SSR181507, a dopamine D2 receptor antagonist and 5-HT1A receptor agonist. II: Behavioral profile predictive of an atypical antipsychotic activity.

SSR181507 ((3-exo)-8-benzoyl-N-(((2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl)methyl)-8-azabicyclo(3.2.1)octane-3-methanamine monohydrochloride) is a novel tropanemethanamine benzodioxane that displays antagonist activity at dopamine D(2) receptors and agonist activity at 5-HT(1A) receptors. SSR181507 antagonized apomorphine-induced climbing in mice and stereotypies in rats (ED(50) of 2 and 3.4 mg/kg i.p., respectively) and blocked D-amphetamine-induced hyperlocomotion in rats at lower doses (0.3-1 mg/kg i.p.). At 1-10 mg/kg, it was found to disrupt active avoidance in mice. SSR181507 did not induce catalepsy in rats (MED>60 mg/kg i.p.) and antagonized (3-10 mg/kg i.p.) haloperidol-induced catalepsy. SSR181507 was also active in two models sensitive to antidepressant/anxiolytic drugs: in a guinea-pig pup/mother separation test, it decreased (1-3 mg/kg i.p.) the time spent vocalizing during the separation episode, and in a lithium-induced taste aversion procedure in rats, it partially reversed (3 mg/kg i.p.) the decrease of intake of a saccharin solution. Furthermore, SSR181507 increased (3 mg/kg i.p.) the latency time to paradoxical sleep in rats, an effect commonly observed with antidepressants. Coadministration of the selective 5-HT(1A) blocker SL88.0338 produced catalepsy and antagonized the effects of SSR181507 in the depression/anxiety tests, confirming the view that activation of 5-HT(1A) receptors confers an atypical profile on SSR181507, and is responsible for its antidepressant/anxiolytic properties. Finally, SSR181507 (1-3 mg/kg) did not affect memory performance in a Morris water maze task in rats. The pharmacological profile of SSR181507 suggests that it should control the symptoms of schizophrenia, in the absence of extrapyramidal signs and cognitive deficits, with the additional benefit of antidepressant/anxiolytic activities.     

Dual activation by lisuride of central serotonin 5-HT(1A) and dopamine D(2) receptor sites: drug discrimination and receptor binding studies

To investigate central serotonergic (5-HT) and dopaminergic (DA) actions of lisuride, the discriminative properties of lisuride (0.05mg/kg, i.p.) in rats were investigated, in addition to the radioligand binding of the compound to 5-HT and DA receptor subtypes. Lisuride was found to possess high affinities for 5-HT(1A) receptor sites (Ki=0.5nM) and D(2) receptor sites (Ki=2.0nM). The autoradiographic binding pattern of 4nM [(3)H]lisuride in rat brain showed high densities of sites displaceable by the 5-HT(1A) agonist 8-OH-DPAT in hippocampus, lateral septal nucleus and amygdala, as well as those displaceable by the D(2) antagonist sulpiride in striatum, nucleus accumbens and olfactory tubercle. In drug discrimination tests, the mixed, D(1)/D(2) agonist apomorphine, the partial D(2) receptor agonists (-)-3-PPP and terguride and the 5-HT(1A) agonist 8-OH-DPAT substituted for lisuride. The D(1) agonist SKF38393, the D(1) antagonist SCH23390, the D(2) antagonist sulpiride, the 5HT(1B) agonist m-CPP and the 5-HT(2) agonist DOI were not generalized to the lisuride cue. In antagonism tests, the D(2) antagonist haloperidol and the 5-HT antagonist methysergide both induced partial but significant antagonism to the lisuride cue, but the D(1) antagonist SCH23390 did not. These results indicate that discriminative stimulus properties of lisuride are mediated by the dual activation of 5-HT(1A) and D(2) receptor sites in brain.     

Putative antipsychotics with pronounced agonism at serotonin 5-HT1A and partial agonist activity at dopamine D2 receptors disrupt basal PPI of the startle reflex in rats

INTRODUCTION: Prepulse inhibition (PPI) of the startle reflex has been extensively studied because it is disrupted in several psychiatric diseases, most notably schizophrenia. In rats, and to a lesser extent, in humans, PPI can be diminished by dopamine (DA) D(2)/D(3) and serotonin 5-HT(1A) receptor agonists. A novel class of potential antipsychotics (SSR181507, bifeprunox, and SLV313) possess partial agonist/antagonist properties at D(2) receptors and various levels of 5-HT(1A) activation. MATERIALS AND METHODS: It thus appeared warranted to assess, in Sprague-Dawley rats, the effects of these antipsychotics on basal PPI. RESULTS: SSR181507, sarizotan, and bifeprunox decreased PPI, with a near-complete abolition at 2.5-10 mg/kg; SLV313 had a significant effect at 0.16 mg/kg only. Co-treatment with the 5-HT(1A) receptor antagonist WAY100,635 (0.63 mg/kg) showed that the 5-HT(1A) agonist activity of SSR181507 was responsible for its effect. By contrast, antipsychotics with low affinity and/or efficacy at 5-HT(1A) receptors, such as aripiprazole (another DA D(2)/D(3) and 5-HT(1A) ligand), and established typical and atypical antipsychotics (haloperidol, clozapine, risperidone, olanzapine, quetiapine, and ziprasidone) had no effect on basal PPI (0.01-2.5 to 2.5-40 mg/kg). DISCUSSION: The present data demonstrate that some putative antipsychotics with pronounced 5-HT(1A) agonist activity, coupled with partial agonist activity at DA D(2) receptors, markedly diminish PPI of the startle reflex in rats. CONCLUSIONS: These data raise the issue of the influence of such compounds on sensorimotor gating in humans.     

