NLX-112, a highly selective 5-HT1A receptor biased agonist, does not exhibit misuse potential in male rats or macaques.

NLX-112 (a.k.a. F13640 or befiradol) exhibits nanomolar affinity, exceptional selectivity and biased agonism at serotonin 5-HT1A receptors. NLX-112 displays robust analgesic activity in a number of rodent models of pain, and is currently developed as a treatment for l-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD) patients. Noteworthy, PD patients can suffer from comorbid chronic pain, thus necessitating the use of analgesic drugs, such as opioids, which have potential for misuse. Additionally, dopamine agonists used to treat PD can produce cocaine-like effects in preclinical assays of misuse potential. The present study investigated whether NLX-112 possesses misuse potential of its own using two behavioural assays routinely used for this purpose: intracranial self-stimulation (ICSS) in rats, and cocaine discrimination in macaque monkeys. In rats, low doses of NLX-112 (0.03 and 0.1 mg/kg p.o.) did not alter ICSS frequency-rate curves, while higher doses (0.3 and 1.0 mg/kg) shifted the curve to the right and flattened it, i.e., reduced ICSS. As expected, cocaine (10 mg/kg i.p.) shifted the curve to the left, i.e., facilitated ICSS, but NLX-112 (0.03 and 0.1 mg/kg p.o.) did not further enhance cocaine-induced facilitation of ICSS. In monkeys trained to discriminate cocaine (0.4 mg/kg i.m.) from saline, NLX-112 (0.01-0.1 mg/kg p.o.) did not substitute for cocaine. Taken together, these results suggest that NLX-112, at doses displaying anti-dyskinetic activity in rat, marmoset and macaque models of LID, is free from misuse potential. From a translational perspective, this is a desirable property for a compound destined to be used in PD patients, who can suffer from comorbid chronic pain necessitating the use of potentially misused analgesic drugs.

NLX-101, a highly selective 5-HT1A receptor biased agonist, mediates antidepressant-like activity in rats via prefrontal cortex 5-HT1A receptors.

NLX-101 (also known as F15599) exhibits nanomolar affinity, exceptional selectivity and biased agonist activation of serotonin 5-HT1A receptors. Given systemically, it displays antidepressant-like activity in the rat forced swim test (FST), and preferentially activates 5-HT1A post-synaptic heteroreceptors in the prefrontal cortex (PFC), a brain region involved in the control of mood. Here, we assessed the ability of NLX-101 to produce antidepressant-like activity in the FST following in-situ PFC unilateral microinjection. (+)8-OH-DPAT and F13714, two 5-HT1A receptor agonists that do not display cortical biased agonism, were tested as comparators. NLX-101 decreased time spent in immobility in a bi-modal manner, with a first MED of 0.25 µg (immobility reduced from 160 to 80 s) but immobility returned to control levels at the next dose (1 µg). At higher doses, immobility decreased monotonically, with a second MED of 16 µg and a maximal effect (36 s) at 32 µg. (+)8-OH-DPAT and F13714 also diminished immobility but, unlike NLX-101, they did so in a unimodal manner, with MEDs of 1 and 4 µg, and maximal responses of 31 and 4 s, for (+)8-OH-DPAT and F13714, respectively. The effects of (+)8-OH-DPAT (16 µg) and of both active doses of NLX-101 (0.25 and 16 µg) were prevented by the 5-HT1A receptor antagonist WAY-100,635 (0.63 mg/kg s.c.). In conclusion, activation of 5-HT1A receptors in the PFC by NLX-101 produces robust antidepressant-like effects in the rat FST, with a distinctive bimodal dose-response pattern. These data suggest that NLX-101 may target specific 5-HT1A receptor subpopulations in PFC, likely located on GABAergic and/or glutamatergic neurons.

The selective 5-HT1A receptor agonist, NLX-112, exerts anti-dyskinetic effects in MPTP-treated macaques.

