Central muscarinic acetylcholine receptor availability in patients treated with clozapine

Repurposing neuroleptics: clozapine as a novel, adjuvant therapy for melanoma brain metastases

The blood-brain barrier and the distinct brain immunology provide challenges in translating commonly used chemotherapeutics to treat intracranial tumors. Previous reports suggest anti-tumoral effects of antipsychotics, encouraging investigations into potential treatment effects of neuroleptics on brain metastases. For the first time, the therapeutic potential of the antipsychotic drug clozapine in treating melanoma brain metastases (MBM) was investigated using three human MBM cell lines. Through in vitro cell culture and viability experiments, clozapine displayed potent anti-tumoral effects on MBM cells with an exploitable therapeutic window when compared to normal human astrocytes or rat brain organoids. Further, it was shown that clozapine inhibited migration, proliferation, and colony formation in a dose-dependent manner. Through flow cytometry and proteome screening, we found that clozapine induced apoptosis in MBM cells and potentially altered the tumor immunological environment by upregulating proteins such as macrophage inflammatory protein-1 alpha (MIP-1α) and interleukin-8 (IL-8). In conclusion, clozapine shows significant and selective anti-tumoral effects on MBM cell lines in vitro. Further in vivo experiments are warranted to translate these results into clinical use.

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

Basal-Forebrain Cholinergic Nuclei Alterations are Associated With Medication and Cognitive Deficits Across the Schizophrenia Spectrum

BACKGROUND AND HYPOTHESIS

The cholinergic system is altered in schizophrenia. Particularly, patients' volumes of basal-forebrain cholinergic nuclei (BFCN) are lower and correlated with attentional deficits. It is unclear, however, if and how BFCN changes and their link to cognitive symptoms extend across the schizophrenia spectrum, including individuals with at-risk mental state for psychosis (ARMS) or during first psychotic episode (FEP).

STUDY DESIGN

To address this question, we assessed voxel-based morphometry (VBM) of structural magnetic resonance imaging data of anterior and posterior BFCN subclusters as well as symptom ratings, including cognitive, positive, and negative symptoms, in a large multi-site dataset (n = 4) comprising 68 ARMS subjects, 98 FEP patients (27 unmedicated and 71 medicated), 140 patients with established schizophrenia (SCZ; medicated), and 169 healthy controls.

RESULTS

In SCZ, we found lower VBM measures for the anterior BFCN, which were associated with the anticholinergic burden of medication and correlated with patients' cognitive deficits. In contrast, we found larger VBM measures for the posterior BFCN in FEP, which were driven by unmedicated patients and correlated at-trend with cognitive deficits. We found no BFCN changes in ARMS. Altered VBM measures were not correlated with positive or negative symptoms.

CONCLUSIONS

Results demonstrate complex (posterior vs. anterior BFCN) and non-linear (larger vs. lower VBM) differences in BFCN across the schizophrenia spectrum, which are specifically associated both with medication, including its anticholinergic burden, and cognitive symptoms. Data suggest an altered trajectory of BFCN integrity in schizophrenia, influenced by medication and relevant for cognitive symptoms.

Lower cholinergic basal forebrain volumes link with cognitive difficulties in schizophrenia

A potential pathophysiological mechanism of cognitive difficulties in schizophrenia is a dysregulated cholinergic system. Particularly, the cholinergic basal forebrain nuclei (BFCN), the source of cortical cholinergic innervation, support multiple cognitive functions, ranging from attention to decision-making. We hypothesized that BFCN structural integrity is altered in schizophrenia and associated with patients' attentional deficits. We assessed gray matter (GM) integrity of cytoarchitectonically defined BFCN region-of-interest in 72 patients with schizophrenia and 73 healthy controls, matched for age and gender, from the COBRE open-source database, via structural magnetic resonance imaging (MRI)-based volumetry. MRI-derived measures of GM integrity (i.e., volumes) were linked with performance on a symbol coding task (SCT), a paper-pencil-based metric that assesses attention, by correlation and mediation analysis. To assess the replicability of findings, we repeated the analyses in an independent dataset comprising 26 patients with schizophrenia and 24 matched healthy controls. BFCN volumes were lower in patients (t(139)=2.51, p = 0.01) and significantly associated with impaired SCT performance (r = 0.31, p = 0.01). Furthermore, lower BFCN volumes mediated the group difference in SCT performance. When including global GM volumes, which were lower in patients, as covariates-of-no-interest, these findings disappeared, indicating that schizophrenia did not have a specific effect on BFCN relative to other regional volume changes. We replicated these findings in the independent cohort, e.g., BFCN volumes were lower in patients and mediated patients' impaired SCT performance. Results demonstrate lower BFCN volumes in schizophrenia, which link with patients' attentional deficits. Data suggest that a dysregulated cholinergic system might contribute to cognitive difficulties in schizophrenia via impaired BFCN.