PD 158771, a potential antipsychotic agent with D(2)/D(3) partial agonist and 5-HT(1A) agonist actions. I. Neurochemical effects

The neurochemical effects of a novel dopamine (DA) D(2)-like and serotonin (5-HT) 5-HT(1A) agonist, PD 158771, are described. PD 158771 exhibited affinities for human D(2L), D(3) and D(4.2) receptors expressed in Chinese hamster ovary (CHO)-K1 cells with K(i) (nM) values of 5.2, 13.7 and 34.8 respectively. PD 158771 showed high affinity for cloned human 5-HT(1A) (K(i) = 2.6 nM) and rat hippocampal 5-HT(1A) receptors (K(i) = 3.5 nM). Weaker affinities were observed at alpha 1-adrenergic (K(i) = 43 nM), histamine H(1) (IC(50) = 30 nM), 5-HT(2A) (K(i) = 24.5 nM) and sigma (sigma) -1 binding sites (K(i) = 24.5 nM). In measures of in vitro functional activity, PD 158771 stimulated [(3)H]thymidine uptake in CHO p-5 cells transfected with hD(3) receptors with a maximal effect of 23% relative to quinpirole. In hD(2)L, the corresponding value was 60% with an EC(50) of 29 nM, again indicating partial DA agonist action of PD 158771. In vivo, PD 158771 produced a dose-related decrease in DA synthesis in the striatum and mesolimbic regions of rat brain treated with gamma-butyrolactone (GBL), indicating a DA autoreceptor agonist action. In animals not treated with GBL, PD 158771 produced a dose-related decrease in DA synthesis and extracellular DA. A decrease in 5-HT synthesis in several brain areas was observed consistent with an agonist response. Further support for DA autoreceptor agonist action is that PD 158771 produced a partial inhibition of the firing of substantia nigra zona compacta DA neurons, an effect reversed by haloperidol. In conclusion, PD 158771 exhibited affinities for DA and 5-HT receptors, appears to possess DA and 5-HT agonist actions; and it could provide improved antipsychotic profile with minimal side effects.     

Actions of novel antipsychotic agents on apomorphine-induced PPI disruption: influence of combined serotonin 5-HT1A receptor activation and dopamine D2 receptor blockade.

The dopamine D1/D2 agonist apomorphine (0.63 mg/kg) disrupted prepulse inhibition (PPI) of acoustic startle in rats, a model of sensorimotor gating deficits observed in schizophrenia. All current antipsychotics, which antagonize D2 receptors, prevent this apomorphine-induced deficit. A novel class of antipsychotics possesses, in addition to D2 antagonist property, various levels of 5-HT1A agonist activity. Considering that the latter itself produces PPI deficits, it appeared necessary to assess the potential of this novel class of antipsychotics to reverse apomorphine-PPI deficits. Potent D2 antagonists, like haloperidol (0.63-2.5 mg/kg), risperidone (0.63-10 mg/kg), and olanzapine (0.63-40 mg/kg) prevented apomorphine PPI disruption. The atypical antipsychotics, clozapine (40 mg/kg), nemonapride (0.01-2.5 mg/kg), ziprasidone (10 mg/kg), and aripiprazole (0.01 and 10 mg/kg), which all exhibit 5-HT1A agonist properties, reversed PPI deficits at some doses only, whereas the anti-dyskinetic agent sarizotan (0.16-10 mg/kg), an efficacious 5-HT1A agonist, did not. New generation antipsychotics with marked 5-HT1A agonist properties, such as SLV313 and SSR181507 (0.0025-10 mg/kg and 0.16-10 mg/kg, respectively) did not reverse these deficits whereas bifeprunox (0.04-2.5 mg/kg) did. To reveal the contribution of 5-HT1A agonist properties in the lack of effects of SLV313 and SSR181507, we pretreated rats with the 5-HT1A antagonist WAY100635 (0.63 mg/kg). Under these conditions, significant reversal of PPI deficit was observed, indicating that D2 antagonist properties of SLV313 and SSR181507 are now sufficient to overcome the disruptive effects of apomorphine. To summarize, antipsychotics possessing agonist efficacy at 5-HT1A receptors exhibit diverse profiles against apomorphine-induced PPI deficits, depending on the balance between D2 and 5-HT1A activities, suggesting that they may display distinct activity on some aspects of gating deficits in schizophrenic patients.     

