Aripiprazole,a novel antipsychotic,is a high-affinity partial agonist at human dopamine D2 receptors

Aripiprazole is the first next-generation atypical antipsychotic with a mechanism of action that differs from currently marketed typical and atypical antipsychotics. Aripiprazole displays properties of an agonist and antagonist in animal models of dopaminergic hypoactivity and hyperactivity, respectively. This study examined the interactions of aripiprazole with a single population of human D2 receptors to clarify further its pharmacologic properties. In membranes prepared from Chinese hamster ovary cells that express recombinant D2L receptors, aripiprazole bound with high affinity to both the G protein-coupled and uncoupled states of receptors. Aripiprazole potently activated D2 receptor-mediated inhibition of cAMP accumulation. Partial receptor inactivation using the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) significantly reduced the maximum effect of aripiprazole on inhibition of cAMP accumulation. This effect was seen with concentrations of EEDQ that did not alter the maximal inhibitory effect of dopamine. Consistent with the expected effects of a partial agonist, increasing concentrations of aripiprazole blocked the action of dopamine with maximal blockade equal to the agonist effect of aripiprazole alone. The efficacy of aripiprazole relative to that of dopamine varied from 25% in cells that lacked spare receptors for dopamine to 90% in cells with receptor reserve. These results, together with previous studies demonstrating partial agonist activity at serotonin 5-hydroxytryptamine (5-HT)1A receptors and antagonist activity at 5-HT2A receptors, support the identification of aripiprazole as a dopamine-serotonin system stabilizer. The receptor activity profile may underlie the unique activity of aripiprazole in animals and its antipsychotic activity in humans.     

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: I. Cellular, electrophysiological, and neurochemical profile in comparison with ropinirole

S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] displayed marked affinity for cloned, human (h)D(3) receptors (pK(i), 8.1) at which, in total G-protein ([(35)S]GTPgammaS binding, guanosine-5'-O-(3-[(35)S]thio)-triphosphate), Galpha(i3) (antibody capture/scintillation proximity), and mitogen-activated protein kinase (immunoblot) activation procedures, it behaved as an agonist: pEC(50) values, 8.7, 8.6, and 8.5, respectively. These actions were blocked by haloperidol and the selective D(3) receptor antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl) benzamide)]. S32504 showed lower potency at hD(2S) and hD(2L) receptors in [(35)S]GTPgammaS binding (pEC(50) values, 6.4 and 6.7) and antibody capture/scintillation proximity (hD(2L), pEC(50), 6.6) procedures. However, reflecting signal amplification, it potently stimulated hD(2L) receptor-coupled mitogen-activated protein kinase (pEC(50), 8.6). These actions were blocked by haloperidol and the selective D(2) receptor antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol]. The affinity of S32504 for hD(4) receptors was low (5.3) and negligible for hD(1) and hD(5) receptors (pK(i), <5.0). S32504 showed weak agonist properties at serotonin(1A) ([(35)S]GTPgammaS binding, pEC(50), 5.0) and serotonin(2A) (G(q), pEC(50), 5.2) receptors and low affinity for other (>50) sites. In anesthetized rats, S32504 (0.0025-0.01 mg/kg, i.v.) suppressed electrical activity of ventrotegmental dopaminergic neurons. Correspondingly, S32504 (0.0025-0.63 mg/kg, s.c.) potently reduced dialysis levels (and synthesis) of dopamine in striatum, nucleus accumbens, and frontal cortex of freely moving rats, actions blocked by haloperidol and L741,626 but not by S33084. In contrast, S32504 only weakly inhibited serotonergic transmission and failed to affect noradrenergic transmission. Actions of S32504 were expressed stereospecifically versus its less active enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbomoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine]. Although the D(3)/D(2) agonist and antiparkinsonian agent ropinirole mimicked the profile of S32504, it was less potent. In conclusion, S32504 is a potent and selective agonist at dopamine D(3) and D(2) receptors.     

Extrastriatal dopamine D2 receptors: distribution, pharmacological characterization and region-specific regulation by clozapine.

