Novel, potent, selective, and brain penetrant phosphodiesterase 10A inhibitors.

Herein we report the discovery of a novel series of phosphodiesterase 10A inhibitors. Optimization of a HTS hit (17) resulted in potent, selective, and brain penetrant 23 and 26; both exhibited much lower clearance in vivo and decreased volume of distribution (rat PK) and have thus the potential to inhibit the PDE10A target in vivo at a lower efficacious dose than the reference compound WEB-3.

The effects of PDE10 inhibition on attentional set-shifting do not depend on the activation of dopamine D1 receptors.

Inhibitors of phosphodiesterase 10A (PDE10A) represent a novel class of potential antipsychotic compounds. These principles increase the level of cAMP and cGMP in the medium spiny neurons of the striatum and resemble the neurochemical consequences of dopamine D2 receptor inhibition and dopamine D1 receptor stimulation. Cognitive dysfunctions, including an impaired ability to shift perceptual attentional set, are core features of schizophrenia. In the present study, we investigated the involvement of D1 receptors in the procognitive action of the PDE10A inhibitor using the attentional set-shifting task in rats. The performance of the rats in the extradimensional shift stage of the attentional set-shifting task was taken as an index of cognitive flexibility. We first assessed the effects of the D1 agonist in otherwise untreated animals and in animals pretreated with the D1 receptor antagonist. We then investigated the procognitive effects of the PDE10A inhibitor, MP-10, in otherwise untreated animals and in animals pretreated with the D1 receptor antagonist. The dopamine D1 receptor antagonist SCH-23390 produced cognitive impairment at the dose of 0.0125 mg/kg, but not at 0.0063 mg/kg. The D1 receptor agonist, SKF-81,297, produced a procognitive effect that was abolished by 0.0063 mg/kg of SCH-23390. The compound MP-10 produced a procognitive effect at the dose of 0.3 mg/kg, but not at 0.1 mg/kg. Rat pretreatment with 0.0063 mg/kg of SCH-23390 did not block the procognitive effect of 0.3 mg/kg of MP-10. The present study demonstrates that the blockade of dopamine D1 receptors is unlikely to affect the procognitive effects of PDE10A inhibition.

Rescue of infralimbic mGluR2 deficit restores control over drug-seeking behavior in alcohol dependence.

A key deficit in alcohol dependence is disrupted prefrontal function leading to excessive alcohol seeking, but the molecular events underlying the emergence of addictive responses remain unknown. Here we show by convergent transcriptome analysis that the pyramidal neurons of the infralimbic cortex are particularly vulnerable for the long-term effects of chronic intermittent ethanol intoxication. These neurons exhibit a pronounced deficit in metabotropic glutamate receptor subtype 2 (mGluR(2)). Also, alcohol-dependent rats do not respond to mGluR(2/3) agonist treatment with reducing extracellular glutamate levels in the nucleus accumbens. Together these data imply a loss of autoreceptor feedback control. Alcohol-dependent rats show escalation of ethanol seeking, which was abolished by restoring mGluR(2) expression in the infralimbic cortex via viral-mediated gene transfer. Human anterior cingulate cortex from alcoholic patients shows a significant reduction in mGluR(2) transcripts compared to control subjects, suggesting that mGluR(2) loss in the rodent and human corticoaccumbal neurocircuitry may be a major consequence of alcohol dependence and a key pathophysiological mechanism mediating increased propensity to relapse. Normalization of mGluR(2) function within this brain circuit may be of therapeutic value.

Acute inhibition of 11beta-hydroxysteroid dehydrogenase type-1 improves memory in rodent models of cognition.