Novel antipsychotic agents with 5-HT(1A) agonist properties: role of 5-HT(1A) receptor activation in attenuation of catalepsy induction in rats

Compounds possessing 5-HT(1A) agonist properties attenuate catalepsy induced by D(2) receptor blockade. Here we examined the role of 5-HT(1A) receptor agonism in the reduced cataleptogenic potential of several novel antipsychotic agents in the crossed leg position (CLP) and the bar catalepsy tests in rats. When administered alone, ziprasidone produced marked catalepsy, whereas aripiprazole, bifeprunox, SLV313, SSR181507 and sarizotan did not. However, when 5-HT(1A) receptors were blocked with the selective antagonist, WAY100635 (0.63 mg/kg, SC), robust cataleptogenic properties of SLV313, bifeprunox and sarizotan were unmasked and the catalepsy induced by ziprasidone was accentuated. In contrast, only modest catalepsy was induced by aripiprazole and SSR181507, even following a higher dose of WAY100635 (2.5 mg/kg). This suggests that these compounds possess other anti-cataleptic properties, such as partial agonism at dopamine D(2) receptors. The capacity to reverse neuroleptic-induced catalepsy was investigated in interaction studies with haloperidol (2.5 mg/kg, SC). Whereas ziprasidone and aripiprazole did not markedly reduce the effects of haloperidol, SLV313 and sarizotan attenuated CLP catalepsy. In contrast, SSR181507 and bifeprunox potently inhibited both CLP and bar catalepsy. Taken together, these data show that 5-HT(1A) receptor activation reduces the cataleptogenic potential of novel antipsychotic agents but indicate marked diversity in the contribution of 5-HT(1A) and/or other mechanisms to the profiles of the drugs.     

Partial agonist properties of the antipsychotics SSR181507, aripiprazole and bifeprunox at dopamine D2 receptors: G protein activation and prolactin release

Dopamine D2 receptor antagonists induce hyperprolactinemia depending on the extent of D2 receptor blockade. We compared the effects of the new antipsychotic agents SSR181507 ((3-exo)-8-benzoyl-N-[[(2 s)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl]methyl]-8-azabicyclo[3.2.1]octane-3-methanamine monohydrochloride), bifeprunox (DU127090: 1-(2-Oxo-benzoxazolin-7-yl)-4-(3-biphenyl)methylpiperazinemesylate) and SLV313 (1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine) with those of aripiprazole (7-{4-[4-(2,3-dichlorophenyl)-1-piperazinyl]-butyloxy)-3,4-dihydro-2(1 H)-quinolinone), clozapine and haloperidol, on functional measures of dopamine D2 receptor activity in vitro and in vivo: [35S]-GTPgammaS binding to membranes from Sf9 insect cells expressing human dopamine D2 Long (hD2 L) receptors, and serum prolactin levels in the rat. All compounds antagonized apomorphine-induced G protein activation at dopamine hD2 L receptors. Antagonist potencies of aripiprazole, bifeprunox and SLV313 were similar to haloperidol (pK(b) = 9.12), whereas SSR181507 (8.16) and clozapine (7.35) were less potent. Haloperidol, SLV313 and clozapine were silent antagonists but SSR181507, bifeprunox and aripiprazole stimulated [35S]-GTPgammaS binding by 17.5%, 26.3% and 25.6%, respectively, relative to 100 microM apomorphine (Emax = 100%). pEC50s were: SSR181507, 8.08; bifeprunox, 8.97; aripiprazole, 8.56. These effects were antagonized by raclopride. Following oral administration in vivo, the drugs increased prolactin release to different extents. SLV313 and haloperidol potently (ED50 0.12 and 0.22 mg/kg p.o., respectively) stimulated prolactin release up to 86 and 83 ng/ml. Aripiprazole potently (ED50 0.66 mg/kg p.o.) but partially (32 ng/ml) induced prolactin release. SSR181507 (ED50 4.9 mg/kg p.o.) also partially (23 ng/ml) enhanced prolactin release. Bifeprunox only weakly increased prolactin at high doses (13 ng/ml at 40 mg/kg) and clozapine only affected prolactin at the highest dose tested (41 ng/ml at 40 mg/kg). Prolactin levels of the corresponding vehicle-treated animals were <4.3 ng/ml. These data show that (1) SSR181507, aripiprazole and bifeprunox, but not SLV313, are partial agonists at dopamine hD2 L receptors in vitro; (2) SSR181507, bifeprunox and aripiprazole exhibit reduced prolactin release in vivo compared with drugs that are neutral antagonists at dopamine D2 receptors.     

