dose increased the percent of animals showing antinociceptive activity (defined as 4 or less abdominal licks during the antigen infusion). The NK2 receptor antagonist SR 48968 reduced the antigen-induced abdominal licking behavior at IP doses of 3 and 10 mg./kg. but was ineffective at 1 mg./kg. The NK3 receptor antagonist SB 235375 (30 mg./kg., IP) did not reduce this behavior. These results suggest a role for activation of NK1 and NK2, but not NK3 receptors, by tachykinins released from capsaicin-sensitive nerves, in the increased abdominal licking behavior response of guinea pigs to intravesical antigen infusion.

Further evidence that 5-HT-induced relaxation of pig pulmonary artery is mediated by endothelial 5-HT(2B) receptors.

The endothelial 5-HT receptor mediating relaxation of pig pulmonary artery has been characterized using the selective 5-HT(2B) receptor agonist BW 723C86 and a variety of structurally diverse 5-HT receptor antagonists. If arterial rings with intact endothelium were precontracted with prostaglandin F(2alpha) (3 microM), BW 723C86 caused concentration-dependent relaxation with a pEC(50)=8.21+/-0.03 and E(max)=89+/-4% relative to 5-HT. The relaxant responses to BW 723C86 were inhibited by the 5-HT(2B) receptor antagonist SB 204741, the 5-HT(2B/2C) receptor antagonist SB 206553 and the antimigraine drug pizotifen, yielding pA(2) values of 6.68, 7.20 and 8.32, respectively. The pA(2) values against BW 723C86 were similar to those determined against 5-HT. The relaxant effect of 5-HT was antagonized by a variety of 22 compounds of diverse chemical structures. Based on the calculated mean pA(2) values the order of the most potent antagonists was ritanserin (9.38) > methysergide (8. 86) > pizotifen (8.47) >/= methiothepin (8.32) > LY 53857 (7.84) >/= amoxapine (7.80) >/= loxapine (7.73) >/= metergoline (7.64) >/= mianserin (7.51) >/= rauwolscine (7.39). Compounds with weak blocking potency were yohimbine (6.37), spiperone (5.88) and ketanserin (5.85). Correlation analysis between the affinities of the antagonists in pig pulmonary artery and those from radioligand binding studies at human and rat 5-HT(2B) receptors showed a highly significant correlation (r=0.95 and 0.84, P<0.002 and <0.005). Correlation with 5-HT(2C) receptors was much lower (r=0.57, P=0.035), and no correlations were obtained with 5-ht(6) and 5-HT(7) receptors. It is concluded that the 5-HT receptor mediating endothelium-dependent relaxation of pig pulmonary artery is of the 5-HT(2B) subtype.

Leukotriene receptor antagonists and synthesis inhibitors reverse survival in eosinophils of asthmatic individuals.

Eosinophilia is a feature of airway inflammation associated with asthma. Leukotriene antagonists provide therapeutic benefit in asthma, but their potential antiinflammatory actions have not been fully explored. We have examined the role of eosinophil-derived cysteinyl leukotrienes in the maintenance of eosinophil survival, and the involvement of leukotrienes in the paracrine stimulation of eosinophil survival by mast cells and lymphocytes. We obtained eosinophils and autologous lymphocytes from peripheral blood of asthmatic subjects. Leukotriene (LT)-B(4), LTC(4) and LTD(4), granulocyte-macrophage colony-stimulating factor (GM-CSF), and fibronectin promoted eosinophil survival. LTD(4) (10(-)(6) M) was as effective as GM-CSF (5 ng/ml) and fibronectin (400 ng/ml) in promoting survival. Lymphocytes and conditioned medium from a human mast cell line (HMC-1) induced eosinophil survival. Blockade of cysteinyl leukotriene receptors with SKF 104353 (pobilukast, 3 nM), and inhibition of 5-lipoxygenase (5-LO) with BW A4C (1 microM) and of 5-LO activating protein with MK 886 (1 microM), all increased basal rates of eosinophil apoptosis and reversed GM-CSF-induced eosinophil survival. Fifty percent reversal of GM-CSF- induced survival was achieved with SKF 104353 at 0.3 nM. The potency of SKF 104353 was two orders of magnitude greater than that of the LTB(4) receptor antagonist SB 201146. Mast cell- and lymphocyte-induced eosinophil survival were completely reversed by SB 201146, SKF 104353, BW A4C, and MK 886. These findings provide evidence for the involvement of an autocrine cysteinyl leukotriene pathway that supports eosinophil survival in response to a range of survival stimuli. They also suggest that LTB(4) could act as a paracrine stimulus of eosinophil survival.