Mounting evidence suggests excessive glucocorticoid activity may contribute to Alzheimer's disease (AD) and age-associated memory impairment. 11ß-hydroxysteroid dehydrogenase type-1 (HSD1) regulates conversion of glucocorticoids from inactive to active forms. HSD1 knock-out mice have improved cognition, and the nonselective inhibitor carbenoxolone improved verbal memory in elderly men. Together, these data suggest that HSD1 inhibition may be a potential therapy for cognitive deficits, such as those associated with AD. To investigate this, we characterized two novel and selective HSD1 inhibitors, A-918446 and A-801195. Learning, memory consolidation, and recall were evaluated in mouse 24 h inhibitory avoidance. Inhibition of brain cortisol production and phosphorylation of cAMP response element-binding protein (CREB), a transcription factor involved in cognition, were also examined. Rats were tested in a short-term memory model, social recognition, and in a separate group cortical and hippocampal acetylcholine release was measured via in vivo microdialysis. Acute treatment with A-801195 (10-30 mg/kg) or A-918446 (3-30 mg/kg) inhibited cortisol production in the ex vivo assay by ∼ 35-90%. Acute treatment with A-918446 improved memory consolidation and recall in inhibitory avoidance and increased CREB phosphorylation in the cingulate cortex. Acute treatment with A-801195 significantly improved short-term memory in rat social recognition that was not likely due to alterations of the cholinergic system, as acetylcholine release was not increased in a separate set of rats. These studies suggest that selective HSD1 inhibitors work through a novel, noncholinergic mechanism to facilitate cognitive processing.

Pharmacological properties and procognitive effects of ABT-288, a potent and selective histamine H3 receptor antagonist.

Blockade of the histamine H(3) receptor (H(3)R) enhances central neurotransmitter release, making it an attractive target for the treatment of cognitive disorders. Here, we present in vitro and in vivo pharmacological profiles for the H(3)R antagonist 2-[4'-((3aR,6aR)-5-methyl-hexahydro-pyrrolo[3,4-b]pyrrol-1-yl)-biphenyl-4-yl]-2H-pyridazin-3-one (ABT-288). ABT-288 is a competitive antagonist with high affinity and selectivity for human and rat H(3)Rs (K(i) = 1.9 and 8.2 nM, respectively) that enhances the release of acetylcholine and dopamine in rat prefrontal cortex. In rat behavioral tests, ABT-288 improved acquisition of a five-trial inhibitory avoidance test in rat pups (0.001-0.03 mg/kg), social recognition memory in adult rats (0.03-0.1 mg/kg), and spatial learning and reference memory in a rat water maze test (0.1-1.0 mg/kg). ABT-288 attenuated methamphetamine-induced hyperactivity in mice. In vivo rat brain H(3)R occupancy of ABT-288 was assessed in relation to rodent doses and exposure levels in behavioral tests. ABT-288 demonstrated a number of other favorable attributes, including good pharmacokinetics and oral bioavailability of 37 to 66%, with a wide central nervous system and cardiovascular safety margin. Thus, ABT-288 is a selective H(3)R antagonist with broad procognitive efficacy in rodents and excellent drug-like properties that support its advancement to the clinical area.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions.

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Effects of a positive allosteric modulator of group II metabotropic glutamate receptors, LY487379, on cognitive flexibility and impulsive-like responding in rats.

Orthosteric group II metabotropic glutamate receptor (mGluR) agonists are regarded as novel, effective medications for all major symptom domains of schizophrenia, including cognitive disturbances. mGluR2s also can be affected in a more subtle way by positive allosteric modulators (PAMs) characterized by a unique degree of subtype selectivity and neuronal frequency-dependent activity. Because currently available treatments for schizophrenia do not improve cognitive dysfunction, the main aim of the present study was to examine the effects of a mGluR2 PAM, N-(4-(2-methoxyphenoxy)-phenyl-N-(2,2,2-trifluoroethylsulfonyl)-pyrid-3-ylmethylamine (LY487379), on rat cognitive flexibility and impulsive-like responding, assessed in an attentional set-shifting task (ASST) and a differential reinforcement of low-rate 72 s (DRL72) schedule of food reinforcement. In addition, in vivo microdialysis was used to assess the drug's impact on cortical levels of dopamine, norepinephrine, serotonin, and glutamate. Rats treated with LY487379 (30 mg/kg) required significantly fewer trials to criteria during the extradimensional shift phase of the ASST. Under a DRL72 schedule, LY487379 (30 mg/kg) decreased the response rate and increased the number of reinforcers obtained. These effects were accompanied by the shift of the frequency distribution of responses toward longer inter-response time durations. LY487379 significantly enhanced extracellular norepinephrine and serotonin levels in the medial prefrontal cortex. In summary, the present study demonstrates that a mGluR2 PAM, LY487379, promotes cognitive flexibility and facilitates behavioral inhibition. These procognitive effects may contribute to the therapeutic efficacy of agents stimulating mGluR2 in schizophrenia.