LY367265, an inhibitor of the 5-hydroxytryptamine transporter and 5-hydroxytryptamine(2A) receptor antagonist: a comparison with the antidepressant, nefazodone

The potential antidepressant, LY367265 (1-[2-[4-(6-fluoro-1H-indol-3-yl)-3, 6-dihydro-1(2H)-pyridinyl]ethyl]-5,6-dihydro-1H,4H-[1,2, 5]thiadiazolo[4.3.2-ij]quinoline-2,2,-dioxide) has been shown to have a higher affinity for the 5-hydroxytryptamine (5-HT) transporter (K(i)=2.3 nM) and 5-HT(2A) (K(i)=0.81 nM) receptor than the clinically effective antidepressant, nefazodone. It is a potent inhibitor of [3H]5-HT uptake into rat cortical synaptosomes (IC(50)=3.1 nM) and shows selectivity over that for [3H]noradrenaline (IC(50)>1000 nM). It potentiates potassium-induced [3H]5-HT outflow from prelabelled guinea pig cortical slices both in the presence (EC(50)=950 nM) and absence (EC(50)=250 nM) of a saturating concentration of the 5-HT transport inhibitor, paroxetine, indicating a low level of activity at the 5-HT(1B/1D) autoreceptor. These studies indicate that LY367265 is a putative antidepressant which, because of its 5-HT(2A) receptor antagonist activity, has the potential to produce less sleep disturbance and sexual dysfunction than selective serotonin uptake inhibitors.     

In vivo evidence that 5-HT2C receptor antagonist but not agonist modulates cocaine-induced dopamine outflow in the rat nucleus accumbens and striatum.

During recent years, much attention has been devoted at investigating the modulatory role of central 5-HT(2C) receptors on dopamine (DA) neuron activity, and it has been proposed that these receptors modulate selectively DA exocytosis associated with increased firing of DA neurons. In the present study, using in vivo microdialysis in the nucleus accumbens (NAc) and the striatum of halothane-anesthetized rats, we addressed this hypothesis by assessing the ability of 5-HT(2C) agents to modulate the increase in DA outflow induced by haloperidol and cocaine, of which the effects on DA outflow are associated or not with an increase in DA neuron firing, respectively. The intraperitoneal administration of cocaine (10-30 mg/kg) induced a dose-dependent increase in DA extracellular levels in the NAc and the striatum. The effect of 15 mg/kg cocaine was potentiated by the mixed 5-HT(2C/2B) antagonist SB 206553 (5 mg/kg i.p.) and the selective 5-HT(2C) antagonist SB 242084 (1 mg/kg i.p.) in both brain regions. The mixed 5-HT(2C/2B) agonist, Ro 60-0175 (1 mg/kg i.p.), failed to affect cocaine-induced DA outflow, but reduced significantly the increase in DA outflow induced by the subcutaneous administration of 0.1 mg/kg haloperidol. The obtained results provide evidence that 5-HT(2C) receptors exert similar effects in both the NAc and the striatum, and they modulate DA exocytosis also when its increase occurs independently from an increase in DA neuron impulse activity. Furthermore, they show that 5-HT(2C) agonists, at variance with 5-HT(2C) antagonists, exert a preferential control on the impulse-stimulated release of DA.     

Effects of novel antipsychotics with mixed D(2) antagonist/5-HT(1A) agonist properties on PCP-induced social interaction deficits in the rat

Considerable interest has arisen in identifying antipsychotic agents with improved efficacy against negative symptoms, such as social withdrawal. In rats, a social interaction deficit can be induced by the NMDA antagonist phencyclidine (PCP). Here, we examined the effects of antipsychotics, reported to exert dual 5-HT(1A)/D(2) actions, on PCP-induced social interaction deficits. Drugs were administered daily for 3 days in combination with either vehicle or PCP (2.5mg/kg, SC) and social interaction was measured on the last day of drug treatment. Pairs of unfamiliar rats receiving the same treatment were placed in a large open field for 10 min and the number of social behaviors were scored. The results indicate that: (1) PCP significantly reduced social interaction by over 50% compared with vehicle-treated controls; (2) haloperidol (0.0025-0.16 mg/kg, SC) and clozapine (0.04-10mg/kg, IP) did not reverse PCP-induced social interaction deficits; (3) the substituted benzamide remoxipride reversed PCP-induced deficits at 0.63 and 2.5mg/kg (4) the 5-HT(1A) agonist 8-OH-DPAT was inactive (at 0.01-0.63 mg/kg, SC); (5) among compounds reported to exert dual 5-HT(1A)/D(2) actions, SSR181507 (at 0.16 mg/kg, SC) and aripiprazole (at 0.04 and 0.16 mg/kg, IP), but not ziprasidone (0.04-2.5mg/kg, IP), SLV313 (0.0025-0.16 mg/kg, SC) or bifeprunox (0.01-0.63 mg/kg, IP), significantly reversed PCP-induced social interaction deficits; and (6) the 5-HT(1A) receptor antagonist WAY100635 blocked the effects of SSR181507 and aripiprazole. These findings indicate that the balance of activity at 5-HT(1A) and D(2) receptors profoundly influences the activity of antipsychotics in this model of social withdrawal, and their potential benefit on at least some of the negative symptoms of schizophrenia.     

