David L. McKinzie

Selective muscarinic receptor agonist xanomeline as a novel treatment approach for schizophrenia

OBJECTIVE There are significant unmet needs in the treatment of schizophrenia, especially for the treatment of cognitive impairment, negative syndrome, and cognitive function. Preclinical data suggest that agonists with selective affinity for acetylcholine muscarinic receptors provide a potentially new mechanism to treat schizophrenia. The authors studied xanomeline, a relatively selective muscarinic type 1 and type 4 (M(1) and M(4)) receptor agonist, to determine if this agent is effective in the treatment of schizophrenia. METHOD In this pilot study, the authors examined the efficacy of xanomeline on clinical outcomes in subjects with schizophrenia (N=20) utilizing a double-blind, placebo-controlled, 4-week treatment design. Outcome measures included the Positive and Negative Syndrome Scale (PANSS) for schizophrenia, the Brief Psychiatric Rating Scale (BPRS), the Clinical Global Impression (CGI) scale, and a test battery designed to measure cognitive function in patients with schizophrenia. RESULTS Subjects treated with xanomeline did significantly better than subjects in the placebo group on total BPRS scores and total PANSS scores. In the cognitive test battery, subjects in the xanomeline group showed improvements most robustly in measures of verbal learning and short-term memory function. CONCLUSIONS These results support further investigation of xanomeline as a novel approach to treating schizophrenia.

3-(4-Chloro-2-Morpholin-4-yl-Thiazol-5-yl)-8-(1-Ethylpropyl)-2,6-Dimethyl-Imidazo[1,2-b]Pyridazine: A Novel Brain-Penetrant, Orally Available Corticotropin-Releasing Factor Receptor 1 Antagonist with Efficacy in Animal Models of Alcoholism

We describe a novel corticotropin-releasing factor receptor 1 (CRF1) antagonist with advantageous properties for clinical development, and its in vivo activity in preclinical alcoholism models. 3-(4-Chloro-2-morpholin-4-yl-thiazol-5-yl)-8-(1-ethylpropyl)-2,6-dimethyl-imidazo[1,2-b]pyridazine (MTIP) inhibited 125I-sauvagine binding to rat pituitary membranes and cloned human CRF1 with subnanomolar affinities, with no detectable activity at the CRF2 receptor or other common drug targets. After oral administration to rats, MTIP inhibited 125I-sauvagine binding to rat cerebellar membranes ex vivo with an ED50 of ∼1.3 mg/kg and an oral bioavailability of 91.1%. Compared with R121919 (2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine) and CP154526 (N-butyl-N-ethyl-4,9-dimethyl-7-(2,4,6-trimethylphenyl)-3,5,7-triazabicyclo[4.3.0]nona-2,4,8,10-tetraen-2-amine), MTIP had a markedly reduced volume of distribution and clearance. Neither open-field activity nor baseline exploration of an elevated plus-maze was affected by MTIP (1–10 mg/kg). In contrast, MTIP dose-dependently reversed anxiogenic effects of withdrawal from a 3 g/kg alcohol dose. Similarly, MTIP blocked excessive alcohol self-administration in Wistar rats with a history of dependence, and in a genetic model of high alcohol preference, the msP rat, at doses that had no effect in nondependent Wistar rats. Also, MTIP blocked reinstatement of stress-induced alcohol seeking both in postdependent and in genetically selected msP animals, again at doses that were ineffective in nondependent Wistar rats. Based on these findings, MTIP is a promising candidate for treatment of alcohol dependence.

Cholinergic dilation of cerebral blood vessels is abolished in M 5 muscarinic acetylcholine receptor knockout mice

The M5 muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M1-M5) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M5 receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M5 receptor-deficient mice (M5R−/− mice). M5R−/− mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M5 receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M5 receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M5 receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R−/− mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R−/− mice. Our findings provide direct evidence that M5 muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimers disease and focal cerebral ischemia, cerebrovascular M5 receptors may represent an attractive therapeutic target.

Regional heterogeneity for the intracranial self-administration of ethanol within the ventral tegmental area of female Wistar rats.

