Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs.

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Comparative Pro-cognitive and Neurochemical Profiles of Glycine Modulatory Site Agonists and Glycine Reuptake Inhibitors in the Rat: Potential Relevance to Cognitive Dysfunction and Its Management.

Frontocortical NMDA receptors are pivotal in regulating cognition and mood, are hypofunctional in schizophrenia, and may contribute to autistic spectrum disorders. Despite extensive interest in agents potentiating activity at the co-agonist glycine modulatory site, few comparative functional studies exist. This study systematically compared the actions of the glycine reuptake inhibitors, sarcosine (40-200 mg/kg) and ORG24598 (0.63-5 mg/kg), the agonists, glycine (40-800 mg/kg), and D-serine (10-160 mg/kg) and the partial agonists, S18841 (2.5 mg/kg s.c.) and D-cycloserine (2.5-40 mg/kg) that all dose-dependently prevented scopolamine disruption of social recognition in adult rats. Over similar dose ranges, they also prevented a delay-induced impairment of novel object recognition (NOR). Glycine reuptake inhibitors specifically elevated glycine but not D-serine levels in rat prefrontal cortical (PFC) microdialysates, while glycine and D-serine markedly increased levels of glycine and D-serine, respectively. D-Cycloserine slightly elevated D-serine levels. Conversely, S18841 exerted no influence on glycine, D-serine, other amino acids, monamines, or acetylcholine. Reversal of NOR deficits by systemic S18841 was prevented by the NMDA receptor antagonist, CPP (20 mg/kg), and the glycine modulatory site antagonist, L701,324 (10 mg/kg). S18841 blocked deficits in NOR following microinjection into the PFC (2.5-10 µg/side) but not the striatum. Finally, in rats socially isolated from weaning (a neurodevelopmental model of schizophrenia), S18841 (2.5 and 10 mg/kg s.c.) reversed impairment of NOR and contextual fear-motivated learning without altering isolation-induced hyperactivity. In conclusion, despite contrasting neurochemical profiles, partial glycine site agonists and glycine reuptake inhibitors exhibit comparable pro-cognitive effects in rats of potential relevance to treatment of schizophrenia and other brain disorders where cognitive performance is impaired.

5-HT(6) receptor recruitment of mTOR as a mechanism for perturbed cognition in schizophrenia.

Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5-HT(6) receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5-HT(6) receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5-HT(6) receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5-HT(6) agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post-weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5-HT(6) antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5-HT(6) receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control.

S32212, a novel serotonin type 2C receptor inverse agonist/α2-adrenoceptor antagonist and potential antidepressant: I. A mechanistic characterization.

Although most antidepressants suppress serotonin (5-HT) and/or noradrenaline reuptake, blockade of 5-HT(2C) receptors and α(2)-adrenoceptors likewise enhances monoaminergic transmission. These sites are targeted by the urea derivative N- [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3-H-benzo[e]indole-3-carboxamide (S32212). S32212 was devoid of affinity for monoamine reuptake sites, yet displayed pronounced affinity (pK(i), 8.2) for constitutively active human 5-HT(2CINI) (h5-HT(2CINI)) receptors, behaving as an inverse agonist in reducing basal Gα(q) activation, [(3)H]inositol-phosphate production, and the spontaneous association of h5-HT(2CINI)-Renilla luciferase receptors with ß-arrestin2-yellow fluorescent protein. Furthermore, upon 18-h pretreatment, S32212 enhanced the plasma membrane expression of h5-HT(2CINI) receptors as visualized by confocal microscopy and quantified by enzyme-linked immunosorbent assay. Its actions were prevented by the neutral antagonist 6-chloro-5-methyl-N-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl]indoline-1-carboxamide (SB242,084), which also impeded the induction by long-term exposure to S32212 of otherwise absent Ca(2+) mobilization in mouse cortical neurones. In vivo, S32212 blunted the inhibitory influence of the 5-HT(2C) agonist 2-(3-chlorobenzyloxy)-6-(1-piperazinyl)pyrazine (CP809,101) on ventrotegmental dopaminergic neurones. S32212 also blocked 5-HT-induced Gα(q) and phospholipase C activation at the h5-HT(2A) and, less potently, h5-HT(2B) receptors and suppressed the discriminative stimulus properties of the 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane in rats. S32212 manifested marked affinity for human α(2A)- (pK(i) 7.2), α(2B)- (pK(i) 8.2), and α(2C)- (pK(i) 7.4) adrenoceptors, at which it abolished noradrenaline-induced recruitment of Gα(i3), Gα(o), adenylyl cyclase, and extracellular-regulated kinase1/2. Moreover, S32212 dose-dependently abolished the discriminative stimulus effects of the α(2)-adrenoceptor agonist (S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(1',2',3',4'-tetrahydronaphthalene)] (S18616). Finally, S32212 displayed negligible affinity for α(1A)-adrenoceptors, histamine H(1) receptors, and muscarinic M(1) receptors. In conclusion, S32212 behaves as an inverse agonist at h5-HT(2C) receptors and as an antagonist at human α(2)-adrenoceptors (and h5-HT(2A) receptors). Its promising profile in preclinical models potentially relevant to the treatment of depression is described in J Pharmacol Exp Ther 340:765-780, 2012.