Molecular imaging of schizophrenia: Neurochemical findings in a heterogeneous and evolving disorder

The past four decades have seen enormous efforts placed on a search for molecular markers of schizophrenia using positron emission tomography (PET) and single photon emission computed tomography (SPECT). In this narrative review, we cast a broad net to define and summarize what researchers have learned about schizophrenia from molecular imaging studies. Some PET studies of brain energy metabolism with the glucose analogue FDGhave have shown a hypofrontality defect in patients with schizophrenia, but more generally indicate a loss of metabolic coherence between different brain regions. An early finding of significantly increased striatal trapping of the dopamine synthesis tracer FDOPA has survived a meta-analysis of many replications, but the increase is not pathognomonic of the disorder, since one half of patients have entirely normal dopamine synthesis capacity. Similarly, competition SPECT studies show greater basal and amphetamine-evoked dopamine occupancy at post-synaptic dopamine D_2/3 receptors in patients with schizophrenia, but the difference is likewise not pathognomonic. We thus propose that molecular imaging studies of brain dopamine indicate neurochemical heterogeneity within the diagnostic entity of schizophrenia. Occupancy studies have established the relevant target engagement by antipsychotic medications at dopamine D_2/3 receptors in living brain. There is evidence for elevated frontal cortical dopamine D₁ receptors, especially in relation to cognitive deficits in schizophrenia. There is a general lack of consistent findings of abnormalities in serotonin markers, but some evidence for decreased levels of nicotinic receptors in patients. There are sparse and somewhat inconsistent findings of reduced binding of muscarinic, glutamate, and opioid receptors ligands, inconsistent findings of microglial activation, and very recently, evidence of globally reduced levels of synaptic proteins in brain of patients. One study reports a decline in histone acetylase binding that is confined to the dorsolateral prefrontal cortex. In most contexts, the phase of the disease and effects of past or present medication can obscure or confound PET and SPECT findings in schizophrenia.

Serum anticholinergicity is associated with reduced prefrontal brain function in early course schizophrenia

Increased anticholinergic activity resulting from pharmacotherapies used to treat schizophrenia is associated with poorer cognition. However the neural mechanisms underlying this effect are unknown. In this study of 39 early course schizophrenia outpatients, we demonstrate that increased serum anticholinergic activity is associated with reduced activation across the prefrontal cortex, including the dorsolateral, anterior, and medial prefrontal cortices, during two tasks of cognitive control. Lower activation in the dorsolateral and anterior prefrontal cortices mediated the association between increased anticholinergicity and poorer neurocognitive function. Such findings provide preliminary insight into how anticholinergic medications may impact cognition through reduced prefrontal cortical function in schizophrenia.

Allosteric modulation of cholinergic system: Potential approach to treating cognitive deficits of schizophrenia

Schizophrenia is a psychiatric disorder affecting approximately 1% of the population worldwide and is characterised by the presence of positive and negative symptoms and cognitive deficits. Whilst current therapeutics ameliorate positive symptoms, they are largely ineffective in improving negative symptoms and cognitive deficits. The cholinergic neurotransmitter system heavily influences cognitive function and there is evidence that implicates disruption of the central cholinergic system in schizophrenia. Historically, targeting the cholinergic system has been impeded by poor selectivity leading to intolerable side effects warranting the need to develop more targeted therapeutic compounds. In this review we will summarise evidence supporting the roles of the cholinergic system, particularly the muscarinic M₁ receptor, in the pathophysiology of schizophrenia and discuss the potential of a promising new class of candidate compounds, allosteric ligands, for addressing the difficulties involved in targeting this system. The body of evidence presented here highlights the dysfunction of the cholinergic system in schizophrenia and that targeting this system by taking advantage of allosteric ligands is having clinically meaningful effect on cognitive deficits.

Effects of anticholinergic challenge on psychopathology and cognition in drug-free patients with schizophrenia and healthy volunteers

RATIONALE

Many aspects of the neurobiology of schizophrenia, especially the physiological basis of the negative symptoms and associated cognitive deficits, remain inadequately understood. Tandon and Greden (1989) postulated a central role of dopaminergic/cholinergic imbalance in schizophrenia.

OBJECTIVE/METHODS

In light of this hypothesis, we elected to investigate the effects of anticholinergic challenge on psychopathology, cognition and attention in 12 unmedicated patients with schizophrenia and 12 healthy controls. The first examination occurred before any pharmacological intervention; the second examination was carried out immediately following an intravenous infusion of 5 mg biperiden, a centrally acting antimuscarinergic agent.

RESULTS

The biperiden challenge provoked a considerable increase in PANSS scores in both groups which was significantly more pronounced in patients (repeated measures analysis of variance (ANOVA) (rmANOVA): F(df) = 6.4(1,22); p = 0.019). The increase in the PANSS scores showed a significant negative correlation with age in patients. Biperiden caused considerable cognitive impairments in both groups. A significant group difference (rmANOVA) could be observed for TMT-B (F(df) = 11.29(1,22); p = 0.003).

