Derivatives of (R)-2-amino-5-methoxytetralin: antagonists and inverse agonists at the dopamine D2A receptor.

A series of N-arylmethyl substituted (R)-5-methoxy-2-(propylamino)tetralins has been prepared and evaluated for affinity and efficacy at dopamine (DA) D2A receptors. The novel compounds appeared to be antagonists or inverse agonists. (R)-2-[(Benzyl)propylamino]-5-methoxytetralin (7) was characterized as a potent inverse agonists at DA D2A receptors in a [35S]GTPgammaS binding assay.

Novel (R)-2-amino-5-fluorotetralins: dopaminergic antagonists and inverse agonists.

A series of secondary and tertiary N-alkyl derivatives of (R)-2-amino-5-fluorotetralin have been prepared. The affinities of the compounds for [3H]raclopride-labeled cloned human dopamine (DA) D2 and D3 receptors as well as [3H]-8-OH-DPAT-labeled rat hippocampal 5-HT1A receptors were determined. In order to selectively determine affinities for the high-affinity agonist binding site at DA D2 receptors, the agonist [3H]quinpirole was used. The intrinsic activities of the compounds at DA D2 and D3 receptors were evaluated in a [35S]GTP gamma S binding assay. The novel compounds were characterized as dopaminergic antagonists or inverse agonists. The antagonist (R)-2-(butylpropylamino)-5-fluorotetralin (16) bound with high affinity (Ki = 4.4 nM) to the DA D3 receptor and was the most D3-selective compound (10-fold). (R)-2-[[4-(8-Aza-7, 9-dioxospiro[4.5]decan-8-yl)butyl]propylamino]-5-fluorote tralin (18) bound with very high affinity to both DA D3 and 5-HT1A receptors (Ki = 0.2 nM) and was also characterized as a dopaminergic antagonist. (R)-2-(Benzylpropylamino)-5-fluorotetralin (10) behaved as an inverse agonist at both DA D2 and D3 receptors. It decreased the basal [35S]GTP gamma S binding and potently inhibited the DA-stimulated [35S]GTP gamma S binding. It is apparent that the intrinsic activity of a 2-aminotetralin derivative may be modified by varying the N-alkyl substituents.

Derivatives of (R)- and (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin: synthesis and interactions with 5-HT1A receptors.

Analogs of the 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin [(S)-1,(S)-UH301] have been prepared. The C8-substituent has been varied, and in some derivatives one of the N-propyl groups has been exchanged for a 4-(8-aza-7,9-dioxospiro[4.5]decan-8-yl)-butyl group. The novel compounds have been evaluated for affinity to rat brain 5-HT1A receptors in competition experiments with [3H]-8-OH-DPAT. In addition, the efficacy of the compounds was assessed by their ability to inhibit the VIP-stimulated cAMP formation in GH4ZD10 cells expressing rat 5-HT1A receptors. Varying degrees of intrinsic activity was revealed among the compounds tested, i.e., the profiles ranged from full agonists to antagonists. All R-enantiomers are characterized as full agonists at 5-HT1A receptors, whereas partial agonists or antagonists were found among the corresponding S-enantiomers. Substitution of one of the N-propyl groups for a 4-(8-aza-7,9-dioxospiro[4.5]decan-8-yl)butyl group seems to increase efficacy as well as affinity for 5-HT1A receptors. A favorable interaction with an accessory binding site by the N-4-(8-aza-7,9-dioxospiro[4.5]decan-8-yl)butyl group may contribute to the increased affinity.

The 5-HT1A receptor antagonist (S)-UH-301 augments the increase in extracellular concentrations of 5-HT in the frontal cortex produced by both acute and chronic treatment with citalopram.

