Nina Mohell

Binding characteristics of remoxipride and its metabolites to dopamine D2 and D3 receptors

The substituted benzamide, remoxipride, is a new atypical antipsychotic agent with good clinical efficacy and low extrapyramidal side-effect potential. In the present study, the in vitro receptor binding properties of remoxipride and several of its metabolites to rat striatal dopamine D2 and cloned human dopamine D2A and D3 receptors were investigated. Remoxipride bound to [3H]raclopride-labelled dopamine D2 receptors in rat striatum with an affinity (Ki) of 113 nM. The significantly lower affinities of remoxipride reported when [3H]spiperone was used as a radioligand are suggested to be due to methodological problems associated with the use of very high-affinity radioligands. Some of the phenolic metabolites of remoxipride found mainly in rat exhibited considerably higher affinities to dopamine D2 and D3 receptors than remoxipride itself. The pyrrolidone metabolites found mainly in the human had very low dopamine D2 and D3 affinities. The present in vitro results suggest that the behavioural effects of remoxipride in rats may reflect the effect of remoxipride and some of its high-affinity metabolites.

Time course of bromocriptine induced excitation in the rat: behavioural and biochemical studies

The aim of the present study was to further investigate the behavioural and biochemical pharmacology of the directly acting dopamine (DA) receptor agonist bromocriptine (BRC).BRC produced an initial depression of locomotion followed after about an hour by a weak but significant locomotor stimulation. The stimulation was potentiated by concomitant administration of the D1 agonist SKF38393.Ex vivo biochemical determinations indicated that reductions in dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels occurred in the striatum after BRC injection without a significant change in DA levels, indicating a reduced DA turnover. An increase in 5-hydroxytryptamine (5HT) and 5-hydroxyindoleacetic acid (5HIAA) levels occurred in the striatum leading to a significant increase in turnover (i.e. ratio of 5HIAA to 5HT). Noradrenaline concentrations increased in the striatum. In the cortex, sharp falls in HVA and DOPAC levels without a corresponding change in DA were observed. While there was no significant change in noradrenaline levels in this brain region, an increase in 5HIAA, but not in 5HT, levels occurred. These changes indicate an increase in 5HT turnover (ratio of 5HIAA to 5HT).In vivo dialysis indicated that extracellular levels of DA, DOPAC and HVA in the striata of freely moving rats were sharply reduced for at least 6 h after injection.In vitro binding studies showed that BRC exhibited high (Ki values in low nanomolar range) affinities for DA D2A, D2B, D3, α1 and α2 adrenergic receptors together with unexpectedly high affinity (about 1 nM) for 5HT1A receptors.The data indicate that the initial behavioural depression and later locomotor stimulation induced by BRC are accompanied by a sharp monophasic fall in striatal extracellular DA levels as indicated by dialysis studies. Since the behavioural stimulation was augmented by concomitant D1 receptor stimulation, the data suggest that the reduced DA turnover is influencing the amount of DA available to stimulate postsynaptic D1 receptors. However, the biochemical studies indicated that BRC has a high affinity for 5HT1A receptors and affects the turnover of 5HT in the brain. Thus, the behavioural effects of BRC may depend not only on effects on the DA system but also on 5HT systems.[/p]

Differential serotoninergic and dopaminergic activities of the (R)- and the (S)-enantiomers of 2-(di-n-propylamino) tetralin