In vivo pharmacological characterization of a novel selective alpha7 neuronal nicotinic acetylcholine receptor agonist ABT-107: preclinical considerations in Alzheimer's disease.

We previously reported that alpha7 nicotinic acetylcholine receptor (nAChR) agonism produces efficacy in preclinical cognition models correlating with activation of cognitive and neuroprotective signaling pathways associated with Alzheimer's disease (AD) pathology. In the present studies, the selective and potent alpha7 nAChR agonist 5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole (ABT-107) was evaluated in behavioral assays representing distinct cognitive domains. Studies were also conducted to address potential issues that may be associated with the clinical development of an alpha7 nAChR agonist. Specifically, ABT-107 improved cognition in monkey delayed matching to sample, rat social recognition, and mouse two-trial inhibitory avoidance, and continued to improve cognitive performance at injection times when exposure levels continued to decline. Rats concurrently infused with ABT-107 and donepezil at steady-state levels consistent with clinical exposure showed improved short-term recognition memory. Compared with nicotine, ABT-107 did not produce behavioral sensitization in rats or exhibit psychomotor stimulant activity in mice. Repeated (3 days) daily dosing of ABT-107 increased extracellular cortical acetylcholine in rats, whereas acute administration increased cortical extracellular signal-regulated kinase and cAMP response element-binding protein phosphorylation in mice, neurochemical and biochemical events germane to cognitive function. ABT-107 increased cortical phosphorylation of the inhibitory residue (Ser9) of glycogen synthase kinase-3, a primary tau kinase associated with AD pathology. In addition, continuous infusion of ABT-107 in tau/amyloid precursor protein transgenic AD mice reduced spinal tau hyperphosphorylation. These findings show that targeting alpha7 nAChRs may have potential utility for symptomatic alleviation and slowing of disease progression in the treatment AD, and expand the understanding of the potential therapeutic viability associated with the alpha7 nAChR approach in the treatment of AD.

The drug-induced helplessness test: an animal assay for assessing behavioral despair in response to neuroleptic treatment.

RATIONALE: Neuroleptic dysphoria encompasses a range of unpleasant subjective responses and, as a result, is difficult to study in preclinical animal models. OBJECTIVE: Based on the learned helplessness model of depression, increases in escape failures (EFs) in the drug-induced helplessness test (DH) are proposed to reflect drug-induced depressive-like state, a contributing factor to neuroleptic dysphoria in humans. MATERIALS AND METHODS: Effects of the typical antipsychotic haloperidol and the atypical antipsychotics risperidone, olanzapine, aripiprazole, quetiapine, and clozapine were investigated in the DH test. We further characterized this test by examining compounds affecting motor function, cognition, anxiety, and those with antidepressant activity. RESULTS: The antipsychotics haloperidol, risperidone, aripiprazole, and olanzapine, all increased EFs, while quetiapine had no effect, and clozapine reduced EFs. Amphetamine, diazepam, and ciproxifan, had no effect on EFs. Scopolamine significantly reduced EFs and MK-801 showed a trend toward reducing EFs at doses not significantly sti mulating locomotor activity. Subchronic, but not acute, imipramine and subchronic fluoxetine significantly reduced EFs at doses significantly suppressing locomotor activity. Dissociation appears to exist between performance in the DH test and compound effects on catalepsy or locomotor activity. CONCLUSIONS: After discussing potential alternative interpretations of the drug-induced changes of EFs, we propose the DH test as a useful test for assessing a drug-induced, depressive-like state that may contribute to neuroleptic dysphoria.

The dopamine D3 receptor plays an essential role in alcohol-seeking and relapse.