In vivo effects of the 5-HT(6) antagonist SB-271046 on striatal and frontal cortex extracellular concentrations of noradrenaline, dopamine, 5-HT, glutamate and aspartate.

Although the 5-HT(6) receptor subtype was identified some 5 years ago, very little is known about its function within the brain. Here we demonstrate, for the first time, the neurochemical effects of a selective 5-HT(6) receptor ligand. Using in vivo microdialysis in the freely moving rat, we evaluated the effects of the selective 5-HT(6) receptor antagonist SB-271046 by simultaneous measurement of 5-hydroxytryptamine (5-HT), dopamine (DA), noradrenaline (NA), glutamate and aspartate from the striatum and frontal cortex. SB-271046 did not alter basal levels of 5-HT, DA and NA in either brain region. Similarly, there was no change basal levels of either of the excitatory amino acids within the striatum. In contrast, administration of SB-271046 (10 mg kg(-1) s.c.) produced a significant (P<0.05), tetrodotoxin-dependent, increase in extracellular levels of both glutamate and aspartate within the frontal cortex, reaching maximum values of 375.4+/-82.3 and 215. 3+/-62.1% of preinjection values, respectively.

Role of uptake inhibition and autoreceptor activation in the control of 5-HT release in the frontal cortex and dorsal hippocampus of the rat.

1. Using brain microdialysis, we compared the relative role of 5-hydroxytryptamine (5-HT; serotonin) blockade and somatodendritic 5-HT(1A) and/or terminal 5-HT(1B) autoreceptor activation in the control of 5-HT output. 2. Fluoxetine (10 mg kg(-1) i.p.) doubled the 5-HT output in frontal cortex and dorsal hippocampus. The 5-HT(1A) receptor antagonist WAY 100635, (0.3 mg kg(-1) s.c.) potentiated the effect of fluoxetine only in frontal cortex (to approximately 500 % of baseline). 3. Methiothepin (10 mg kg(-1) s.c.) further enhanced the 5-HT rise induced by fluoxetine+WAY 100635, to 835+/-179% in frontal cortex and 456+/-24% in dorsal hippocampus. Locally applied, methiothepin potentiated the fluoxetine-induced 5-HT rise more in the former area. 4. The selective 5-HT(1B) receptor antagonist SB-224289 (4 mg kg(-1) i.p.) enhanced the effect of fluoxetine (10 mg kg(-1) i.p.) in both areas. As with methiothepin, SB-224289 (4 mg kg(-1) i.p.) further enhanced the 5-HT increase produced by fluoxetine+WAY 100635 more in frontal cortex (613+/-134%) than in dorsal hippocampus (353+/-59%). 5. Locally applied, fluoxetine (10 - 300 microM; EC(50)=28 - 29 microM) and citalopram (1 - 30 microM; EC(50)=1.0 - 1.4 microM) increased the 5-HT output two to three times more in frontal cortex than in dorsal hippocampus. These data suggest that the comparable 5-HT increase produced by systemic fluoxetine in frontal cortex and dorsal hippocampus results from a greater effect of reuptake blockade in frontal cortex that is offset by a greater autoreceptor-mediated inhibition of 5-HT release. As a result, 5-HT autoreceptor antagonists preferentially potentiate the effect of fluoxetine in frontal cortex.