Racemic 2-(di-n-propylamino)tetralin ((R,S)-DPAT), which lacks phenolic or other aromatic substituents, induces both dopaminergic (sniffing, licking and gnawing) and serotoninergic (forepaw treading and flat body posture) behavioural responses. The present study shows that s.c. administration of (R)-DPAT induces typical 5-HT1A receptor agonist behaviours. These effects are blocked by the 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin ((S)-UH-301). Administration of (S)-DPAT induces dopaminergic behaviours, which are fully antagonised by raclopride, a dopamine D2 receptor antagonist. Both enantiomers induce hypothermia, (R)-DPAT being antagonised by (S)-UH-301, whereas (S)-DPAT is antagonised by raclopride. The accumulation of 5-hydroxytryptophan and DOPA (3,4-dihydroxyphenylalanine) after decarboxylase inhibition that reflects presynaptic actions on 5-HT (5-hydroxytryptamine, serotonin) and dopamine neurons, respectively, are inhibited by both enantiomers of DPAT. (R)-DPAT is more potent than (S)-DPAT as an inhibitor of 5-hydroxytryptophan accumulation whereas (S)-DPAT is more potent than (R)-DPAT as an inhibitor of DOPA accumulation. Thus, in functional tests of postsynaptic actions (R)-DPAT behaves as a 5-HT1A receptor agonist and (S)-DPAT as a dopamine D2 receptor agonist. Presynaptically, (R)-DPAT shows selectivity for 5-HT1A receptors and (S)-DPAT for dopamine D2 receptors. Receptor binding studies, utilizing [3H]8-hydroxy-2-(di-n-propylamino)tetralin and [3H]quinpirole as radioligands for 5-HT1A and dopamine D2 receptors, respectively, showed (R)-DPAT to have a 3-fold higher affinity than (S)-DPAT for 5-HT1A receptors, whereas (S)-DPAT had a 6-fold higher affinity than (R)-DPAT for dopamine D2 receptors. Thus, the results from receptor binding studies support the conclusion that (R)- and (S)-DPAT are agonists showing selectivity for 5-HT1A and dopamine D2 receptors, respectively. Taken together, these findings may explain previous controversies with regard to the pharmacology of racemic DPAT and re-emphasise the necessity to study pure enantiomers of chiral compounds.

Interactions of ligands with active and inactive conformations of the dopamine D2 receptor

The affinities of 19 pharmacologically diverse dopamine D2 receptor ligands were determined for the active and inactive conformations of cloned human dopamine D2 receptors expressed in Ltk cells. The agonist [3H]quinpirole was used to selectively label the guanine nucleotide-binding protein-coupled, active receptor conformation. The antagonist [3H]raclopride, in the presence of the non-hydrolysable GTP-analogue Gpp(NH)p and sodium ions and in the absence of magnesium ions, was used to label the free inactive receptor conformation. The intrinsic activities of the ligands were determined in a forskolin-stimulated cyclic AMP assay using the same cells. An excellent correlation was shown between the affinity ratios (KR/KRG) of the ligands for the two receptor conformations and their intrinsic activity (r=0.96). The ligands included eight structurally related and enantiopure 2-aminotetralin derivatives; the enantiomers of 5-hydroxy-2-(dipropylamino)tetralin, 5-methoxy-2-(dipropylamino)tetralin, 5-fluoro-2-(dipropylamino)tetralin and 2-(dipropylamino)tetralin. The (S)-enantiomers behaved as full agonists in the cyclic AMP assay and displayed a large KR/KRG ratio. The (R)-enantiomers were classified as partial agonists and had lower ratios. The structure-affinity relationships of these compounds at the active and the inactive receptor conformations were analysed separately, and used in conjunction with a homology based receptor model of the dopamine D2 receptor. This led to proposed binding modes for agonists, antagonists and partial agonists in the 2-aminotetralin series. The concepts used in this study should be of value in the design of ligands with predetermined affinity and intrinsic activity.

Receptor binding characteristics of [3H]NAD-299, a new selective 5-HT1A receptor antagonist.

In vitro receptor binding properties of the novel tritiated 5-hydroxytryptamine1A (5-HT1A) receptor antagonist (R)-3-N,N-dicyclobutylamino-8-fluoro-[6-3H]-3,4-dihydro-2H-1-benzo pyran-5-carboxamide ([3H]NAD-299, generic name robalzotan) were evaluated and compared with those of the agonist 8-hydroxy-2-[2,3-3H]di-n-(propylamino)tetralin ([3H]8-OH-DPAT). [3H]NAD-299 binding displayed a Kd value of 0.17 nM and a Bmax value of 26.7 pmol/g wet weight of rat hippocampus. Same binding affinity (Kd = 0.16 nM) was found to cloned human 5-HT1A receptors. Addition of the nonhydrolyzable GTP analog guanylylimidodiphosphate had no effect on the binding characteristics of [3H]NAD-299, while it significantly decreased both the affinity and density of receptors labeled with [3H]8-OH-DPAT. The rank order of potency of various compounds to inhibit [3H]NAD-299 binding is consistent with the labeling of 5-HT1A receptors. This newly developed high-affinity and selective antagonist radioligand provides a valuable tool for studies of 5-HT1A receptors both in vitro and in vivo.