CONCLUSIONS

The anticholinergic intervention caused more pronounced psychopathological and cognitive deteriorating effects in patients suffering from schizophrenia than in healthy volunteers. This could be related to the disrupted cholinergic transmission in schizophrenia. Our findings speak on behalf of the need of a more restrictive use of anticholinergics in psychiatric patients. The age-related attenuation of PANSS score increases in patients could be related to the age-dependent changes in dopamine dynamics and also to the age-associated decline of the availability of muscarinic receptors. Our results emphasise the need for further investigation of cholinergic disturbances in schizophrenia.

Prefrontal gray matter morphology mediates the association between serum anticholinergicity and cognitive functioning in early course schizophrenia

Antipsychotic and other medications used in the treatment of schizophrenia place a burden on the cholinergic subsystems of the brain, which have been associated with increased cognitive impairment in the disorder. This study sought to examine the neurobiologic correlates of the association between serum anticholinergic activity (SAA) and cognitive impairments in early schizophrenia. Neurocognitive performance on measures of memory and executive function, structural magnetic resonance imaging (MRI) scans, and SAA assays were collected from 47 early course, stabilized outpatients with schizophrenia or schizoaffective disorder. Voxel-based morphometry analyses employing general linear models, adjusting for demographic and illness-related confounds, were used to investigate the associations between SAA, gray matter morphology, and neurocognitive impairment. SAA was related to working memory and executive function impairments. Higher SAA was significantly associated with lower gray matter density in broad regions of the frontal and medial-temporal lobes, including the dorsolateral prefrontal cortex (DLPFC), hippocampus, and striatum. Lower gray matter volume in the left DLPFC was found to significantly mediate the association between SAA and working memory impairment. Disease- and/or medication-related cholinergic dysfunction may be associated with brain volume abnormalities in early course schizophrenia, which may account for the association between SAA and cognitive dysfunction in the disorder.

Cholinergic pathways and cognition in patients with schizophrenia: a pilot study

BACKGROUND

Cognitive deficits are core features in schizophrenia. Disruption in cholinergic neurotransmission has been associated with executive dysfunction in animals and humans. The objective of this study was to evaluate the impact of compromised cholinergic pathways on executive versus non-executive cognitive functions of patients with schizophrenia.

METHODS

62 patients with schizophrenia and 62 age- and sex-matched non-psychiatric control subjects ("controls") were assessed and compared using: clinical measures, cognitive measures of global cognition, executive function, and memory; and an MRI-based visual rating scale that assesses damage strategically localized within the cholinergic pathways.

RESULTS

11 of the 62 patients with schizophrenia (17.7%) and 6 of the 62 controls (9.7%) had compromised cholinergic pathways. These proportions were not statistically significant. Patients and controls with compromised cholinergic pathways were more impaired on measures related to executive function than patients or controls without compromised pathways.

CONCLUSIONS

Patients with schizophrenia have worse executive function than controls. Compromised cholinergic pathways appear to worsen the executive dysfunction observed in schizophrenia. If these preliminary findings are replicated, they could lead to the identification of a subgroup of patients with schizophrenia who could specifically benefit from interventions enhancing cholinergic neurotransmission.

Imaging as tool to investigate psychoses and antipsychotics

The results of imaging studies have played an important role in the formulation of hypotheses regarding the etiology of psychosis and schizophrenia, as well as in our understanding of the mechanisms of action of antipsychotics. Since this volume is primarily directed to molecular aspects of psychosis and antipsychotics, only the results of molecular imaging techniques addressing these topics will be discussed here.One of the most consistent findings of molecular imaging studies in schizophrenia is an increased uptake of DOPA in the striatum, which may be interpreted as an increased synthesis of L-DOPA. Also, several studies reported an increased release of dopamine induced by amphetamine in schizophrenia patients. These findings played an important role in reformulating the dopamine hypothesis of schizophrenia. To study the roles of the neurotransmitters γ-aminobutyric acid (GABA) and glutamate in schizophrenia, SPECT as well as MR spectroscopy have been used. The results of preliminary SPECT studies are consistent with the hypothesis of NMDA receptor dysfunction in schizophrenia. Regarding the GABA deficit hypothesis of schizophrenia, imaging results are inconsistent. No changes in serotonin transporters were demonstrated in imaging studies in schizophrenia, but studies of several serotonin receptors showed conflicting results. The lack of selective radiotracers for muscarinic receptors may have hampered examination of this system in schizophrenia as well as its role in the induction of side effects of antipsychotics. Interestingly, preliminary molecular imaging studies on the cannabinoid-1 receptor and on neuroinflammatory processes in schizophrenia have recently been published. Finally, a substantial number of PET/SPECT studies have examined the occupancy of receptors by antipsychotics and an increasing number of studies is now focusing on the effects of these drugs using techniques like spectroscopy and pharmacological MRI.

