Multimodal imaging study of the 5-HT1A receptor biased agonist, NLX-112, in a model of L-DOPA-induced dyskinesia.

INTRODUCTION: The leading treatment for motor signs of Parkinson's disease is L-DOPA, but, upon extended use, it can lead to levodopa-induced dyskinesia (LID). Serotonergic neurons are involved in LID etiology and previous pre-clinical studies have shown that NLX-112, a 5-HT1A biased agonist, has robust antidyskinetic effects. Here, we investigated its effects in hemiparkinsonian (HPK) rats with a unilateral nigrostriatal 6-OHDA lesion. METHODS: We compared HPK rats with LID (i.e., sensitized to the dyskinetic effects of chronic L-DOPA) and without LID (HPK-non-LID), using [18F]FDG PET imaging and fMRI functional connectivity following systemic treatment with saline, L-DOPA, NLX-112 or L-DOPA + NLX-112. RESULTS: In HPK-non-LID rats, [18F]FDG PET experiments showed that L-DOPA led to hypermetabolism in motor areas (cerebellum, brainstem, and mesencephalic locomotor region) and to hypometabolism in cortical regions. L-DOPA effects were also observed in HPK-LID rats, with the additional emergence of hypermetabolism in raphe nuclei and hypometabolism in hippocampus and striatum. NLX-112 attenuated L-DOPA-induced raphe hypermetabolism and cingulate cortex hypometabolism in HPK-LID rats. Moreover, in fMRI experiments NLX-112 partially corrected the altered neural circuit connectivity profile in HPK-LID rats, through activity in regions rich in 5-HT1A receptors. CONCLUSION: This neuroimaging study sheds light for the first time on the brain activation patterns of HPK-LID rats. The 5-HT1A receptor agonist, NLX-112, prevents occurrence of LID, likely by activating pre-synaptic autoreceptors in the raphe nuclei, resulting in a partial restoration of brain metabolic and connectivity profiles. In addition, NLX-112 also rescues L-DOPA-induced deficits in cortical activation, suggesting potential benefit against non-motor symptoms of Parkinson's disease.

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, display ketamine-like RAAD and anti-TRD activities in rat CMS models.

OBJECTIVES: NLX-101 and NLX-204 are highly selective serotonin 5-HT1A 'biased' agonists, displaying potent and efficacious antidepressant-like activity upon acute administration in models such as the forced swim test. METHODS: we compared the effects of repeated administration of NLX-101, NLX-204 and ketamine in the chronic mild stress (CMS) model of depression, considered to have high translational potential, on sucrose consumption (anhedonia measure), novel object recognition (NOR; working memory measure) and elevated plus maze (EPM; anxiety measure) in male Wistar and Wistar-Kyoto rats (the latter being resistant to classical antidepressants). RESULTS: in Wistar rats, NLX-204 and NLX-101 (0.08-0.16 mg/kg i.p.), like ketamine (10 mg/kg i.p.) dose-dependently reversed CMS-induced sucrose intake deficit from treatment Day 1, with nearly full reversal observed at the higher dose at Days 8 and 15. These effects persisted for 3 weeks following treatment cessation. In the NOR test, both doses of NLX-101/NLX-204, and ketamine, rescued the deficit in discrimination index caused by CMS on Days 3 and 17; all three compounds increased time spent in open arms (EPM) but only NLX-204 achieved statistical significance on Days 2 and 16. In Wistar-Kyoto rats, all 3 compounds were also active in the sucrose test and, to a lesser extent, in the NOR and EPM. In non-stressed rats (both strains), the three compounds produced no significant effects in all tests. CONCLUSIONS: these observations further strengthen the hypothesis that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve rapid-acting/sustained antidepressant effects combined with activity against TRD, in addition to providing beneficial effects against memory deficit and anxiety in depressed patients.

The 5-HT1A receptor biased agonist, NLX-204, shows rapid-acting antidepressant-like properties and neurochemical changes in two mouse models of depression.