S32212, a novel serotonin type 2C receptor inverse agonist/α2-adrenoceptor antagonist and potential antidepressant: II. A behavioral, neurochemical, and electrophysiological characterization.

The present studies characterized the functional profile of N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3-H-benzo[e]indole-3-carboxamide) (S32212), a combined serotonin (5-HT)(2C) receptor inverse agonist and α(2)-adrenoceptor antagonist that also possesses 5-HT(2A) antagonist properties (J Pharmacol Exp Ther 340:750-764, 2012). Upon parenteral and/or oral administration, dose-dependent (0.63-40.0 mg/kg) actions were observed in diverse procedures. Both acute and subchronic administration of S32212 reduced immobility time in a forced-swim test in rats. Acutely, it also suppressed marble burying and aggressive behavior in mice. Long-term administration of S32212 was associated with rapid (1 week) and sustained (5 weeks) normalization of sucrose intake in rats exposed to chronic mild stress and with elevated levels of mRNA encoding brain-derived neurotrophic factor in hippocampus and amygdala (2 weeks). S32212 accelerated the firing rate of adrenergic perikarya in the locus coeruleus and elevated dialysis levels of noradrenaline in the frontal cortex and hippocampus of freely moving rats. S32212 also elevated the frontocortical levels of dopamine and acetylcholine, whereas 5-HT, amino acids, and histamine were unaffected. These neurochemical actions were paralleled by "promnemonic" properties: blockade of scopolamine-induced deficits in radial maze performance and social recognition and reversal of delay-induced impairments in social recognition, social novelty discrimination, and novel object recognition. It also showed anxiolytic actions in a Vogel conflict procedure. Furthermore, in an electroencephalographic study of sleep architecture, S32212 enhanced slow-wave and rapid eye movement sleep, while decreasing waking. Finally, chronic administration of S32212 neither elevated body weight nor perturbed sexual behavior in male rats. In conclusion, S32212 displays a functional profile consistent with improved mood and cognitive performance, together with satisfactory tolerance.

Agomelatine suppresses locomotor hyperactivity in olfactory bulbectomised rats: a comparison to melatonin and to the 5-HT(2c) antagonist, S32006.

The novel melatonergic agonist/5-HT(2C) antagonist agomelatine displays robust antidepressant properties in humans and is active in pre-clinical models predictive of antidepressant effects. In this study, we investigated its potential influence on the locomotor hyperactivity displayed by olfactory bulbectomised rats, a putative measure of potential antidepressant activity. In addition, we compared the actions of agomelatine to those of melatonin and S32006, a selective antagonist at 5-HT(2C) receptors. Vehicle, agomelatine (10 and 50mg/kg), melatonin (10 and 50mg/kg), S32006 (0.16mg/kg to 10mg/kg) and the prototypical tricyclic antidepressant, imipramine (10mg/kg), were administered by intraperitoneal injection for 14days to male, Sprague-Dawley sham-operated and bulbectomised rats. In agreement with previous studies, imipramine was active in the model and both the lower and higher doses of agomelatine also significantly and markedly reversed the bulbectomy-induced hyperactivity to a level comparable to that seen in sham operated animals, in which agomelatine exerted no effect. Similarly the 5-HT(2C) antagonist, S32006, dose-dependently and significantly attenuated hyperactivity of bulbectomised animals, albeit with a maximal effect somewhat less marked than that of agomelatine. On the other hand, melatonin did not affect the locomotor behaviour of bulbectomised rats. The activity of agomelatine in the model is consistent with its known antidepressant properties in the clinic.

S41744, a dual neurokinin (NK)1 receptor antagonist and serotonin (5-HT) reuptake inhibitor with potential antidepressant properties: a comparison to aprepitant (MK869) and paroxetine.