The distribution of dopamine D2 receptors in the rat brain was determined by quantitative autoradiography of the binding of [125I]epidepride and the effects of chronic drug administration on regulation of receptors in striatal and extrastriatal brain regions were characterized. [125I]Epidepride (2200 Ci/mmol) bound with high affinity to coronal tissue sections from the rat brain (Kd = 78 pM), and specific binding was detected in a number of discrete layers, nuclei or regions of the hippocampus, thalamus, cerebellum and other extrastriatal sites. Pharmacological analysis of radioligand binding to hippocampal and cerebellar membranes indicated binding to dopamine D2 receptors, and approximately 10% of the binding appeared to represent low affinity idazoxan-displaceable binding to alpha-2 adrenoceptors. The binding to extrastriatal regions resembled previously reported radioligand binding to dopamine D2 receptors in striatal and cortical membranes. Chronic (14 day) administration of two dopamine D2 receptor antagonists, either the typical neuroleptic haloperidol (1.5 mg/kg i.p.) or the atypical neuroleptic clozapine (30 mg/kg i.p.), caused a significant increase in the density of [125I]epidepride binding sites in the medial prefrontal cortex and parietal cortex. Only haloperidol caused a significant increase in the density of [3H]spiperone and [125I]epidepride binding sites in the striatum and a slight increase in [125I]epidepride binding sites in the hippocampus. Similar administration of amphetamine (5 mg/kg i.p.) had no significant effect on the density of dopamine D2 receptors in any brain region examined. In addition, no drug-induced changes in the characteristics of dopamine D2 receptors in discrete areas of the cerebellum were observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: III. Actions in models of potential antidepressive and anxiolytic activity in comparison with ropinirole

In forced-swim tests in mice and rats, the novel D(3)/D(2) receptor agonist S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] dose-dependently (0.04-2.5 mg/kg) and stereospecifically suppressed immobility compared with its enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth-[1,2-b]-1,4-oxazine]. Ropinirole was less potent than S32504 in this procedure, and it was likewise less potent than S32504 (0.04-2.5 mg/kg) in attenuating motor-suppressant properties of the alpha(2)-adrenoceptor agonist S18616 [(S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(1',2',3',4'-tetrahydronaphthalene)]]. In a learned helplessness paradigm, S32504 (0.08-2.5 mg/kg) suppressed escape failures. Furthermore, in a chronic mild stress model of anhedonia, S32504 (0.16-2.5 mg/kg) rapidly restored the suppression of sucrose consumption. S32504 inhibited marble-burying behavior in mice (0.04-0.16 mg/kg) and aggressive behavior in isolated mice (0.04-2.5 mg/kg): only higher doses of ropinirole mimicked these actions of S32504. In tests of anxiolytic activity, S32504 was more potent (0.0025-0.16 mg/kg) than ropinirole in suppressing fear-induced ultrasonic vocalizations, and S32601 was inactive. Furthermore, in contrast to ropinirole, S32504 modestly enhanced punished responses in a Vogel conflict procedure and increased open-arm entries in a plus-maze. At doses active in the above-described procedures, S32504 did not elicit hyperlocomotion. In the forced-swim, marble-burying, and ultrasonic vocalization models, actions of S32504 were blocked by the D(2)/D(3) antagonists haloperidol and raclopride and by the D(2) antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol], but not by the D(3) receptor antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl)benzamide. Finally, chronic administration of S32504 did not, in contrast to venlafaxine, modify corticolimbic levels of serotonin(2A) receptors or brain-derived neurotrophic factor. In conclusion, S32504 displays a broad and distinctive profile of activity in models of potential antidepressive and anxiolytic properties. Its actions are more pronounced than those of ropinirole and principally involve engagement of D(2) receptors.     