In a recent study, utilizing single cell recording techniques, we have shown that administration of 5-HT1A receptor antagonists, e.g. (S)-UH-301, to rats concomitantly treated, acute or chronically, with the selective serotonin reuptake inhibitor (SSRI) citalopram significantly increases the activity of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN). Here we report correlative experiments using microdialysis in freely moving animals to measure extracellular levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex, a major projection area for DRN-5-HT neurons. Acute administration of (S)-UH-301 (2.5 mg/kg s.c.) or citalopram (2.0 mg/kg s.c.) increased 5-HT concentrations with a maximum of about 70% and 185%, respectively, above baseline. However, when (S)-UH-301 was administered 30 min before citalopram the maximal increase in 5-HT levels was approximately 400%. In rats chronically treated with citalopram (20 mg/kg/day i.p. for 14 days) basal 5-HT concentrations in the frontal cortex were significantly increased and 5-HIAA concentrations were decreased when measured 10-12 h, but not 18-20 h, after the last injection of citalopram, as compared to basal 5-HT and 5-HIAA concentrations in chronic saline-treated rats. When (S)-UH-301 (2.5 mg/kg s.c.) was administered 12 h, but not 20 h, after the last dose of citalopram it produced a significantly larger increase in extracellular concentrations of 5-HT than in control rats. However, in rats pretreated with a single, very high dose of citalopram, 20 mg/kg i.p., administration of (S)-UH-301 at 12 h after citalopram did not increase 5-HT levels. The augmentation by (S)-UH-301 of the increase in brain 5-HT output produced by acute administration of citalopram is probably due to antagonism of the citalopram induced feedback inhibition of 5-HT cells in the DRN, as previously suggested. However, the capacity of (S)-UH-301 to further increase the already elevated extracellular concentrations of 5-HT in brain in animals maintained on a chronic citalopram regimen, in which significant tolerance to the initial feedback inhibition of DRN-5-HT cells and developed, represents a novel finding. Generally, the reduced feedback inhibition of 5-HT neurons obtained with chronic citalopram treatment, and the associated elevation of brain 5-HT concentrations, may be related to functional desensitization of somatodendritic 5-HT1A autoreceptors in the DRN. This phenomenon may also largely explain the larger increase in 5-HT output produced by (S)-UH-301 in chronic citalopram treated animals as compared to its effect in control animals. Yet, a contributory factor may be a slight, remaining feedback inhibition of the 5-HT cells caused by residual citalopram at 12, but not 20 h after its last administration. Previous clinical studies suggest that addition of a 5-HT1A receptor antagonist to an SSRI in the treatment of depression may accelerate the onset of clinical effects. Moreover, in therapy-resistant cases maintained on SSRI treatment, addition of a 5-HT1A receptor antagonist may improve clinical efficacy. Since the therapeutic effect of SSRIs in depression has been found to be critically linked to the availability of 5-HT in brain, our experiments results support, in principle, both of the above clinically based notions.

The 5-HT1A receptor antagonist (S)-UH-301 decreases dopamine release in the rat nucleus accumbens and striatum.

In this study we employed in vivo microdialysis to examine the effects of the selective 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin [(S)-UH-301] on extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens (NAC) and dorsal striatum of awake freely moving rats. Systemic administration of (S)-UH-301 (1.25, 2.5, 5.0 mg/kg s.c.) dose-dependently decreased extracellular concentrations of DA, DOPAC and HVA in the NAC. (S)-UH-301 (2.5 mg/kg s.c.) also decreased DA, but not DOPAC and HVA, concentrations in the striatum. Infusion of low concentrations (1, 10 microM) of (S)-UH-301 into either the NAC or the striatum did not affect DA levels, while only the highest concentration (1,000 microM) significantly decreased DA levels in both areas. Similarly, infusion of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT] only in high concentrations (100, 1,000 microM) decreased DA levels in both regions. These data suggest that (S)-UH-301 decreases DA release both in the NAC and the striatum probably indirectly via its purported DA-D2/D3 receptor agonistic properties. However, the observed inhibitory effect of (S)-UH-301 on DA release in the studied brain regions may also be explained, at least partly, by a serotonergic influence on the DA systems, acting at 5-HT1A receptor sites located elsewhere in the brain.