Though neurokinin(1) (NK(1)) receptors are implicated in depressed states and their treatment, selective antagonists have disappointed in clinical trials. Accordingly, we designed a novel ligand, S41744 (2-piperazin-1-yl-indan-2-carboxylic-acid-(3-chloro-5-fluoro-benzyl)-methyl-amide), which both blocks NK(1) receptors and interferes with serotonin (5-HT) reuptake. S41744 mimicked the selective antagonist aprepitant in binding human (h)NK(1) receptors and in antagonising Substance-P-mediated Extracellular-Regulated-Kinase phosphorylation (pK(B), 7.7). Further, it dose-dependently (0.63-40.0 mg/kg, i.p.) displaced ex vivo [(3)H]-[Sar(9),Met(O(2))(11)]-Substance P binding to gerbil striatum, attenuated formalin-induced hind-paw licking in gerbils, and antagonised locomotion induced by i.c.v. administration of the NK(1) agonist GR73632 to guinea pigs. Like paroxetine, S41744 recognised h5-HT transporters, reduced synaptosomal uptake of 5-HT (pK(B), 7.9), and dose-dependently (0.63-10.0 mg/kg) elevated dialysis levels of 5-HT in the hippocampus and frontal cortex of freely-moving guinea pigs. Further, S41744 increased extracellular levels of 5-HT in frontal cortex and hippocampus of rats to a greater extent than paroxetine, and its inhibitory influence upon serotonergic perikarya was blunted relative to its affinity for 5-HT transporters. S41744 more potently blocked stress-induced vocalizations in guinea pigs than aprepitant and paroxetine, and it was active in forced-swim and marble-burying procedures of putative antidepressant properties in mice. While aprepitant displayed anxiolytic actions in stress-induced foot-tapping and social interaction tests in gerbils, paroxetine was anxiogenic and S41744 "neutral", reflecting balanced NK(1) antagonism and suppression of 5-HT reuptake. Moreover, S41744 shared anxiolytic actions of aprepitant in the rat Vogel Conflict Test. In conclusion, S41744 is an innovative NK(1) antagonist/5-HT reuptake inhibitor justifying further evaluation for treatment of stress-related disorders.

Neurokinin1 antagonists potentiate antidepressant properties of serotonin reuptake inhibitors, yet blunt their anxiogenic actions: a neurochemical, electrophysiological, and behavioral characterization.

Though neurokinin(1) (NK(1)) receptor antagonists are active in experimental models of depression, clinical efficacy has proven disappointing. This encourages interest in association of NK(1) receptor blockade with inhibition of serotonin (5-HT) reuptake. The selective NK(1) antagonist, GR205171, dose-dependently enhanced citalopram-induced elevations of extracellular levels of 5-HT in frontal cortex, an action expressed stereospecifically vs its less active distomer, GR226206. Further, increases in 5-HT levels in dorsal hippocampus, basolateral amygdala, nucleus accumbens, and striatum were likewise potentiated, and GR205171 similarly facilitated the influence of fluoxetine upon levels of 5-HT, as well as dopamine and noradrenaline. In parallel electrophysiological studies, the inhibitory influence of citalopram and fluoxetine upon raphe-localized serotonergic neurones was stereospecifically blunted by GR205171. Antidepressant actions of citalopram in a forced-swim test in mice were stereospecifically potentiated by GR205171, and it also enhanced attenuation by citalopram of stress-related ultrasonic vocalizations in rats. Further, GR205171 and citalopram additively abrogated the advance in circadian rhythms provoked by exposure to light in hamsters. By contrast, GR205171 stereospecifically blocked anxiogenic actions of citalopram in social interaction procedures in rats and gerbils, and stereospecifically abolished facilitation of fear-induced foot tapping by fluoxetine in gerbils. By analogy to GR205171, a further NK(1) antagonist, RP67580, enhanced the influence of citalopram upon frontocortical levels of 5-HT and potentiated its actions in the forced swim test. In conclusion, NK(1)receptor blockade differentially modulates functional actions of SSRIs: antidepressant properties are reinforced, whereas anxiogenic effects are attenuated. Combined NK(1) receptor antagonism/5-HT reuptake inhibition may offer advantages in the management of depressed and anxious states.

Discriminative stimulus properties of the atypical antidepressant, mirtazapine, in rats: a pharmacological characterization.