AHR-16303B, a novel antagonist of 5-HT2 receptors and voltage-sensitive calcium channels.

In vivo and in vitro methods were used to characterize AHR-16303B, a novel compound with antagonistic action at 5-HT2 receptors and voltage-sensitive calcium channels. The 5-HT2 receptor-antagonistic properties of AHR-16303B were demonstrated by inhibition of (a) [3H]ketanserin binding to rat cerebral cortical membranes (IC50 = 165 nM); (b) 5-hydroxytryptamine (5-HT)-induced foot edema in rats (minimum effective dose, (MED) = 0.32 mg/kg orally, p.o.); (c) 5-HT-induced vasopressor responses in spontaneously hypertensive rats (SHR) (ID50 = 0.18 mg/kg intravenously (i.v.), 1.8 mg/kg p.o.), (d) 5-HT-induced antidiuresis in rats (MED = 1 mg/kg p.o.), and (e) platelet aggregation induced by 5-HT + ADP (IC50 = 1.5 mM). The calcium antagonist properties of AHR-16303B were demonstrated by inhibition of (a) [3H]nimodipine binding to voltage-sensitive calcium channels on rabbit skeletal muscle membranes (IC50 = 15 nM), (b) KCl-stimulated calcium flux into cultured PC12 cells (IC50 = 81 nM), and (c) CaCl2-induced contractions of rabbit thoracic aortic strips (pA2 = 8.84). AHR-16303B had little or no effect on binding of radioligands to dopamine2 (DA2) alpha 1, alpha 2, H1, 5-HT1 alpha, beta 2, muscarinic M1, or sigma opioid receptors; had no effect on 5-HT3 receptor-mediated vagal bradycardia; and had only minor negative inotropic, chronotropic, and dromotropic effects on isolated guinea pig atria. In conscious SHR, 30 mg/kg p.o. AHR-16303B completely prevented the vasopressor responses to i.v. 5-HT, and decreased blood pressure (BP) by 24% 3 h after dosing.     

Lu 35-138 ((+)-(S)-3-{1-[2-(1-acetyl-2,3-dihydro-1H-indol-3-yl)ethyl]-3,6-dihydro-2H-pyridin-4-yl}-6-chloro-1H-indole), a dopamine D4 receptor antagonist and serotonin reuptake inhibitor: characterisation of its in vitro profile and pre-clinical antipsychotic potential

The present study describes the pharmacological profile of the putative antipsychotic drug Lu 35-138 ((+)-(S)-3-{1-[2-(1-acetyl-2,3-dihydro-1H-indol-3-yl)ethyl]-3,6-dihydro-2H-pyridin-4-yl}-6-chloro-1H-indole). The in vitro receptor profile of Lu 35-138 revealed high affinity (K(i)=5 nM) and competitive antagonism (K(b)=8 nM) at dopamine D(4) receptors combined with potent 5-HT uptake inhibition (IC(50)=3.2 nM) and moderate alpha(1)-adrenoceptor affinity (K(i)=45 nM). In vivo, Lu 35-138 selectively counteracted hyperlocomotion induced by d-amphetamine (0.5 mg/kg; ED(50)=4.0 mg/kg, s.c.) in rats and phencyclidine (PCP; 2.5 mg/kg; ED(50)=13 mg/kg, s.c.) in mice. Lu 35-138 was unable to affect hyperlocomotion induced by a high dose of d-amphetamine (2.0 mg/kg), which indicates a preferential action on limbic versus striatal structures. A similar limbic selectivity of Lu 35-138 was indicated in voltammetric measure of dopamine output in the core and shell subdivisions of the nucleus accumbens in rats. Furthermore, a relatively large dose of Lu 35-138 (18 mg/kg, s.c.) counteracted d-amphetamine-induced disruption of pre-pulse inhibition in rats and repeated administration of Lu 35-138 (0.31 or 1.25 mg/kg, p.o. once daily for 3 weeks) reduced the number of spontaneously active dopamine neurones in the ventral tegmental area, underlining its antipsychotic-like profile. Lu 35-138 failed to induce catalepsy in rats or dystonia in Cebus apella monkeys and did not deteriorate spatial memory in rats as assessed by water maze performance. Collectively, these results suggest that Lu 35-138 possesses antipsychotic activity combined with a low extrapyramidal and cognitive side effect liability.     