BACKGROUND: Long-term treatment of Parkinson's disease (PD) with l-DOPA typically leads to development of l-DOPA induced dyskinesia (LID). Amantadine, an NMDA antagonist, attenuates LID, but with limited efficacy and considerable side-effects. NLX-112 (also known as befiradol or F13640), a highly selective and efficacious 5-HT1A receptor agonist, reduced LID when tested in rodent and marmoset models of PD. METHODS: The effects of NLX-112 (0.03, 0.1 and 0.3 mg/kg PO) on established LID evoked by acute challenge with l-DOPA (27.5 ± 3.8 mg/kg PO) were assessed in MPTP-treated cynomolgus macaques. Amantadine (10 mg/kg PO) was tested as a positive control. Plasma exposure of NLX-112 (0.1 mg/kg PO) was determined. RESULTS: NLX-112 significantly and dose-dependently reduced median LID levels by up to 96% during the first hour post-administration (0.3 mg/kg). Moreover, NLX-112 reduced the duration of 'bad on-time' associated with disabling LID by up to 48% (0.3 mg/kg). In contrast, NLX-112 had negligible impact on the anti-parkinsonian benefit of l-DOPA. NLX-112 exposure peaked at ~50 ng/ml at 30 min post-administration but decreased to ~15 ng/ml at 2h. Amantadine reduced by 42% 'bad on-time' associated with l-DOPA, thereby validating the model. CONCLUSION: These data show that, in MPTP-lesioned cynomolgus macaques, NLX-112 exerts robust anti-dyskinetic effects, without reducing the anti-parkinsonian benefit of l-DOPA. These observations complement previous findings and suggest that selective and high efficacy activation of 5-HT1A receptors by NLX-112 may constitute a promising approach to combat LID in PD, providing an alternative for patients in whom amantadine is poorly tolerated or without useful effect.

NLX-112, a highly selective 5-HT1A receptor agonist, mediates analgesia and antidepressant-like activity in rats via spinal cord and prefrontal cortex 5-HT1A receptors, respectively.

NLX-112 (a.k.a. F13640 or befiradol) possesses marked activity in a variety of animal models of pain and of neuropsychiatric disorders; it exhibits nanomolar affinity, exceptional selectivity and high agonist efficacy at 5-hydroxytryptamine1A (5-HT1A) receptors. Although NLX-112 has been shown to activate 5-HT1A postsynaptic heteroreceptors in the prefrontal cortex (PFC), a brain region involved in the control of depressive states, the influence of NLX-112 on spinal cord 5-HT1A receptors (implicated in the control of pain) has not been described. Here we report on the ability, in rats, of NLX-112 to elicit analgesia in the intraplantar formalin model of nociceptive pain following intrathecal (i.t.) administration, and its ability to produce antidepressant-like activity in the forced swim test (FST) following in situ PFC microinjection. NLX-112, injected i.t. (L5-L6 region) induced analgesic effects in the formalin model of tonic nociceptive pain. At 20 µg, it almost abolished the effect of formalin on both the paw licking and paw elevation measures, and in both the early (0-5 min after formalin administration, reflecting acute pain) and the late (22.5-27.5 min, reflecting inflammatory pain) phases. The effects of NLX-112 (20 µg i.t.) were reversed by co-administration of 20 µg i.t. of the 5-HT1A receptor antagonist, WAY100635. Furthermore, the analgesic effects of systemically administered NLX-112 (0.63 mg/kg i.p.) were reversed by i.t. administration of WAY100635 (20 µg), most notably on paw licking. Finally, microinjection of NLX-112, bilaterally in the PFC, dose-dependently (MED 4 µg) and markedly reduced immobility in the FST (circa 90% reduction at 32 µg). In conclusion, the present data demonstrate that activation of spinal cord-located 5-HT1A receptors is sufficient for NLX-112 to mediate its analgesic effects in a rat model of tonic nociceptive pain. The data also highlight the involvement of PFC 5-HT1A receptors in the antidepressant-like activity of NLX-112 in the FST. Overall, the study suggests that highly selective and high efficacy 5-HT1A receptors agonists, such as NLX-112, could be useful to treat painful conditions associated with depressive states, through activation of different sub-populations of 5-HT1A receptors.

NLX-112, a highly selective 5-HT1A receptor agonist: Effects on body temperature and plasma corticosterone levels in rats.