Metabolic activity in the brain of juvenile and adult rats with a neonatal ventral hippocampal lesion

Longitudinal studies on patients for schizophrenia suggest that functional brain perturbations precede the onset of symptoms. Rats with a neonatal ventral hippocampal lesion (NVHL) are considered as a heuristic neurodevelopmental model of schizophrenia. We characterized basal metabolic changes observed in NVHL rats before and after the age when known behavioral alterations have been reported. Male pups were lesioned with ibotenic acid at postnatal day 7 (PD7). We measured local cerebral metabolic rates for glucose (LCMRglc) by the quantitative autoradiographic [(14)C]2-deoxyglucose technique at pre- (PD21) and postpubertal (PD42) ages when NVHL rats do not express abnormal dopamine related behaviors, and at adulthood (PD70). We observed a widespread increase in LCMRglcs in PD21 NVHL indicative of an ongoing intense reorganization of the brain while at PD42, increases were less extended. At PD70, changes in glucose metabolism were restricted to specific systems, such as the auditory system, the cerebellum, the serotonergic median raphe, and median septum. These data show in a heuristic animal model of schizophrenia that functional metabolic changes within the brain could precede the onset of dopamine-related behavioral alterations and lead to a distinct ensemble of functional changes in adulthood in systems that may be relevant to schizophrenia.

Proapoptotic and prepulse inhibition (PPI) disrupting effects of Hypericum perforatum in rats

ETHNOPHARMACOLOGICAL RELEVANCE

St. John's wort extract is commonly used as a wound healing, anti-inflammatory, anxiolytic, diuretic, antibiotic, antiviral and cancer chemoprotective agent. It also has nootropic and/or antiamnestic effects.

AIM OF THE STUDY

Prepulse inhibition (PPI) of startle response is a valuable paradigm for sensorimotor gating processes. A previous study indicated that single administration of St. John's wort extract (500 mg/kg) caused PPI disruption in rats. The effect of antiamnestic doses of the extract on PPI has not been investigated despite the coexistence of impaired memory and PPI deficit in some neurological disorders.

MATERIALS AND METHODS

The effects of acute (500 mg/kg) and chronic (200mg/kg for 3 days) administration of St. John's wort extract were investigated for its antiamnestic activity. The effects of administration of the antiamnestic dose of the extract and hyperforin, its main active component, were tested on PPI of an acoustic startle response in rats. This study also investigated the proapoptotic effect of hyperforin in animals, demonstrating PPI deficit, by electrophoresis of DNA isolated from selected brain areas.

RESULTS

Disruption of PPI resulted after treatment of rats with an antiamnestic dose of the extract (200mg/kg for 3 days) and with hyperforin. Gel electrophoresis showed DNA fragmentation of the cortices of hyperforin-treated animals exhibiting PPI deficit.

CONCLUSIONS

The exacerbating effect of St. John's wort extract on PPI deficit may provide a limitation for using the extract to manage cognitive disturbance in psychotic and Huntington's disease patients manifesting PPI deficit.

Lack of association between antipsychotic-induced Parkinsonism or its subsymptoms and rs4606 SNP of RGS2 gene in African-Caribbeans and the possible role of the medication: the Curacao extrapyramidal syndromes study X

Recent studies demonstrate an association between antipsychotic-induced parkinsonism (AIP) and rs4606 SNP of RGS2 gene in Jewish and African-Americans. The current study investigates the association between rs4606 and AIP or its subsymptoms (rest tremor, rigidity, and bradykinesia) in 112 psychiatric inpatients of African-Caribbean origin. Presence of AIP, rigidity, bradykinesia, and tremor was measured by the UPDRS. We applied chi(2) (or Fisher Exact) and logistic regression analyses in several models including rs4606, age, gender, dose of antipsychotics, and anticholinergics, and two other putatively functional SNPs in DRD2 (-141CIns/Del) and HTR2C (Cys23Ser) genes. In contrast to recent literature, we find no evidence for an association between rs4606 and AIP or any of its subsymptoms. We hypothesize that the observed lack of association is due probably to differences in serotonin 2A-receptor affinities of the antipsychotics utilized (in contrast to the other published studies, the majority of our patients utilized typical antipsychotics).

Insulin, IGF-1, and muscarinic agonists modulate schizophrenia-associated genes in human neuroblastoma cells

BACKGROUND

Genes associated with energy metabolism are decreased in schizophrenia brain and human and rodent diabetic skeletal muscle. These and other similarities between diabetes and schizophrenia suggest that an insulin signaling deficit may underlie schizophrenia. We determined with human SH-SY5Y neuroblastoma and astrocyte cell lines whether insulin or other molecules could modulate genes opposite to their change reported in schizophrenia brain.

METHODS

Both cell lines were treated with insulin, insulin-like growth factor (IGF)-1, IGF-2, or brain-derived neurotrophic factor (BDNF). Genes whose expression was found with microarrays to be changed by insulin in a reciprocal manner to their change in schizophrenia were used in a 16-gene miniarray to identify small molecules that might mimic insulin.

