Brain Muscarinic Receptors Research Articles

Effects of the Partial M1 Muscarinic Cholinergic Receptor Agonist CDD-0102A on Stereotyped Motor Behaviors and Reversal Learning in the BTBR Mouse Model of Autism.

BackgroundAutism spectrum disorders (ASD) are a set of neurodevelopmental disorders marked by a lack of social interaction, restrictive interests, and repetitive behaviors. There is a paucity of pharmacological treatments to reduce core ASD symptoms. Various lines of evidence indicate that reduced brain muscarinic cholinergic receptor activity may contribute to an ASD phenotype.MethodsThe present experiments examined whether the partial M1 muscarinic receptor agonist, 5-(3-ethyl-1,2,4-oxadiazol-5-yl)-1,4,5,6-tetrahydropyrimidine hydrochloride (CDD-0102A), alleviates behavioral flexibility deficits and/or stereotyped motor behaviors in the BTBR mouse model of autism. Behavioral flexibility was tested using a reversal learning test. Stereotyped motor behaviors were measured by eliciting digging behavior after removal of nesting material in a home cage and by measuring repetitive grooming.ResultsCDD-0102A (0.2 and 0.6 mg/kg but not 1.2 mg/kg) injected prior to reversal learning attenuated a deficit in BTBR mice but did not affect performance in B6 mice. Acute CDD-0102A treatment (1.2 and 3 mg/kg) reduced self-grooming in BTBR mice and reduced digging behavior in B6 and BTBR mice. The M1 muscarinic receptor antagonist VU0255035 (3 mg/kg) blocked the effect of CDD-0102A on grooming behavior. Chronic treatment with CDD-0102A (1.2 mg/kg) attenuated self-grooming and digging behavior in BTBR mice. Direct CDD-0102A infusions (1 µg) into the dorsal striatum reduced elevated digging behavior in BTBR mice. In contrast, CDD-0102A injections in the frontal cortex were not effective.ConclusionsThe results suggest that treatment with a partial M1 muscarinic receptor agonist may reduce repetitive behaviors and restricted interests in autism in part by stimulating striatal M1 muscarinic receptors.

PET Imaging of brain muscarinic receptors with 18F-Fluorobenzyl-Dexetimide: A first in human study

M1 and M4 muscarinic receptor (mAChR) agonists are under development for the treatment of schizophrenia, Alzheimer's and Parkinson's disease. We performed first-in-human PET imaging of mAChR with 18F-Fluorobenzyl-Dexetimide (FDEX) in 10 healthy participants (29.4±4.3yrs). Four underwent dynamic brain scanning for 240 min, and then six underwent static brain scans at 120 and 160-min post injection of 250 MBq of FDEX. Gjedde-Patlak graphical analysis was applied to determine the influx constant (Ki). Regional tissue ratios (SUVR) were calculated using the cerebellar cortex as the reference region. No adverse events were observed. The tracer showed good brain entry (∼4.2% ID at 5 min) but irreversible distribution kinetics over four hours in regions of high mAChR. Binding was consistent with the distribution of mAChR receptors with striatum > cortex > hippocampus >> thalamus >>> cerebellum with low variance in regional binding between subjects. Ki was 0.42±0.04 in the putamen, 0.27±0.01 in frontal cortex, 0.25±0.02 in the hippocampus and 0.10±0.01 in the thalamus. SUVR at 120 and 240 min. were highly correlated with these Ki values with R2 of 0.91 and 0.99 respectively. FDEX yields high quality brain images with uptake in the known distribution of mAChR with remarkably little variance between normal subjects.

Xanomeline suppresses excessive pro-inflammatory cytokine responses through neural signal-mediated pathways and improves survival in lethal inflammation.