NLX-112 (a.k.a. F13640 or befiradol), exhibits nanomolar affinity, exceptional selectivity and high agonist efficacy at 5-hydroxytryptamine 5-HT1A receptors. It possesses marked activity in a variety of animal models of depression, pain and L-DOPA-induced dyskinesia. However, its influence on translational biomarkers of central 5-HT1A receptor activation has not been previously described. Here, we report on the activity, in rats, of NLX-112 to increase plasma corticosterone levels and produce hypothermia, two responses which are also elicited by 5-HT1A receptor agonists in humans. NLX-112 elicited dose-dependent hypothermia (minimal effective dose, MED: 0.31mg/kg p.o.) and also increased plasma corticosterone both by oral and intraperitoneal routes (MED: 0.63mg/kg in both cases). The increase in corticosterone induced by NLX-112 (0.63mg/kg p.o.) was abolished by co-administration of the selective 5-HT1A receptor antagonist, WAY100635. Additionally, NLX-112 also dose-dependently induced flat body posture, forepaw treading and lower lip retraction (MEDs 0.31-0.63mg/kg p.o.). The doses of NLX-112 which induce hypothermia or corticosterone release were similar to those inducing serotonergic behaviors but greater than those reported previously in models of therapeutic-like activity (range 0.04 to 0.16mg/kg). Overall, the present study provides information for clinical dose estimations of NLX-112 and suggests that therapeutic effects may occur at doses below those at which biomarker responses are observed.

NLX-112, a novel 5-HT1A receptor agonist for the treatment of L-DOPA-induced dyskinesia: Behavioral and neurochemical profile in rat.

L-DOPA is the gold-standard treatment for Parkinson's disease (PD), but induces troublesome dyskinesia after prolonged treatment. This is associated with the 'false neurotransmitter' conversion of L-DOPA to dopamine by serotonin neurons projecting from the raphe to the dorsal striatum. Reducing their activity by targeting pre-synaptic 5-HT1A receptors should thus be an attractive therapeutic strategy, but previous 5-HT1A agonists have yielded disappointing results. Here, we describe the activity of a novel, highly selective and potent 5-HT1A agonist, NLX-112 (also known as befiradol or F13640) in rat models relevant to PD and its associated affective disorders. NLX-112 (0.16 mg/kg, i.p.) potently and completely reversed haloperidol-induced catalepsy in intact rats and abolished L-DOPA-induced Abnormal Involuntary Movements (AIMs) in hemiparkinsonian rats, an effect that was reversed by the selective 5-HT1A antagonist, WAY100635. In microdialysis experiments, NLX-112 profoundly decreased striatal 5-HT extracellular levels, indicative of inhibition of serotonergic function. NLX-112 also blunted the L-DOPA-induced surge in dopamine levels on the lesioned side of the brain, an action that likely underlies its anti-dyskinetic effects. NLX-112 (0.16 mg/kg, i.p.) robustly induced rotations in hemiparkinsonian rats, suggesting that it has a motor facilitatory effect. Rotations were abolished by WAY100635 and were ipsilateral to the lesioned side, suggesting a predominant stimulation of the dopamine system on the non-lesioned side of the brain. NLX-112 also efficaciously reduced immobility time in the forced swim test (75% reduction at 0.16 mg/kg, i.p.) and eliminated stress-induced ultrasonic vocalization at 0.08 mg/kg, i.p., effects consistent with potential antidepressant- and anxiolytic-like properties. In other tests, NLX-112 (0.01-0.16 mg/kg, i.p.) did not impair the ability of L-DOPA to rescue forepaw akinesia in the cylinder test but decreased rotarod performance, probably due to induction of flat body posture and forepaw treading which are typical of 5-HT1A agonists upon acute administration. However, upon repeated administration of NLX-112 (0.63 mg/kg, i.p., twice a day), flat body posture and forepaw treading subsided within 4 days of treatment. Taken together, these observations suggest that NLX-112 could exhibit a novel therapeutic profile, combining robust anti-dyskinetic properties without impairing the therapeutic properties of L-DOPA, and with additional beneficial effects on non-motor (affective) symptoms.

Levomilnacipran (F2695), a norepinephrine-preferring SNRI: profile in vitro and in models of depression and anxiety.