RESULTS

Insulin phosphorylated its receptor in the neuroblastoma cells but not in astrocytes and, like IGF-1, increased ERK1/2 and Akt phosphorylation. Insulin and IGF-1 increased the expression of genes decreased in schizophrenia, including those involved in mitochondrial functions, glucose and energy metabolism, hydrogen ion transport, and synaptic function. These gene effects were confirmed and shown to be dose related with the 16-gene miniarrays. Most of 1940 pharmacologically unique compounds failed to alter gene expression, with the exception of muscarinic agonists, which mimicked insulin and IGF-1, and which were blocked by the muscarinic antagonists atropine and telenzepine.

CONCLUSIONS

Stimulation of muscarinic and insulin/IGF-1 receptors alter genes associated with metabolic and synaptic functions in a manner reciprocal to their changes in schizophrenia. Pharmacologic activation of these receptors may normalize genomic alterations in schizophrenia and better address root causes of this disease.

Muscarinic agonists for the treatment of cognition in schizophrenia

It is widely accepted that cholinergic activity at muscarinic receptors is required to maintain cognitive functions, including learning and memory. Memory domains are especially impaired in schizophrenia, which may explain difficulties in psychosocial rehabilitation of individuals with this illness. However, little is known about the mechanism of this impairment. To understand our current knowledge, we reviewed the literature since 1990 via a PubMed search for the terms "muscarinic", "schizophrenia", "cognition", "memory", "learning", and "agonist" in combination. We found 89 basic science/laboratory studies, case reports/series, case-control studies, cross-sectional studies, standardized controlled animal trials, standardized controlled human trials, and reviews. Although further research is required to fully understand the neuropharmacology of the cholinergic system in cognitive function in schizophrenia, we have examined the data currently available. In general, these data suggest that agonist activity at acetylcholine muscarinic type 1 (M1) receptors would enhance memory and learning in schizophrenia. We present an overview of likely side effects of muscarinic agonists. We outline the anticholinergic activity of several available antipsychotics and review the available M1 muscarinic agonists.

Decreased M1 muscarinic receptor density in rat amphetamine model of schizophrenia is normalized by clozapine, but not haloperidol

There is increasing evidence supporting the involvement of the muscarinic-cholinergic system in schizophrenia. We examined the M1 muscarinic receptor density and mRNA expression in brains of a rat amphetamine model of schizophrenia. We also assessed the effect of the model and chronic treatment with haloperidol and clozapine on brain M1 receptor density and gene expression. A significant decrease of about 20% in the density of M1 receptor was detected in the cortex and in the striatum of amphetamine model rats. A significant increase of 33% in the density of the M1 receptor was found in the cortex and striatum of rats treated chronically with clozapine (0.5 mg/kg), but not with haloperidol (25 mg/kg). Chronic clozapine, but not haloperidol, normalized the decrease in M1 receptors observed in amphetamine model rats, in both cortex and striatum. Regulation of the M1 receptor may occur in a post-transcriptional phase. Our findings suggest involvement of both dopaminergic and cholinergic-muscarinic systems in the pathophysiology and pharmacotherapy of schizophrenia.

New biological therapies from the human genome

Over the past 20 years, the biotechnology industry has been extraordinarily successful in bringing a wide variety of new products to the market, including recombinant versions of natural proteins such as growth hormone, insulin and the gonadotropins. The availability of the human genome sequence has given us the chance to identify the entire catalogue of human secreted proteins, often called the secretome. One of the challenges of biotechnology research has been to identify the biological activities of these proteins and to identify if any of them could have a therapeutic or pharmacologic use. The paradigm has effectively been reversed, in that it used to be easy to know the biological activity, but difficult to clone, whereas now the contrary is true. Five years on, it is clear that finding new biological activities is a very difficult process. Much of the ground gained in this period has either been through the development of antibodies as therapies or by the use of protein engineering to produce better versions of the proteins that are already being produced.

Decreased prepulse inhibition and increased sensitivity to muscarinic, but not dopaminergic drugs in M5 muscarinic acetylcholine receptor knockout mice

RATIONALE

Schizophrenic patients show decreased measures of sensorimotor gating, such as prepulse inhibition of startle (PPI). In preclinical models, these measures may be used to predict antipsychotic activity. While current antipsychotic drugs act largely at dopamine receptors, the muscarinic acetylcholine receptors offer promising novel pharmacotherapy targets. Of these, the M(5) receptor gene was recently implicated in susceptibility to schizophrenia. Due to the lack of selective ligands, muscarinic receptor knockout mice have been generated to elucidate the roles of the five receptor subtypes (M(1)-M(5)).

OBJECTIVES

Here, we used M(5) receptor knockout (M(5)-/-) mice to investigate the involvement of M(5) receptors in behavioral measures pertinent to schizophrenia. We tested the hypothesis that disruption of M(5) receptors affected PPI or the effects of muscarinic or dopaminergic agents in PPI or psychomotor stimulation.