Inflammatory conditions characterized by excessive immune cell activation and cytokine release, are associated with bidirectional immune system-brain communication, underlying sickness behavior and other physiological responses. The vagus nerve has an important role in this communication by conveying sensory information to the brain, and brain-derived immunoregulatory signals that suppress peripheral cytokine levels and inflammation. Brain muscarinic acetylcholine receptor (mAChR)-mediated cholinergic signaling has been implicated in this regulation. However, the possibility of controlling inflammation by peripheral administration of centrally-acting mAChR agonists is unexplored. To provide insight we used the centrally-acting M1 mAChR agonist xanomeline, previously developed in the context of Alzheimer's disease and schizophrenia. Intraperitoneal administration of xanomeline significantly suppressed serum and splenic TNF levels, alleviated sickness behavior, and increased survival during lethal murine endotoxemia. The anti-inflammatory effects of xanomeline were brain mAChR-mediated and required intact vagus nerve and splenic nerve signaling. The anti-inflammatory efficacy of xanomeline was retained for at least 20h, associated with alterations in splenic lymphocyte, and dendritic cell proportions, and decreased splenocyte responsiveness to endotoxin. These results highlight an important role of the M1 mAChR in a neural circuitry to spleen in which brain cholinergic activation lowers peripheral pro-inflammatory cytokines to levels favoring survival. The therapeutic efficacy of xanomeline was also manifested by significantly improved survival in preclinical settings of severe sepsis. These findings are of interest for strategizing novel therapeutic approaches in inflammatory diseases.

Positive allosteric modulation of M1 muscarinic acetylcholine receptors suppresses lethal peripheral inflammation (P5082)

Abstract Peripheral inflammation can be regulated by activation of brain muscarinic acetylcholine receptor (mAChR)-dependent signaling functionally associated with a vagus-nerve mediated anti-inflammatory circuit (Proc Natl Acad Sci USA, 2006, 5219; Brain Behav Immun, 2009, 23:41). Here, we studied the specific role of the M1 mAChR subtype in this regulation by utilizing BQCA, a highly specific allosteric M1 mAChR activator that crosses the blood-brain barrier. Single drug (5,10 or 20 mg/kg, i.p) administration in mice 1h prior to endotoxin (8 mg/kg, i.p.) dose-dependently reduced serum and splenic TNF levels and significantly improved survival in mice as compared to vehicle-treated controls. Pharmacological blockade of brain mAChRs significantly abolished BQCA anti-inflammatory effects. Furthermore, BQCA (20 mg/kg, i.p.) significantly reduced serum and splenic TNF levels in wild type mice, but failed to alter TNF levels in M1 KO mice. Together these results indicate the anti-inflammatory role of increased functional activity of endogenous acetylcholine on the M1 mAChR by selective allosteric receptor activation. Our findings are of interest for further development of BQCA and other centrally-acting allosteric activators of the M1 mAChR as a novel class of experimental anti-inflammatory therapeutics.

Antipsychotic Medications: Linking Receptor Antagonism to Neuropsychological Functioning in First Episode Psychosis

Antipsychotic medications can contribute to neurocognitive and motor impairments, but specific links to individualized pharmacological treatment regimens are unclear. In 68 participants with stabilized first-episode psychosis (FEP), we investigated the links between neuropsychological functions and an established anticholinergic potency index and a new D(2) antagonist potency index developed in our lab. Each participant's psychiatric medication regimen was converted into estimated receptor antagonist loads based upon specific medication dosage(s) and reported in vitro brain muscarinic cholinergic and D(2) receptor antagonism. In addition to the global neuropsychological impairments of FEP participants, the findings supported the hypothesized links between receptor antagonist loads and specific deficits. Higher anticholinergic load was associated with poorer delayed verbal memory but was not related to motor functioning. In contrast, higher D(2) load was associated with poorer motor functioning but not verbal memory. These selective antagonist load associations explained 19% of the variance in motor functioning and 17% of the variance in delayed verbal memory. Evidently, some of the neuropsychological impairments found in persons with FEP are selectively related to the specific pharmacodynamics and the dosing of their medication regimens. Moreover, these effects can be readily estimated from practical and inexpensive indices.