Our study aimed to identify new candidate genes, which might be involved in alcohol craving and relapse. To find changes in gene expression after long-term alcohol consumption, we studied gene expression profiles in the striatal dopamine system by using DNA microarrays of two different alcohol-preferring rat lines (HAD and P). Our data revealed an up-regulation of the dopamine D3 receptor (D3R) after 1 yr of voluntary alcohol consumption in the striatum of alcohol preferring rats that was confirmed by qRT-polymerase chain reaction. This finding was further supported by the finding of up-regulated striatal D3R mRNA in nonselected Wistar rats after long-term alcohol consumption when compared with age-matched control animals. We further examined the role of the D3R in mediating alcohol relapse behavior using the alcohol deprivation effect (ADE) model in long-term alcohol drinking Wistar rats and the model of cue-induced reinstatement of alcohol-seeking behavior using the selective D3R antagonist SB-277011-A (0, 1, 3, and 10 mg/kg) and the partial agonist BP 897 (0, 0.1, 1, and 3 mg/kg). Both treatments caused a dose-dependent reduction of relapse-like drinking in the ADE model as well as a decrease in cue-induced ethanol-seeking behavior. We conclude that long-term alcohol consumption leads to an up-regulation of the dopamine D3R that may contribute to alcohol-seeking and relapse. We therefore suggest that selective antagonists of this pharmacological target provide a specific treatment approach to reduce alcohol craving and relapse behavior.

Effects of antipsychotics and selective D3 antagonists on PPI deficits induced by PD 128907 and apomorphine.

Rats treated with apomorphine and amphetamine display sensorimotor gating impairments, as measured by prepulse inhibition (PPI), and these impairments can be reversed by antipsychotic treatment. However, it remains unknown whether the dopamine (DA) D(3) receptor plays a role in mediating these effects on PPI, as none of these DA agonists or antipsychotics are exclusively selective at either D(2) or D(3) receptors. To address this question, the current study was designed to investigate whether antipsychotic drugs and selective D(3) antagonists could block the PPI-disruptive effects of PD 128907 (a preferential D(3) agonist) and apomorphine. We found that the effect of PD 128907 on PPI in rats could be antagonized by risperidone, clozapine, and the selective D(3) antagonists SB 277011 and A-691990, but not by raclopride or haloperidol, while the apomorphine-induced PPI deficit could be reversed by risperidone, clozapine and haloperidol, but not by SB 2770111 and A-691990. These results suggest that the D(3) receptor does not mediate apomorphine-induced disruption of PPI in rats, however, given the findings that PD 128907 elicited a PPI-disruptive effect that was blocked by selective D(3) antagonists, a role of D(3) receptor in mediating PPI in rats cannot be ruled out. The possible mechanisms of D(3) receptor involvement in PPI are discussed.

Discovery of Novel and Highly Selective Inhibitors of Calpain for the Treatment of Alzheimer's Disease: 2-(3-Phenyl-1H-pyrazol-1-yl)-nicotinamides.

Calpain overactivation has been implicated in a variety of pathological disorders including ischemia/reperfusion injury, cataract formation, and neurodegenerative diseases such as Alzheimer's disease (AD). Herein we describe our efforts leading to the identification of ketoamide-based 2-(3-phenyl-1H-pyrazol-1-yl)nicotinamides as potent and reversible inhibitors of calpain with high selectivity versus related cysteine protease cathepsins, other proteases, and receptors. Broad efficacy in a set of preclinical models relevant to AD suggests that inhibition of calpain represents an attractive approach with potential benefit for the treatment of AD.

Effect of dopamine D3 antagonists on PPI in DBA/2J mice or PPI deficit induced by neonatal ventral hippocampal lesions in rats.