2-Aminotetralin-derived substituted benzamides with mixed dopamine D2, D3, and serotonin 5-HT1A receptor binding properties: A novel class of potential atypical antipsychotic agents

A new chemical class of potential atypical antipsychotic agents, based on the pharmacological concept of mixed dopamine D2 receptor antagonism and serotonin 5-HT1A receptor agonism, was designed by combining the structural features of the 2-(N,N-di-n-propylamino)tetralins (DPATs) and the 2-pyrrolidinylmethyl-derived substituted benzamides in a structural hybrid. Thus, a series of 35 differently substituted 2-aminotetralin-derived substituted benzamides was synthesized and the compounds were evaluated for their ability to compete for [3H]-raclopride binding to cloned human dopamine D2A and D3 receptors, and for [3H]-8-OH-DPAT binding to rat serotonin 5-HT1A receptors in vitro. The lead compound of the series, 5-methoxy-2-[N-(2-benzamidoethyl)-N-n-propylamino]tetralin (12a), displayed high affinities for the dopamine D2A receptor (Ki = 3.2 nM), the dopamine D3 receptor (Ki = 0.58 nM) as well as the serotonin 5-HT1A receptor (Ki = 0.82 nM). The structure-affinity relationships of the series suggest that the 2-aminotetralin moieties of the compounds occupy the same binding sites as the DPATs in all three receptor subtypes. The benzamidoethyl side chain enhances the affinities of the compounds for all three receptor subtypes, presumably by occupying an accessory binding site. For the dopamine D2 and D3 receptors, this accessory binding site may be identical to the binding site of the 2-pyrrolidinylmethyl-derived substituted benzamides.

In Vitro Receptor Binding Characteristics of the New Dopamine D2 Antagonist [125I]NCO-298: Methodological Considerations of High Affinity Binding

The substituted benzamide [125I]NCQ-298 is a recently developed radioligand that has been shown to bind with high affinity and selectivity to dopamine D2 receptors. The present studies were designed to optimize the in vitro receptor binding method of [125I]NCQ-298 and to determine whether it labels the same receptor population as the widely used benzamide [3H]raclopride. Rat striatal D2 receptors and cloned human D2 and D3 receptors were used. We found that due to the high affinity of [125I]NCQ-298 (Kd approximately 20 pmol/l), long incubation time (4 hrs at 30 degrees C) and low receptor concentration (approximately 2 pmol/l) were necessary in order to reach equilibrium and avoid ligand depletion. The optimal composition of the incubation buffer for rat striatal [125I]NCQ-298 binding assays was (in mM): 50 Tris-HCI, 120 NaCl, 5 KCl, 1 MgCl2, 0.01 pargyline, 0.1 EDTA, 0.05 protease inhibitors (PMSF and bacitracin) and 0.01% ascorbic acid. It is concluded that, when studied under correct experimental conditions, [125I]NCQ-298 is an excellent high-affinity D2 receptor radioligand which labels the same receptor population as [3H]raclopride (Bmax values; 32 +/- 3 and 36 +/- 1 pmol/g w.w., respectively).

Derivatives of 2-arylcyclopropylamine: Synthesis and interactions with 5-HT1A receptors.

Abstract A series of cis- and trans -derivatives of2-aryl- N , N -dipropylcyclopropylamines and 1-(2-arylcyclopropyl)- N , N -dipropylmethylamines were synthesized and evaluated for affinity at the 5-HT 1A receptor. The key step in the syntheses was a cyclopropanation of cis- and trans -3-arylpropenoic esters with diazomethane which proceeds with retention of the stereochemistry. cis -1-[2-(3-Methoxyphenyl)cyclopropyl]- N , N -dipropylmethylamine ( 32 ) had the highest 5-HT 1A -receptor affinity (K i = 58 nM) of the novel derivatives.

Novel derivatives of 3-(dipropylamino)chroman. Interactions with 5-HT1A and D2A receptors.

Novel 8-aryl and 8-aroyl substituted derivatives of 3-(dipropylamino)chroman are described. The compounds have been prepared by a palladium catalyzed reaction of iodoarenes and a stannylated derivative of [eta6-3-(dipropylamino)chroman]Cr(CO)3. Several of the compounds have high affinity for 5-HT1A receptors whereas the affinity for D2A receptors is lower, the 8-arylated derivatives being slightly more potent than the 8-aroylated analogues.