Biochemical and electrophysiological evidence that RO 60-0175 inhibits mesolimbic dopaminergic function through serotonin 2C receptors

In vivo microdialysis and electrophysiological techniques were used to elucidate the role of the 5-HT 2 receptor family on the control of mesolimbic dopaminergic system exerted by serotonin (5-HT). Administration of RO 60-0175 (1 mg/kg, i.p.), a selective 5-HT 2C receptor agonist, significantly decreased dopamine (DA) release by 26±4% (below baseline) 60 min after injection. Moreover, RO 60-0175 (80–320 μg/kg, i.v.) dose-dependently decreased the basal firing rate of DA neurons in the ventral tegmental area (VTA), reaching its maximal inhibitory effect (53.9±15%, below baseline) after the dose of 320 μg/kg. The selective 5-HT 2C receptor antagonist SB 242084 completely blocked the inhibitory action of RO 60-0175 on accumbal DA release and on the firing rate of VTA DA cells. On the contrary, both (±)-DOI, a mixed 5-HT 2A/2C receptor agonist, and the selective 5-HT 2B agonist BW 723C86, did not affect either DA release in the nucleus accumbens or the firing rate of VTA DA cells. Taken together, these data confirm that central 5-HT system exerts an inhibitory control on the mesolimbic DA system and that 5-HT 2C receptors are involved in this effect.

The role of 5-hydroxytryptamine 3 and 5-hydroxytryptamine 4 receptors in the regulation of gut motility in the ferret

The role of 5-hydroxytryptamine (5-HT) 3 and 5-HT 4 receptors in the regulation of gut motility in the ferret was investigated. The selective 5-HT 3 receptor antagonist ramosetron (1 – 10 μg kg s.c.) prolonged the interval of gastric antral migrating motor complex, but had only slight effect on small intestinal and colonic motility in unfed animals. The selective 5-HT 4 receptor antagonist SB 204070 did not affect motility throughout gut in unfed animals. Neither ramosetron nor SB 204070 affected the motility throughout gut in fed animals. In conclusion, neither 5-HT 3 nor 5-HT 4 receptors tonically regulate ferret gut motility except that S-HT 3 receptors have a key role in the occurrence of migrating motor complex specifically in the stomach. The role of 5-HT 3 and 5-HT 4 receptor system in the regulation of gut motility in ferrets is similar to that in other mammalian species studied, including humans. This similarity suggests that the ferret is a suitable model animal to study gut motor functions in humans.

Evidence for accelerated desensitisation of 5-HT 2C receptors following combined treatment with fluoxetine and the 5-HT 1A receptor antagonist, WAY 100,635, in the rat

Both pre-clinical and clinical studies suggest that additional treatment with 5-HT 1A receptor antagonists may accelerate the antidepressant efficacy/onset of selective serotonin re-uptake inhibitors (SSRIs). Given that chronic SSRI treatment has been shown to desensitise 5-HT 2C receptor mediated responses, we have used the rat social interaction test to determine if combined treatment with WAY 100,635, a selective 5-HT 1A receptor antagonist, will accelerate this effect. In pairs of unfamiliar rats, acute administration of the 5-HT 2C receptor agonist m-chlorophenylpiperazine (mCPP) or fluoxetine decreased the time spent in social interaction, responses which were reversed by the 5-HT 2C/2B receptor antagonists SB 200646A and SB 221284. Similar reductions in social interaction were observed in rats treated with fluoxetine (10 mg/kg, i.p. daily) for 4, 7 and 14 days but was no longer apparent after 28 days of treatment. In contrast, only 7 days of combined treatment with WAY 100,635 (1 mg/kg/s.c./day) and fluoxetine were needed to reverse this response. The decrease in social interaction induced by an acute challenge of mCPP (1 mg/kg, i.p.) was also reduced after 6 days co-treatment with WAY 100,635 and fluoxetine. Thus, WAY 100,635 accelerates SSRI-induced desensitisation of 5-HT 2C receptors, suggesting that this response might contribute towards the therapeutic effects of SSRIs in man.