Levomilnacipran (LVM; F2695) is the more active enantiomer of the serotonin/norepinephrine (5-HT/NE) reuptake inhibitor (SNRI) milnacipran and is currently under development for the treatment of major depressive disorder. LVM was benchmarked against two other SNRIs, duloxetine and venlafaxine, in biochemical, neurochemical and pharmacological assays. LVM exhibited high affinity for human NE (Ki = 92.2 nM) and 5-HT (11.2 nM) transporters, and potently inhibited NE (IC50 = 10.5 nM) and 5-HT (19.0 nM) reuptake (human transporter) in vitro. LVM had 2-fold greater potency for norepinephrine relative to serotonin reuptake inhibition (i.e. NE/5-HT potency ratio: 0.6) and 17 and 27 times higher selectivity for NE reuptake inhibition compared with venlafaxine and duloxetine, respectively. LVM did not exhibit affinity for 23 off-target receptors. LVM (i.p.) increased cortical extracellular levels of 5-HT, and NE (minimal effective doses: MEDs = 20 and 10 mg/kg, respectively). In anti-depressive/anti-stress models, i.p. LVM diminished immobility time in the mouse forced swim (MED = 20 mg/kg) and tail suspension (MED = 2.5 mg/kg) tests, and reduced shock-induced ultrasonic vocalizations in rats (MED = 5 mg/kg). Duloxetine and venlafaxine were less potent (MEDs ≥ 10 mg/kg). At doses active in these three therapeutically-relevant models, LVM (i.p.) did not significantly affect spontaneous locomotor activity. In summary, LVM is a potent, selective inhibitor of NE and 5-HT transporters with preferential activity at the former. It is efficacious in models of anti-depressive/anti-stress activity, with minimal potential for locomotor side effects.

Comparison of milnacipran, duloxetine and pregabalin in the formalin pain test and in a model of stress-induced ultrasonic vocalizations in rats.

Milnacipran and duloxetine, serotonin/noradrenalin reuptake inhibitors, and pregabalin, a alpha(2)-delta(1) Ca(2+) channel blocker, are efficacious against fibromyalgia, a condition characterized by diffuse chronic pain and associated with stress. We compared these compounds (i.p. route), in rat models of acute/inflammatory pain (2.5% intraplantar formalin) and stress-induced ultrasonic vocalization (USV: 22kHz calls following presentation of a conditioned stimulus previously associated with foot-shocks). In the formalin test, milnacipran dose-dependently attenuated paw elevation and licking (minimal effective dose, MED: 2.5mg/kg for licking/late phase). Duloxetine was slightly more potent (MED=0.63). Pregabalin also reduced paw licking/late phase (MED=0.63), but was inactive up to 160mg/kg for paw elevation (both phases) and paw licking (early phase). Milnacipran dose-dependently reduced USV (MED=10, near total inhibition at 20mg/kg); duloxetine was less potent (MED=20). Pregabalin (2.5-80mg/kg) was only significantly active at 40mg/kg. Milnacipran, duloxetine and pregabalin possess analgesic activity in the formalin test on paw licking/late phase (corresponding to inflammatory pain with a central sensitization component). In the stress-induced USV model, milnacipran was the most potent and efficacious compound. To summarize, reduction of formalin-induced paw licking/late phase might constitute a useful indicator of potential activity against inflammatory/centrally sensitized pain, as might be expressed in fibromyalgia.

Chronic restraint stress induces mechanical and cold allodynia, and enhances inflammatory pain in rat: Relevance to human stress-associated painful pathologies.

Whereas acute stress often results in analgesia, chronic stress can trigger hyperalgesia/allodynia. This influence of long-term stress on nociception is relevant to numerous painful pathologies, such as fibromyalgia (FM), characterized by diffuse muscular pain (hyperalgesia) and/or tenderness (allodynia). Hence, there is a need for pre-clinical models integrating a chronic-stress dimension to the study of pain. Here, we assessed the effects of protracted/intermittent stress produced by daily, 1h restraint periods in cylinders, 4 days/week over 5 weeks, on eight models of hyperalgesia and allodynia in rats. This type of stress potentiated chemical hyperalgesia in the formalin model (160 and 76% increase of pain score above controls, during the early and late phases, respectively). It also produced thermal allodynia in response to cold (paw acetone test: 200% increase of allodynia score during week 3-5) and heat (42 degrees C tail immersion test: 15% decrease of withdrawal threshold, from week 2 onward). This stress also resulted in mechanical allodynia in the von Frey filaments model (60% decrease in threshold during week 2-5). However, such a stress regimen had no influence in the Randall-Selitto test of mechanical hyperalgesia, and in the tail immersion models of cold (4 degrees C) or hot (48 degrees C) thermal hyperalgesia, as well as cold (15 degrees C) allodynia. This model of prolonged/intermittent restraint stress may be useful in investigating the mechanisms linking stress and pain, and provide an assay to assess the potential therapeutic efficacy of drugs targeted against painful pathologies with a strong stress component, including but not restricted to FM.

5-HT1A receptors are involved in the effects of xaliproden on G-protein activation, neurotransmitter release and nociception.