Activation of cortical serotonin 5-HT1A receptors may be a promising strategy to achieve rapid-acting antidepressant (RAAD) activity. NLX-204 is a selective 5-HT1A receptor biased agonist that, in naïve mice, robustly decreases immobility in the forced swim test (FST), and preferentially phosphorylates extracellular signal-regulated kinase (ERK1/2), involved in antidepressant activity. Here, we evaluated the properties of NLX-204 in two mouse models of depression. Male CD-1 mice were subjected to unpredictable chronic mild stress (UCMS) for 4-weeks or to repeated corticosterone (CORT, 20 mg/kg s.c./day) for 3-weeks before receiving acute administration of NLX-204 (2 mg/kg, p.o.). Depressive-like behavior was assessed in the FST, anhedonia-like behavior in the sucrose preference test (SPT) and locomotor activity was also recorded. Phosphorylation of ERK1/2 (pERK1/2) and cAMP response binding element (pCREB) were measured ex vivo in hippocampus and prefrontal cortex (PFC). UCMS or CORT treatment increased immobility in the FST, elicited a sucrose preference deficit, and decreased pERK1/2 and pCREB levels in PFC and hippocampus. NLX-204 reduced depressive-like behavior in the FST in CORT and UCMS mice, and normalized sucrose preference in CORT mice, suggesting anti-anhedonic activity. NLX-204 increased pERK1/2 levels in PFC of UCMS mice. NLX-204 also increased pCREB levels in PFC of CORT mice. These data suggest that NLX-204 has RAAD-like properties not only in naïve mice, but also in mice in a "depressive-like" state, and that these involve changes in PFC and hippocampal pERK1/2 and pCREB levels. These data provide additional evidence that activation of 5-HT1A receptors by selective biased agonists, such as NLX-204, may constitute a promising RAAD strategy.

Neural serotonergic circuits for controlling long-term voluntary alcohol consumption in mice.

Alcohol-use-disorders are chronic relapsing illnesses, often co-morbid with anxiety. We have previously shown using the "drinking-in-the-dark" model in mice that the stimulation of the serotonin receptor 1A (5-HT1A) reduces ethanol binge-drinking behaviour and withdrawal-induced anxiety. The 5-HT1A receptor is located either on Raphe neurons as autoreceptors, or on target neurons as heteroreceptors. By combining a pharmacological approach with biased agonists targeting the 5-HT1A auto- or heteroreceptor and a chemogenetic approach (DREADDs), here we identified that ethanol-binge drinking behaviour is dependent on 5-HT1A autoreceptors and 5-HT neuronal function, with a transition from DRN-dependent regulation of short-term (6 weeks) ethanol intake, to MRN-dependent regulation after longer ethanol exposure (12 weeks). We further identified a serotonergic microcircuit (5-HTMRN→DG) originating from the MRN and projecting to the dentate gyrus (DG) of the hippocampus, that is specifically affected by, and modulates long-term ethanol consumption. The present study indicates that targeting Raphe nuclei 5-HT1A autoreceptors with agonists might represent an innovative pharmacotherapeutic strategy to combat alcohol abuse.

Dissecting the contribution of 5-HT1A auto- and heteroreceptors in sucrose overconsumption in mice.