Hypotensive effect of endothelin-1 via endothelin-B-receptor pathway on pulmonary circulation is enhanced in rats with pulmonary hypertension.

The pharmacological characterization of endothelin-1 (ET-1) in the pulmonary circulation in pulmonary hypertension (PH) is not known precisely. We investigated the effect of intravenous injection of ET-1 (1000 pmol/kg) on right ventricular systolic pressure (RVSP) (which is equal to systolic pulmonary arterial pressure) in rats with monocrotaline-induced PH. ET-1 decreased RVSP in PH rats; however, ET-1 did not alter RVSP in control rats, suggesting that ET-1 causes dilatation of the pulmonary artery in PH rats. Under pretreatment with the endothelin-A- (ET(A)) receptor antagonist BMS 193884, ET-1 decreased RVSP in PH rats more than in control rats, suggesting that pulmonary vasodilator action of ET-I mediated via the ET(B)-receptor pathway is augmented in PH rats. Under pretreatment with the ET(A/B)-receptor antagonist SB 209670, the effect of ET-1 in lowering pulmonary arterial pressure was abolished in both groups of rats. These results suggest that the hypotensive effect of ET-1 on pulmonary circulation mediated via the ET(B)-receptor pathway is enhanced in PH rats compared with control normal rats. It is considered that the blockade of only the ET(A)-receptor pathway is preferable to the blockade of both the ET(A)- and ET(B)-receptor pathways in the treatment of PH.

NK 3 receptor blockade prevents hyperalgesia and the associated spinal cord substance P release in monoarthritic rats

Previous studies in vitro have shown that NK 3 receptors exist on primary afferent terminals in rat spinal cord and mediate potentiation of the depolarisation-evoked substance P (SP) release. In the present study we have investigated the role of the NK 3 receptor-mediated SP release system in a model of inflammatory pain. Monoarthritis was induced in rats by unilateral injection of complete Freund's adjuvant (CFA); withdrawal latencies to a thermal stimulus were subsequently measured at various times following CFA. The CFA-treated paw displayed hyperalgesia as early as 4 h after CFA injection and hyperalgesia was maintained until day 4 but had disappeared by day 21. The thermal hyperalgesia was associated with an increase in basal SP release from spinal cord synaptosomes. The possible involvement of endogenous neurokinin B acting at NK 3 receptors was tested by using SB 223412-A [(S)-(−)- N-(α-ethylbenzyl)-3-hydroxy-2-phenylquinoline-4-carboxamide hydrochloride], a novel, potent ( K i=30 nM) and selective ( K i>10 000 nM for NK 1 and NK 2 receptors), non-peptidic NK 3 receptor antagonist. In vitro SB 223412-A antagonised the potentiation of SP release produced by senktide in spinal cord synaptosomes. Administered systemically to monoarthritic rats (50 mg/kg, p.o., b.i.d., for 4 days), the NK 3 receptor antagonist SB 223412-A significantly reduced thermal hyperalgesia and normalised the basal release of SP from spinal cord synaptosomes. The data suggest that neurokinin B acting at NK 3 receptors that mediate SP release within the spinal cord play a role in inflammation. These NK 3 receptors may represent, therefore, appropriate targets in the therapy of inflammatory pain.