MATERIALS AND METHODS

We measured PPI in M(5)-/-, heterozygous and wild-type mice without drugs, and with clozapine (0.56-3.2 mg/kg) or haloperidol (0.32-3.2 mg/kg) alone, and as pretreatment to D: -amphetamine. In addition, we evaluated locomotor stimulation by the muscarinic antagonist trihexyphenidyl (0.56-56 mg/kg) and by cocaine (3.2-56 mg/kg).

RESULTS

The M(5)-/- mice showed decreased PPI relative to wild-type mice, and clozapine appeared to reduce this difference, while haloperidol increased PPI regardless of genotype. The M(5)-/- mice also showed more locomotor stimulation by trihexyphenidyl than wild-type mice, while cocaine had similar effects between genotypes.

CONCLUSIONS

These data suggest that disruption of the M(5) receptor gene affected sensorimotor gating mechanisms, increased sensitivity to clozapine and to the psychostimulant effects of muscarinic antagonists without modifying the effect of dopaminergic drugs.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Evaluation of [18F]fluoroxanomeline {5-{4-[(6-[18F]fluorohexyl)oxy]-1,2,5-thiadiazol-3-yl}-1-methyl-1,2,3,6-tetrahydropyridine} in muscarinic knockout mice

INTRODUCTION

We set out to develop a muscarinic M1-selective agonist (based on the structure of the functionally M1-selective xanomeline) that could be radiolabeled with fluorine-18 for use as an imaging agent for positron emission tomography.

METHODS

The radiochemical synthesis was achieved, employing the arts of organic and radiochemical syntheses. Binding selectivity studies employed biodistribution studies, using autoradiography and/or tissue dissection, in wild-type or muscarinic receptor knockout mice.

RESULTS

[(18)F]Fluoroxanomeline shows rather uniform uptake in all mouse brain regions and high specific binding, with a brain-to-blood ratio of 32 at 60 min postinjection. In addition, the specific binding is demonstrated by a 58% to 75% decrease in brain uptake upon coinjection with 5 nmol of unlabeled fluoroxanomeline or xanomeline. Brain uptake studies with [(3)H]xanomeline in muscarinic knockout mice show decreased uptake in M1 (17-34%) and M2 (2-20%) knockout mice compared with control. However, statistical significance was observed in only a few regions. Comparison of [(18)F]fluoroxanomeline in knockout mice showed no difference in M1 or M4 knockout mice but a general decrease in M2 (2-24%) knockout mice. The decrease of [(18)F]fluoroxanomeline uptake in M2 knockout mice reached statistical significance in brain stem, cerebellum, frontal cortex, hippocampus, inferior colliculus and superior colliculus.

CONCLUSION

Although xanomeline displays highly selective M1 agonist activity in functional assays, little selectivity for muscarinic subtype binding was observed for xanomeline or its fluorine-containing analogue, fluoroxanomeline. This emphasizes the lack of correlation between functional selectivity and binding selectivity.

Alterations of muscarinic and GABA receptor binding in the posterior cingulate cortex in schizophrenia

The posterior cingulate cortex (PCC), a key component of the limbic system, has been implicated in the pathology of schizophrenia because of its sensitivity to NMDA receptor antagonists. Recent studies have shown that the PCC is dysfunctional in schizophrenia, and it is now suspected to be critically involved in the pathogenesis of schizophrenia. Studies also suggest that there are abnormalities in muscarinic and GABAergic neurotransmission in schizophrenia. Therefore, in the present study we used quantitative autoradiography to investigate the binding of [(3)H]pirenzepine, [(3)H]AF-DX 384 and [(3)H]muscimol, which respectively label M1/4 and M2/4 muscarinic and GABA(A) receptors, in the PCC of schizophrenia and control subjects matched for age and post-mortem interval. The present study found that [(3)H]pirenzepine binding was significantly decreased in the superficial (-24%, p=0.002) and deep (-35%, p<0.001) layers of the PCC in the schizophrenia group as compared with the control group. In contrast, a dramatic increase in [(3)H]muscimol binding was observed in the superficial (+112%, p=0.001) and deep layers (+100%, p=0.017) of the PCC in the schizophrenia group. No difference was observed for [(3)H]AF-DX 384 binding between the schizophrenia and control groups. The authors found a significant inverse correlation between [(3)H]pirenzepine binding in the deep cortical layers and [(3)H]muscimol binding in the superficial layers (rho=-0.732, p=0.003). In addition, negative correlations were also found between age and [(3)H]pirenzepine binding in both superficial and deep cortical layers (rho=-0.669 p=0.049 and rho=-0.778, p=0.014), and between age of schizophrenia onset and [(3)H]AF-DX 384 binding (rho=-0.798, p=0.018). These results for the first time demonstrated the status of M1/M4, M2/M4 and GABA(A) receptors in the PCC in schizophrenia. Whilst the exact mechanism causing these alterations is not yet known, a possible increased acetylcholine and down regulated GABA stimulation in the PCC of schizophrenia is suggested.

