The inhibition of nicotine-evoked relaxation of the guinea-pig isolated basilar artery by some analgesic drugs and progesterone.

1. The purpose of this study was to investigate the mechanism of nicotine-evoked relaxation of the guinea-pig isolated basilar artery and to study the effects of drugs associated with the aetiology or treatment of migraine on the nicotine response. 2. The guinea-pig isolated basilar artery, pre-contracted with prostaglandin F2alpha (PGF2alpha), in the presence of atropine (3 microM) and guanethidine (3 microM), relaxed on addition of nicotine (0.1 mM) in approximately 50% of preparations. The responses to nicotine were of short duration and blocked in preparations pre-treated for 10 min with capsaicin (1 microM) and are therefore probably a consequence of the stimulation of trigeminal C fibre terminals. 3. Responses to nicotine were reduced in the presence of 5-carboxamidotryptamine, 5-hydroxytryptamine and sumatriptan in that order of potency. This is consistent with a 5-HT1 receptor mechanism. These agonists evoked small additional contractions in vessels pre-contracted with PGF2alpha. 4. Indomethacin (0.3-10 microM), aspirin (10-30 microM), and nitro-L-arginine methyl ester (L-NAME, 0.1 mM) reduced nicotine-evoked relaxation of the basilar artery, suggesting the involvement of both nitric oxide and cyclo-oxygenase products in this response. 5. Progesterone (1 microM) markedly reduced the response to nicotine, a possible reflection of the ion channel blocking activity of high concentrations of this compound. 6. The guinea-pig basilar artery is a preparation in which the effects of drugs on responses to stimulation of trigeminal nerve terminals can be studied in vitro and may thus be of interest in assessing the actions of drugs used in treatment of headache.

The enantiomers of zacopride: an intra-species comparison of their potencies in functional and anxiolytic models.

1. The 5-HT3 receptor antagonist, zacopride, and its enantiomers, R(+)-zacopride and S(-)-zacopride, were examined in three pharmacological models: (i) 5-HT-induced depolarization of the mouse isolated vagus nerve preparation, (ii) the 5-HT-evoked von Bezold-Jarisch reflex in the mouse, and (iii) the mouse light:dark box model of anxiety. Other standard 5-HT3 receptor antagonists were also included for comparison in these studies. 2. Racemic zacopride, and both of the enantiomers, displayed potent 5-HT3 receptor antagonist activity in the isolated vagus nerve and in the von Bezold-Jarisch model. No 5-HT3 receptor agonist or partial agonist effects of these compounds were detected. 3. In the isolated vagus nerve, R(+)-zacopride and ondansetron were surmountable 5-HT3 receptor antagonists (pA2 values of 9.3 and 8.3, respectively), whereas racemic zacopride, S(-)-zacopride and tropisetron were insurmountable antagonists, markedly suppressing the maximum response to 5-HT. 4. In vivo, racemic zacopride, R(+)-zacopride, S(-)-zacopride and WAY100289 were potent antagonists of the 5-HT-evoked von Bezold-Jarisch reflex, with minimum effective doses (lowest dose required to reduce the reflex by > or = 85%; MED85) of 1.0, 3.0, 0.3 and 3.0 micrograms kg-1, s.c., respectively. 5. Racemic zacopride, R(+)-zacopride and S(-)-zacopride were active in the mouse light:dark box model of anxiety, with similar potencies (minimum effective dose 1 microgram kg-1, s.c.) and similar active dose-ranges (1-1000 micrograms kg-1, s.c.). 6. The doses of racemic zacopride, R( + )-zacopride and S(-)-zacopride required to block 5-HT3receptors in vivo correlated reasonably well with their potencies in an anxiety model within the same species. In these studies, there was no evidence of a marked difference between the anxiolytic potencies ofR( + )-zacopride and S(-)-zacopride.

Further characterization of the putative 5-HT4 receptor mediating depolarization of the rat isolated vagus nerve.