5-Hydroxytryptamine- and U46619-mediated vasoconstriction in bovine pulmonary conventional and supernumerary arteries: effect of endogenous nitric oxide

We compared 5-hydroxytryptamine (5-HT)- and U46619-mediated contractions in bovine pulmonary conventional arteries (CA) and supernumerary arteries (SA). The effects of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) (100 microM) and the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ) (10 microM) on the responses of CA and SA to 5-HT and U46619 were also examined. In addition, the effects of the 5-HT(2B) receptor antagonist SB 200646 (1 nM-1 microM) on the responses to 5-HT in SA and CA were studied. Tissue cGMP levels were measured in the absence and presence of L-NAME, ODQ, 5-HT and U46619. 5-HT was approximately 30 times more potent in SA ¿-log[EC(50) (M)] (pEC(50)) 6.32+/-0.13¿ than in CA (5.05+/-0.14). U46619 displayed a similar potency in both CA (pEC(50) 7.80+/-0.07) and SA (7.75+/-0. 12). L-NAME did not significantly alter the resting tone of CA or SA. In contrast, ODQ produced a transient increase in the tone of both CA and SA. Neither L-NAME nor ODQ altered the responses to 5-HT or U46619 in CA. In addition, neither L-NAME nor ODQ altered the responses to U46619 in SA, but both L-NAME and ODQ increased the magnitude of the response to 5-HT in SA without changing the sensitivity. Inhibition of the 5-HT(2B) receptor with SB 200646 did not alter the response to 5-HT in SA or CA. Basal levels of cGMP (pmol/mg of protein) were similar in CA (1.16+/-0.33) and SA (0. 8+/-0.51), and were not significantly changed in the presence of 5-HT or U46619. L-NAME and ODQ reduced the basal levels of cGMP in both SA and CA. The results suggest that endogenous NO selectively attenuates the vasoconstrictor response to 5-HT in SA, but not in CA. These results also suggest that the NO/cGMP pathway may have a role in maintaining low vascular tone, but that other mechanisms are able to compensate for the absence of this pathway.

A highly enantioselective conjugate reduction of 3-arylinden-1-ones using bakers' yeast for the preparation of (S)-3-arylindan-1-ones.

[formula: see text] The bakers' yeast reduction of 3-(1,3-benzodioxol-5-yl)-6-propoxy-1H-inden-1-one 4 has been shown to give (S)-3-(1,3-benzodioxol-5-yl)-2,3-dihydro-6-propoxy-1H-indan-1-one 6 in 65% yield with high enantioselectivity (> 99.0% ee), a key intermediate for the synthesis of the endothelin receptor antagonist SB 217242. In addition, the substituted 3-arylinden-1-ones 10a-e gave equally high enantioselectivity for the 3-arylindan-1-one products 13a-e. Mechanistic studies of the reaction indicate the operative pathway to be an asymmetric conjugate reduction, wherein the hydride transfer from NAD(P)H occurs from the Re-face of the indenone substrate.

Action of the selective 5-HT4 receptor antagonist SB 204070A in the rat kindling model of epilepsy

Although there has been increasing evidence that the serotonin (5-HT) system plays an important role in regulating clinical and experimentally-induced seizures, no studies have examined the direct action of agents modulating 5-HT4 receptors in seizure models. The present study assessed the effects of the potent and selection 5-HT4 receptor antagonist, (1-butyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate hydrochloride (SB 204070A), on kindled seizures and on the kindling development originating from the rat amygdala. A subcutaneous injection of 1 mg/kg SB 204070A significantly reduced the duration of bilateral forelimb clonus of fully amygdala-kindled seizures. In contrast, daily treatment with SB 204070A at the same dose prior to each electrical stimulation produced no significant effects on behavioral or electrographic seizure development, although the duration of bilateral forelimb clonus of the first generalized seizure was significantly reduced. These results suggest that blockade of 5-HT4 receptors can inhibit the expression of kindled generalized seizures, but not the developmental seizure process of kindling.

Action of the selective 5‐HT4 receptor antagonist SB 204070A in the rat kindling model of epilepsy

Action of the selective 5‐HT4 receptor antagonist SB 204070A in the rat kindling model of epilepsy

Pharmacological characterisation of 5-HT receptors positively coupled to adenylyl cyclase in the rat hippocampus.