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: II. Actions in rodent, primate, and cellular models of antiparkinsonian activity in comparison to ropinirole

These studies evaluated the potential antiparkinsonian properties of the novel dopamine D(3)/D(2) receptor agonist S32504 [(+)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] in comparison with those of the clinically employed agonist ropinirole. In rats with a unilateral, 6-hydroxydopamine lesion of the substantia nigra, S32504 (0.0025-0.04 mg/kg, s.c.) more potently elicited contralateral rotation than S32601 [(-)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth-[1,2-b]-1,4-oxazine (its less active enantiomer)], ropinirole, and l-3,4-dihydroxyphenylalanine (l-DOPA). Rotation elicited by S32504 was blocked by the D(2)/D(3) receptor antagonists haloperidol and raclopride and by the D(2) antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol], but not by the D(3) antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl)benzamide]. As assessed by dialysis in both lesioned and nonlesioned animals, S32504 (0.04-2.5 mg/kg, s.c.) reduced striatal levels of acetylcholine. This effect was blocked by raclopride, haloperidol, and L741,626 but not S33084. In rats treated with reserpine, hypolocomotion was reversed by S32504 and, less potently, by ropinirole. In "unprimed" marmosets treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, both s.c. (0.01-0.04 mg/kg) and p.o. (0.04-1.25 mg/kg) administration of S32504 dose-dependently and rapidly (within 10 min) increased locomotor activity and reduced disability. Furthermore, S32504 dose-dependently reversed bradykinesia and improved posture in "L-DOPA-primed" animals, whereas eliciting less pronounced dyskinesia than l-DOPA. Finally, in terminally differentiated SH-SY5Y cells presenting a dopaminergic phenotype, S32504, but not S32601, abrogated the neurotoxic effects of 1-methyl-4-phenylpyridinium, an action inhibited by raclopride and S33084 but not L741,626. Ropinirole was weakly neuroprotective in this model. In conclusion, S32504 displays potent and stereospecific activity in rodent, primate, and cellular models of antiparkinsonian properties. Although activation of D(2) receptors is crucial to the motor actions of S32504, engagement of D(3) receptors contributes to its neuroprotective properties.     

Effects of chronic SCH23390 treatment on the biochemical and behavioral properties of D1 and D2 dopamine receptors: potentiated behavioral responses to a D2 dopamine agonist after selective D1 dopamine receptor upregulation

Chronic treatment of rats with SCH23390 (0.5 mg/kg/day s.c.), a D1 dopamine receptor antagonist, for 21 days resulted in an increase in D1 dopamine receptors but produced no change in D2 dopamine receptors. During habituation to locomotor activity cages the rats treated chronically with SCH23390 showed significantly higher locomotor activity than controls treated chronically with saline. When injected with the selective D1 dopamine receptor agonist SKF38393 (3 mg/kg), rats treated chronically with SCH23390 showed significantly greater stereotypy and locomotor activity responses. Surprisingly, rats treated chronically with SCH23390 also showed significantly higher locomotor activity and stereotypy responses when treated with the selective D2 dopamine receptor agonist, quinpirole (LY171555) (0.3 mg/kg). These results indicate that a selective increase in D1 receptors may not be necessary, but is sufficient, to lead to an enhanced behavioral response to either selective D1 or D2 dopamine receptor agonists. If, indeed, an enhanced stereotypy and locomotor activity response to dopaminergic agonists in rats after a brief chronic treatment with a neuroleptic drug is predictive of tardive dyskinesia potential in the clinical setting, these results can suggest that SCH23390 may also induce tardive dyskinesia in humans. Adenylate cyclase activity stimulated by guanine nucleotides, forskolin or dopamine was enhanced after chronic treatment with SCH23390. However, dopamine-stimulated adenylate cyclase activity was not potentiated detectably by the increase in receptor number over the more general increase in guanine nucleotide-stimulated cyclic AMP production. Additionally, no change was observed in dopamine competition for [3H]SCH23390 binding, with dopamine's RH/RL ratio remaining unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

The endocannabinoid noladin ether acts as a full agonist at human CB2 cannabinoid receptors