The rise in obesity prevalence has been linked to overconsumption of high-sugar containing food and beverages. Recent evidence suggests that chronic sucrose consumption leads to changes in serotonergic neuroplasticity within the neural circuits involved in feeding control. Although there is a relationship between serotonin signalling in the brain and diet-induced obesity, the specific serotonin (5-HT) receptors or pathways involved remain unknown. The 5-HT1A receptor subtype plays a role in regulating mood, anxiety, and appetite, and has been associated with reversing addiction to substances of abuse. However, the respective role of 5-HT1A auto- vs heteroreceptors in sucrose consumption has not been examined. Mice were given controlled access to either 5%, 10% or 25% w/v sucrose, or water as a control, for 12 weeks using the well-established "drinking in the dark" protocol (n = 6-8 mice per group). Ligands selectively targeting 5-HT1A auto- and/or heteroreceptors (NLX-112, unbiased 5-HT1A receptor agonist; NLX-101, preferential heteroreceptor agonist; F13714, preferential autoreceptor agonist) were administered i.p. acutely after 6 and 12 weeks of sucrose consumption. The specific involvement of 5-HT1A receptors in these effects was verified by blockade with the selective 5-HT1A receptors antagonist WAY-100,635. The specific subpopulation of 5-HT1A receptors involved in sucrose consumption was dependent on the concentration of sucrose solution and the duration of exposure to sucrose (6 weeks vs 12 weeks). Long-term sucrose consumption leads to accentuated 5-HT1A autoreceptor function. Thus, targeting 5-HT1A autoreceptors might represent an effective therapeutic strategy to combat the rise in obesity resulting from the overconsumption of high-sugar diet.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders.

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with 18F to produce the first agonist PET radiopharmaceutical (known as [18F]-F13640) for investigation of the active state of 5-HT1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT1A receptors and new profiles of cellular signaling bias, notably for ß-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

The selective 5-HT1A receptor agonist NLX-112 displays anxiolytic-like activity in mice.

Anxiety is amongst the commonest neuropsychiatric disorders, and there is a large body of evidence to suggest that abnormalities in serotonergic function are involved in its pathogenesis. Several studies have implicated 5-HT1A receptor activation in mitigating anxiety disorders, so this study investigated the acute effects of a highly selective, potent and efficacious 5-HT1A receptor full agonist, NLX-112 (a.k.a. befiradol, F13640), in middle-aged C57bl/6 J male mice. Video tracking was used to measure several parameters including time spent in the open and closed arms of an elevated plus maze (EPM), distance travelled and thigmotaxis in an open field test (OFT). At 0.1 to 1.0 mg/kg s.c., NLX-112 markedly decreased thigmotaxis and increased exploratory behaviour in the OFT and EPM assays. Hence, at 0.3 mg/kg, NLX-112 augmented locomotor activity in the centre of an open field arena by 164% and increased the time spent in the open arms of the EPM by 119% of control. These results indicate that anxiety-like behaviours in mice are significantly diminished with low doses of NLX-112. NLX-112 may therefore possess anxiolytic properties which complement its known activity in models of movement disorders.

Discriminative stimulus properties of the 5-HT1A receptor biased agonists NLX-101 and F13714, in rats trained to discriminate 8-OH-DPAT from saline.

NLX-101 and F13714 are selective, full efficacy, biased agonists of the serotonin (5-HT1A) receptor. NLX-101 preferentially activates cortical postsynaptic 5-HT1A receptors, whereas F13714 preferentially activates raphe nuclei presynaptic 5-HT1A receptors. We compared NLX-101 and F13714 for their efficacy and potency to substitute for the discriminative cue produced by the prototypical, nonbiased 5-HT1A receptor agonist 8-OH-DPAT (racemate). Male and female Sprague-Dawley rats were trained to discriminate 8-OH-DPAT (0.1 mg/kg i.p., 20 min pretreatment) from saline using a classical two-lever drug-discrimination procedure. 8-OH-DPAT (0.01 and 0.05 mg/kg i.p.) dose-dependently substituted for the training dose, with about 50% responding on the 8-OH-DPAT-associated lever at 0.05 mg/kg. F13714 fully and very potently substituted for the training dose of 8-OH-DPAT from 0.018 mg/kg i.p., whereas NLX-101 only achieved full substitution at 0.5 mg/kg i.p., a dose which is known to also activate presynaptic 5-HT1A receptors. The 5-HT1A receptor partial agonist, buspirone, partially substituted (~80%) at 1 and 2 mg/kg i.p., doses which also decreased response rates. F13714 decreased response rates at 0.05 mg/kg. The selective 5-HT1A receptor antagonist WAY-100 635 (1 mg/kg s.c., 40 min pretreatment) elicited almost no responding on the 8-OH-DPAT-associated lever by itself, but blocked the discriminative stimulus effects produced by administration (20 min pretreatment) of 8-OH-DPAT (0.1 mg/kg), F13714 (0.025 mg/kg), NLX-101 (0.5 mg/kg) or buspirone (1 mg/kg). These data suggest that the discriminative cue produced by 0.1 mg/kg i.p. 8-OH-DPAT results from activation of presynaptic 5-HT1A receptors. They also further demonstrate the distinct profiles in behavioral models of 5-HT1A receptor-biased agonists.