The pharmacological properties of 5-hydroxytryptamine (5-HT) receptors positively coupled to adenylyl cyclase in the rat hippocampus were investigated using selective agonists and antagonists. 5-HT (0.008-125 microM) stimulated cyclic AMP formation in homogenates of rat hippocampus in a concentration-dependent manner. The maximal increase in cyclic AMP formation occurred at 1 microM (141+/-6%) and the half-maximal effect (EC50) at 50+/-22 nM. Cyclic AMP accumulation induced by 1 microM 5-HT was partly inhibited by the selective 5-HT1A receptor antagonist WAY 100,635 (1 microM), the selective 5-HT4 receptor antagonist SB 203,186 (1 microM), and the 5-HT2A/c/ 5-HT7 receptor antagonist mesulergine (25 microM). WAY 100,635, SB 203,186 and mesulergine inhibited the effect of 5-HT (1 microM) by 47%, 33% and 49%, respectively. The combination of WAY 100,635 (1 microM) with SB 203,186 (1 microM) or mesulergine (25 microM) resulted in stronger inhibition than with each antagonist alone, and the combination of all three antagonists produced almost total blockade (95%) of 5-HT-induced cyclic AMP accumulation. 5-Carboxamidotryptamine (5-CT; 0.008-125 microM), a 5-HT1/5-HT7 receptor agonist, and SDZ 216-454 (0.008-125 microM), a selective 5-HT4 receptor agonist, concentration-dependently stimulated cyclic AMP formation, but the maximal effect of each agonist was smaller than that of 5-HT alone. SDZ 216-454 (5 microM) and 5-CT (5 microM) in combination stimulated cyclic AMP formation in an additive manner. 8-OH-PIPAT and 8-OH-DPAT, two selective 5-HT1A agonists, produced a small but significant increase in cyclic AMP formation at concentrations above 0.04 microM and 10 microM, respectively. These findings suggest that at least three 5-HT receptor subtypes, i.e. 5-HT1A, 5-HT7 and 5-HT4 receptors, are involved in mediating 5-HT-induced cyclic AMP formation in rat hippocampus.

The Effect of Chronic Hypoxia on Endothelin Receptor Subtype-mediated Responses in Rat Isolated Airways

Contractile responses to endothelin-1 (ET-1) were investigated in isolated trachea from rats previously exposed to chronic hypoxia (10% O2) or room air for 14 days. Concentration-response curves were constructed to ET-1 (10−11–3×10−7m) in the presence and absence of the ETAreceptor antagonist FR 139317 (10−8, 10−7and 10−6m), the ETBreceptor antagonist BQ 788 (10−6and 3×10−6m), the non-selective ET receptor antagonist SB 209670 (10−7and 10−6m) and a combination of FR 139317 (10−6m) and BQ 788 (10−6m). Concentration–response curves were also conducted to the ETBreceptor agonist sarafotoxin S6c (10−11–3×10−7m). In addition, responses to ET-1 (10−11–3×10−7m) were examined in the presence and absence of the nitric oxide synthase inhibitor, L-NAME. In control rat trachea, both FR 139317 and BQ 788 failed to inhibit ET-1-induced contractions and, indeed, FR 139317 (10−8m) and BQ 788 actually potentiated responses. In trachea from chronic hypoxic rats, FR 139317 did not alter ET-1 responses whereas BQ 788 again potentiated ET-1-induced contractions. The non-selective ET receptor antagonist SB 209670 attenuated ET-1-evoked contractions in trachea from control and chronically hypoxic rats. A combination of FR 139317 (10−6m) and BQ 788 (10−6m) also attenuated ET-1 responses in control rat trachea, but not trachea from chronically hypoxic rats. In trachea from both control and chronically hypoxic rats, L-NAME significantly potentiated responses to ET-1. To investigate ET receptor-mediated relaxation, tissues were preconstricted with methacholine and concentration–response curves were conducted to ET-1(10−13–10−8m) in the presence and absence of BQ 788 (10−6m) and to the ETBreceptor agonist sarafotoxin S6c (10−13–10−8m). In trachea from control and chronic hypoxic rats, ET-1 and sarafotoxin S6c evoked only very small, non-reproducible relaxatory responses.