Pharmacological profile of SB-357134: a potent, selective, brain penetrant, and orally active 5-HT(6) receptor antagonist

N-(2,5-Dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulfonamide (SB-357134) potently inhibited [125I]SB-258585 and [3H]LSD binding in a HeLa cell line expressing human 5-HT(6) receptors (pK(i)=8.6 and 8.54, respectively). Furthermore, SB-357134 inhibited [125I]SB-258585 binding in human caudate--putamen and in rat and pig striatum membranes (pK(i)=8.82, 8.44, and 8.61, respectively). SB-357134 displayed over 200-fold selectivity for the 5-HT(6) receptor versus 72 other receptors and enzymes. 5-HT-stimulated cyclic AMP (cAMP) accumulation in human 5-HT(6) receptors was competitively antagonised by SB-357134 (pA(2)=7.63). SB-357134 inhibited ex vivo [125I]SB-258585 binding in the rat with an ED(50) of 4.9 +/- 1.3 mg/kg po, 4 h postdose. In the rat maximal electroshock seizure threshold (MEST) test, SB-357134 produced a potent and dose-dependent increase in seizure threshold, with a minimum effective dose of 0.1 mg/kg po. At 10 mg/kg po, maximum activity occurred between 4 and 6 h postdose. Good exposure was observed with SB-357134 at 10 mg/kg po, reaching maximal blood and brain concentrations of 4.3 +/- 0.2 and 1.3 +/- 0.06 microM, respectively, 1 h postdose. In addition, SB-357134 (10 mg/kg po) enhanced memory and learning following chronic administration (twice a day for 7 days) in the rat water maze. Overall, these studies demonstrate that SB-357134 is a potent, selective, brain penetrant, and orally active 5-HT(6) receptor antagonist.     

Characterization of SB-269970-A, a selective 5-HT(7) receptor antagonist

The novel 5-HT(7) receptor antagonist, SB-269970-A, potently displaced [(3)H]-5-CT from human 5-HT(7(a)) (pK(i) 8.9+/-0.1) and 5-HT(7) receptors in guinea-pig cortex (pK(i) 8.3+/-0.2). 5-CT stimulated adenylyl cyclase activity in 5-HT(7(a))/HEK293 membranes (pEC(50) 7.5+/-0.1) and SB-269970-A (0.03 - 1 microM) inhibited the 5-CT concentration-response with no significant alteration in the maximal response. The pA(2) (8.5+/-0.2) for SB-269970-A agreed well with the pK(i) determined from [(3)H]-5-CT binding studies. 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes (pEC(50) of 8.4+/-0.2) was inhibited by SB-269970-A (0.3 microM) with a pK(B) (8.3+/-0.1) in good agreement with its antagonist potency at the human cloned 5-HT(7(a)) receptor and its binding affinity at guinea-pig cortical membranes. 5-HT(7) receptor mRNA was highly expressed in human hypothalamus, amygdala, thalamus, hippocampus and testis. SB-269970-A was CNS penetrant (steady-state brain : blood ratio of ca. 0.83 : 1 in rats) but was rapidly cleared from the blood (CLb=ca. 140 ml min(-1) kg(-1)). Following a single dose (3 mg kg(-1)) SB-269970 was detectable in rat brain at 30 (87 nM) and 60 min (58 nM). In guinea-pigs, brain levels averaged 31 and 51 nM respectively at 30 and 60 min after dosing, although the compound was undetectable in one of the three animals tested. 5-CT (0.3 mg kg(-1) i.p.) induced hypothermia in guinea-pigs was blocked by SB-269970-A (ED(50) 2.96 mg kg(-1) i.p.) and the non-selective 5-HT(7) receptor antagonist metergoline (0.3 - 3 mg kg(-1) s.c.), suggesting a role for 5-HT(7) receptor stimulation in 5-CT induced hypothermia in guinea-pigs. SB-269970-A (30 mg kg(-1)) administered at the start of the sleep period, significantly reduced time spent in Paradoxical Sleep (PS) during the first 3 h of EEG recording in conscious rats.     

EMD 281014, a new selective serotonin 5-HT2A receptor antagonist

The 5-HT2A receptor ligand 7-[4-[2-(4-fluoro-phenyl)-ethyl]-piperazine-1-carbonyl]-1H-indole-3-carbonitrile HCl (EMD 281014) selectively binds to human (h) and rat 5-HT2A receptors (IC50 values 0.35 and 1 nM, respectively; vs. 1334 nM for h5-HT2C) and inhibited 5-HT-stimulated [35S]guanosine 5'-O-3-thiotriphosphate (GTPgammaS)-accumulation in h5-HT2A transfected Chinese hamster ovary cells (IC50 9.3 nM). EMD 28014 counteracted the N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced decrease of [3H]ketanserin binding in rat frontal cortex (ID50 0.4 mg/kg p.o.) and R-(-)-1-(2,5-dimethoxy-4-iodophenyl)-aminopropane (DOI)-induced head-twitch behaviour in mice (ID50 0.01 mg/kg s.c., 0.06 mg/kg p.o.), demonstrating unique selectivity and efficacy.     