Synthesis and pharmacology of the enantiomers of UH301: opposing interactions with 5-HT1A receptors.

The (S)-enantiomer of 5-fluoro-8-hydroxy-2-(dipropylamino) tetralin [(S)-2a; (S)-UH301] was the first reported 5-HT1A receptor antagonist. We now give a full account on the synthetic effort leading to the preparation of the racemate and the enantiomers of 2a. The crystal and molecular structure of 2a. HBr has been determined by X-ray diffraction and the absolute configuration has been deduced using statistical tests of the crystallographic R values. The unit cell is tetragonal (P4(1)2(1)2) with a = b = 13.2235(2), c = 39.560(1) A and contains two crystallographically independent molecules in each asymmetric unit. The two solid state conformers differ in the conformation of the N-propyl groups. The pharmacological characterization of the enantiomers was done by use of in vivo biochemical and behavioural assays in rats. The (R)-enantiomer of 2a is a 5-HT1A receptor agonist of low potency while (S)-2a does not exhibit any agonist properties at 5-HT1A receptors. As a consequence of the opposing effects of the enantiomers, the racemate, rac-2a, does not produce any clear-cut effects in rats. The reduced efficacy of (S)-2a as compared to the well known 5-HT1A receptor agonist 8-hydroxy-2-(dipropylamino) tetralin (1;8-OH-DPAT) may be due to the fluoro-substituent induced negative potential of the aromatic ring.

5-HT1A receptor antagonists increase the activity of serotonergic cells in the dorsal raphe nucleus in rats treated acutely or chronically with citalopram.

In this study we have examined the acute effects of systemic administration of the selective serotonin reuptake inhibitor (SSRI), citalopram, in combination with either of the two selective 5-HT1A receptor antagonists, [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] or (+)-N-tertbutyl 3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenyl-propionamide dihydrochloride [(+)-WAY100135], on the activity of single 5-HT neurons in the dorsal raphe nucleus (DRN) of anesthetized rats using extracellular recording techniques. Acute administration of citalopram (0.3 mg/kg i.v.) significantly decreased the firing rate of DRN-5-HT cells most likely as a result of indirect stimulation of inhibitory somatodendritic 5-HT1A autoreceptors located on 5-HT cells in the DRN. This effect of citalopram was completely reversed by (S)-UH-301 (0.5 mg/kg i.v.) and partly by (+)-WAY100135 (0.5 mg/kg i.v.). Furthermore, the inhibitory effect of citalopram on the activity of 5-HT neurons was significantly attenuated by pretreatment with (S)-UH-301 (0.25 mg/kg i.v.) or (+)-WAY100135 (0.25 mg/kg i.v.). We have also studied the effects of (S)-UH-301 (0.03-0.50 mg/kg i.v.) on the firing rate of single DRN-5-HT cells in rats chronically treated with citalopram (20 mg/kg/day i.p. x 14 days). Administration of (S)-UH-301 significantly and dose-dependently increased the activity of 5-HT cells in citalopram-treated rats, but did not affect these neurons in saline-treated (1 ml/kg/day i.p. x 14 days), control rats. Our results thus suggest that 5-HT1A receptor antagonists can augment both the acute and chronic effects of citalopram on central serotonergic neurotransmission.(ABSTRACT TRUNCATED AT 250 WORDS)

The 5-HT1A receptor antagonist (S)-UH-301 blocks the qR)-8-OH-DPAT-induced inhibition of serotonergic dorsal raphe cell firing in the rat.