NLX-101, a cortical 5-HT1A receptor biased agonist, reverses scopolamine-induced deficit in the delayed non-matching to position model of cognition.

NLX-101 is a selective, high efficacy, biased agonist at post-synaptic cortical 5-HT1A receptors. We have previously shown that it opposes deficits produced by blockade of NMDA receptors and has pro-cognitive activity of its own. Based on the strong interaction between 5-HT1A receptors and the central cholinergic system, we tested NLX-101 on scopolamine-induced impairment of cognition in a delayed non-matching to position (DNMTP) model. The cholinesterase inhibitor, tacrine, was used as a comparator. In operant chambers with two retractable levers, male rats were trained to press one randomly presented lever during a "sample" phase. Following a time delay of either 1, 5 or 10 s, both levers were then presented, the rat being required to press the correct lever (i.e. the one not previously presented) to receive a food pellet reward. Scopolamine (0.16 mg/kg i.p.) significantly impaired accuracy (i.e. choice of correct lever) at 5 and 10 s delays. In contrast, NLX-101 (0.04, 0.16, 0.63 mg/kg i.p.) did not worsen accuracy, except at 0.63 mg/kg. Moreover, NLX-101 (0.04 and 0.16 mg/kg) dose-dependently and significantly opposed scopolamine-induced impairment for 5 and 10 s delays, with near-total reversal at 10 s. The acetylcholinesterase inhibitor, tacrine, also opposed scopolamine-induced impairment but was less potent and efficacious, with a single significant effect at 2.5 mg/kg and 5 s delay only. The present data suggest that biased agonism at post-synaptic, cortical 5-HT1A receptors could prove useful in neurological or neuropsychiatric pathologies characterized by cognitive deficits consecutive to a reduced central cholinergic tone.

Marble Burying in NMRI Male Mice Is Preferentially Sensitive to Pre- Versus Postsynaptic 5-HT1A Receptor Biased Agonists.

Obsessive compulsive disorder (OCD) is a psychiatric disorder characterized by excessive and repetitive thoughts and gestures, mainly treated pharmacologically with selective serotonin reuptake inhibitors (SSRIs). The marble burying test in mice is commonly used to model OCD and has been shown to be sensitive to SSRIs, which decrease burying behavior. The activity of SSRIs in this model is mediated through activation of 5-hydroxytryptamine (5-HT) 1A receptors, but the respective implication of pre- versus postsynaptic 5-HT1A receptors has not been elucidated. Here, we investigated marble burying behavior by male NMRI mice following acute administration of 3 biased agonists, which preferentially activate presynaptic 5-HT1A receptors (F13714) or postsynaptic receptors (NLX-101) or which exhibit balanced activation of both pre- and postsynaptic 5-HT1A receptors (NLX-112). When administered at the dose of 2.5 mg/kg i.p., all 3 biased agonists completely or nearly completely abolished marble burying behavior. However, they varied in their potency with minimal effective doses of 0.16, 0.63, and 2.5 mg/kg i.p., for F13714, NLX-112, and NLX-101, respectively. The selective 5-HT1A receptor antagonist, WAY100,635 was inactive up to 2.5 mg/kg. These results suggest that marble burying behavior in male NMRI mice is preferentially sensitive to activation of pre- versus postsynaptic 5-HT1A receptors. Moreover, they suggest that targeting 5-HT1A receptors with biased agonists could provide an innovative therapeutic approach to combat OCD.