Memory defect induced by beta-amyloid plus glutamate receptor agonist is alleviated by catalpol and donepezil through different mechanisms

Our previous studies demonstrate that a non-cholinesterase inhibitor (AChEI) compound catalpol, purified from a traditional Chinese medicinal herb Rehmannia glutinosa, could improve the symptoms and pathological changes in animal and cellular models of memory related neurodegenerative diseases. In this study, we compared catalpol with the most commonly used AChEI donepezil in respect to their mechanism of action on the neurodegenerative changes in an animal model induced by beta-amyloid (Aβ) plus glutamate receptor agonist. It was found that the model mice showed significant deficit in the learning ability and memory in Y maze avoidance test, and meanwhile both donepezil and catalpol greatly improve the learning ability and memory after 2 to 3months' administration. At the selected doses, donepezil only partially raised the declined brain muscarinic acetylcholine receptor (M receptor) density and choline acetyltransferase (ChAT) activity resulting in these levels still lower than normal control, while catalpol completely retrieved these two parameters. ELISA revealed that catalpol, instead of donepezil, possessed the capability of elevating the declined brain BDNF level of the animal model. The ELISA results on the BDNF protein level was confirmed by quantitative RT-PCR measurement of BDNF mRNA in Aβ25–35-treated primary culture of forebrain neurons. In combination with our previous work, we think the neuroprotective effects of donepezil and catalpol are mediated through different mechanisms. Since BDNF has been proved to be an important intrinsic factor in protecting neurodegenerative diseases, catalpol may be a hopefully effective compound against neurodegenerative changes induced by Aβ and glutamate receptor agonist.

Characterization of Bladder Selectivity of Antimuscarinic Agents on the Basis ofIn VivoDrug-Receptor Binding

The in vivo muscarinic receptor binding of antimuscarinic agents (oxybutynin, solifenacin, tolterodine, and imidafenacin) used to treat urinary dysfunction in patients with overactive bladder is reviewed. Transdermal administration of oxybutynin in rats leads to significant binding of muscarinic receptors in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin shows significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M3 subtype. Oral tolterodine binds more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding of oral imidafenacin in rats is more selective and longer-lasting in the bladder than in other tissues such as the submaxillary gland, heart, colon, lung, and brain, suggesting preferential muscarinic receptor binding in the bladder. In vivo quantitative autoradiography with (+)N-[11C]methyl-3-piperidyl benzilate in rats shows significant occupancy of brain muscarinic receptors with the intravenous injection of oxybutynin, solifenacin, and tolterodine. The estimated in vivo selectivity in brain is significantly greater for solifenacin and tolterodine than for oxybutynin. Imidafenacin occupies few brain muscarinic receptors. Similar findings for oral oxybutynin were observed with positron emission tomography in conscious rhesus monkeys with a significant disturbance of short-term memory. The newer generation of antimuscarinic agents may be advantageous in terms of bladder selectivity after systemic administration.

Selective Binding of Bladder Muscarinic Receptors in Relation to the Pharmacokinetics of a Novel Antimuscarinic Agent, Imidafenacin, to Treat Overactive Bladder

The binding of orally administered imidafenacin, used to treat overactive bladders, to muscarinic receptors in rat tissue was characterized based on pharmacokinetics. The binding in six tissues including bladder tissue was measured using [N-methyl-(3)H] scopolamine methyl chloride ([(3)H]NMS). Pharmacokinetic parameters were estimated from measurements of the concentration of imidafenacin in serum, the bladder, and the submaxillary gland by liquid chromatography-mass spectrometry/mass spectrometry. The receptor binding affinity of imidafenacin in vitro was significantly lower in the bladder than submaxillary gland or colon. The oral administration of imidafenacin (0.79, 1.57, and 6.26 μmol/kg) was characterized by a more selective and longer-lasting binding to muscarinic receptors in the bladder than other tissues. Imidafenacin showed little binding to brain muscarinic receptors, consistent with its minor effect on the central nervous system. Pharmacokinetic data showed that orally administered imidafenacin was distributed at a higher concentration in the bladder than the serum or submaxillary gland of rats. After the intravesical instillation of imidafenacin, there was significant binding of muscarinic receptors in the bladder. Furthermore, a significant level of imidafenacin was detected in the urine of rats given a 1.57 μmol/kg concentration of this agent. The present study demonstrated that imidafenacin administered orally distributes predominantly to the bladder and exerts more selective and longer-lasting effect on the bladder than other tissues, such as the submaxillary gland, colon, and brain. Furthermore, the imidafenacin excreted in urine may play an important role in pharmacokinetic and pharmacological selectivity.