Schizophrenic patients typically exhibit impairment of sensorimotor gating, which can be modeled in animal models such as the test of prepulse inhibition of startle response (PPI) in rodents. It has been found that antipsychotics enhanced PPI in DBA mice and reversed the PPI deficit induced by neonatal ventral hippocampal (NVH) lesions in rats. However, the relative involvement of D(3) and D(2) receptors in these effects is unknown since all antipsychotics are D(2)/D(3) antagonists with limited binding preference at D(2) receptors. Therefore, in the current study, we investigated the influence of several dopamine antagonists with higher selectivity at D(3) vs D(2) receptors on PPI in DBA/2J mice and in NVH-lesioned rats. The PPI in DBA/2J mice was enhanced by the nonselective D(2)/D(3) antagonists, haloperidol at 0.3-3 mg/kg, or risperidone at 0.3-1 mg/kg, while PPI-enhancing effects were observed after the administration of higher doses of the preferential D(3)/D(2) antagonist, BP 897 at 8 mg/kg, and the selective D(3) antagonists, SB 277011 at 30 mg/kg and A-437203 at 30 mg/kg. No effect was observed following the treatment with the selective D(3) antagonist, AVE 5997 up to 30 mg/kg. The PPI deficits induced by NVH lesions were reversed by haloperidol but not by the more selective D(3) antagonists, A-437203 and AVE 5997. BP 897 enhanced PPI nonselectivity, that is, in both lesioned and nonlesioned rats. In summary, the present study indicates that PPI-enhancing effects induced by antipsychotics in DBA/2J mice and in NVH-lesioned rats are unlikely to be mediated by D(3) receptors.

An ELISA-based method for the quantification of incorporated BrdU as a measure of cell proliferation in vivo.

In this study, we describe a new rapid and versatile method to determine the BrdU content of DNA in brain tissues dissected from BrdU-treated rats. Different to already existing BrdU ELISAs the method is suitable for the assessment of BrdU incorporation in ex vivo experiments as it is based on the analysis of tissue extracts instead of immobilized cells. The method comprises the preparation of DNA extracts from dissected tissues, the immobilization of BrdU-containing DNA with an anti-BrdU antibody and quantification of the incorporated BrdU by a peroxidase-conjugated anti-BrdU antibody. Validating the new assay in vitro, we found a clear-cut dependency of the ELISA signal from the time SKNSH neuroblastoma cells had been exposed to BrdU. Parallel studies with existing ELISAs and a parallel immunocytochemical determination of BrdU positive cells revealed comparable results. In vivo experiments showed a virtually linear relationship between the BrdU immunoreactivity in the hippocampus and the time rats have been exposed to BrdU. Repeating the determination of the BrdU content of the same set of tissue samples revealed reproducible relative differences of the ELISA signals. This was true for protocols using purified DNA as well as crude DNA extracts. For the sensitivity and reproducibility of the method heat denaturation of the DNA prior to the analysis in the ELISA was crucial. In rats treated with electroconvulsion the BrdU content of the hippocampus, determined by the new ELISA, was increased to 225% of controls. In a parallel immunohistochemical study, the number of BrdU positive cells was comparably increased to 251% of controls. The assay thus provides a rapid method to detect changes of cell proliferation in dissected brain tissues and other proliferative tissues. With appropriate protocols, the assay may also be used to assess the generation of particular cell types like neurons in neurogenic areas.

Absence of amino acid-induced glomerular hyperfiltration in dopamine D3 receptor knockout mice.

Pharmacological inhibition of receptors of the dopamine D2-like family has been shown to abolish the glomerular hyperfiltration in response to amino acids. To further discriminate between the receptor subtypes within the D2-like family, we investigated the effects of amino acid infusion on renal function in dopamine D3 receptor knockout (-/-) mice. In clearance experiments pentobarbital-anesthetized D3 receptor (-/-) and wild-type (+/+) mice were infused with Ringer solution at baseline, followed by a continuous infusion of mixed amino acids (10%). Baseline glomerular filtration rate (GFR), assessed by renal clearance of [3H]-inulin, was the same in D3 receptor (-/-) mice (0.56+/-0.08 ml/min per g kidney weight) and wild-type animals (0.56+/-0.04 ml/min per g kw). During infusion of amino acids, GFR was significantly elevated by 50% in D3 receptor (+/+) mice. In contrast, this amino acid-induced response of GFR was abolished in D3 receptor (-/-) mice. Baseline urinary water and sodium excretion was not significantly different in both groups of mice. As observed in GFR, these renal excretory parameters were significantly elevated during amino acid infusion in D3 receptor (+/+) but not in D3 receptor (-/-) mice. Time controls, constantly infused with Ringer solution, did not show significant changes in GFR, renal water or sodium excretion during the entire experiment, indicating stable experimental conditions. Taken together, the data underline the involvement of dopamine D2-like receptors in the renal response to amino acid infusion and, in addition, attribute this effect to the dopamine D3 receptor subtype.