Noladin ether (NE) is a putative endogenously occurring cannabinoid demonstrating agonist activity at CB1 receptors. Because of reported selective affinity for CB1 receptors, the pharmacological actions of NE at CB2 receptors have not been examined. Therefore, the purpose of this study was to characterize the binding and functional properties of NE at human CB2 receptors stably expressed in Chinese hamster ovary (CHO) cells as well as in HL-60 cells, which express CB2 receptors endogenously. Surprisingly, in transfected CHO cells, NE exhibits a relatively high nanomolar affinity for CB2 receptors (K(i) = 480 nM), comparable to that observed for the endocannabinoid 2-arachidonoyl glycerol (2-AG) (K(i) = 1016 nM). Furthermore, NE activates G proteins and inhibits the intracellular effector adenylyl cyclase with equivalent efficacy relative to the full cannabinoid agonists 2-AG and CP 55,940 (CP) [(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol]. The rank order of potency for G protein activation and effector regulation by the three agonists is similar to their apparent affinity for CB2 receptors; CP > NE > or = 2-AG. Regulation of adenylyl cyclase activity by all agonists is inhibited by pertussis toxin pretreatment or by coincubation with AM630 [6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)-methanone], a CB2 antagonist. Chronic treatment with NE or CP results in CB2 receptor desensitization and down-regulation. All agonists also inhibit adenylyl cyclase activity in HL-60 cells. Together, these data indicate that NE acts as a full agonist at human CB2 receptors and thus might have important physiological functions at peripheral cannabinoid receptors.     

Evaluation of histamine H1-, H2-, and H3-receptor ligands at the human histamine H4 receptor: identification of 4-methylhistamine as the first potent and selective H4 receptor agonist

The histamine H(4) receptor (H(4)R) is involved in the chemotaxis of leukocytes and mast cells to sites of inflammation and is suggested to be a potential drug target for asthma and allergy. So far, selective H(4)R agonists have not been identified. In the present study, we therefore evaluated the human H(4)R (hH(4)R) for its interaction with various known histaminergic ligands. Almost all of the tested H(1)R and H(2)R antagonists, including several important therapeutics, displaced less than 30% of specific [(3)H]histamine binding to the hH(4)R at concentrations up to 10 microM. Most of the tested H(2)R agonists and imidazole-based H(3)R ligands show micromolar-to-nanomolar range hH(4)R affinity, and these ligands exert different intrinsic hH(4)R activities, ranging from full agonists to inverse agonists. Interestingly, we identified 4-methylhistamine as a high-affinity H(4)R ligand (K(i) = 50 nM) that has a >100-fold selectivity for the hH(4)R over the other histamine receptor subtypes. Moreover, 4-methylhistamine potently activated the hH(4)R (pEC(50) = 7.4 +/- 0.1; alpha = 1), and this response was competitively antagonized by the selective H(4)R antagonist JNJ 7777120 [1-[(5-chloro-1H-indol-2-yl)-carbonyl]-4-methylpiperazine] (pA(2) = 7.8). The identification of 4-methylhistamine as a potent H(4)R agonist is of major importance for future studies to unravel the physiological roles of the H(4)R.     

123I]epidepride binding to cerebellar dopamine D2/D3 receptors is displaceable: implications for the use of cerebellum as a reference region

The low density of cerebellar dopamine D(2)/D(3) receptors provides the basis for using the cerebellum as a representation of free- and non-specifically bound radioligand in positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies. With the development of ultra high-affinity dopamine D(2)/D(3) ligands like [(123)I]epidepride, [(18)F]fallypride, and [(11)C]FLB-457, quantification of extrastriatal low density receptor populations including the cerebellum is possible with important implications for calculation of binding parameters. [(123)I]epidepride-SPECT was performed in 23 patients with schizophrenia before and after 3 months of antipsychotic treatment with either risperidone (n=14) or zuclopenthixol (n=9). In the unblocked situation and partially blocked situation, the average distribution volumes were 5.2+/-1.3 mL/mL and 4.0+/-0.8 mL/mL, respectively. The paired distribution volumes were reduced by 22+/-15% (mean+/-SD) after antipsychotic treatment (p<0.0001, paired Student's t-test). From the paired distribution volumes in cerebellum and extrastriatal regions, the average distribution volume representing free and non-specifically bound [(123)I]epidepride was calculated to be 3.3+/-0.8 mL/mL. Both the % [(123)I]epidepride fraction of plasma radioactivity (p>0.76) and the plasma [(123)I]epidepride concentration (p>0.45) were unchanged after antipsychotic treatment (paired Student's t-test). These results strongly suggest the presence of "non-negligible" specific [(123)I]epidepride binding to dopamine D(2)/D(3) receptors in the cerebellum. Using the cerebellum as a representation of free and non-specifically bound radioligand and neglecting the specifically bound component may lead to results that erroneously imply that antipsychotic drugs bind to extrastriatal dopamine D(2)/D(3) receptors with a higher affinity than to striatal dopamine D(2)/D(3) receptors.     