Noninvasive evaluation of brain muscarinic receptor occupancy of oxybutynin, darifenacin and imidafenacin in rats by positron emission tomography

Aims The current study was conducted to evaluate, by the noninvasive positron emission tomography (PET), the binding of antimuscarinic agents used to treat overactive bladder (OAB) to muscarinic receptors in rat brain. Main methods Muscarinic receptor occupancy in the rat brain after the intravenous (i.v.) injection of oxybutynin, darifenacin and imidafenacin was evaluated by using a small animal PET system, and compared with the results by ex vivo autoradiographic and ex vivo radioligand binding experiments. Key findings In PET study, the i.v. injection of oxybutynin but not darifenacin or imidafenacin at pharmacological doses decreased significantly binding potential (BP) of (+) N-[ 11C]methyl-3-piperidyl benzilate ([ 11C](+)3-MPB) in the rat cerebral cortex and corpus striatum in a dose-dependent manner. Similarly, in the in vivo autoradiographic experiment, oxybutynin dose-dependently reduced binding of [ 11C](+)3-MPB in the brain, whereas darifenacin and imidafenacin did not. Following the i.v. injection of oxybutynin, darifenacin and imidafenacin, there was a similar degree of binding to muscarinic receptors in the bladder as demonstrated by a significant increase in apparent dissociation constant ( K d ) values for specific [N-methyl- 3H]scopolamine methyl chloride ([ 3H]NMS) binding. Significant binding of muscarinic receptors in the brain was observed after the injection of oxybutynin but not darifenacin or imidafenacin. Significance Oxybutynin but not darifenacin or imidafenacin has potential side effects on the central nervous system (CNS) in patients with OAB. The results reveal the noninvasive characterization of brain receptor occupancy by PET to be a powerful tool for precise evaluation of adverse CNS effects of antimuscarinic agents in pre-clinical and clinical evaluations.

Cholinesterase inhibitors and stress: Effects on brain muscarinic receptor density in mice

Exposure to the reversible cholinesterase inhibitor, pyridostigmine bromide (PB), in conjunction with stress, has been suggested as a possible cause of Gulf War Syndrome. This work explores the hypothesis that PB exposure coupled with stress will alter cholinergic receptor density based on the rationale that prolonged exposure to PB and stress will lead to increased stimulation of cholinergic receptors due to the reduced capacity to degrade acetylcholine, leading to changes in receptor levels. Male C57Bl6 mice were exposed to PB (3 or 10 mg/kg/day) or physostigmine (2.88 mg/kg/day) for 7 days via ALZET mini-osmotic pumps implanted subcutaneously. The mice were stressed by shaking at random intervals (avg of 2 min/30 min) for 1 week, which was sufficient to increase blood cortisol levels. Brain tissue for autoradiographic analysis was collected on day 7 of treatment. While we examined many brain regions, analysis revealed that most of the significant changes ( p < 0.05) were seen in cholinergic nuclei. Stress typically increased muscarinic receptor density, while PB and PHY generally decreased muscarinic receptor density.

The Forefront for Novel Therapeutic Agents Based on the Pathophysiology of Lower Urinary Tract Dysfunction: Bladder Selectivity Based on In Vivo Drug–Receptor Binding Characteristics of Antimuscarinic Agents for Treatment of Overactive Bladder

We have reviewed the binding of antimuscarinic agents, used to treat urinary dysfunction in patients with overactive bladder, to muscarinic receptors in target and non-target tissues in vivo. Transdermal administration of oxybutynin in rats led to significant binding in the bladder without long-term binding in the submaxillary gland and the abolishment of salivation evoked by oral oxybutynin. Oral solifenacin showed significant and long-lasting binding to muscarinic receptors in mouse tissues expressing the M3 subtype. Oral tolterodine bound more selectively to muscarinic receptors in the bladder than in the submaxillary gland in mice. The muscarinic receptor binding activity of oral darifenacin in mice was shown to be pronounced and long-lasting in the bladder, submaxillary gland, and lung. In vivo quantitative autoradiography using (+)N-[11C] methyl-3-piperidyl benzilate in rats showed significant occupancy of brain muscarinic receptors on intravenous injection of oxybutynin, propiverine, solifenacin, and tolterodine. The estimated in vivo bladder selectivity compared to brain was significantly greater for solifenacin and tolterodine than oxybutynin. Darifenacin occupied few brain muscarinic receptors. Similar findings were also observed with positron emission tomography in conscious rhesus monkeys. The newer generation of antimuscarinic agents may be advantageous in the bladder selectivity after systemic administration.