A putative 5-HT4 receptor-mediated depolarization of the rat isolated vagus nerve has been studied using a grease-gap extracellular recording technique. Ondansetron (1 microM) was used to block the predominant 5-HT3 receptor mediated depolarization in this preparation and the effects of the 5-HT4 receptor antagonists DAU 6285 (endo-8-methyl-8-azabicyclo [3.2.1] oct-3-yl-2,3-dihydro-6-methoxy-2-oxo-1H-benzimidazole-1- carboxylate HCl); 0.3, 1.0 or 3.0 microM and SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino)-ethyl ester HCl); 0.1, 0.3 or 1.0 microM were studied on the residual, ondansetron-resistant, component of the response. The effects of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) and of forskolin on the ondansetron-resistant response were also studied. Both DAU 6285 and SDZ 205-557 acted as competitive antagonists of the ondansetron-resistant response to 5-HT with pA2 values of 6.8 (6.7-7.1, n = 12) and 7.1 (6.9-7.5, n = 12) respectively. The vagus nerve was depolarized by IBMX (100 microM) or forskolin (10 microM), the effects being similar to the maximum response to 5-HT. In the presence of IBMX (100 microM) or forskolin (10 microM) the ondansetron-resistant component of the response to 5-HT was enhanced and the 5-HT3 receptor-mediated component reduced. These results with DAU 6285 and SDZ 205-557 are consistent with a 5-HT4 receptor-mediated mechanism of the ondansetron-resistant depolarizing response to 5-HT.

The antagonist actions of WAY-100135 and its enantiomers on 5-HT1A receptor-mediated hyperpolarization of the rat isolated superior cervical ganglion.

A grease-gap extracellular recording technique was used to detect 5-HT1A receptor-mediated hyperpolarizing responses to 5-hydroxytryptamine (5-HT) in the rat isolated superior cervical ganglion. In the presence of the novel 5-HT1A receptor antagonist, WAY-100135 [N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropan amide], the responses to 5-HT were antagonised in a competitive manner with a pA2 value of 7.2 (6.9-8.5) and Schild plot slope of 1.0 (0.4-1.6), n = 20. The antagonist activity was greater in the (+) than the (-)enantiomer of WAY-100135. The pA2 value of the (+)enantiomer was 7.5 (7.2-8.0), Schild plot slope 1.2 (0.8-1.6), n = 17. In contrast the (-)enantiomer had weak antagonist activity (pA2 6.3 +/- 0.25, n = 3). No agonist activity of WAY-100135 or its enantiomers were observed in this study.

Characterization of the 5-hydroxytryptamine receptor mediating the positive inotropic response in guinea-pig isolated left atria.

1. 5-Hydroxytryptamine (5-HT), in the presence of propranolol (1 microM), atropine (3 microM) and ketanserin (1 microM), induced a positive inotropic response of guinea-pig isolated electrically paced left atria (pEC50 = 7.52). The positive inotropic response was mimicked by alpha-methyl-5-HT (pEC50 = 7.26) and 5-carboxamidotryptamine (5-CT; pEC50 = 6.56) but not by sumatriptan or 1-(m-chlorophenyl) piperazine (m-CPP). 2. The 5-HT induced positive inotropic response was competitively antagonized by both mesulergine (pA2 = 7.68) and methiothepin (pA2 = 6.67). Methysergide was a surmountable antagonist at 3 nM producing a rightward shift in the 5-HT concentration-response curve giving an apparent pA2 = 9.2 with no significant reduction in the maximum. At higher concentrations, methysergide behaved as an insurmountable antagonist, significantly reducing the maximum response to 5-HT as well as producing rightward shifts in the 5-HT concentration-response curves. 3. The 5-HT-induced positive inotropic response was not antagonized by either tropisetron (10 microM) or yohimbine (10 microM). 4. The guinea-pig atrial 5-HT receptor does not satisfy the criteria for any of the currently recognised 5-HT receptor subtypes and appears to have some similarities to the atypical 5-HT receptors previously described in other peripheral tissues.

A component of 5-HT-evoked depolarization of the rat isolated vagus nerve is mediated by a putative 5-HT4 receptor.

This study describes a component of 5-HT-evoked depolarization of the rat isolated vagus nerve which was unaffected by the 5-HT3 receptor antagonist ondansetron. A grease-gap extracellular recording technique was used. Ondansetron (10-100 nmol/l) displaced the 5-HT concentration-response curve to the right yielding a pA2 value of 8.6 (8.5-8.8), consistent with 5-HT3 receptor antagonism, and revealing a component of the 5-HT response which was resistant to ondansetron blockade. In the presence of ondansetron (100 nmol/l) the maximum depolarization in the resistant phase was 15.5 (12.6-19.2)% of the initial maximum response to 5-HT and the pEC50 value was 7.0 (6.7-7.3). The mechanism of the ondansetron-resistant component of the 5-HT response resembled a 5-HT4-receptor-effect in being absent in preparations equilibrated with 5-methoxytryptamine (10 mumol/l) and antagonised by ICS 205930 (tropisetron, pA2 6.4). 5-Methoxytryptamine alone was an agonist in the vagus nerve with a maximum response similar to that of the ondansetron resistant phase of the 5-HT response. Similarly renzapride alone evoked small depolarizations of this preparation but antagonized the ondansetron resistant phase of the 5-HT response (pA2 7.3-7.4). These effects of 5-methoxytryptamine and renzapride are also consistent with a 5-HT4 receptor mechanism. Ketanserin (1 mumol/l) and methysergide (1 mumol/l) had little effect on responses to 5-HT. The depolarization evoked by this putative 5-HT4 receptor mechanism was small but prolonged and appears to mask and after-hyperpolarizing phase of the 5-HT response in this tissue.