RATIONALE: Though interoceptive properties of antidepressants have been described, discriminative stimulus (DS) properties of mirtazapine, which does not affect monoamine reuptake, remain uncharacterized. OBJECTIVES: The objectives of the study are to train rats to recognize a mirtazapine DS, then perform substitution studies with other antidepressants and drugs acting at sites occupied by mirtazapine. MATERIALS AND METHODS: Using a two-lever, fixed-ratio 10 schedule, rats were trained to discriminate mirtazapine (2.5 mg/kg, i.p.) from saline. RESULTS: Sessions, 63 +/- 8, were necessary to reach the criterion for 14 rats that all subsequently recognized (100%) mirtazapine at the training dose. Mirtazapine blocks serotonin (5-HT)(2C) receptors, and the 5-HT(2C) antagonists, SB242,084, SB243,213 and S32006, revealed dose-dependent and full (> or =80%) substitution at doses of 2.5, 2.5, and 0.63 mg/kg, respectively. By contrast, the 5-HT(2A) antagonists, MDL100,907 and SR46349-B, the 5-HT(2B) antagonist, SB204,741, and the 5-HT(3) antagonist, ondansetron, showed no significant substitution. Though mirtazapine indirectly recruits 5-HT(1A) receptors, the 5-HT(1A) agonists, buspirone and 8-OH-DPAT, did not substitute. Mirtazapine blocks alpha(2)-adrenoceptors, but several alpha(2)-adrenoceptor antagonists (yohimbine, RX821,002 and atipamezole) failed to substitute. Despite blockade by mirtazapine of histamine H(1) receptors, no substitution was seen with the selective H(1) antagonist, pyrilamine. Finally, the selective noradrenaline reuptake inhibitor, reboxetine (0.16), fully substituted for mirtazapine, whereas the 5-HT/noradrenaline reuptake inhibitors, duloxetine and S33005, several 5-HT reuptake inhibitors (citalopram, fluvoxamine, and paroxetine) and the dopamine reuptake inhibitors, bupropion and GBR12,935, did not substitute. CONCLUSION: Mirtazapine elicits a DS in rats for which selective antagonists at 5-HT(2C) receptors display dose-dependent substitution, whereas drugs acting at other sites recognized by mirtazapine are ineffective.

Cellular and behavioural profile of the novel, selective neurokinin1 receptor antagonist, vestipitant: a comparison to other agents.

This study characterized the novel neurokinin (NK)(1) antagonist, vestipitant, under clinical evaluation for treatment of anxiety and depression. Vestipitant possessed high affinity for human NK(1) receptors (pK(i), 9.4), and potently blocked Substance P-mediated phosphorylation of Extracellular-Regulated-Kinase. In vivo, it occupied central NK(1) receptors in gerbils (Inhibitory Dose(50), 0.11 mg/kg). At similar doses, it abrogated nociception elicited by formalin in gerbils, and blocked foot-tapping and locomotion elicited by the NK(1) agonist, GR73632, in gerbils and guinea pigs, respectively. Further, vestipitant attenuated fear-induced foot-tapping in gerbils, separation-induced distress-vocalizations in guinea pigs, marble-burying behaviour in mice, and displayed anxiolytic actions in Vogel conflict and fear-induced ultrasonic vocalization procedures in rats. These actions were mimicked by CP99,994, L733,060 and GR205,171 which acted stereoselectively vs its less active isomer, GR226,206. In conclusion, vestipitant is a potent NK(1) receptor antagonist: its actions support the utility of NK(1) receptor blockade in the alleviation of anxiety and, possibly, depression.

S32006, a novel 5-HT2C receptor antagonist displaying broad-based antidepressant and anxiolytic properties in rodent models.

RATIONALE: Serotonin (5-HT)(2C) receptors are implicated in the control of mood, and their blockade is of potential interest for the management of anxiodepressive states. OBJECTIVES: Herein, we characterized the in vitro and in vivo pharmacological profile of the novel benzourea derivative, S32006. MATERIALS AND METHODS: Standard cellular, electrophysiological, neurochemical, and behavioral procedures were used. RESULTS: S32006 displayed high affinity for human (h)5-HT(2C) and h5-HT(2B) receptors (pK (i)s, 8.4 and 8.0, respectively). By contrast, it had negligible (100-fold lower) affinity for h5-HT(2A) receptors and all other sites examined. In measures of Gq-protein coupling/phospholipase C activation, S32006 displayed potent antagonist properties at h5-HT(2C) receptors (pK (B) values, 8.8/8.2) and h5-HT(2B) receptors (7.8/7.7). In vivo, S32006 dose-dependently (2.5-40.0 mg/kg, i.p. and p.o.) abolished the induction of penile erections and a discriminative stimulus by the 5-HT(2C) receptor agonist, Ro60,0175, in rats. It elevated dialysis levels of noradrenaline and dopamine in the frontal cortex of freely moving rats, and accelerated the firing rate of ventrotegmental dopaminergic and locus ceruleus adrenergic neurons. At similar doses, S32006 decreased immobility in a forced-swim test in rats, reduced the motor depression elicited by 5-HT(2C) and alpha(2)-adrenoceptor agonists, and inhibited both aggressive and marble-burying behavior in mice. Supporting antidepressant properties, chronic (2-5 weeks) administration of S32006 suppressed "anhedonia" in a chronic mild stress procedure and increased both expression of BDNF and cell proliferation in rat dentate gyrus. Finally, S32006 (0.63-40 mg/kg, i.p. and p.o) displayed anxiolytic properties in Vogel conflict and social interaction tests in rats. CONCLUSION: S32006 is a potent 5-HT(2C) receptor antagonist, and possesses antidepressant and anxiolytic properties in diverse rodent models.

The melanin-concentrating hormone1 receptor antagonists, SNAP-7941 and GW3430, enhance social recognition and dialysate levels of acetylcholine in the frontal cortex of rats.