Flaws in the Serum Anticholinergic Activity Assay: Implications for the Study of Delirium

Flaws in the Serum Anticholinergic Activity Assay: Implications for the Study of Delirium

In Vivo Quantitative Autoradiographic Analysis of Brain Muscarinic Receptor Occupancy by Antimuscarinic Agents for Overactive Bladder Treatment

We evaluated the effects of five clinically used antimuscarinic agents for overactive bladder (OAB) treatment on in vivo muscarinic receptor binding in rat brain by quantitative autoradiography. There was a dose-related decrease in in vivo specific +N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) binding in each brain region of rats 10 min after i.v. injection of oxybutynin, propiverine, solifenacin, and tolterodine. Rank order of the i.v. dose for 50% receptor occupancy (RO(50)) of antimuscarinic agents in rat brain regions was propiverine > solifenacin > tolterodine, oxybutynin. There was a good linear relationship between in vivo (pRO(50) values in the rat hippocampus) and in vitro (pK(i) values in human M(1) receptors) receptor binding activities of propiverine, solifenacin, and tolterodine. The observed RO(50) value of oxybutynin was approximately five times smaller than the predicted in vitro K(i) value. The dose ratios of antimuscarinic agents for the brain receptor occupancy (RO(50)) to the inhibition of carbachol- and volume-induced increases in intravesical pressure (ID(50)), which reflects in vivo selectivity for the urinary bladder over the brain, were greater for solifenacin, tolterodine, and propiverine than oxybutynin. Darifenacin displayed only a slight decrease in specific [11C](+)3-MPB binding in the rat brain regions, and it was not dose-related. In conclusion, in vivo quantitative autoradiographic analysis of brain muscarinic receptor occupancy may provide fundamental basis for managing central nervous system (CNS) side effects in antimuscarinic therapy for OAB. It is suggested that in the treatment of OAB, CNS side effects can be avoided by antimuscarinic agents with high selectivity for the urinary bladder over the brain.

Prenatal exposure to nicotine with associated in utero hypoxia decreased fetal brain muscarinic mRNA in the rat

Prenatal exposure to nicotine can be associated with fetal abnormal development and brain damage. This study determined the effect of administration of nicotine with associated in utero hypoxia in maternal rats from early, middle, and late gestation on fetal blood hemoglobin, and expression of cholinergic receptor subtypes in the fetal brain. Our results demonstrated that maternal subcutaneous nicotine from the early gestation increased fetal hemoglobin and hematocrit, associated with reduction of PO 2. Although exposure to nicotine during late gestation had no effects on fetal brain weight, nicotine administration from the early gestation significantly decreased fetal brain muscarinic receptor (M1, M2, M3, and M4) mRNA expression, associated with restricted brain growth. Nicotine-altered muscarinic receptor subtype expression in the fetal forebrain and hindbrain showed regional differences. In addition, there were gestational differences for fetal brain muscarinic suppression by prenatal nicotine. Together, the results demonstrate that nicotine-induced in utero hypoxia is associated with poor development of muscarinic receptors in the fetal brain and restricted brain growth, and that either prolonged prenatal exposure to nicotine or critical “window” period for the brain development during pregnancy may play a role in prenatal nicotine-induced fetal muscarinic-receptor deficiency in the fetal brain.

Effects of Typical and Atypical Antipsychotic Drugs on Rat Brain Muscarinic Receptors

Quantitative in vitro autoradiography was used to examine changes in muscarinic M1/M4 and M2/M4 receptors (targeted with [3H]pirenzepine and [3H]AF-DX384 respectively), in rats treated with the typical (haloperidol) and atypical (clozapine and olanzapine) antipsychotic medications for a period of 36 days. Rats were sacrificed at either 2 h or 48 h after the last drug administration to examine immediate effects as well as the effects at 48 h after drug withdrawal. Haloperidol significantly increased [3H]pirenzepine binding in the dentate gyrus (37%) and in the CA1 region of the hippocampus (34%) in animals sacrificed 2 h after the last drug administration compared to controls. Similarly, clozapine significantly increased [3H]pirenzepine binding in dentate gyrus (29%) in rats sacrificed 2 h after the last drug administration compared to controls. Haloperidol decreased [3H]AF-DX384 binding in the basolateral nucleus of the amygdala (20%) in the rats sacrificed 48 h after the last drug administration compared to controls. These findings suggest that muscarinic receptors and limbic brain regions such as hippocampus and amygdala might represent common targets that mediate beneficial clinical effects of antipsychotic drugs.