Novel antagonists of the 5-HT3 receptor. Synthesis and structure-activity relationships of (2-alkoxybenzoyl)ureas.

A series of benzoylureas derived from bicycle amines were prepared and evaluated for 5-HT3 antagonist activity on the rat isolated vagus nerve. From among these compounds, those analogues which were ortho substituted by an alkoxy group on the benzoyl function were shown to be potent 5-HT3 antagonists with similar or greater potency than the standard agent ondansetron. NMR and X-ray crystallography studies showed these o-alkoxy compounds to exist as a planar, hydrogen-bonded, tricyclic ring system. In molecular modeling studies on endo-N-[[(8-methyl-8-azabicyclo[3.2.1]octan-3-yl-amino] carbonyl]-2-(cyclopropylmethoxy)benzamide (30) the central hydrogen-bonded ring was able to mimic an aromatic ring present in previously reported 5-HT3 antagonists.

Synthesis and alpha 2-adrenoceptor antagonist activity of some disulfonamidobenzoquinolizines.

A series of disulfonamidobenzo[a]quinolizines were synthesized and evaluated for their alpha 2- and alpha 1-adrenoceptor antagonist activity on the rat vas deferens and anococcygeus muscle, respectively. N-((2 beta,11b alpha)-1,3,4,6,7,11b-Hexahydro-2H-benzo[a]quinolizin-2-yl)-N- [2-[(methylsulfonyl)amino]ethyl]methanesulfonamide (4) and its N-[2-[(methylsulfonyl)amino]ethyl]ethanesulfonamide (22), N-[2-[(ethylsulfonyl)amino]ethyl]ethanesulfonamide (27), and N-[2-[(methylsulfonyl)amino]ethyl]-4-methylbenzenesulfonamide (30) analogues showed 400-fold or greater selectivity in favor of alpha 2- over alpha 1-adrenoceptor blockade.

Synthesis and structure-activity relationships of 2-sulfonamido-1,3,4,6,7,11b alpha-hexahydro-2H-benzo[a]quinolizines as alpha 2-adrenoceptor antagonists.

A series of 2-sulfonamido-1,3,4,6,7,11b alpha-hexahydro-2H-benzo[a]quinolizines were synthesized and examined for alpha 2- and alpha 1-adrenoceptor antagonist activity on the rat vas deferens and anococcygeus muscle, respectively. A number of compounds in this series were shown to be potent and selective alpha 2-adrenoceptor antagonists. Studies on the resolved enantiomers of compounds 6, 10, and 16 showed that alpha 2-adrenoceptor antagonist activity resided primarily in the 2R,11bS isomers, related to the absolute configuration of the alpha 2-antagonist yohimbine, such that the benzene ring and sulfonamide groups in this series were superimposable on the pyrrole and ester groups of yohimbine.

The effects of some alpha-adrenoceptor antagonists on the responses of the canine saphenous vein to B-HT 933, UK-14304 and methoxamine.

The benzoquinolizines Wy 25309, Wy 26703 and Wy 27127, previously reported as potent antagonists at presynaptic alpha 2-adrenoceptors were also potent antagonists of B-HT 933 in isolated saphenous veins of the dog confirming their activity at post synaptic alpha 2-adrenoceptors. Yohimbine was a more potent antagonist of B-HT 933 in isolated saphenous vein than were the Wy compounds or idazoxan contrasting with the reported potencies of these compounds at presynaptic sites in rat vas deferens and raising the possibility of differences between pre- and postsynaptic alpha 2-adrenoceptors. Contractions of the saphenous vein were observed with high concentrations of idazoxan.

Some pharmacological properties of Wy 27127 a more selective alpha 2:alpha 1-adrenoceptor antagonist than idazoxan in vitro.