Effects of Donepezil Adjunctive Treatment to Ziprasidone on Cognitive Deficits in Schizophrenia

The objective of this study was to examine the effects of adjunctive treatment with the acetylcholinesterase inhibitor, donepezil, on cognitive deficits and psychopathology in schizophrenic patients treated with the antipsychotic, ziprasidone. The design of the study was double blind, placebo controlled, and longitudinal. Patients were treated with ziprasidone for 8 weeks, thereafter randomized to 4 months of double-blind adjunctive treatment with either donepezil (dose, 5-10 mg) or placebo. The severity of psychopathology (PANSS) and the cognitive deficits were examined at baseline and after 4 months. A total of 21 schizophrenic patients were enrolled, of whom 11 patients completed the trial (donepezil, n = 7; placebo, n = 4). There were no within- or between-group differences in changes on the Positive and Negative Syndrome Scale scores or a global cognitive score. Within-group improvements (all at trend level P = 0.07) were seen in the placebo group on Trail-Making Test B, immediate verbal recall, and set-shifting errors. The donepezil group showed a significant deterioration on planning efficiency (P = 0.04). Between-group differences were found between the lack of improvement in immediate verbal recall in the donepezil group and the improvement in the placebo group (P = 0.02), and between the deterioration of planning efficiency in the donepezil group and the stability in the placebo group (trend level, P = 0.07). Linear regression analyses showed that neither baseline psychopathology scores, baseline levels of cognitive deficits, nor psychopathology changes over time accounted for these changes in cognitive scores. The study found no evidence of improved cognition after treatment with donepezil, although the conclusions that can be drawn are limited by the small sample size.

A model of anticholinergic activity of atypical antipsychotic medications

BACKGROUND

Atypical antipsychotics clozapine, olanzapine, and quetiapine have significant affinity for the muscarinic receptors in vitro, while aripiprazole, risperidone, and ziprasidone do not. Dissimilarity in binding profiles may contribute to the reported differences in the anticholinergic effects of these antipsychotics. However, it is difficult with the available data to predict the likelihood of anticholinergic effects occurring with various doses of an atypical antipsychotic.

METHODS

We developed a model to assess the potential anticholinergic activity (AA) of atypical antipsychotics at therapeutic doses. A radioreceptor assay was used to measure in vitro AA at 6 clinically relevant concentrations of aripiprazole, clozapine, olanzapine, quetiapine, risperidone, and ziprasidone. Using published pharmacokinetic data, in combination with the measured in vitro AA, dose-AA curves were generated.

RESULTS

Clozapine, and to a lesser extent olanzapine and quetiapine showed dose-dependent increases in AA. At therapeutic doses, the AA (in pmol/mL of atropine equivalents) was estimated to range from 27-250, 1-15, and 0-5.4 pmol/mL for clozapine, olanzapine, and quetiapine, respectively. Aripiprazole, risperidone, and ziprasidone did not demonstrate AA at any of the concentrations studied.

CONCLUSIONS

Therapeutic doses of clozapine, olanzapine, and, to a lesser extent, quetiapine are associated with clinically relevant AA.

Galantamine and donepezil attenuate pharmacologically induced deficits in prepulse inhibition in rats

Acetylcholinesterase inhibitors (AChEIs) are currently being evaluated as adjunctive therapy for the cognitive dysfunction of schizophrenia. This core symptom of schizophrenia has often been attributed to impaired attention and abnormal sensory motor gating, features that are also found in Huntington's Disease, autism, and several other psychiatric and neurological disorders. The ability to improve prepulse inhibition (PPI) of the acoustic startle response may predict the efficacy of compounds as cognitive enhancers. In this study, PPI was disrupted in Wistar rats in three pharmacologic models: dopamine receptor agonism by apomorphine, NMDA receptor antagonism by MK801, or muscarinic acetylcholine receptor antagonism by scopolamine. We then evaluated the commonly used AChEIs, donepezil (0.5, 1.0, or 2.0mg/kg) and galantamine (0.3, 1.0, or 3.0mg/kg) for the capacity to improve PPI in each model. Under vehicle conditions, the prepulse stimuli (75, 80 and 85dB) inhibited the startle response to a 120dB auditory stimulus in a graded fashion. Galantamine (depending on dose) improved PPI deficits in all three PPI disruption models, whereas donepezil ameliorated PPI deficits induced by scopolamine and apomorphine, but was not effective in the MK801 model. These results indicate that some AChEIs may have the potential to improve cognition in schizophrenia by improving auditory sensory gating.