Melanin-concentrating hormone (MCH)1 receptors are widely expressed in limbic structures and cortex. Their inactivation is associated with anxiolytic and antidepressive properties but little information is available concerning cognition. This issue was addressed using the selective antagonists, SNAP-7941 and GW3430, in a social recognition paradigm in rats. The muscarinic blocker, scopolamine (1.25 mg/kg s.c.), reduced social recognition, an action dose-dependently blocked by SNAP-7941 and GW3430 (0.63-10.0 and 20.0-80.0 mg/kg i.p., respectively) which did not themselves display amnesic properties. Further, in a protocol where a spontaneous deficit was induced by a prolonged inter-session delay, SNAP-7941 and GW3430 dose-dependently enhanced social recognition. In dialysis studies, SNAP-7941 (0.63-40.0 mg/kg i.p.) and GW3430 (10.0-40.0 mg/kg i.p.) elevated extracellular levels of acetylcholine (ACh) in the frontal cortex (FCX) of freely moving rats. The SNAP-7941 effect was specific, as it did not increase levels of ACh in ventral and dorsal hippocampus: moreover, it did not modify levels of noradrenaline, dopamine, serotonin and glutamate in FCX. Active doses of SNAP-7941 and GW3430 corresponded to doses (2.5-40.0 and 10.0-80.0 mg/kg i.p., respectively) exerting anxiolytic properties in Vogel conflict and ultrasonic vocalization tests, and antidepressant actions in forced swim, isolation-induced aggression and marble-burying procedures. In contrast to SNAP-7941 and GW3430, the benzodiazepine, diazepam, decreased social recognition and dialysate levels of ACh, while the tricyclic, imipramine, reduced social recognition and failed to enhance cholinergic transmission. In conclusion, at anxiolytic and antidepressant doses, SNAP-7941 and GW3430 improve social recognition and elevate extracellular ACh levels in FCX. This profile differentiates MCH1 receptor antagonists from conventional anxiolytic and antidepressant agents.

S33138 (N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1] benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenyl-acetamide), a preferential dopamine D3 versus D2 receptor antagonist and potential antipsychotic agent: III. Actions in models of therapeutic activity and induction of side effects.

In contrast to clinically available antipsychotics, the novel benzopyranopyrrolidine derivative, S33138 (N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenyl-acetamide), behaves as a preferential antagonist of D(3) versus D(2) receptors and does not interact with histamine H(1) and muscarinic receptors. In contrast to haloperidol, clozapine, olanzapine, and risperidone, S33138 (0.16-2.5 mg/kg s.c.) did not disrupt performance in passive-avoidance and five-choice serial reaction time procedures. Furthermore, upon either systemic administration (0.04-2.5 mg/kg s.c.) or introduction into the frontal cortex (0.04-0.63 mug/side), S33138 potently attenuated the perturbation of social recognition by scopolamine or a prolonged intersession delay. Over a comparable and low-dose range, S33138 (0.04-0.63 mg/kg s.c.) elevated dialysis levels of acetylcholine in the frontal cortex of freely moving rats. At higher doses (2.5-10.0 mg/kg s.c.), S33138 also increased frontocortical levels of histamine, whereas monoamines, glutamate, glycine, and GABA were unaffected. By analogy to the other antipsychotics, S33138 (0.63-10.0 mg/kg s.c.) inhibited conditioned avoidance responses in rats, apomorphine-induced climbing in mice, and hyperlocomotion elicited by amphetamine, cocaine, dizocilpine, ketamine, and phencyclidine in rats. S33138 (0.16-2.5 mg/kg s.c.) also blocked the reduction of prepulse inhibition elicited by apomorphine. In comparison with the above actions, only "high" doses of S33138 (10.0-40.0 mg/kg s.c.) elicited catalepsy. To summarize, reflecting preferential blockade of D(3) versus D(2) receptors, S33138 preserves and/or enhances cognitive function, increases frontocortical cholinergic transmission, and is active in models of antipsychotic properties at doses well below those inducing catalepsy. In comparison with clinically available agents, S33138 displays, thus, a distinctive and promising profile of potential antipsychotic properties.

Pro-cognitive effects of 5-HT6 receptor antagonists in the social recognition procedure in rats: implication of the frontal cortex.