Daily LSD administration selectively decreases serotonin2 receptor binding in rat brain

The effect of ten daily injections of saline or d-lysergic acid diethylamide (LSD) (260 micrograms/kg i.p.) on serotonin1 (5-hydroxytryptamine1, 5-HT1) and 5-HT2 receptor binding was determined in brain membranes from rats killed 24 h after the last injection. [3H]LSD (3.0 nM) was used with either 30.0 nM 5-HT or 70.0 nM cinanserin to estimate 5-HT1 and 5-HT2 receptors, respectively. LSD administration decreased 5-HT2 binding in cortex, striatum, hippocampus, and diencephalon/midbrain without altering 5-HT1 or total specific binding.     

The role of the 5-HT1D receptor as a presynaptic autoreceptor in the guinea pig

The present study investigated the role of the 5-hydroxytryptamine (5-HT, serotonin)1D receptor as a presynaptic autoreceptor in the guinea pig. In keeping with the literature, the 5-HT1B selective antagonist, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydrospiro [furo[2,3-f]indole-3,4'-piperidine]oxalate (SB224289) potentiated [3H]5-HT outflow from pre-labelled slices of guinea pig cerebral cortex confirming its role as a presynaptic autoreceptor in this species. In addition, the 5-HT1D receptor-preferring antagonists, 1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-2H-pyridin-1-yl]-ethyl]-3-pyridin-4-yl-methyl-tetrahydro-pyrimidin-2-one (LY367642), (R)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456219), (S)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456220) and 1-[2-[4-(4-fluoro-benzoyl)-piperidin-1-yl]-ethyl]-3,3-dimethyl-1,2-dihydro-indol-2-one (LY310762), potentiated [3H]5-HT outflow from this preparation with potencies (EC50 values=31-140 nM) in the same range as their affinities for the guinea pig 5-HT1D receptor (Ki values=100-333 nM). The selective 5-HT1D receptor agonist, R-2-(4-fluoro-phenyl)-2-[1-[3-(5-[1,2,4]triazol-4-yl-1H-indol-3-yl)-propyl]-piperidin-4-ylamino]-ethanol dioxylate (L-772,405), inhibited [3H]5-HT outflow. In microdialysis studies, administration of either SB224289 or LY310762 at 10 mg/kg by the intraperitoneal (i.p.) route, potentiated the increase in extracellular 5-HT concentration produced by a maximally effective dose of the selective serotonin re-uptake inhibitor, fluoxetine (at 20 mg/kg i.p.). In addition, the 5-HT1D receptor-preferring antagonist and 5-HT transporter inhibitor, LY367642 (at 10 mg/kg i.p.), elevated extracellular 5-HT concentrations to a greater extent than a maximally effective dose of fluoxetine. It is concluded that the 5-HT1D receptor, like the 5-HT1B receptor, may be a presynaptic autoreceptor in the guinea pig.     

Evidence that imipramine activates 5-HT1C receptor function.

The anti-immobility effect of imipramine (15 mg/kg) in the forced swimming test in mice was antagonized by the non-selective 5-hydroxytryptamine (5-HT) antagonist, metitepine (0.5 mg/kg), by the 5-HT1C/5-HT2 antagonist, mesulergine (15 mg/kg), and by the dopamine D2 antagonist, d,l-sulpiride (50 mg/kg). These three antagonists did not alter the behaviour of imipramine-treated mice in an open-field and did not reduce imipramine brain levels. The 5-HT2 antagonist, ritanserin (0.06 mg/kg), the 5-HT1A/5-HTB antagonist, l-propranolol (20 mg/kg), and the 5-HT3 antagonists, endo-2,3-dihydro-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2-oxo-1H- benzimidazole-1-carboxamide hydrochloride (DAU 6215; 0.1 mg/kg) and 1,2,3,9-tetrahydro-9-methyl-3[(2-methyl-1H-imidazol-1-yl)methyl]-4H- carbazol-4-one, HCl.2H2O) (GR 38032F; 0.1 mg/kg), failed to reduce imipramine-induced anti-immobility. Subthreshold doses of 8-hydroxy-2-(di-n-propylamino)tetralin hydrochloride (8-OH-DPAT; 0.5 mg/kg) and imipramine (7.5 mg/kg) did not synergize in reducing immobility. d,l-Sulpiride, but not mesulergine, antagonized the effect of desipramine (15 mg/kg) in the forced swimming test. All compounds were administered i.p. 6 min before imipramine or desipramine, given i.p. 30 min before the testing. Imipramine produced 50% inhibition of [3H]mesulergine binding to 5-HT1C receptors at 10 microM, a concentration below that obtained following i.p. imipramine administration. The results suggest a contribution of 5-HT1C receptors in the mechanism of the imipramine effect in the forced swimming test.     