Antidepressant-like effect of D(2/3) receptor-, but not D(4) receptor-activation in the rat forced swim test.

Dopamine plays a role in the pathophysiology of depression and therapeutic effects of antidepressants but the contribution of individual D(2)-like receptor subtypes (D(2), D(3), D(4)) to depression is not known. We present evidence that activation of D(2)/D(3), but not D(4) receptors, can affect the outcome in the rat forced swim test (FST). Nomifensine, a dopamine uptake inhibitor (7, 14, and 28 micromol/kg); quinpirole, a D(2)-like receptor and agonist (0.4, 1.0, and 2.0 micromol/kg); PD 12,8907, a preferential D(3) receptor agonist (0.17, 0.35, and 0.7 micromol/kg); PD 168077 (0.1, 0.3, and 1.0 micromol/kg) and CP 226269 (0.3, 1.0, and 3.0 micromol/kg), both selective D(4) receptor agonists, were administered s.c. 24, 5, and 0.5/1 h before testing. Nomifensine, quinpirole at all doses and PD 128907 at the highest dose decreased immobility time in FST. PD 168077 and CP 226269 had no effect on the model. To further clarify what type of dopamine receptors were involved in the anti-immobility effect of quinpirole, we tested different antagonists. Haloperidol, a D(2)-like receptor antagonist (0.27 micromol/kg), completely blocked the effect of quinpirole; A-437203 (LU-201640), a selective D(3) receptor antagonist (17.46 micromol/kg), showed a nonsignificant trend to attenuate the effect of the low dose of quinpirole, and L-745,870, a selective D(4) receptor antagonist (1.15 micromol/kg), had no effect. The pharmacological selectivity of the compounds tested suggests that the antidepressant-like effects of quinpirole are most likely mediated mainly by D(2) and to a lesser extent by D(3) but not D(4) receptors.

Impaired cognition after stimulation of P2Y1 receptors in the rat medial prefrontal cortex.

We hypothesize that cortical ATP and ADP accumulating in the extracellular space, eg during prolonged network activity, contribute to a decline in cognitive performance in particular via stimulation of the G protein-coupled P2Y1 receptor (P2Y1R) subtype. Here, we report first evidence on P2Y1R-mediated control of cognitive functioning in rats using bilateral microinfusions of the selective agonist MRS2365 into medial prefrontal cortex (mPFC). MRS2365 attenuated prepulse inhibition of the acoustic startle reflex while having no impact on startle amplitude. Stimulation of P2Y1Rs deteriorated performance accuracy in the delayed non-matching to position task in a delay dependent manner and increased the rate of magazine entries consistent with both working memory disturbances and impaired impulse control. Further, MRS2365 significantly impaired performance in the reversal learning task. These effects might be related to MRS2365-evoked increase of dopamine observed by microdialysis to be short-lasting in mPFC and long-lasting in the nucleus accumbens. P2Y1Rs were identified on pyramidal cells and parvalbumin-positive interneurons, but not on tyrosine hydroxylase-positive fibers, which argues for an indirect activation of dopaminergic afferents in the cortex by MRS2365. Collectively, these results suggest that activation of P2Y1Rs in the mPFC impairs inhibitory control and behavioral flexibility mediated by increased mesocorticolimbic activity and local disinhibition.

Dopamine D3 receptor antagonism--still a therapeutic option for the treatment of schizophrenia.