Abstract  Rationale: Because current findings indicate that the selectively bred alcohol-preferring P line of rats self-administers 50–200 mg% ethanol (EtOH) directly into the ventral tegmental area (VTA), whereas the alcohol-nonpreferring NP line does not, it is important to determine whether unselected, common stock rats would self-administer EtOH directly into the VTA. In addition, because neuroanatomical and self-administration studies indicate that the VTA may be functionally heterogeneous, the present study was designed to determine whether there were subregional differences within the VTA for the intracranial self-administration (ICSA) of EtOH. Objectives: The objective of this study was to employ the ICSA technique to determine whether adult female Wistar rats would self-administer EtOH directly into the VTA, and whether regional heterogeneity existed for EtOH self-infusion within the VTA. Methods: Following surgery to implant guide cannulae aimed at either the posterior or anterior VTA, subjects were placed in standard experimental chambers equipped with an ’active lever’ [fixed ratio (FR)1 schedule of reinforcement], which caused the delivery of the infusate, and an ’inactive lever’, which had no programmed consequence. Subjects were assigned to groups that self-administered either artificial cerebrospinal fluid (aCSF) throughout, or 100–400 mg% EtOH for the first four sessions (acquisition), aCSF in sessions 5 and 6 (extinction), and EtOH again during session 7 (reinstatement). Results: During the four acquisition sessions, rats with posterior VTA placements readily self-administered 200 mg% and 250 mg% EtOH and discriminated between the active and inactive levers. These subjects also demonstrated extinction, when aCSF was substituted for EtOH, and reinstatement when EtOH was reintroduced. Rats with posterior VTA placements self-infused 300 mg% and 400 mg% EtOH, and demonstrated lever discrimination only during the initial acquisition sessions. In contrast, rats with anterior VTA placements did not self-administer EtOH. Conclusions: The findings suggest that EtOH is reinforcing within the posterior VTA of Wistar rats, and the VTA is a functionally heterogeneous structure with regard to EtOH reinforcement.

Use of M1-M5 muscarinic receptor knockout mice as novel tools to delineate the physiological roles of the muscarinic cholinergic system

In this review we report recent findings on the physiological role of the five known muscarinic acetylcholine receptors (mAChRs) as shown by gene targeting technology. Using knockout mice for each mAChRs subtype, the role of mAChRs subtypes in a number of physiological functions was confirmed and new activities were discovered. The M1 mAChRs modulate neurotransmitter signaling in cortex and hippocampus. The M3 mAChRs are involved in exocrine gland secretion, smooth muscle contractility, pupil dilation, food intake, and weight gain. The role of the M5 mAChRs involves modulation of central dopamine function and the tone of cerebral blood vessels. mAChRs of the M2 subtype mediate muscarinic agonist-induced bradycardia, tremor, hypothermia, and autoinhibition of release in several brain regions. M4 mAChRs modulate dopamine activity in motor tracts and act as inhibitory autoreceptors in striatum. Thus, as elucidated by gene targeting technology, mAChRs have widespread and manifold functions in the periphery and brain.

Allosteric modulation of the muscarinic M4 receptor as an approach to treating schizophrenia

Current antipsychotics provide symptomatic relief for patients suffering from schizophrenia and related psychoses; however, their effectiveness is variable and many patients discontinue treatment due to side effects. Although the etiology of schizophrenia is still unclear, a leading hypothesis implicates an imbalanced dopaminergic system. Muscarinic acetylcholine (ACh) receptors regulate dopamine levels in key areas of the brain involved in psychosis, with the M4 subtype emerging as a key regulator of dopaminergic hyperactivity. Unfortunately, no selective small molecule tools exist to provide pharmacological validation of this hypothesis. Here, we describe the discovery of a small molecule modulator, LY2033298, that is highly selective for human M4 receptors by virtue of targeting an allosteric site on this receptor. Pharmacological assays confirmed the selectivity of LY2033298 for the M4 receptor and revealed the highest degree of positive allosteric enhancement of ACh potency thus far identified. Radioligand binding assays also show this compound to directly potentiate agonist binding while having minimal effects on antagonist binding. Mutational analysis identified a key amino acid (D432) in the third extracellular loop of the human M4 receptor to be critical for selectivity and agonist potentiation by LY2033298. Importantly, LY2033298 was active in animal models predictive of clinical antipsychotic drug efficacy indicating its potential use as a first-in-class, selective, allosteric muscarinic antipsychotic agent.

Metabotropic glutamate 2 receptor potentiators: receptor modulation, frequency-dependent synaptic activity, and efficacy in preclinical anxiety and psychosis model(s)