Comparative Evaluation of Central Muscarinic Receptor Binding Activity by Oxybutynin, Tolterodine and Darifenacin Used to Treat Overactive Bladder

We characterized muscarinic receptor binding in the mouse cerebral cortex after oral administration of anticholinergic agents used to treat overactive bladder. Muscarinic receptors in the mouse cerebral cortex and bladder after oral administration of anticholinergic agents were measured using [(3)H]N-methylscopolamine. In vitro binding affinities of tolterodine and its metabolite 5-hydroxymethyl metabolite in the mouse cerebral cortex and bladder were considerably greater than those of oxybutynin and darifenacin. Also, muscarinic receptor binding affinity of oxybutynin and its metabolite N-desethyl-oxybutynin in the cerebral cortex compared with that in the bladder was 2 to 3 times higher, whereas that of tolterodine and 5-hydroxymethyl metabolite was approximately 2 times lower. Oral administration of oxybutynin (76.1 micromol/kg), tolterodine (6.31 micromol/kg) and darifenacin (59.1 micromol/kg) showed binding activity that was approximately equal to that of bladder muscarinic receptors. Oral administration of oxybutynin (76.1 micromol/kg) showed significant binding of cerebral cortical muscarinic receptors in mice, as indicated by about a 2-fold increase in K(d) values for specific [(3)H]N-methylscopolamine binding 0.5 and 2 hours later. On the other hand, tolterodine and darifenacin given at oral doses that would exert a similar extent of bladder receptor binding activity as oxybutynin showed only a low level of binding activity of central muscarinic receptors in mice. Significant binding of brain muscarinic receptors in mice was observed by the oral administration of oxybutynin but not tolterodine and darifenacin.

Estrogen Therapy and brain muscarinic receptor density in healthy females: A SPET study

Estrogen Therapy (ET) may protect against age-related cognitive decline and neuropsychiatric disorders (e.g. Alzheimer's disease). The biological basis for this putative neuroprotective effect is not fully understood, but may include modulation of cholinergic systems. Cholinergic dysfunction has been implicated in age-related memory impairment and Alzheimer's disease. However, to date no one has investigated the effect of long-term ET on brain cholinergic muscarinic receptor aging, and related this to cognitive function. We used Single Photon Emission Tomography (SPET) and ( R, R)[ 123I]-I-QNB, a novel ligand with high affinity for m 1/m 4 muscarinic receptors, to examine the effect of long-term ET and age on brain m 1/m 4 receptors in healthy females. We included 10 younger premenopausal subjects and 22 postmenopausal women; 11 long-term ET users (all treated following surgical menopause) and 11 ET never-users (surgical menopause, n = 2). Also, verbal memory and executive function was assessed in all postmenopausal subjects. Compared to young women, postmenopausal women (ET users and never-users combined) had significantly lower muscarinic receptor density in all brain regions examined. ET users also had higher muscarinic receptor density than ET never-users in all the brain regions, and this reached statistical significance in left striatum and hippocampus, lateral frontal cortex and thalamus. Moreover, in ET users, ( R, R)[ 123I]-I-QNB binding in left hippocampus and temporal cortex was significantly positively correlated with plasma estradiol levels. We also found evidence for improved executive function in ET users as compared to ET never-users. However, there was no significant relationship between receptor binding and cognitive function within any of the groups. In healthy postmenopausal women use of long-term ET is associated with reduced age-related differences in muscarinic receptor binding, and this may be related to serum estradiol levels.