BACKGROUND AND PURPOSE: Xaliproden (SR57746A) is a 5-HT(1A) receptor agonist and neurotrophic agent that reduces oxaliplatin-mediated neuropathy in clinical trials. The present study investigated its profile on in vitro transduction, neurochemical responses and acute nociceptive pain tests in rats. EXPERIMENTAL APPROACH: Xaliproden was tested on models associated with 5-HT(1A) receptor activation including G-protein activation, extracellular dopamine and 5-HT levels measured by microdialysis and formalin-induced pain. Activation of 5-HT(1A) receptors was confirmed by antagonism with WAY100635. KEY RESULTS: Xaliproden exhibited high affinity for rat (r) and human (h) 5-HT(1A) receptors (pK(i)= 8.84 and 9.00). In [(35)S]GTPgammaS (guanosine 5'-O-(3-[(35)S]thio)triphosphate) assays it activated both hippocampal r5-HT(1A)[pEC(50)/E(MAX) of 7.58/61% (%5-HT)] and recombinant h5-HT(1A) receptors (glioma C6-h5-HT(1A): 7.39/62%; HeLa-h5-HT(1A): 7.24/93%). In functional [(35)S]GTPgammaS autoradiography, xaliproden induced labelling in structures enriched with 5-HT(1A) receptors (hippocampus, lateral septum, prefrontal and entorhinal cortices). Xaliproden inhibited in vivo binding of [(3)H]WAY100635 to 5-HT(1A) receptors in mouse frontal cortex and hippocampus (ID(50): 3.5 and 3.3 mg x kg(-1), p.o. respectively). In rat, it increased extracellular dopamine levels in frontal cortex and reduced hippocampal 5-HT levels (ED(50): 1.2 and 0.7 mg x kg(-1), i.p. respectively). In a rat pain model, xaliproden inhibited paw licking and elevation (ED(50): 1 and 3 mg x kg(-1), i.p. respectively) following formalin injection in the paw. All effects were reversed by pretreatment with WAY100635. CONCLUSIONS AND IMPLICATIONS: These results indicate that activation of 5-HT(1A) receptors is the principal mechanism of action of xaliproden and provide further support for the utility of 5-HT(1A) receptor activation as an anti-nociceptive strategy.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms.

BACKGROUND AND PURPOSE: The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia. EXPERIMENTAL APPROACH: F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems. KEY RESULTS: Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating. CONCLUSIONS AND IMPLICATIONS: The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. II. Activity in models of positive symptoms of schizophrenia.

BACKGROUND AND PURPOSE: F15063 is a high affinity D(2)/D(3) antagonist, D(4) partial agonist, and high efficacy 5-HT(1A) agonist, with little affinity (40-fold lower than for D(2) receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects. EXPERIMENTAL APPROACH: Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and 'serotonin syndrome'. KEY RESULTS: F15063 potently (ED(50)s: 0.23 to 1.10 mg kg(-1) i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED(50)=0.30 mg kg(-1) i.p.). F15063, owing to its 5-HT(1A) agonism, did not produce (ED(50)>40 mg kg(-1) i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg(-1) p.o. Furthermore, F15063 did not induce the 'serotonin syndrome' in rats (flat body posture and forepaw treading: ED(50) >32 mg kg(-1) i.p.). CONCLUSIONS AND IMPLICATIONS: F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper).

SSR181507, a dopamine D(2) receptor antagonist and 5-HT(1A) receptor agonist, alleviates disturbances of novelty discrimination in a social context in rats, a putative model of selective attention deficit.

