Valérie Audinot

Clozapine inhibits serotoninergic transmission by an action at α1-adrenoceptors not at 5-HT1A receptors

This study examined the mechanism underlying the influence of clozapine upon serotoninergic transmission in the rat. In vitro, clozapine manifested weak affinity at 5-HT1A receptors (pKi = 6.5) as compared to the agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (9.0), but high affinity at alpha 1-adrenoceptors (8.2) as compared to the alpha 1-adrenoceptor antagonist, prazosin (9.7). Ex vivo, clozapine (inhibitory dose (ID)50 = 0.7 mg/kg s.c.) mimicked prazosin (0.5) in potently occupying central alpha 1-adrenoceptors whereas, as compared to 8-OH-DPAT (0.2), it failed to occupy 5-HT1A receptors (> 10.0). The firing of serotoninergic neurones in the dorsal raphe nucleus was abolished by 8-OH-DPAT, clozapine and prazosin with ID50 values of 0.006, 0.09 and 0.07 mg/kg i.v., respectively. At comparable doses, they reduced striatal turnover of 5-HT. While the 5-HT1A receptors antagonists, (-)-tertatolol (2.0 mg/kg i.v.) and spiperone (0.63 mg/kg i.v.), blocked the action of 8-OH-DPAT upon dorsal raphe nucleus firing, they failed to modify the effect of clozapine and prazosin. In contrast, the alpha 1-adrenoceptor agonist, cirazoline (0.005 mg/kg i.v.) prevented the actions of clozapine and prazosin, but not that of 8-OH-DPAT. It is concluded that clozapine only weakly interacts with 5-HT1A receptors and that its potent alpha 1-adrenoceptor antagonist properties underlie inhibition of serotoninergic transmission.

Modulation of mesolimbic dopamine release by the selective dopamine D3 receptor antagonist, (+)-S 14297

In Chinese hamster ovary cells stably transfected with recombinant rat dopamine D2 or D3 receptors, the naphthofurane antagonist, (+)-S 14297 [(+)-[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro- naphtho(2,3b)dihydro,2,3-furane]], displayed a pronounced preference for dopamine D3 versus D2 receptors: Ki values = 13 and 365 nM, respectively. In contrast, its distomer, (-)-S 17777, showed low affinity (296 versus 3403 nM). The aminotetralin agonist, (+)-7-OH-DPAT (7-hydroxy-2-(di-n-propylamino)tetralin), also showed high affinity at dopamine D3 (1.8 nM) versus D2 (96 nM) receptors while its (-)-isomer showed low affinity (71 and 1461 nM). In freely moving rats, (+)-7-OH-DPAT (0.16 mg/kg s.c.)-but not (-)-7-OH-DPAT-decreased dialysate levels of dopamine in the nucleus accumbens. (+)-S 14297 (1.25 mg/kg s.c.) markedly inhibited the action of (+)-7-OH-DPAT without influencing dopamine levels alone. Further, this action was stereospecific in that (-)-S 17777 (20.0 mg/kg s.c.) was inactive. In conclusion, data obtained with the novel, selective dopamine D3 receptor antagonist, (+)-S 14297 suggest that dopamine D3 autoreceptors modulate the release of dopamine from mesolimbic dopaminergic neurones.

Novel benzopyrano[3,4-c]pyrrole derivatives as potent and selective dopamine D3 receptor antagonists

A new series of benzopyrano[3,4-c]pyrrole derivatives were synthesized and evaluated for their interaction with dopamine D3 versus D2 receptors. Amongst these compounds, 4x (S 33084) was found to be a potent and selective dopamine D3 receptor antagonist.

S 14297, a novel selective ligand at cloned human dopamine D3 receptors, blocks 7-OH-DPAT-induced hypothermia in rats

The selective dopamine D3 receptor agonist, 7-OH-DPAT ((+)-7-hydroxy-2-(di-n-propylamino)tetralin) and the novel naphthofurane, S 14297 ((+)-[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro- naphtho(2,3b)dihydro,2,3-furane]), bound with high affinity and selectivity to recombinant, human dopamine D3 versus D2 receptors stably transfected into Chinese hamster ovary cells: Ki values = 2 versus 103 nM for 7-OH-DPAT and 13 versus 297 nM for S 14297. In contrast, the putative dopamine D3 receptor antagonist, AJ 76 (cis-(+)-5-methoxy-1-methyl-2-(n- propylamino)tetralin), displayed low affinity and selectivity for dopamine D3 versus D2 sites (70 versus 154 nM). 7-OH-DPAT (0.01-0.16 mg/kg s.c.) provoked hypothermia in rats, an action abolished by S 14297 (0.04-0.63 mg/kg s.c.) and, less potently, by AJ 76 (0.16-2.5 mg/kg s.c.). S 14297 (20.0 mg/kg s.c.) did not modify prolactin secretion. These data suggest that dopamine D3 receptors mediate hypothermia in the rat and that S 14297 acts as a selective antagonist at these sites.