RATIONALE: 5-HT6 receptor antagonists improve cognitive processes in rodents. However, their site(s) of action remains unexplored and their influence upon social memory has been little investigated. OBJECTIVES: We examined the influence of 5-HT6 receptor ligands upon social memory in rats by use of systemic or local administration into the frontal cortex (FCX), striatum, or nucleus basalis magnocellularis (NBM). MATERIALS AND METHODS: The social recognition test is based upon the ability of an adult rat to recognize a younger conspecific during the second of two 5-min sessions. In a procedure without an inter-session interval, the actions of drugs alone and the ability to reverse "amnesia" induced by the muscarinic antagonist, scopolamine (1.25 mg/kg, s.c.), were examined. The potential promnesic effect of drugs was also investigated in another procedure where a spontaneous deficit of recognition was induced by a 120-min inter-session interval. RESULTS: The 5-HT6 receptor agonist, WAY-181187 (10.0 mg/kg, i.p.), significantly impaired social recognition. This effect was abolished by the 5-HT6 receptor antagonists, SB-271046 (20.0 mg/kg, i.p.) and SB-258585 (10.0 mg/kg, i.p.). These agents also abolished scopolamine-induced amnesia (10.0 and 2.5 mg/kg, i.p., respectively) and reversed the delay-induced deficit (10.0-20.0 and 2.5-10.0 mg/kg, i.p., respectively). WAY-181187 into the FCX significantly impaired social recognition (0.16-0.63 microg/side). Conversely, SB-271046 into the FCX (2.5-5.0 microg/side), but neither into the striatum nor the NBM, significantly reversed spontaneous deficit. CONCLUSION: These results indicate that 5-HT6 receptors modulate social recognition by actions in the FCX and underpin their pertinence as targets for the treatment of psychiatric disorders in which cognitive function is compromised.

S33138 [N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]-benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide], a preferential dopamine D3 versus D2 receptor antagonist and potential antipsychotic agent. II. A neurochemical, electrophysiological and behavioral characterization in vivo.

The novel benzopyranopyrrolidine, S33138 [N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide], is a preferential antagonist of cloned human D(3) versus D(2L) and D(2S) receptors. In mice, S33138 (0.04-2.5 mg/kg i.p.) increased levels of mRNA encoding c-fos in D(3) receptor-rich Isles of Calleja and nucleus accumbens more potently than in D(2) receptor-rich striatum. Furthermore, chronic (3 weeks) administration of S33138 to rats reduced the number of spontaneously active dopaminergic neurones in the ventral tegmental area (0.16-10.0 p.o.) more potently than in the substantia nigra (10.0). In primates treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, antiparkinson actions of the D(3)/D(2) agonist, ropinirole, were potentiated by low doses of S33138 (0.01-0.16 p.o.) but diminished by a high dose (2.5). Consistent with antagonism of postsynaptic D(3)/D(2) sites, S33138 attenuated hypothermia and yawns elicited by the D(3)/D(2) agonist 7-OH-DPAT [(+)-7-dihydroxy-2-(di-n-propylamino)-tetralin] in rats, and it blocked (0.01-0.63, s.c.) discriminative properties of PD128,907 [(+)-(4aR,10bR)-3,4, 4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol; trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolininecarboxamide]. Suggesting antagonist properties at D(3)/D(2) autoreceptors, S33138 prevented (0.16-2.5 s.c.) the inhibitory influence of PD128,907 upon dopamine release in frontal cortex, nucleus accumbens, and striatum and abolished (0.004-0.25 i.v.) its inhibition of ventral tegmental dopaminergic neuron firing. At higher doses, antagonist actions of S33138 (0.5-4.0 i.v.) at alpha(2C)-adrenoceptors were revealed by an increased firing rate of adrenergic perikarya. Finally, antagonism of 5-hydroxytryptamine (5-HT(2A) and 5-HT(7)) receptors was shown by blockade of 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane-induced head twitches (0.63-10.0 s.c.) and 5-carboxytryptamine-induced hypothermia (2.5-20.0 i.p.), respectively. In conclusion, S33138 displays modest antagonist properties at central alpha(2C)-adrenoceptors, 5-HT(2A) and 5-HT(7) receptors. Furthermore, in line with its in vitro actions, it more potently blocks cerebral populations of D(3) versus D(2) receptors.

Selective blockade of dopamine D(3) versus D(2) receptors enhances frontocortical cholinergic transmission and social memory in rats: a parallel neurochemical and behavioural analysis.