Brain regional acetylcholinesterase activity and muscarinic acetylcholine receptors in rats after repeated administration of cholinesterase inhibitors and its withdrawal

Activity of acetylcholinesterase (AChE) and specific binding of [(3)H]quinuclidinyl benzilate (QNB), [(3)H]pirenzepine (PZP) and [(3)H]AF-DX 384 to muscarinic acetylcholine receptor (mAChR) preparations in the striatum, hippocampus and cortex of rats were determined 1, 6 and 11 days after the last treatment with an organophosphate DDVP, a carbamate propoxur or a muscarinic agonist oxotremorine as a reference for 7 and 14 days. AChE activity was markedly decreased in the three regions 1 day after the treatment with DDVP for 7 and 14 days with a gradual recovery 6 to 11 days, and much less decreased 1, 6 and 11 days after the treatment with propoxur for 7 days but not for 14 days in the hippocampus and cortex. The binding of [(3)H]-QNB, PZP and AF-DX 384 in the three regions was generally decreased by the treatment with DDVP for 7 and 14 days. Such down-regulations were generally restored 6 or 11 days after the treatment for 7 but not for 14 days. The down-regulation or up-regulation as measured by [(3)H]-QNB, PZP and AF-DX 384 was observed 1, 6 or 11 days after treatment with propoxur for 7 days and/or 14 days. Repeated treatment with oxotremorine produced similar effects except AChE activity to DDVP. These results suggest that repeated inhibition of AChE activity may usually cause down-regulation of mAChRs with some exception in the hippocampus when a reversible antiChE propoxur is injected.

In vivo imaging of muscarinic receptors in the aging female brain with ( R, R)[ 123I]-I-QNB and single photon emission tomography

The effect of age on brain muscarinic receptor density is unclear. Some in vivo neuroimaging studies have reported a large age-related reduction in muscarinic receptor density; however, others have reported increases or no change. The variability in these results most likely arises because of the heterogeneity of the populations studied, differences in quantification methods employed, and a paucity of subtype selective ligands. Thus, we used the m 1/m 4 selective probe ( R, R)[ 123I]-I-QNB to investigate age-related differences in brain muscarinic receptors in healthy females. We included 10 younger subjects (age range 26–37) and 22 older women (age range 57–82 years). The older women had significantly lower ( R, R)[ 123I]-I-QNB binding in widespread brain regions including cerebral cortex and hippocampus. Across all subjects, regional binding was significantly negatively correlated with age. Thus, in this population of healthy women, there was an age-related reduction in muscarinic receptor density. This may contribute to age-related differences in cognitive function and risk for Alzheimer's disease.

Association of anticholinergic load with impairment of complex attention and memory in schizophrenia.

The authors' goals were 1) to establish a clinically useful standard index of the relative anticholinergic potency of psychiatric medications; 2) to determine which cognitive functions are most affected by the administration of anticholinergic medications to patients with schizophrenia; and 3) to compare in vitro and clinically derived indexes of anticholinergic load in predicting these cognitive impairments. One hundred six clinically stable patients with schizophrenia were given a brief neuropsychological battery and evaluated on a standard symptom rating scale. The anticholinergic load associated with their psychiatric medications was estimated by using 1) a pharmacological index, calculated from a compilation of published studies reporting in vitro brain muscarinic receptor antagonism, and 2) a clinical index, based on clinician ratings of the anticholinergic side effects of medications. The authors analyzed the correlations of both indexes with the neuropsychological measures and with summary neuropsychological factor scores. The clinical and pharmacological anticholinergic indexes were highly correlated with each other and showed virtually identical associations with neuropsychological measures. Anticholinergic load was associated with lower scores on measures of attention and declarative memory, including several measures of auditory and visual memory and two tests of complex attention, but was unrelated to intelligence, simple attention, working memory, executive functions, conceptual fluency, or motor speed. This pattern of cognitive impairment with central cholinergic antagonism is consistent with emerging models of the functional anatomy of ascending forebrain cholinergic subsystems. Both pharmacological and clinical indexes show utility in predicting the effects of anticholinergic load on cognition in schizophrenia. Doses of psychiatric medication within the range of routine pharmacotherapy practice may have clinically significant effects on memory and complex attention in patients with schizophrenia; these effects may contribute as much as one-third to two-thirds of the memory deficit typically seen in patients with schizophrenia.