Antipsychotic drug action: targets for drug discovery with neurochemical imaging

Schizophrenia is a serious lifelong mental illness for which current treatments may only be partially effective. All antipsychotic medications available at present are thought to exert their main antipsychotic effect through antagonism of dopamine D2 receptors. Clozapine is the most effective antipsychotic drug currently available, but it can cause serious side effects, including agranulocytosis and diabetes. Pharmacologic factors that distinguish clozapine from other antipsychotic drugs have been studied to try to develop safer drugs with similar efficacy to clozapine. These have met with limited success. Neurochemical imaging techniques, such as positron emission tomography, single photon emission tomography and magnetic resonance spectroscopy, have been used to study antipsychotic drug action in living human subjects. These techniques shed a great deal of light on the mechanisms of antipsychotic action and have revealed a number of novel targets for future drug development in schizophrenia. Next-generation antipsychotic medications will aim to improve on the efficacy and tolerability of currently available medications. The authors believe that they are likely to achieve this through drug action at non-D2 sites. Future research and drug development, including the development of medications to prevent progression from the prepsychotic stage to schizophrenia, will rely heavily on neurochemical imaging methods at all stages in the drug-discovery pipeline.

Weaving single photon imaging into new drug development

The specific aim of this review is to assess the potential contribution of single photon emitting radiopharmaceutical technologies to new drug development. For each phase of therapeutic drug development, published literature was sought that shows single photon emitters can add value by quantifying pharmacokinetics, visualizing mechanisms of drug action, estimating therapeutic safety indices, or measuring dose-dependent pharmacodynamic effects. Not any published reports were found that describe using nuclear medicine techniques to help manage the progress of a new drug development program. As a consequence, most of the case in favor of weaving single photon imaging into the process had to be built on extrapolations from studies that showed feasibility post hoc. The strongest evidence of potential value was found for drug candidates that hope to influence diseases characterized by cell proliferation or cell death, particularly in the fields of oncology, cardiology, nephrology, and inflammation. Receptor occupancy studies were observed to occasionally offer unique advantages over analogous studies with positron emission tomography (PET). Enough hard data sets were found to justify the costs of using single photon imaging in a variety of new drug development paradigms.

The highly efficacious actions of N-desmethylclozapine at muscarinic receptors are unique and not a common property of either typical or atypical antipsychotic drugs: is M1 agonism a pre-requisite for mimicking clozapine's actions?

RATIONALE

Recent studies have suggested that the salutary actions of clozapine in schizophrenia may be due to selective activation of M(1) muscarinic receptors by clozapine and/or its major active metabolite N-desmethylclozapine.

OBJECTIVE

We systematically tested this hypothesis by screening a large number of psychoactive compounds, including many atypical antipsychotic drugs, for agonist activity at cloned, human M(1), M(3) and M(5) muscarinic receptors.

RESULTS

Only three of the 14 atypical antipsychotic drugs we tested were found to possess partial agonist actions at M(1) muscarinic receptors (fluperlapine, JL13, clozapine). A few additional miscellaneous compounds had a modest degree of M(1) agonist actions. Only carbachol and N-desmethylclozapine had appreciable M(3) muscarinic agonism at M(3) muscarinic receptors, although several were M(5) partial agonists including MK-212, N-desmethylclozapine and xanomeline.

CONCLUSION

Although M(1) muscarinic receptor-selective partial agonists have shown promise in some preclinical antipsychotic drug models, these studies indicate that it is unlikely that the salutary actions of clozapine and similar atypical antipsychotic drugs are mediated solely by M(1) muscarinic receptor activation. It is possible, however, that the M(1) agonism of N-desmethylclozapine contributes to the uniquely beneficial actions of clozapine. Thus, these results are consistent with the notion that a balanced degree of activity at multiple biogenic amine receptors, including M(1) muscarinic agonism, is responsible for the uniquely beneficial actions of clozapine.

SPET imaging of central muscarinic receptors with (R,R)[123I]-I-QNB: methodological considerations

Investigations on the effect of normal healthy ageing on the muscarinic system have shown conflicting results. Also, in vivo determination of muscarinic receptor binding has been hampered by a lack of subtype selective ligands and differences in methods used for quantification of receptor densities. Recent in vitro and in vivo work with the muscarinic antagonist (R,R)-I-QNB indicates this ligand has selectivity for m(1) and m(4) muscarinic receptor subtypes. Therefore, we used (R,R)[(123)I]-I-QNB and single photon emission tomography to study brain m(1) and m(4) muscarinic receptors in 25 healthy female subjects (11 younger subjects, age range 26-32 years and 14 older subjects, age range 57-82 years). Our aims were to ascertain the viability of tracer administration and imaging within the same day, and to evaluate whether normalization to whole brain, compared to normalization to cerebellum, could alter the clinical interpretation of results. Images were analyzed using the simplified reference tissue model and by two ratio methods: normalization to whole brain and normalization to cerebellum. Significant correlations were observed between kinetic analysis and normalization to cerebellum, but not to whole brain. Both the kinetic analysis and normalization to cerebellum showed age-related reductions in muscarinic binding in frontal, orbitofrontal, and parietal regions. Normalization to whole brain, however, failed to detect age-related changes in any region. Here we show that, for this radiotracer, normalizing to a region of negligible specific binding (cerebellum) significantly improves sensitivity when compared to global normalization.