Though dopaminergic mechanisms modulate cholinergic transmission and cognitive function, the significance of specific receptor subtypes remains uncertain. Here, we examined the roles of dopamine D(3) versus D(2) receptors. By analogy with tacrine (0.16-2.5 mg/kg, s.c.), the selective D(3) receptor antagonists, S33084 (0.01-0.63) and SB277,011 (0.63-40.0), elicited dose-dependent, pronounced and sustained elevations in dialysis levels of acetylcholine (ACh) in the frontal cortex, but not the hippocampus, of freely-moving rats. The actions of these antagonists were stereospecifically mimicked by (+)S14297 (1.25), whereas its inactive distomer, (-)S17777, was ineffective. The preferential D(2) receptor antagonist, L741,626 (10.0), failed to modify levels of ACh. S33084 (0.01-0.63) and SB277,011 (0.16-2.5) also mimicked tacrine (0.04-0.63) by dose-dependently attenuating the deleterious influence of scopolamine (1.25) upon social memory (recognition by an adult rat of a juvenile conspecific). Further, (+)S14297 (1.25) versus (-)S17777 stereospecifically blocked the action of scopolamine. Using an intersession interval of 120 min (spontaneous loss of recognition), S33084 (0.04-0.63), SB277,011 (0.16-10.0) and (+)S14297 (0.63-10.0) likewise mimicked tacrine (0.16-2.5) in enhancing social memory. In contrast, L741,626 (0.16-10.0) displayed amnesic properties. In conclusion, selective blockade of D(3) receptors facilitates frontocortical cholinergic transmission and improves social memory in rats. These data support the pertinence of D(3) receptors as a target for treatment of disorders in which cognitive function is compromised.

Discriminative stimulus properties of S32504, a novel D3/D2 receptor agonist and antiparkinson agent, in rats: attenuation by the antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine, and by selective antagonists at dopamine D2 but not D3 receptors.

RATIONALE: Drug-discrimination studies have proven instructive in the characterization of psychotropic agents, a procedure applied herein to the novel antiparkinson agent, S32504. This highly selective agonist at dopamine D(3) and (less potently) D(2) receptors displays potent antiparkinson, neuroprotective and antidepressant properties (Millan et al., J Pharmacol Exp Ther 309:936-950, 2004a; Millan et al., J Pharmacol Exp Ther 309:903-920, 2004b; Millan et al., J Pharmacol Exp Ther 309:921-935, 2004c). OBJECTIVES: To generate a discriminative stimulus (DS) with S32504 and undertake substitution/antagonism studies with diverse antiparkinson and antipsychotic agents. MATERIALS AND METHODS: Using a two-lever, fixed-ratio 10 schedule, rats were trained to recognize S32504 (0.04 mg/kg, s.c.) from saline. RESULTS: S32504 displayed dose-dependent and stereospecific substitution in comparison to its less active racemic form, (+/-) S31411, and to its inactive (-) distomer, S32601. Apomorphine, and the selective D(3)/D(2) receptor agonists, ropinirole, PD128,907, 7-OH-DPAT and CGS15855A, fully (=80%) substituted for S32504, whereas D(4) and D(1)/D(5) receptor agonists were ineffective. The selective D(3) vs D(2) receptor partial agonist, BP897, did not substitute for S32504 and the selective D(3) receptor antagonists, S33084, SB277,011, GR218,231, PNU99194A and S14297, did not block its DS properties. By contrast, S32504 lever selection was blocked by the preferential D(2) vs D(3) receptor antagonists, L741,626 and S23199, and by the D(2)/D(3) antagonists, raclopride and haloperidol. The D(2)/D(3) receptor partial agonists and antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine and preclamol did not substitute for S32504: indeed, they dose-dependently attenuated its DS properties. CONCLUSION: The antiparkinson agent, S32504, displays DS properties principally mediated by high-efficacy activation of D(2) receptors Antipsychotics known to act as partial agonists at D(2)/D(3) receptors attenuate DS properties of S32504, actions reflecting their low efficacy at these sites.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats: a characterization with alpha/beta-adrenoceptor subtype selective ligands, antidepressants, and antagonists at neuropeptide receptors.

Although little information is available concerning discriminative stimulus (DS) properties of antidepressants, rats can be trained to recognize the selective norepinephrine (NE) reuptake inhibitor, reboxetine (2.5 mg/kg i.p.). By analogy to reboxetine (effective dose50, 1.1), 'full' (80%) substitution dose50 was obtained with the NE reuptake inhibitors, nisoxetine (4.9), nomifensine (0.5) and BW1555,U88 (1.0). Full substitution was also attained with the NE/serotonin (5-HT) reuptake inhibitors, S33005 (0.3), venlafaxine (4.8) and duloxetine (26.8), and the tricyclics, imipramine (2.5) and clomipramine (2.9). In contrast, the 5-HT reuptake inhibitors, citalopram, sertraline and paroxetine (all >2.5), and the 5-HT reuptake inhibitors/5-HT2 receptor antagonists, nefazodone and trazodone (both >10.0), did not substitute for reboxetine. The 'atypical' antidepressants, mirtazapine (>10.0) and mianserin (>2.5), similarly failed to substitute. DS properties of reboxetine were dose-dependently blocked by the alpha1-adrenoceptor (AR) antagonists, prazosin (inhibitory dose50, 0.3) and WB4101 (0.5), but resistant to the alpha2-AR antagonists, atipamezole (>0.63), idazoxan (>2.5) and RX821,002 (>0.08), and to the beta1-AR and beta2-AR antagonists, betaxolol (>2.5) and ICI118,551 (>10.0). Interestingly, the neurokinin-1 receptor antagonist, GR205,171, stereospecifically substituted for reboxetine (1.1) compared to its less active isomer, GR226,206 (>10.0). The corticotrophin-releasing factor-1 antagonists, DMP695 (>40), CP154,526 (>10.0) and SN003 (>40.0), and the melanin-concentrating hormone-1 antagonist, SNAP-7941 (>40.0), failed to substitute for reboxetine. In conclusion, DS properties of reboxetine are mimicked by antidepressants recognizing NE transporters, and require functionally intact alpha1-ARs for their expression. The neurokinin-1 antagonist, GR205,171, mimics the interoceptive properties of reboxetine, possibly reflecting its elevation of extracellular levels of NE in corticolimbic structures.