G protein coupling and ligand selectivity of the D2L and D3 dopamine receptors

The human dopamine D(2L) receptor couples promiscuously to multiple members of the Galpha(i/o) subfamily. Despite the high homology of the D(2L) and D(3) receptors, the G protein coupling specificity of the human D(3) receptor is less clearly characterized. The primary aim of this study, then, was the parallel characterization of the G protein coupling specificity of the D(2L) and D(3) receptors. By using both receptor-G protein fusion proteins and stable cell lines in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins were expressed in an inducible fashion, we demonstrated highly selective coupling of the D(3) receptor to Galpha(o1). Furthermore, by using the fusion proteins to ensure identical stoichiometry of receptor to G protein for each pairing, a range of ligands displayed higher potency and, for partial agonists, higher efficacy at the D(3) receptor when coupled to Galpha(o1) compared with the D(2L) receptor. The second aim of this study was to investigate the molecular basis of the above differential G protein coupling specificity. The importance of a 12-amino acid sequence from the C-terminal end of the third intracellular loop of the D(2L) receptor in providing promiscuity in G protein coupling was demonstrated using a chimeric D(3)/D(2) receptor in which the equivalent region of the D(3) receptor was exchanged for this sequence. This chimera displayed D(3)-like affinity for [(3)H]spiperone and potency for agonists but gained D(2)-like ability to couple to each of Galpha(i1-3) as well as Galpha(o1).     

Differential regulation of dopamine D2 and D3 receptors by chronic drug treatments

Regulation of the expression of dopamine D2 and D3 receptors in the rat brain was examined using quantitative autoradiography after chronic (14 day) drug treatments designed to increase or decrease dopamine receptor stimulation. Reserpine treatment depleted endogenous dopamine by more than 90% and significantly increased the binding of [(125)I]NCQ 298 to D2 receptors in the nucleus accumbens, ventral pallidum, and substantia nigra. In contrast, this treatment significantly decreased the binding of [(125)I]7-OH-PIPAT to D3 receptors in each of these regions. Chronic stimulation of D2-like receptors with quinpirole (1 mg/kg/day) or 7-OH-DPAT (1 mg/kg/day) produced decreases in [(125)I]NCQ 298 binding in the nucleus accumbens, ventral pallidum, and substantia nigra as expected. As with depletion, chronic stimulation elicited an opposite response from D3 receptors with significant increases observed in the ventral pallidum and substantia nigra. D3 receptor expression in the nucleus accumbens was unchanged. Baclofen (30 mg/kg/day) or continuous administration of the psychomotor stimulant cocaine (20 mg/kg/day) produced no significant changes in D2 or D3 receptor binding in any region examined. Acute administration of the irreversible antagonist EEDQ (10 mg/kg) nearly eliminated D2 receptor binding in all regions, but inactivated D3 receptors only in the VP and SN, suggesting subtype-specific and region-specific differences in receptor occupancy. The existence of regional and subtype-specific heterogeneities in the regulation of these receptors supports the contention that despite their similar pharmacological profiles, D2 and D3 receptors may mediate different functional responses.     

Effects of dopamine D1 ligands on eye blinking in monkeys: efficacy, antagonism, and D1/D2 interactions

Dopamine D1 ligands have been classified and ordered according to efficacy in both in vitro and in vivo studies. In the present experiments, dopamine D1 ligands reported to differ in in vitro efficacy were evaluated for efficacy-related effects on eye blinking in squirrel monkeys. Additional comparisons were made with the effects of D2 receptor agonists and indirect dopamine agonists. The results show that D1 agonists increased eye blinking in an efficacy-related manner, whereas the D1 receptor blocker SCH 39166 [(-)-trans-6,7,7alpha,8,9,13beta-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo[d]naphtho[2,1-b]azepine] only decreased rates of eye blinking. D1 high-efficacy agonists induced rates of eye blinking that were 2- to 3-fold greater than observed with dopamine D2 agonists and indirect agonists. In drug combination experiments, increases in eye blinking induced by the D1 high-efficacy agonist R-(+)-6-Br-APB [R-(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide] were antagonized by both the D1 antagonist SCH 39166 and the lower efficacy agonist SKF 83959 [6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide], consistent with dopamine D1 receptor mediation of these behavioral effects. The dopamine D2 agonist (+)-PHNO [(+)-N-propyl-hydroxynaphthoxazine], which selectively activates dopamine D2 receptors, also attenuated D1 agonist-induced increase in eye blinking, suggesting that D2 receptor actions may inhibit D1-mediated increases in eye blinking. Overall, eye blink rate appears to be a robust behavioral measure that can be used to measure changes in dopaminergic D1 signaling and as a functional assay of agonist efficacy at dopamine D1 receptors.     