The potential of D(3) receptor antagonism to treat positive, negative, and cognitive symptoms of schizophrenia is reviewed on the basis of preclinical results and preliminary clinical data. Dopamine D(3) receptors are expressed in mesencephalic, limbic, and cortical areas relevant to psychotic and cognitive symptoms of schizophrenia. As expected, selective dopamine D(3) receptor antagonists are not effective in antipsychotic animal models, reflecting D(2) receptor antagonism. However, selective D(3) receptor antagonists affect electrical activity of dopamine neurons in the ventral tegmental area similar to atypical antipsychotics, counteract effects produced by NMDA glutamate receptor blockade, and enhance cortical dopamine and acetylcholine in microdialysis. In contrast to dopamine D(2) receptor antagonists, D(3) antagonists positively influence a variety of social and cognitive behaviors in rodents, including tests representing cognitive flexibility and executive function, which are both impaired in schizophrenia patients. Despite considerable affinity for D(3) receptors, the second-generation antipsychotics clozapine, risperidone, and olanzapine when administered to patients with schizophrenia seem not to occupy D(3) receptors sufficiently to derive any conclusion on a D(3)-mediated therapeutic benefit. ABT-925, the first selective D(3) receptor antagonist, was recently studied in patients with schizophrenia. It produced cognitive signals but did not achieve sufficient D(3) receptor occupancy to test the hypothesis that D(3) receptor antagonism is of therapeutic value to treat symptoms of schizophrenia. Based on mechanistic and experimental considerations and due to the fact that D(3) receptor antagonism can inhibit extrapyramidal symptoms and produce neither anhedonia nor metabolic adverse effects, the development and clinical testing of newer D(3) receptor antagonists with high potency at D(3) receptors, enabling sufficient receptor occupancy, is highly warranted.

Assessment of the abuse liability of ABT-288, a novel histamine H3 receptor antagonist.

RATIONALE: Histamine H3 receptor antagonists, such as ABT-288, have been shown to possess cognitive-enhancing and wakefulness-promoting effects. On the surface, this might suggest that H3 antagonists possess psychomotor stimulant-like effects and, as such, may have the potential for abuse. OBJECTIVES: The aim of the present study was to further characterize whether ABT-288 possesses stimulant-like properties and whether its pharmacology gives rise to abuse liability. METHODS: The locomotor-stimulant effects of ABT-288 were measured in mice and rats, and potential development of sensitization was addressed. Drug discrimination was used to assess amphetamine-like stimulus properties, and drug self-administration was used to evaluate reinforcing effects of ABT-288. The potential development of physical dependence was also studied. RESULTS: ABT-288 lacked locomotor-stimulant effects in both rats and mice. Repeated administration of ABT-288 did not result in cross-sensitization to the stimulant effects of d-amphetamine in mice, suggesting that there is little overlap in circuitries upon which the two drugs interact for motor activity. ABT-288 did not produce amphetamine-like discriminative stimulus effects in drug discrimination studies nor was it self-administered by rats trained to self-administer cocaine. There were no signs of physical dependence upon termination of repeated administration of ABT-288 for 30 days. CONCLUSIONS: The sum of these preclinical data, the first of their kind applied to H3 antagonists, indicates that ABT-288 is unlikely to possess a high potential for abuse in the human population and suggests that H3 antagonists, as a class, are similar in this regard.

Preclinical characterization of A-582941: a novel alpha7 neuronal nicotinic receptor agonist with broad spectrum cognition-enhancing properties.

Among the diverse sets of nicotinic acetylcholine receptors (nAChRs), the alpha7 subtype is highly expressed in the hippocampus and cortex and is thought to play important roles in a variety of cognitive processes. In this review, we describe the properties of a novel biaryl diamine alpha7 nAChR agonist, A-582941. A-582941 was found to exhibit high-affinity binding and partial agonism at alpha7 nAChRs, with acceptable pharmacokinetic properties and excellent distribution to the central nervous system (CNS). In vitro and in vivo studies indicated that A-582941 activates signaling pathways known to be involved in cognitive function such as ERK1/2 and CREB phosphorylation. A-582941 enhanced cognitive performance in behavioral models that capture domains of working memory, short-term recognition memory, memory consolidation, and sensory gating deficit. A-582941 exhibited a benign secondary pharmacodynamic and tolerability profile as assessed in a battery of assays of cardiovascular, gastrointestinal, and CNS function. The studies summarized in this review collectively provide preclinical validation that alpha7 nAChR agonism offers a mechanism with potential to improve cognitive deficits associated with various neurodegenerative and psychiatric disorders.