RationaleTo increase subtype selectivity and provide a novel means to alter receptor function, we discovered and characterization potentiators for the metabotropic glutamate 2 receptor (mGlu2).Methods and resultsA class of 3-pyridylmethylsulfonamides (e.g., 3-MPPTS; 2,2,2-trifluoro-N-[3-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide) were found to be potent, subtype-selective potentiators of human and rat mGlu2. The sulfonamides increased agonist potency in functional assays but did not displace orthosteric radiolabeled antagonist or agonist binding to cloned mGlu2 receptors. Rather, the modulators increased the affinity of most of the orthosteric agonists including glutamate, DCG-IV (2S,2′R,3′R)-2-(2′,3′-dicarboxylcyclopropyl)glycine), and LY354740 (1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicaroxylate monohydrate). In striatal brain slices, LY354740 inhibited evoked excitatory postsynaptic potentials (EPSPs) equally well following either a low- (0.06 Hz) or high (4 Hz)-frequency stimulation of corticostriatal afferents. In contrast, the mGlu2 potentiator cyPPTS (2,2,2-trifluoro-N-[3-(cyclopentyloxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide) inhibited striatal EPSPs only at higher frequencies of stimulation (2 and 4 Hz). Several sulfonamides including 4-MPPTS, 4-APPES (N-[4-(4-carboxamidophenoxy)phenyl]-N-(3-pyridinylmethyl)-ethanesulfonamide hydrochloride monohydrate) and/or CBiPES N-[4′-cyano-biphenyl-3-yl)-N-(3-pyridinylmethyl)-ethanesulfonamide hydrochloride) were tested in mGlu2/3 agonist-sensitive rodent model(s) of anxiety and psychosis. As seen with LY354740, both 4-MPPTS and 4-APPES were efficacious in a rat fear-potentiated startle paradigm. Likewise in mice, CBiPES attenuated a stress-induced hyperthermia and PCP-induced hyperlocomotor activity. Furthermore, CBiPES mediated alteration in PCP-induced hyperlocomotor activity was sensitive to mGlu2/3 antagonist pretreatment.ConclusionsTaken together, the data indicate mGlu2 receptor potentiators have a unique use-dependent effect on presynaptic glutamate release, and show efficacy in several mGlu2/3-sensitive animal models of psychiatric disorders.

Cocaine Is Self-Administered into the Shell but Not the Core of the Nucleus Accumbens of Wistar Rats

The rewarding properties of cocaine have been postulated to be regulated, in part, by the mesolimbic dopamine system. However, the possibility that the rewarding properties of cocaine are mediated by direct activation of this system has yielded contradictory findings. The intracranial self-administration technique is used to identify specific brain regions involved in the initiation of response-contingent behaviors for the delivery of a reinforcer. The present study assessed whether adult Wistar rats would self-administer cocaine directly into the nucleus accumbens shell (AcbSh) and core (AcbC). For each subregion, subjects were placed in standard two-lever operant chambers and randomly assigned to one of five groups for each site that were given either artificial cerebrospinal fluid (aCSF), or 400, 800, 1200, or 1600 pmol of cocaine/100 nl to self-administer. The data indicate that rats with placements within the AcbSh readily self-administered 800 to 1600 pmol of cocaine/100 nl and responded significantly more on the active than inactive lever. These subjects also decreased responding on the active lever when aCSF was substituted for cocaine and reinstated responding on the active lever when cocaine was reintroduced. Coinfusion of the D2-like receptor antagonist sulpiride inhibited cocaine self-infusion in the AcbSh. In contrast to the AcbSh data, rats failed to self-administer any tested dose of cocaine into the AcbC or areas ventral to the AcbSh. These findings suggest that the AcbSh is a neuroanatomical substrate for the reinforcing effects of cocaine and that activation of D2-like receptors is involved.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinsons disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

Molecular mechanisms of action and in vivo validation of an M4 muscarinic acetylcholine receptor allosteric modulator with potential antipsychotic properties.

We recently identified LY2033298 as a novel allosteric potentiator of acetylcholine (ACh) at the M4 muscarinic acetylcholine receptor (mAChR). This study characterized the molecular mode of action of this modulator in both recombinant and native systems. Radioligand-binding studies revealed that LY2033298 displayed a preference for the active state of the M4 mAChR, manifested as a potentiation in the binding affinity of ACh (but not antagonists) and an increase in the proportion of high-affinity agonist–receptor complexes. This property accounted for the robust allosteric agonism displayed by the modulator in recombinant cells in assays of [35S]GTPγS binding, extracellular regulated kinase 1/2 phosphorylation, glycogen synthase kinase 3β phosphorylation, and receptor internalization. We also found that the extent of modulation by LY2033298 differed depending on the signaling pathway, indicating that LY2033298 engenders functional selectivity in the actions of ACh. This property was retained in NG108-15 cells, which natively express rodent M4 mAChRs. Functional interaction studies between LY2033298 and various orthosteric and allosteric ligands revealed that its site of action overlaps with the allosteric site used by prototypical mAChR modulators. Importantly, LY2033298 reduced [3H]ACh release from rat striatal slices, indicating retention of its ability to allosterically potentiate endogenous ACh in situ. Moreover, its ability to potentiate oxotremorine-mediated inhibition of condition avoidance responding in rodents was significantly attenuated in M4 mAChR knockout mice, validating the M4 mAChR as a key target of action of this novel allosteric ligand.