Migraine patients show an increased density of dopamine D3 and D4 receptors on lymphocytes

Recent studies have revealed peculiar functional and genetic features of dopamine receptors in migraine. As peripheral blood lymphocytes (PBL) may represent a tool for peripheral detection of neuroreceptors, we compared the expression of dopamine D3 (DRD3) and D4 (DRD4) receptors on PBL in migraine patients and in healthy controls using radioligand binding assay techniques in the presence of antidopamine D2-like receptor antibodies. The dopamine D2-like receptor agonist [3H]7-OH-DPAT was used as a radioligand. An increased density of both DRD3 (P=0.0006) and DRD4 (P=0.002) on PBL was observed in migraineurs compared with controls. This up-regulation might reflect central and/or peripheral dopamine receptor hypersensitivity due to hypofunction of the dopaminergic system. These findings support the view that dopamine D2-like receptors are involved in the determination of the so-called migraine trait, which may help to elucidate several clinical features of the disease.     

Sumanirole, a highly dopamine D2-selective receptor agonist: in vitro and in vivo pharmacological characterization and efficacy in animal models of Parkinson's disease

The purpose of this study is to demonstrate that sumanirole is a novel dopamine receptor agonist with high in vitro and in vivo selectivity for the D(2) receptor subtype. Sumanirole, (R)-5,6-dihydro-5-(methylamino)-4H-imidazo[4,5,1-ij]quinolin-2(1H)-one (Z)-2-butenedioate (1:1), is unique; it has greater than 200-fold selectivity for the D(2) receptor subtype versus the other dopamine receptor subtypes in radioligand binding assays. In cell-based assays, sumanirole is a fully efficacious agonist, with EC(50) values between 17 and 75 nM. In animals, sumanirole elicits many physiological responses attributed to D(2)-like receptor function. In rats, sumanirole is a full agonist for elevation of striatal acetylcholine levels (ED(50) = 12.1 micromol/kg i.p.). Sumanirole s.c. dose dependently decreased plasma prolactin levels and depressed dopamine neuron firing rates in the substantia nigra pars compacta with an ED(50) of 2.3 micromol/kg i.v. This high selectivity for D(2) receptors translates into excellent locomotor stimulant activity in animal models of Parkinson's disease. In reserpinized, alpha-methyl-para-tyrosine-treated rats, sumanirole caused a significant and sustained increase in horizontal activity at doses > or =12.5 micromol/kg s.c. In unilateral 6-hydroxydopamine-lesioned rats, sumanirole caused profound, sustained rotational behavior and was substantially more efficacious than any other agonist tested. Sumanirole-stimulated rotational behavior was blocked by the dopamine receptor antagonist haloperidol. Sumanirole dose dependently improved disability scores and locomotor activities of two of three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys. In summary, sumanirole is the first published selective D(2) receptor agonist. The compound has activity in animal models of dopamine hypofunction and has a high level of efficacy in animal models of Parkinson's disease.     

Ro 19-8022, a nonbenzodiazepine partial agonist at benzodiazepine receptors: neuropharmacological profile of a potential anxiolytic.