RATIONALE: Selective attention deficit, characterised by the inability to differentiate relevant from irrelevant information, is considered to underlie many cognitive deficits of schizophrenia, and appears to be only marginally responsive to treatment with current antipsychotics. OBJECTIVES: We compared the activity of the putative atypical antipsychotic SSR181507 (a dopamine D(2) receptor antagonist and 5HT(1A) receptor agonist) with reference compounds, on disturbances of novelty discrimination in a social context in rats, a behavioural paradigm that putatively models selective attention deficit. METHODS: A first (familiar) juvenile rat was presented to an adult rat for a period (P1) of 30 min. A second (novel) juvenile was then introduced at the end of P1 for a period (P2) of 5 min. The ability of the adult rat to discriminate between the two juveniles, presented at the same time, was evaluated by measuring the ratio of the time spent in interaction with the novel vs the familiar juvenile during P2. RESULTS: Adult rats spent more time exploring the novel than the familiar juvenile. This novelty discrimination capacity was disrupted by: (1) parametric modification of the procedure (reduction of time spent in contact with the familiar juvenile during P1); (2) acute injection of psychotomimetics that are known to induce schizophrenia-like symptoms in humans, such as phencyclidine (PCP; 3 mg/kg, i.p.) and d-amphetamine (1 mg/kg, i.p.) and (3) neonatal treatment with PCP (three injections of 10 mg/kg, s.c.), a model based on the neurodevelopmental hypothesis of schizophrenia. The potential atypical antipsychotic SSR181507 (0.03-3 mg/kg, i.p.) and the atypical antipsychotics clozapine (0.1-1 mg/kg, i.p.) and amisulpride (1-3 mg/kg, i.p.) attenuated deficits in novelty discrimination produced by parametric manipulation and by acute or neonatal treatment with PCP. The typical antipsychotic haloperidol (up to 0.3 mg/kg, i.p.) attenuated only deficits in novelty discrimination produced by parametric modification. CONCLUSION: Collectively, these results suggest that SSR181507 can alleviate disturbances of novelty discrimination in a social context in rats, and that this paradigm may represent a suitable animal model of selective attention deficits observed in schizophrenia.

SSR181507, a putative atypical antipsychotic with dopamine D2 antagonist and 5-HT1A agonist activities: improvement of social interaction deficits induced by phencyclidine in rats.

Social behaviour is frequently impaired in schizophrenic patients, and current antipsychotics appear poorly effective in alleviating this deficit. SSR181507 is a selective dopamine D2 receptor antagonist and 5-HT1A receptor agonist [Neuropsychopharmacology 28 (2003) 2064] with an atypical antipsychotic profile and additional antidepressant/anxiolytic activities [Neuropsychopharmacology 28 (2003) 1889]. Here, we sought to assess the efficacy of SSR181507, and of reference antipsychotics and antidepressant/anxiolytics, to counteract phencyclidine (PCP)-induced social interaction deficit in rats. Pairs of unfamiliar rats were placed for 10 min each day into a dimly lit arena, during four consecutive days. On the test day (5th day), each pair was placed into the arena 30 min after i.p. treatment with PCP (or vehicle) and a challenge compound or vehicle (same for both rats, i.p. or s.c.). The time spent in social interaction was scored during 10 min. PCP (1 mg/kg) decreased social interaction time by about 35%. This effect was fully antagonized by pre-treatment with SSR181507 (1 mg/kg). In contrast, neither haloperidol (0.05 and 0.1 mg/kg) nor clozapine (0.3 and 1 mg/kg) antagonized this PCP-induced deficit. The selective 5-HT1A receptor agonist 8-OH-DPAT (0.025 and 0.05 mg/kg s.c.), but not the anxiolytic diazepam (0.75 and 1.5 mg/kg), also improved social interaction impairment in PCP-treated rats: this would indicate that the 5-HT1A receptor agonist properties of SSR181507 are responsible for the reversal of PCP-induced social deficit. These data suggest that, in addition to its atypical antipsychotic profile and antidepressant/anxiolytic activities, SSR181507 has a potential therapeutic activity in another key feature of schizophrenia poorly controlled by current antipsychotics, namely deterioration in social functioning.

Use of negatively reinforcing electrical brain stimulation to detect conventional and nonconventional anxiolytics as well as an anxiogenic drug.

The present study determined whether anxiolytics such as diazepam (DZP), the benzodiazepine (BZD) receptor-selective agonist abecarnil (ABC), or the 5-HT1(A) agent buspirone (BUS) would increase the response latency of rats to switch-off electrical brain stimulation (EBS) of the periaqueductal gray (PAG). We also investigated the effects of pentylenetetrazole (PTZ), a purported anxiogenic. Given acutely, DZP (2.5 and 5 mg/kg, ip) and ABC (0.5 and 1 mg/kg, ip) increased response latency. The BZD receptor antagonist flumazenil (10.0 mg/kg, ip) blocked these effects. Increasing the frequency of EBS reversed the effects of DZP and ABC, suggesting that motor disruption did not account for the increase in latency seen with these drugs. Given acutely, BUS (10.0 mg/kg, ip) also increased response latency, which was likely due to motor disruption because it was not reversed by increasing the frequency of EBS. When BUS (2.5 mg/kg, ip) was given every 8 h for 3 days, an increase in latency was also obtained, which was reversible by increasing the frequency of EBS. Finally, PTZ (10 and 20 mg/kg, ip) shortened the latency to respond. These results (1) suggest that DZP, ABC, and chronic BUS attenuate, whereas PTZ potentiates, the negative reinforcing stimulus (NRS) induced by PAG stimulation, and (2) support the hypothesis that the switch-off procedure accurately detects anxiolytic and anxiogenic drugs.