Tetracyclic analogues of [+]-S 14297: Synthesis and determination of affinity and selectivity at cloned human dopamine D3 vs D2 receptors

Abstract Starting from the structure of the preferential D 3 antagonist S 14297 ( 1 ), we have prepared a series of cis and trans tetracyclic derivatives of general formula I in order to improve potency and selectivity for D 3 receptors. The trans oxazino derivative 2c , showed slightly increased affinity at D 3 receptors and double the selectivity for D 3 over D 2 receptors, in comparison to S 14297. Cis derivatives and compounds where R 1 is aralkyl were totally devoid of activity.

Evidence that dopamine D3 receptors participate in clozapine-induced hypothermia

In analogy to the dopamine D3 receptor agonist, (+)-7-OH-DPAT (7-hydroxy-2-(di-n-propylamino)tetralin) (0.01-0.63 mg/kg s.c.), clozapine dose-dependently (0.63-40.0 mg/kg s.c.) elicited hypothermia in rats. Haloperidol and raclopride, mixed dopamine D2/D3 receptor antagonists, failed, in contrast, to modify core temperature. Further, they dose-dependently inhibited the action of clozapine with inhibitory dose50 values (ID50) of 0.3 mg/kg s.c., in each case. The preferential dopamine D3 versus D2 receptor antagonist, (+)-AJ 76 (cis-(+)-5-methoxy-1-methyl-2-(n-propylamino)tetralin) (ID50 = 2.8), and the selective dopamine D3 versus D2 receptor antagonist, (+/-)-S 11566 ((+/-)(-)[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro-naphtho(2,3b) dihydro,2,3-furane]) (ID50 = 1.6) likewise blocked the action of clozapine without reducing core temperature alone. The action of (+/-)-S 11566 was stereospecific in that its active eutomer, (+)-S 14297 (ID50 = 1.0), also inhibited the action of clozapine whereas its inactive distomer, (-)-S 17777 (ID50 > 10.0), was not effective. Antagonist potency for blockade of clozapine-induced hypothermia correlated powerfully both with potency for blockade of (+)-7-OH-DPAT-induced hypothermia (r = 0.98) and with affinity at cloned human dopamine D3 receptors transfected into Chinese hamster ovary (CHO) cells (r = 0.92). In conclusion, these data suggest that dopamine D3 receptors may be involved in the induction of hypothermia by clozapine in the rat.

Binding profile of the novel 5-HT1B/1D receptor antagonist, [3H]GR 125,743, in guinea-pig brain: a comparison with [3H]5-carboxamidotryptamine

Native brain 5-HT1B/1D) receptors were studied using the novel antagonist, [3H]GR 125,743 (N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyri dyl)benzamide). In guinea-pig striatal membranes, [3H]GR 125,743 displayed rapid association (t1/2 = 4.5 min), high (90%) specific binding and high affinity (K(d) = 0.29 nM), although B(max) values (fmol/mg protein) varied according to brain region-striatum: 199; frontal cortex: 89; hippocampus: 79; cerebellum: 26. In frontal cortex, the B(max) determined with [3H]5-CT ([3H]carboxamidotryptamine) was significantly higher (178; P < 0.05), suggesting that it also labels other binding sites. In striatal membranes, guanylylimidodiphosphate (GppNHp) inhibited [3H]5-CT but not [3H]GR 125,743 binding, suggesting that the latter has antagonist properties. Nevertheless, in competition binding experiments, the pK(i) values obtained with [3H]GR 125,743 and [3H]5-CT for 20 serotonergic ligands, including L 694,247 (2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl ]-1H-indole-3-yl]ethylamine), GR46,611 (3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxybenzyl)acrylami de), sumatriptan and alniditan, were highly correlated (r = 0.99). Ketanserin and ritanserin showed low affinity for [3H]GR 125,743 binding to guinea-pig striatal sites (K(i) = 12600 and 369 nM), suggesting that 5-HT1B (rather than 5-HT1D) receptors are predominantly labelled in this tissue. The present data indicate that [3H]GR 125,743 is a useful tool for studying native 5-HT1B/1D receptors.