Endothelin receptors and calcium translocation pathways in human airways.

Tension and phosphatidyl inositol (PI) turnover experiments were conducted to investigate the receptors and signal transduction pathways responsible for contractions elicited by endothelin (ET) ligands in human bronchus. Nicardipine (1 microM), the L-type calcium channel inhibitor, or incubation in Ca2+-free medium, produced marked inhibition of contractions to the ET(B) receptor-selective agonist, sarafotoxin S6c, and especially those induced by KCl. In contrast, Ca2+-free medium was without appreciable effect against contraction produced by endothelin-1 (ET-1), the non-selective ET(A) and ET(B) receptor agonist. In Ca2+-free medium, ryanodine (10 microM), which inhibits intracellular calcium mobilization, reduced sarafotoxin S6c- and ET-1-induced responses, but was without effect on responses to KCl. Similarly, nickel chloride (Ni2+; 1 mM) caused marked inhibition of contractions induced by sarafotoxin S6c or ET-1, but had no significant effect on KCI concentration-response curves. The mixed ET(A)/ET(B) receptor antagonist SB 209670 (3 microM) inhibited responses to sarafotoxin S6c and ET-1 such that concentration-response curves were shifted rightward, at the 30% maximum response level, by 10.0- and 3.8-fold, respectively, whereas BQ-123 (3 microM), the ET(A) receptor antagonist, was without effect on responses induced by either agonist. ET-1 (1 nM-0.3 microM) caused a concentration-dependent stimulation of PI turnover, whereas sarafotoxin S6c (0.3 nM-0.1 microM) induced only small and variable increases, except at the highest concentration. The increase in PI turnover evoked by ET-1 was inhibited by SB 209670 (3 microM), and also by BQ-123 (3 microM). This is consistent with linkage of ET(A) receptors to activation of inositol phosphate generation in human bronchial smooth muscle cells. Collectively, the data suggest that differences exist in the relative contributions of intracellular and extracellular Ca2+ mobilization mechanisms elicited by ET(A) and ET(B) receptor activation. Thus, sarafotoxin S6c-induced, ET(B) receptor-mediated contraction in human bronchial smooth muscle appears to be dependent, in part, upon extracellular Ca2+, although a significant component of the response was also mediated by intracellular Ca2+ release, including from ryanodine-sensitive stores. ET(A) receptor-mediated contraction of human airway smooth muscle was activated largely via the release of intracellular Ca2+.

Discriminative stimulus properties of the atypical neuroleptic clozapine in rats: tests with subtype selective receptor ligands.