Haloperidol-induced catalepsy is absent in dopamine D(2), but maintained in dopamine D(3) receptor knock-out mice.

We have previously found that mice homozygous for the deletion of the dopamine D(2) receptor gene (D(2)(-/-) mice) do not present spontaneous catalepsy when tested in a "bar test". In the present study, we sought to analyse the reactivity of D(2) receptor mutant mice to the cataleptogenic effects of dopamine D(2)-like or D(1)-like receptor antagonists. In parallel, we assessed the cataleptogenic effects of these antagonists in dopamine D(3) receptor mutant mice. D(2)(-/-) mice were totally unresponsive to the cataleptogenic effects of the dopamine D(2)-like receptor antagonist haloperidol (0.125-2 mg/kg i.p.), while D(2)(+/-) mice, at the highest haloperidol doses tested, showed a level of catalepsy about half that of wild-type controls. The degree of haloperidol-induced catalepsy was thus proportional to the level of striatal dopamine D(2) receptor expression (0.50, 0.30 and 0.08 pmol/mg protein as measured at 0.25 nM [3H]spiperone for D(2)(+/+), D(2)(+/-) and D(2)(-/-) mice, respectively). However, D(2)(-/-) and D(2)(+/-) mice were as sensitive as their wild-type counterparts to the cataleptogenic effects of the dopamine D(1)-like receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390: 0.03-0.6 mg/kg s.c.). Striatal dopamine D(1) receptor expression (as measured using [3H]SCH 23390 binding) was not significantly affected by the genotype. The ability of SCH 23390 to induce catalepsy in D(2)(-/-) mice suggests that their resistance to haloperidol-induced catalepsy is due to the absence of dopamine D(2) receptors, and not to the abnormal striatal synaptic plasticity that has been shown by others to occur in these mice. In agreement with the observation that dopamine D(2) and dopamine D(1) receptor expression was essentially identical in D(3)(+/+), D(3)(+/-) and D(3)(-/-) mice, dopamine D(3) receptor homozygous and heterozygous mutant mice, on the whole, did not differ from their controls in the time spent in a cataleptic position following administration of either haloperidol (0.5-2 mg/kg i.p.) or SCH 23390 (0.03-0.6 mg/kg s.c.). Also, dopamine D(3) receptor mutant mice were no more responsive than wild-type controls when co-administered subthreshold doses of haloperidol (0.125 mg/kg) and SCH 23390 (0.03 mg/kg), suggesting that dopamine D(3) receptor knock-out mice are not more sensitive than wild-types to the synergistic effects of concurrent blockade of dopamine D(2) and dopamine D(1) receptors in this model. These results suggest that the dopamine D(2) receptor subtype is necessary for haloperidol to produce catalepsy, and that the dopamine D(3) receptor subtype appears to exert no observable control over the catalepsy produced by dopamine D(2)-like, D(1)-like and the combination of D(1)-like and D(2)-like receptor antagonists.

Decreased locomotor activity after microinjection of dopamine D2/D3 receptor agonists and antagonists into lobule 9/10 of the cerebellum: a D3 receptor mediated effect?

The restricted localization of dopamine (DA) D3 receptors in the rat cerebellum lobule 9/10 appears to provide a method for investigating the in vivo selectivity of dopaminergic compounds for the D3 receptor subtype. Sprague-Dawley rats implanted with a cannula aimed at lobule 9/10 were microinjected with DA receptor ligands and immediately placed into activity chambers to record their spontaneous locomotor activity for short term (0 to 20 min) and delayed (20 to 40 min) effects. The DA D2/D3 receptor agonists quinelorane (0.1 to 2.5 microg) and 7-OH-DPAT (0.1 to 10 microg) decreased locomotor activity in the first 20 min post-microinjection. In contrast, the DAD1, receptor agonist 6-Br-APB (0.1 to 10 microg) did not affect locomotor activity during this time period, but markedly increased locomotion between 20 and 40 min at the highest dose tested. The DA receptor antagonists haloperidol and raclopride (1 to 10 microg) were also found to reduce locomotor activity. Furthermore, quinelorane and 7-OH-DPAT, but not haloperidol, when microinjected into lobules 1/2 or 6/7 (where no DA D3 receptors have been detected) decreased locomotor scores. These results show that both DA receptor agonists and antagonists decrease locomotor activity when microinjected into lobule 9/10 of the cerebellum. Additionally, DA receptor agonists can reduce spontaneous locomotion when microinjected outside of lobule 9/10. This would suggest that, at least for quinelorane and 7-OH-DPAT, the locomotor decreasing effects following microinjection into cerebellar lobule 9/10 may not be mediated by activity at DA D3 receptors, and that this behavioural assay is unlikely to provide a means for studying the in vivo pharmacology of the DA D3 receptor.