The partial agonist at benzodiazepine receptors, Ro 19-8022, has been characterized as a putative anxiolytic drug with an improved side effect profile. This orally active compound is a representative of a quinolizinone structure class and shows potent anticonflict activity in mice and rats. It protects rodents from convulsions induced by pentylenetetrazol, N-methyl-D-aspartic acid and maximal electroshock, as well as against audiogenic seizures, with an efficacy comparable to that of the full agonist alprazolam. No appreciable sedative or motor-impairing effects could be detected up to a very high dose (100 mg/kg) in the horizontal wire test or the rotarod performance test in mice and rats and in spontaneous behavior in monkeys. Consistent with its characterization as a partial agonist, Ro 19-8022 antagonized the motor impairment induced by the full agonists diazepam or meclonazepam measured in horizontal wire and rotarod tests in rodents, and reduced flunitrazepam-induced effects in squirrel monkeys, with an efficacy comparable to that of the benzodiazepine receptor antagonist flumazenil. After subchronic administration of Ro 19-8022 to mice, antagonist-precipitated withdrawal syndrome was dramatically weaker than after alprazolam treatment, which is indicative of a lower physical dependence liability of Ro 19-8022. Pharmacodynamic effects recorded in convulsion and reversal of motor impairment tests after i.v. administration suggest a long duration of action of this compound. Taken together, such preclinical data suggest that benzodiazepine receptor partial agonists with a neurological and behavioral profile such as that of Ro 19-8022 may offer an innovative therapeutic approach to the treatment of anxiety disorders.     

9 alpha,11 beta-prostaglandin F2, a novel metabolite of prostaglandin D2 is a potent contractile agonist of human and guinea pig airways.

Prostaglandin (PG) D2, the predominant prostanoid released from activated mast cells in humans is initially metabolized by reduction of the C-11 keto function to yield 9 alpha,11 beta-PGF2. In this study the airways effects of 9 alpha,11 beta-PGF2 were compared with those of its epimer 9 alpha,11 alpha-PGF2 (PGF2 alpha) and PGD2. 9 alpha,11 beta-PGF2 was a potent contractile agonist of isolated guinea pig trachea and 4-mm human airways in vitro; the potencies of the PGs relative to PGD2 (= 1.00) being 0.65 (NS) and 4.08 (P less than 0.001) for 9 alpha,11 beta-PGF2, and 0.52 (P less than 0.01) and 2.40 (P less than 0.001) for PGF2 alpha, respectively. When inhaled by asthmatic subjects, 9 alpha,11 beta-PGF2 was a potent bronchoconstrictor agent, being approximately equipotent with PGD2 and 28-32 times more potent than histamine (P less than 0.01). These studies suggest that 9 alpha,11 beta-PGF2 is at least equipotent with PGD2 as a bronchoconstrictor agonist, and in being a major metabolite of PGD2, could contribute to the bronchoconstrictor effect of this mast cell-derived mediator in asthma.     

The dopamine D3/D2 agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] protects against acute and cocaine-kindled seizures in mice: further evidence for the involvement of D3 receptors

Previous findings have demonstrated a protective role for dopamine D(3)/D(2) receptor agonists in the convulsant and lethal effects of acutely administered cocaine. Data are provided here to establish that the protection occurs through a D(3)-linked mechanism and that protection is extended to seizure kindling. The D(3) antagonist SB-277011-A [4-quinolinecarboxamide,N-[trans-4-[2-(6-cyano-3,4-dihydro-2(1H)-isoquinolinyl)ethyl]-cyclohexyl]-(9CI)] prevented the anticonvulsant effect of the D(3)/D(2) receptor agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] on cocaine-induced seizures. The protection afforded by the D(3)/D(2) agonist, (+)-PD-128,907, was eliminated in D(3) receptor-deficient mice. In D(2) receptor knockout mice, the anticonvulsant effects of (+)-PD-128,907 were preserved. (+)-PD-128,907 also prevented the acquisition and expression of cocaine-kindled seizures engendered by repeated daily dosing with 60 mg/kg cocaine. (+)-PD-128,907 also blocked the seizures induced in mice fully seizure kindled to cocaine. Although repeated dosing with cocaine increased the potency of cocaine to produce seizures and lethality (decreased ED(50) values), daily coadministration of (+)-PD-128,907 significantly prevented this potency shift. In mice treated daily with cocaine and (+)-PD-128,907, the density, but not the affinity, of D(3) receptors was increased. The specificity with which (+)-PD-128,907 acts upon this cocaine-driven process was demonstrated by the lack of a significant effect of (+)-PD-128,907 on seizure kindling to a GABA(A) receptor antagonist, pentylenetetrazol. Taken together and with literature findings, the data indicate that dopamine D(3) receptors function in the initiation of a dampening mechanism against the toxic effects of cocaine, a finding that might have relevance to psychiatric disorders of drug dependence, schizophrenia, and bipolar depression.     