The selective 5-HT1A receptor agonist, NLX-112, exerts anti-dyskinetic and anti-parkinsonian-like effects in MPTP-treated marmosets.

l-DOPA is the gold-standard pharmacotherapy for treatment of Parkinson's disease (PD) but can lead to the appearance of troubling dyskinesia which are attributable to 'false neurotransmitter' release of dopamine by serotonergic neurons. Reducing the activity of these neurons diminishes l-DOPA-induced dyskinesia (LID), but there are currently no clinically approved selective, high efficacy 5-HT1A receptor agonists. Here we describe the effects of NLX-112, a highly selective and efficacious 5-HT1A receptor agonist, on LID in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated marmosets, a non-human primate model of PD. NLX-112 exhibited modest plasma half-life (~2h) and marked plasma protein binding (96%). When administered to parkinsonian marmosets with l-DOPA (7 mg/kg p.o.), NLX-112 (0.025, 0.1 and 0.4 mg/kg p.o.) reduced LID scores at early time-points after administration, whilst only minimally interfering with the l-DOPA-induced reversal of motor disability. In contrast, the prototypical 5-HT1A receptor agonist, (+)8-OH-DPAT (0.6 and 2 mg/kg p. o.), reduced LID but also abolished l-DOPA's anti-disability activity. Administered by itself, NLX-112 (0.1, 0.2 mg/kg p.o.) produced very little dyskinesia or locomotor activity, but reduced motor disability scores by about half the extent elicited by l-DOPA, suggesting that it may have motor facilitation effects of its own. Both NLX-112 and (+)8-OH-DPAT induced unusual and dose-limiting behaviors in marmoset that resembled 'serotonin behavioral syndrome' observed previously in rat. Overall, the present study showed that NLX-112 has anti-LID activity at the doses tested as well as reducing motor disability. The data suggest that additional investigation of NLX-112 is desirable to explore its potential as a treatment for PD and PD-LID.

Cortical 5-hydroxytryptamine 1A receptor biased agonist, NLX-101, displays rapid-acting antidepressant-like properties in the rat chronic mild stress model.

BACKGROUND: NLX-101 (also known as F15599) is a highly selective and efficacious 'biased' agonist at cortical 5-hydroxytryptamine 1A (5-HT1A) heteroreceptors. In rodents, it possesses marked antidepressant-like activity, potently and completely abolishing immobility in the forced swim test (FST) with extended duration of action. METHODS: We investigated the antidepressant-like activity of NLX-101 using the rat chronic mild stress (CMS) model of depression, considered to have a higher translational potential than the FST, as it possesses construct, face and predictive validity. The effects of CMS and repeated NLX-101 treatment were tested using sucrose consumption (a measure of anhedonia), novel object recognition (NOR; a measure of working memory) and elevated plus maze (EPM; a measure of anxiety) tests. RESULTS: NLX-101 reversed the CMS-induced decrease of sucrose intake on day 1 of testing, with full reversal observed at the dose of 0.16 mg/kg and a less pronounced but still significant effect at 0.04 mg/kg, both given twice a day intraperitoneally. The effects of NLX-101 were maintained over the 2 week treatment period and persisted for four weeks following cessation of treatment. In the NOR test, both doses of NLX-101 rescued the deficit in discrimination index caused by CMS, without any effect on locomotor activity. However, NLX-101 had no effect on the reduction of open-arms entries produced by CMS in the EPM model. In control, non-stressed rats, NLX-101 produced non-significant effects in all three models. CONCLUSIONS: NLX-101 displayed efficacious activity in the CMS test, with more rapid (1 day) antidepressant-like effects than pharmacological compounds tested previously under the same experimental conditions. These observations suggest that biased agonist targeting of cortical 5-HT1A receptors constitutes a promising strategy to achieve rapid-acting and sustained antidepressant effects.

Serotonin 5-HT1A Receptor Biased Agonists Display Differential Anxiolytic Activity in a Rat Social Interaction Model.