The D3 antagonist PNU-99194A potentiates the discriminative cue produced by the D3 agonist 7-OH-DPAT.

Based on correlations between potencies of various dopamine D2/D3 agonists to substitute for the 7-OH-DPAT discriminative cue and their in vitro (mitogenesis test) potencies, it has been suggested that the 7-OH-DPAT cue is mediated by activity at the D3 subtype. We sought to verify that the 7-OH-DPAT cue could be blocked by PNU-99194A, a commercially available preferential D3 antagonist. Rats were trained (FR10 two-lever, food-reinforced schedule) to press one lever following 7-OH-DPAT (0.1 mg/kg i.p.) and the other lever following saline. Rats were then tested with various doses of 7-OH-DPAT alone or in combination with PNU-99194A. 7-OH-DPAT (0.003 to 0.3 mg/kg) engendered dose-dependent substitution; PNU-99194A (1 to 10 mg/kg) failed to antagonize the cue induced by 0.1 mg/kg of 7-OH-DPAT and, at 10 mg/kg, given in combination with 0.003 to 0.1 mg/kg of 7-OH-DPAT, PNU-99194A markedly shifted the 7-OH-DPAT dose-effect curve to the left, i.e., potentiated the 7-OH-DPAT cue. If PNU-99194A is a preferential D3 antagonist, the present data do not confirm the previous hypothesis that the 7-OH-DPAT cue is mediated by the D3 subtype.

Dopamine D2 receptor knock-out mice are insensitive to the hypolocomotor and hypothermic effects of dopamine D2/D3 receptor agonists.

The dopamine (DA) D2-like family of receptors is comprised of three subtypes, the D2, D3, and D4 receptors. It has been suggested that the potency of DA receptor agonists to produce hypothermia and hypolocomotion in rodents correlates more strongly with the in vitro affinity for, or potency (mitogenesis test) at the D3 than at the D2 subtype. However, it has recently been reported that when tested in DA D3 receptor knock-out mice, several DA D2/D3 receptor agonists (7-OH-DPAT, PD 128907 and quinelorane) induced levels of hypothermia and decreases of locomotor activity similar to those obtained in control (wild-type) mice. These results do not argue in favour of an implication of DA D3 receptors in these in vivo effects. In order to investigate whether the DA D2 receptor is the subtype that mediates hypothermia and hypolocomotion produced by DA D2/D3 receptor agonists, we tested the effects of ip administration of the DA D2/D3 receptor agonists 7-OH-DPAT and PD 128907, on core temperature and locomotor activity in DA D2 receptor knock-out mice (homozygotes: D2(-/-) and heterozygotes: D2(+/-)), and in wild-type (D2(+/+)) mice. 7-OH-DPAT (0.1-3 mg/kg) and PD 128907 (1-10 mg/kg) induced hypothermia and decreased locomotion in D2(+/+) mice, but had no effects in D2(-/-) mice; the magnitude of the hypothermic and locomotor-reducing effects of these two agonists in D2(+/+) mutants was approximately half that of D2(+/+) mice. During the first 10 min in the activity chambers, the level of spontaneous locomotor activity of D2(-/-) individuals was almost 50% below that of D2(+/+) mice; basal locomotor activity of D2(+/-) mice was between that of D2(-/-) and D2(+/+) individuals. Neither type of mutant showed spontaneous catalepsy or deficits in forelimb muscle strength (grip-strength test). These results show that the presence of DA D2 receptors is necessary for the expression of the locomotor- and core temperature-decreasing effects of DA D2/D3 receptor agonists such as 7-OH-DPAT and PD 128907.