PD 158771, a potential antipsychotic agent with D2/D3 partial agonist and 5-HT(1A) agonist actions. II. Preclinical behavioral effects

PD 158771 has been described in receptor binding and biochemical tests as a partial agonist at dopamine (DA) D2 and D3 receptors as well as an agonist at serotonin (5-HT)(1A) receptors. The present studies describe the profile of PD 158771 in rodent and primate behavioral tests. PD 158771 reduced spontaneous locomotor activity in mice (ED(50)=0.38 mg/kg, i.p.) and rats (ED(50) = 1.2 mg/kg, i.p. and 0.16 mg/kg, s.c.), and reduced amphetamine-stimulated locomotion in mice (ED(50) = 0.13 mg/kg, i.p.). At relatively higher doses up to 3 mg/kg, s.c. in rats, PD 158771 did not produce locomotor stimulation or induce stereotypy, indicating a lack of postsynaptic DA agonist activity. PD 158771 reduced apomorphine stimulated locomotion in rats at a dose 4.6-fold greater than those that reduced spontaneous locomotor activity, indicating weak postsynaptic DA antagonist actions; results consistent with a partial agonist profile. PD 158771 produced anxiolytic-like effects in the water-lick (Vogel) conflict test, effects possibly due to the 5-HT(1A) activity. However, PD 158771 was inactive in the water wheel behavioral despair model in rats, indicating lack of antidepressant properties. Similar to known antipsychotics, PD 158771 produced a potent and long-lasting inhibition of conditioned avoidance responding in squirrel monkeys. In contrast to standard antipsychotics, and similar to clozapine, PD 158771 did not cause catalepsy in rats at a dose 20-fold higher than the ED(50) dose for locomotor inhibition. PD 158771 also had a somewhat lower liability than haloperidol to produce extrapyramidal dysfunction in squirrel and cebus monkeys sensitized to the dystonic effects of haloperidol. The data indicate that PD 158771 is a DA partial agonist with weak intrinsic activity that selectively activates brain DA autoreceptors. PD 158771 produced behavioral effects consistent with potential antipsychotic and anxiolytic efficacy, and has an improved profile in the extrapyramidal side effect model when compared to certain currently available antipsychotic agents.     

SB 242084, a selective serotonin2C receptor antagonist, increases dopaminergic transmission in the mesolimbic system.

Electrophysiological techniques and in vivo microdialysis were used to investigate the effect of SB 242084, a potent and selective 5-HT2C receptor antagonist in the control of nigro-striatal and mesolimbic dopaminergic function. Thus, extracellular single unit recordings were performed from neurochemically-identified dopamine (DA) neurons in the substantia nigra, pars compacta (SNc) and the ventral tegmental area (VTA), as well as monitoring of striatal and accumbal basal DA release in anesthetized rats following the administration of SB 242084 and RO 60-0175. Administration of SB 242084 (160-640 microg/kg, i.v.) caused a dose-dependent increase in the basal firing rate of VTA DA neurons, reaching its maximum (27.8+/-6%, above baseline) after 640 microg/kg. Moreover, bursting activity was significantly enhanced by SB 242084 in the VTA. On the other hand, SB 242084 (160-640 microg/kg, i.v.) did not cause any significant change in the basal firing rate and bursting activity of DA neurons in the SNc. Injection of the 5-HT2C receptor agonist RO 60-0175 (80-320% microg/kg, i.v.) dose-dependently decreased the basal firing of DA neurons in the VTA but not in the SNc. RO 60-0175 exerted its maximal inhibitory effect (53.9+/-15.1%, below baseline) in the VTA at the dose of 320 microg/kg. Basal DA release (34.8+/-9%, above baseline) and dihydroxyphenylacetic acid (DOPAC) efflux (19.7+/-7%, above baseline) were significantly enhanced in the nucleus accumbens following the intraperitoneal administration of 10 mg/kg SB 242084. Intraperitoneal injection of 5 mg/kg SB 242084 significantly increased DA release (16.4+/-6%, above baseline) in the nucleus accumbens, but did not affect DOPAC efflux. In the striatum, SB 242084 (5 and 10 mg/kg, i.p.) only slightly increased DA release above baseline (3.5+/-4 and 11.2+/-6%, respectively), without affecting DOPAC efflux in this area. However, the effect of SB 242084 in the striatum was rendered more evident by the fact that injection of the vehicle used to dissolve the drug in a group of control rats, significantly reduced basal DA output by 19.6+/-7%. Stimulation of 5-HT2C receptors by RO 60-0175 (1 mg/kg, i.p.) significantly decreased DA release in the nucleus accumbens by 26.1+/-4% (below baseline) 60 min after injection. On the other hand, RO 60-0175 (1 mg/kg, i.p.) did not cause any significant change of DA release in the striatum. However, DOPAC efflux was reduced by RO 60-0175 (1 mg/kg, i.p.) both in the striatum and the nucleus accumbens. Taken together, these data indicate that the central 5-HT system exerts a tonic and phasic inhibitory control on mesolimbic DA neuron activity and that 5-HT2C receptor subtypes are involved in this effect. Moreover, these findings might open new possibilities for the employment of 5-HT2C receptor antagonists in the treatment of neuropsychiatric disorders related to a hypofunction of central DA neurons.