When placed in an unfamiliar and brightly lit open-field, two adult male rats that have not previously interacted display a low level of social interaction (SI) attributed to an anxiety-like state. The SI test has therefore been used to explore anxiolytic/antistress activity. Here, we investigated the effects of serotonin 5-HT1A receptor agonists displaying various activity profiles, i.e. partial vs full agonist efficacy and pre- versus postsynaptic 5-HT1A receptor preferential activation by "biased agonists". Adult male Sprague-Dawley rats were housed singly before starting the social interaction session. At 30 min before being placed in an open-field, both rats of the dyad were injected (i.p or s.c.) with either vehicle, diazepam (as a reference compound), or one of six 5-HT1A receptor agonists: NLX-101 (a.k.a. F15599), F13714, S15535, flesinoxan, 8-OH-DPAT, and buspirone. Time spent in SI (following, sniffing, playing) was recorded for 10 min. Time spent in SI was inversely correlated with light intensity, with values dropping nearly by half (212.6 ± 18.8 vs 113.7 ± 7.0 s) between 10 and 300 lx (measured at floor level). Under the high light intensity conditions (300 lx), diazepam showed a bell-shaped curve, significantly increasing SI (78% increase in interaction time above control) at 1 mg/kg i.p. only. In the case of 5-HT1A receptor ligands, full agonists, whether nonpreferential (flesinoxan, (±)8-OH-DPAT) or preferential for presynaptic receptors (F13714), showed the strongest activity in this model. The preferential presynaptic receptor partial agonist, S15535, was also active over a wide dose-range, although with lower efficacy than F13714. In contrast, NLX-101, a high-efficacy biased agonist that preferentially activates postsynaptic 5-HT1A receptors, exhibited little activity. The clinical anxiolytic, buspirone, showed a marked effect likely due to its partial agonist activity at 5-HT1A presynaptic receptors. These data support the hypothesis that enhancement of SI in this model is mediated by preferential agonist activation of presynaptic 5-HT1A receptors, and confirm previous studies using local microinjections of (±)8-OH-DPAT. They further support the utility of noninvasive administration of biased agonists for exploring the activity of 5-HT1A receptor subpopulations.

Milnacipran is active in models of irritable bowel syndrome and abdominal visceral pain in rodents.

The role of antidepressants in the treatment of visceral pain has not been extensively examined. Milnacipran, a serotonin/noradrenalin reuptake inhibitor, has recently been approved in the USA for fibromyalgia, a chronic pathology characterized by diffused/chronic musculoskeletal pain, and a high prevalence of irritable bowel syndrome. Here, we determined its antinociceptive efficacy in two visceral pain tests in rodents: the acetic acid-induced writhing model in mice and the butyrate/colonic distension assay in rats, a model of irritable bowel syndrome. Acute milnacipran (5-40 mg/kgi.p.) significantly and dose-dependently reduced writhing (72.2 ± 3.2 versus 17.0 ± 4.1 writhes at 40 mg/kg). Following repeated administration (40 m/kgi.p. for 5 days), milnacipran preserved its ability to significantly reduce writhing (76 ± 8.3 versus 21.1 ± 6.7 writhes). Similarly, in the butyrate model, acute milnacipran (17.5 and 35 mg/kg, i.p.) significantly and dose-dependently increased cramps induction thresholds (from 45.7 ± 5.7 to 66.3 ± 4.8 and 75.6 ± 2.9 mm Hg, for 17.5 and 35 mg/kg, respectively) and reduced the number of cramps (from 3.0 ± 0.8 to 1.2 ± 0.8 and 0.3 ± 0.3 following inflation of an intra-rectal balloon. To summarise, milnacipran was efficacious in the writhing test, after acute and semi-chronic administration. This effect was confirmed after acute administration in a more specific model of colonic hypersensitivity induced by butyrate. This suggests that milnacipran has potential clinical application in the treatment of visceral pain, such as in irritable bowel syndrome, highly co-morbid with fibromyalgia.