Wy 27127 and idazoxan were approximately equipotent as antagonists at alpha 2-adrenoceptors as estimated by their ability to block clonidine-induced inhibition of electrically-evoked contractions of the rat isolated vas deferens. Idazoxan was seven times as potent as Wy 27127, as an antagonist at alpha 1-adrenoceptors as indicated by blockade of methoxamine-induced contractions of the rat isolated anococcygeus muscle. Thus, the alpha 2:alpha 1 selectivity ratio, as calculated from these tests was 407 for Wy 27127 and 76 for idazoxan. Wy 27127 and idazoxan were equipotent in enhancing stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [3H]-noradrenaline as expected for alpha 2-adrenoceptor antagonists. At higher concentrations both compounds reduced the stimulation-evoked contraction of the pulmonary artery but idazoxan was 15 times as potent as Wy 27127 in this respect. Neither compound had marked antagonist actions at 5-hydroxytryptamine (D), muscarinic, presynaptic dopamine or histamine (H1) receptors or at beta 1-adrenoceptors. Thus, idazoxan and Wy 27127 were equipotent alpha 2-adrenoceptor antagonists in vitro, however, the alpha 2:alpha 1 selectivity of Wy 27127 was considerably greater than that of idazoxan by virtue of weaker alpha 1-adrenoceptor antagonism.

A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit.

The alpha 2-adrenoceptor antagonist potencies of the benzoquinolizines (Wy 26 703, Wy 25 309, Wy 26 392), the benzodioxans (RX 781 094, RS 21 361), yohimbine and rauwolscine have been compared at presynaptic alpha 2-adrenoceptors in the isolated vasa deferentia of the rat and rabbit. Yohimbine and rauwolscine are of equal potency as antagonists in both the rat and rabbit against the agonists clonidine or UK 14304. The benzoquinolizines and benzodioxans are very weak antagonists of clonidine or UK 14304 at the presynaptic alpha 2-adrenoceptors of the rabbit vas deferens when compared to their potency at the presynaptic alpha 2-adrenoceptors of the rat vas deferens. This suggests that the presynaptic alpha 2-adrenoceptors present in the rat vasa deferentia may be different from those present in the rabbit vasa deferentia.

Studies of alpha 2-adrenoceptor antagonist potency in vitro: comparisons in tissues from rats, rabbits, dogs and humans.

The potencies of a number of selective alpha 2-adrenoceptor antagonists have been measured in preparations from four species. In the rat vas deferens, the newer synthetic antagonists Wy 25309, Wy 26392, Wy 26703 and RX 781094 were more potent than the alkaloid yohimbine in blocking a clonidine-induced inhibition of an electrically evoked twitch response. In the rabbit vas deferens yohimbine was substantially more potent than the synthetic antagonists in reversing the action of clonidine. Yohimbine was also more potent than the synthetic antagonists in blocking the B-HT 933-induced contractile responses of the dog saphenous vein and preventing adrenaline-induced aggregation of human platelets. Together with previously published data derived from tritium overflow studies on rabbit pulmonary arteries and displacement of [3H]-rauwolscine binding from rat cerebral cortex and human platelets, the results suggest that the adrenoceptor currently labelled alpha 2 may not be a single entity.

Alpha 2-adrenoceptor antagonism and other pharmacological antagonist properties of some substituted benzoquinolizines and yohimbine in vitro.

Comparison of pA2 values for antagonism of clonidine induced inhibition of the electrically evoked contraction of the rat isolated vas deferens (alpha 2-adrenoceptor) and antagonism of contractions to methoxamine on the rat isolated anococcygeus (alpha 1-adrenoceptor) showed a group of substituted benzoquinolizines (Wy 25309, 26392 and 26703) to be more potent and more selective alpha 2-adrenoceptor antagonists than yohimbine. The benzoquinolizines and yohimbine enhanced stimulation-evoked overflow of tritium from rabbit isolated pulmonary arteries preloaded with [3H]-noradrenaline, as expected for alpha 2-adrenoceptor antagonists. In contrast to the results on the rat vas deferens, yohimbine was more potent than the benzoquinolizines. At higher concentrations all the alpha-adrenoceptor antagonists reduced the stimulation-evoked contraction of the pulmonary artery. The benzoquinolizines were competitive antagonists of 5-hydroxytryptamine on the rat isolated ileum. Wy 25309 showed only weak activity (pA2 = 5.21) whereas Wy 26703 was more potent (pA2 = 7.25). Yohimbine was a potent antagonist of 5-hydroxytryptamine. Wy 26703 was the only compound to have histamine antagonist effects in the guinea pig isolated ileum and to antagonise the chronotropic effect of isoprenaline on the isolated atria of the guinea pig and in both instances activity was weak (pA2 values 5.3 and 5.5 respectively). Yohimbine reduced the spontaneous beating of the atria at 3 X 10(-6) M. No compound at 10(-5) M antagonised acetylcholine on the guinea pig ileum. These novel substituted benzoquinolizines should be useful experimental compounds for the study of alpha 2-adrenoceptor mediated responses.