The interoceptive stimulus induced by clozapine (5 mg/kg, i.p.) has been characterized in an operant drug discrimination procedure in the rat using a wide range of receptor subtype-selective agonists and antagonists. Only the muscarinic receptor antagonist scopolamine generalized fully to clozapine (>80%). Partial generalization (defined here as 40% maximal generalization) was seen with the D1 receptor antagonist SCH 23390 (43% maximal generalization), the alpha1-adrenoceptor antagonist prazosin (67%) and the alpha2-adrenoceptor antagonist methoxyidazoxan (42%). All other specific agents tested induced <25% maximal generalization, including the alpha2-adrenoceptor antagonist yohimbine (24%), the histamine H1 receptor antagonist mepyramine (21%), the D2 antagonist typical neuroleptic haloperidol (23%), the D4 receptor antagonist L-745,870 (14%), the 5-hydroxytryptamine-1A (5-HT1A) receptor agonist S-14506 (8%), the 5-HT2A receptor antagonists ketanserin (0%) and M100907 (12%), the 5-HT2B/2C receptor antagonists SB 200646A (8%) and SDZ SER 082 (6%), and the 5-HT3 receptor antagonist ondansetron (0%). The clozapine discriminative stimulus was not blocked by the dopamine D1 receptor antagonist SCH 23390, or by the 5-HT1A receptor antagonist WAY 100635, when given concomitantly with clozapine. Although the results suggest that muscarinic antagonism plays a major role in the clozapine cue, the results have to be considered in the light of the full generalization to clozapine seen with various antipsychotic agents which have very low affinity for muscarinic receptors, including zotepine, quetiapine, JL13 and PNU 96415 (a finding replicated in rats from the same breeding colony as those which generalized to scopolamine). Thus, generalization to clozapine for antipsychotics with multiple affinities but with low muscarinic affinity is probably mediated by additive or perhaps supra-additive actions at other receptors, although extensive studies with various combinations of drug mixtures are required to validate this hypothesis.

The canine external carotid vasoconstrictor 5-HT 1 receptor: blockade by 5-HT 1B (SB224289), but not by 5-HT 1D (BRL15572) receptor antagonists

In vagosympathectomised dogs pre-treated intravenously (i.v.) with mesulergine (300 μg/kg), 1-min intracarotid (i.c.) infusions of 5-hydroxytryptamine (5-HT; 0.3–30 μg/min) and sumatriptan (1–30 μg/min) dose-dependently decreased external carotid blood flow, without affecting mean blood pressure or heart rate. Treatment with the selective 5-HT 1B receptor antagonist SB224289 (2,3,6,7-tetrahydro-1′-methyl-5-[2′-methyl-4′(5-methyl-1,2,4-oxadiazol-3-yl) biphenyl-4-carbonyl]furo[2,3 f]indole-3-spiro-4′-piperidine hydrochloride; 30–300 μg/kg, i.v.) produced a potent, specific and dose-dependent blockade of this response, whereas the selective 5-HT 1D receptor antagonist BRL15572 (1-(3-chlorophenyl)-4-[3,3-diphenyl(2-( S, R) hydroxypropanyl)piperazine]hydrochloride; 30–300 μg/kg, i.v.) was ineffective. It is concluded that mainly 5-HT 1B, but not 5-HT 1D receptors mediate the canine external carotid vasoconstriction by 5-HT and sumatriptan.

Endothelium-derived prostanoids reduce 5-hydroxytryptamine-induced contraction in the human uterine artery.

The contribution of endothelium-linked mechanisms to the contraction induced by 5-hydroxytryptamine (5-HT) was investigated in the isolated human uterine artery. 5-HT contracted the uterine artery concentration-dependently. Removal of the endothelium or treatment with the cyclooxygenase inhibitor indomethacin potentiated the contractile response to 5-HT. The nitric oxide synthase inhibitor L-N(G)-monomethyl-arginine (L-NMMA) did not influence the contraction induced by 5-HT. Indomethacin did not affect the response to 5-HT in endothelium-denuded vessels. The 5-HT1 receptor agonist 5-carboxyamidotryptamine (5-CT) did not relax precontracted arteries. Removal of the endothelium did not change the response to 5-HT in the presence of the 5-HT(1B/D) receptor antagonist GR127935 and the 5-HT1A and 5-HT1B receptor antagonist -pindolol. The 5-HT1B receptor antagonist SB224289 did not affect the contraction induced by 5-HT. The results indicate that the 5-HT-induced contraction in the human uterine artery is accompanied by the release of an endothelium-derived relaxing factor (EDRF). This EDRF seems to be a prostanoid, probably prostacyclin (PGI2). The endothelium-linked mechanism seems to be mediated via a 5-HT1 receptor, but it is not possible to further classify the receptor subtype by the information obtained in this study.