Effects of milnacipran, duloxetine and indomethacin, in polyarthritic rats using the Randall-Selitto model.

Milnacipran, a serotonin and noradrenalin reuptake inhibitor (SNRI), is efficacious in rodents in various models of acute or chronic pain (traumatic, neuropathic, inflammatory, visceral). However, its activity against arthritic pain has never been explored. Here, we assessed the activity of acute treatment with milnacipran in a polyarthritic rat model. Rats were injected in the tail base with complete Freund's adjuvant to induce a state of polyarthritis. Analgesic effects of acute treatment with intraperitoneal administration of milnacipran were then evaluated, using the Randall-Selitto model, against two levels of pressure applied to both hind paws (a lower one, addressing mechanical allodynia and a higher one, addressing mechanical hyperalgesia). The other SNRI duloxetine and the nonsteroidal anti-inflammatory drug indomethacin were tested as positive controls. Milnacipran was significantly and dose dependently active against the decrease of paw withdrawal threshold produced by complete Freund's adjuvant for low (minimum effective dose=5 mg/kg, range tested: 2.5-10 mg/kg) and high (minimum effective dose=10 mg/kg, range tested: 5-20 mg/kg)-pressure levels. Duloxetine (20 mg/kg, intraperitoneally) was significantly active against low pressure only. Indomethacin (3 mg/kg per os) was efficacious against both pressure levels. These rodent data suggest that milnacipran should be efficacious in painful conditions associated with chronic inflammatory states, such as arthritis.

Evaluation of milnacipran, in comparison with amitriptyline, on cold and mechanical allodynia in a rat model of neuropathic pain.

Milnacipran, a serotonin/norepinephrine reuptake inhibitor (SNRI), has shown efficacy against several chronic pain conditions, including fibromyalgia. Here, we evaluated, in rats, its anti-allodynic effects following acute or sub-chronic treatment in a model of neuropathic pain (chronic constriction injury, CCI, of the sciatic nerve). Amitriptyline, a tricyclic antidepressant active pre-clinically and clinically against neuropathic pains, was added as a comparison compound. Upon acute i.p. administration, milnacipran was potently efficacious in the CCI model. It significantly reduced thermal allodynia in the cold (4°C) plate test (MED=2.5mg/kg), and attenuated mechanical allodynia in the von Frey filaments test (MED=10mg/kg). Given sub-chronically (7day, b.i.d.), milnacipran was effective at 10mg/kgi.p. in both tests. Acute amitriptyline (10mg/kgi.p.) was efficacious against mechanical, but less so against cold allodynia; under sub-chronic conditions, it was only active against mechanical allodynia. These data show that milnacipran is as efficacious as the reference compound amitriptyline in a pre-clinical model of injury-induced neuropathy, and demonstrate for the first time that it is active acutely and sub-chronically against cold allodynia. They also suggest that milnacipran has the potential to alleviate allodynia associated with nerve compression-induced neuropathic pain in the clinic (for example following discal hernia, avulsion or cancer-induced tissue damage).

SSR181507, a dopamine D2 receptor and 5-HT(1A) receptor ligand: evidence for mixed anxiolytic- and antidepressant-like activities.

SSR181507, a dopamine D2 receptor antagonist/partial agonist and 5-HT(1A) receptor agonist, is active in animal models of schizophrenia. Furthermore, it shows activity in several anxiety and/or depression models (Depoortere et al. 2003). Presently, we sought to further characterize the latter two activities in rats, using a step-down passive avoidance procedure, a shock-induced ultrasonic vocalization (UV) test in adult subjects and a social interaction test. SSR181507 (0.3 & 1 mg/kg ip), but not the atypical antipsychotics clozapine and olanzapine, decreased the latency time to step-down from a "safety" platform. Effects of SSR181507 were reversed by the selective 5-HT(1A) receptor antagonist SL88.0338. SSR181507 also reduced UV (0.3 & 1 mg/kg ip), an effect not reversed by SL88.0338, and observed with olanzapine, haloperidol, fluoxetine and the 5-HT(1A) receptor agonists 8-OH-DPAT and buspirone, but not diazepam. Furthermore, SSR181507 remained active following 3 weeks of administration (1 mg/kg ip, once daily) in the UV test. Lastly, SSR181507 (3 mg/kg ip) potentiated social interaction, an effect shared by diazepam and buspirone, but not by olanzapine, clozapine, haloperidol and 8-OH-DPAT. These data further strengthen previous findings that the putative atypical antipsychotic SSR181507 has mixed antidepressant and anxiolytic activities.