SK&F 87516, a close analog of fenoldopam, is a partial agonist at dopamine-1 and alpha-2 receptors and produces stimulation of 5-hydroxytryptamine-2 receptors in the cardiovascular system of the rat.

In pentobarbital-anesthetized rats, SK&F 87516 (1.25-80 micrograms/kg/min intravenously over 15 min), the fluoro analog of the selective DA-1 dopamine receptor agonist fenoldopam produced dose-related decreases in carotid artery blood pressure that faded during the infusion period. These effects were abolished by SCH 23390, prolonged by ritanserin, but unchanged by bilateral vagotomy, atenolol, ICI 118,551, idazoxan, methylatropine or S-sulpiride. SK&F 87516 also inhibited the hypotensive effects of clonidine and of the DA-1 receptor agonist fenoldopam, but not of acetylcholine. In pithed rats, SK&F 87516 produced a biphasic vasopressor response. The initial phase was enhanced by SCH 23390 and converted to a transient hypotension by idazoxan. The secondary response was inhibited by ritanserin and enalapril. In pithed, but not in intact rats, SK&F 87516 increased plasma renin activity. In intact rats, SK&F 87516 produced dose-related bradycardic effects that were inhibited (50%) by idazoxan, methylatropine or bilateral vagotomy and abolished by chlorisondamine or pithing. In pithed rats pretreated with either saline or idazoxan, SK&F 87516 reduced the tachycardia to electrical stimulation of preganglionic more than that to postganglionic cardioaccelerator nerve fibers. However, it did not modify heart rate increases evoked by intravenous norepinephrine. In conclusion, SK&F 87516 produces hypotension via vascular DA-1 receptor stimulation. The fading of this effect during the infusion of SK&F 87516 may be related to the partial agonist property of this compound at DA-1 receptors and the stimulation of 5-hydroxytryptamine-2 receptors. SK&F 87516 also behaves as a partial agonist at alpha-2 adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relative selectivity of 6,7-dihydroxy-2-dimethylaminotetralin, N-n-propyl-3-(3-hydroxyphenyl)piperidine, N-n-propylnorapomorphine and pergolide as agonists at striatal dopamine autoreceptors and postsynaptic dopamine receptors

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99), N-n-propyl-3-(3-hydroxyphenylpiperidine [(+/-)-3-PPP], N-n-propylnorapomorphine and pergolide were evaluated for activity on a number of biochemical parameters that are presumed to indicate an agonist effect at dopamine (DA) autoreceptors (antagonism of the gamma-hydroxybutyrate-induced increase in dopa formation), at postsynaptic DA receptors (elevation of acetylcholine levels) or at both types of DA receptors (diminution of DA synthesis and homovanillic acid levels) in rat striatum. All four agents decreased striatal dopa accumulation (in the presence and in the absence of gamma-hydroxybutyrate). N-propylnorapomorphine, pergolide and TL-99 also reduced homovanillic acid levels and increased acetylcholine concentrations in striatum whereas (+/-)-3-PPP was inactive. The compounds were all more potent in diminishing dopa accumulation caused by gamma-hydroxybutyrate treatment than in increasing acetylcholine levels [(+/-)-3-PPP showing the highest dissociation] indicating a preferential agonist activity at DA autoreceptors. The relative selectivity of the compounds for DA autoreceptors and postsynaptic DA receptors was evaluated further by studying the antagonism by these drugs of the activation of striatal dopa formation (index of both DA autoreceptor and postsynaptic DA receptor stimulation) and tyrosine hydroxylase (index of postsynaptic DA receptor stimulation only) induced by haloperidol or reserpine. The DA agonists were all more potent in antagonizing the neuroleptic-induced increase in DA synthesis than in counteracting the drug-induced activation of tyrosine hydroxylase, with (+/-)-3PPP exhibiting the highest dissociation. The present results indicate that the DA agonists studied possess some selectivity for striatal DA autoreceptors, (+/-)-3-PPP being the most selective in this respect.