Activation of dopamine D1 receptors enhances cholinergic transmission and social cognition: a parallel dialysis and behavioural study in rats.

Although dopaminergic mechanisms are known to modulate cognitive function and cholinergic transmission, their pharmacological characterization remains incomplete. Herein, the role of D1 sites was evaluated employing neurochemical and behavioural approaches. By analogy to the acetylcholinesterase inhibitor, galantamine (0.0025-0.63 mg/kg s.c.), the selective and high efficacy D1 receptor agonist, SKF 82958, dose-dependently (0.0025-0.63), robustly and potently enhanced extracellular levels of acetylcholine (ACh) in the frontal cortex and hippocampus of freely moving rats. A further agonist, SKF 81297 (0.04-0.63), mimicked this action whereas the selective antagonist, SCH 23390 (0.00063-0.63), decreased levels of ACh. In the presence of SCH 23390 (0.08), the facilitatory influence of SKF 82958 (0.04) upon ACh levels was abolished. In a model of social memory (recognition of a juvenile by an adult rat), galantamine (0.04-0.63), SKF 82958 (0.01-0.16) and SKF 81297 (0.001-0.16) dose-dependently abrogated amnesic effects of the muscarinic receptor antagonist scopolamine (1.25). Further, under conditions of spontaneous loss of recognition, mimicking the effects of galantamine (0.04-2.5), SKF 82958 (0.01-0.16) and SKF 81297 (0.04-1.25) dose-dependently and specifically facilitated social recognition. Conversely, SCH 23390 (0.0025-0.04) exerted a modest negative influence upon social recognition and, in its presence, the pro-cognitive properties of SKF 82958 were blocked. In conclusion, D1 receptors exert a tonic, facilitatory influence upon cholinergic transmission and social recognition. Although the relationship between these actions awaits further clarification, these data underpin the relevance of D1 receptors to CNS disorders in which cholinergic transmission and social cognition are disrupted.

Antidepressant-like properties of the anti-Parkinson agent, piribedil, in rodents: mediation by dopamine D2 receptors.

The dopamine D2/D3 receptor agonist and alpha2 adrenergic receptor antagonist, piribedil, is used clinically as monotherapy and as an adjunct to L-3,4-dihydroxyphenylalanine in the treatment of Parkinson's disease. As it appears to improve mood, we examined its actions in rodent models of antidepressant properties, in comparison with the prototypical anti-Parkinson agent, apomorphine, the D2/D3 receptor agonist, quinpirole, and the antidepressants, imipramine and fluvoxamine. In the mouse forced-swim test, acute administration of imipramine, fluvoxamine, apomorphine or quinpirole decreased immobility time, actions dose dependently mimicked by piribedil (2.5-10.0 mg/kg, subcutaneously). In rats, acute and subchronic administration of piribedil similarly reduced immobility (0.63-10.0 mg/kg, subcutaneously) and apomorphine, quinpirole and imipramine were also active in this test, whereas fluvoxamine was inactive. Both in mice and in rats, the D2/D3 receptor antagonist, raclopride, and the D2 receptor antagonist, L741,626, dose dependently blocked the antidepressant properties of piribedil, whereas the selective D3 receptor antagonists, S33084 and SB277,011, were ineffective. In a chronic mild stress model in rats, piribedil (2.5-40.0 mg/kg, subcutaneously) restored sucrose intake in stressed animals exerting its actions more rapidly (by week 1) than imipramine. Imipramine, fluvoxamine, apomorphine, quinpirole and piribedil dose dependently (0.63-10.0 mg/kg, subcutaneously) suppressed aggressive and marble-burying behaviour in mice. In the latter procedure, raclopride and L741,626, but not S33084, attenuated the actions of piribedil. Over a dose range (0.63-10.0 mg/kg, subcutaneously) equivalent to those active in models of antidepressant activity, piribedil did not stimulate locomotor behaviour. In conclusion, principally via recruitment of D2 receptors, piribedil exerts robust and specific antidepressant-like actions in diverse rodent models.