The central serotonin 2B receptor: a new pharmacological target to modulate the mesoaccumbens dopaminergic pathway activity.

The function of the serotonin(2B) receptor (5-HT(2B)R) in the mammalian brain is poorly characterized, especially with regard to its influence on dopamine (DA) neuron activity. Here, we assessed this issue by evaluating effects of 5-HT(2B)Rs ligands in the control of striatal and accumbal DA outflow, using in vivo microdialysis in halothane-anesthetized rats, and amphetamine-induced hyperlocomotion in vigil rats. The selective 5-HT(2B)R antagonist 1-[(2-chloro-3,4-dimethoxyphenyl)methyl]-2,3,4,9-tetrahydro-6-methyl-1H-pyrido[3,4-B]indole (LY 266097; 0.16 mg/kg, i.p.) had no influence on basal accumbal and striatal DA outflow but reduced significantly accumbal DA outflow when injected at 0.63 mg/kg. A significant reduction of basal DA outflow in the nucleus accumbens was also observed after i.p. administration of 0.16 mg/kg 2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine, another selective 5-HT(2B)R antagonist. In contrast, the 5-HT(2B)R agonist alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (3 mg/kg, s.c.) had no influence on basal DA outflow in either brain region. The increase in striatal and accumbal DA outflow induced by the 5-HT(2C)R inverse agonist 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole (5 mg/kg, i.p.) was unaltered by LY 266097 (0.63 mg/kg) pre-treatment. Conversely, LY 266097 (0.63 mg/kg) significantly diminished the increase in DA outflow induced by haloperidol (0.01 mg/kg, s.c.) or amphetamine (0.5 mg/kg, i.p.) in the nucleus accumbens, but not in the striatum. Amphetamine-induced hyperlocomotion (1 mg/kg) was also attenuated by LY 266097 (0.63 mg/kg). These findings demonstrate that 5-HT(2B)Rs exert a facilitatory control on mesoaccumbens DA pathway activity, and suggest that they may constitute a new target for improved treatment of DA-related neuropsychiatric disorders.

F15599, a preferential post-synaptic 5-HT1A receptor agonist: activity in models of cognition in comparison with reference 5-HT1A receptor agonists.

We assessed the activity of F15599, a selective and high efficacy 5-HT(1A) agonist that preferentially activates post- versus pre-synaptic receptors, in rat cognition/memory models. F15599 (0.16 mg/kg i.p.) partially alleviated detrimental effects of phencyclidine on working and reference memory deficit in a hole-board model. It also attenuated phencyclidine-induced deficit of cognitive flexibility in a reversal learning task, without effects of its own. F13714 (0.04 mg/kg) a chemical congener of F15599, and 8-OH-DPAT (0.01 or 0.16), were inactive against these phencyclidine-induced deficits, and/or even worsened basal performances. F15599 (0.04-2.5) was less disruptive than F13714 (0.005-0.16) or 8-OH-DPAT (0.01-0.63), on basal performance in models of attention (5-choice serial reaction time task) and working memory (delayed non-matching to position). Finally, unlike either comparator, F15599 reduced PPI with modest potency and only partially. To conclude, F15599, in models of memory/cognition, has a more favourable profile than F13714 and 8-OH-DPAT. This suggests that preferential activation of post-synaptic 5-HT(1A) receptors could prove useful in pathologies characterized by cognitive/memory deficiencies, such as schizophrenia and depression.