(S)-UH-301 [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin, 0.5-4.0 mg/kg i.v.] did not significantly alter the firing rate of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN) as a group, although some individual cells were activated whereas others were depressed. However, (S)-UH-301 (2.0 mg/kg i.v.) consistently reversed the inhibition of DRN-5-HT cells produced by the selective 5-HT1A receptor agonist (R)-8-OH-DPAT (0.5 microgram/kg i.v.) and the dose-response curve for this effect of (R)-8-OH-DPAT was markedly shifted to the right by pretreatment with (S)-UH-301 (1.0 mg/kg i.v.). These results support the notion that (S)-UH-301 acts as an antagonist at central 5-HT1A receptors.

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons.

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Synthesis of (+)-(R)- and (-)-(S)-5-hydroxy-2-methyl-2-dipropylaminotetralin: effects on rat hippocampal output of 5-HT, 5-HIAA, and DOPAC as determined by in vivo microdialysis.

Racemic 5-methoxy-2-methyl-2-dipropylaminotetralin (3) has been prepared by a short synthetic route, in which the N,N-dipropyliminium perchlorate of 5-methoxy-2-tetralone (4) is a key intermediate. Racemic 3 was resolved by crystallization of the corresponding diastereomeric di-p-toluoyltartrates. The enantiomeric excess (%ee) of the phenolic derivatives of (+)-(R)- and (-)-(S)-3 [(+)-(R)- and (-)-(S)-2] was determined by 1H NMR spectroscopic analysis of the corresponding diastereomeric (-)-(R)-1,1'-binaphthyl-2,2'-diylphosphoric acid salts utilizing 13C satellites. X-ray crystallography established the absolute configuration of (-)-(S)-2.HCl. The enantiomers of 2 were tested for hippocampal output of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and dihydroxyphenylacetic acid in rats by use of in vivo microdialysis. The (-)-(S)-enantiomer appeared to affect 5-HT-turnover, whereas (+)-(R)-2 was inactive. Results obtained provide support for the previously reported hypothesis that the inactivity of (-)-(S)-2 at central DA receptors is caused by the steric bulk of the C(2)-methyl group. This makes it possible to define a "DA D2 receptor essential volume."

Neuromedicinal chemistry of 5-HT1A-receptor agonists and antagonists.

Recent progress in a project aiming at developing selective 5-HT1A-receptor agonists and antagonists is reviewed. A large number of analogues of 8-OH-DPAT has been synthesized, and throughout, we have attempted to prepare enantiopure derivatives. Modifications have been concentrated to the N-alkyl substituents, and substitutions in the C1, C2 and C3-positions. The synthetic strategies and procedures are discussed. A number of interesting observations have been made. Affinity, efficacy and stereoselectivity are modified by the various substitutions. The results have been used to deduce a 3D-model for 5-HT1A-receptor agonists. Recently, Pd-catalyzed reactions have been utilized to prepare a number of pharmacologically interesting analogues of 8-OH-DPAT with various C8-substituents. We have also succeeded to convert the agonist 8-OH-DPAT into an antagonist by introduction of a C5-fluoro substituent, producing (S)-UH-301. The pharmacology of this selective 5-HT1A-receptor antagonist is discussed.

Resolved N,N-dialkylated 2-amino-8-hydroxytetralins: stereoselective interactions with 5-HT1A receptors in the brain.

The enantiomers of the N,N-dimethylamino (1), N,N-diethylamino (2), and N,N-dibutylamino (4) derivatives of 8-hydroxy-2-(dipropylamino)tetralin (8-OH DPAT; 3) have been synthesized. The compounds have been tested for activity at central 5-hydroxytryptamine and dopamine receptors by use of biochemical and behavioral tests in rats. In addition, the ability of the enantiomers of 1-4 to displace [3H]-8-OH DPAT from 5-HT1A binding sites was evaluated. Rank order of potencies in the in vivo tests corresponded to that observed in the 5-HT1A binding assay. In all three tests, the enantiomeric potency ratio was about 10 for 1 and 2 and only around 2-4 for 3 and 4. The more potent enantiomer of 1-3 had the R configuration. In contrast, (S)-4 seemed to be slightly more potent than (R)-4.