Heterogeneity of muscarinic autoreceptors and heteroreceptors in the rat brain: effects of a novel M1 agonist, AF102B

AC260584 (4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one), a selective muscarinic M1 receptor agonist, increases acetylcholine and dopamine release in rat medial prefrontal cortex and hippocampus

Both muscarinic and nicotinic receptors are implicated in cognition. We have previously suggested that stimulation of the muscarinic M1 receptor has a beneficial effect on cognition, based upon evidence that the muscarinic M1 receptor agonist of N-desmethylclozapine, the major metabolite of clozapine, may contribute to the ability of clozapine to improve some domains of cognition in schizophrenia. Present study examined the effectiveness of a new muscarinic M1 receptor agonist, 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC260584), to increase the release of acetylcholine and dopamine in the rat medial prefrontal cortex and hippocampus. Using microdialysis in awake, freely moving rats, AC260584, 3 and 10, but not 1 mg/kg (s.c.), significantly increased dopamine release in the medial prefrontal cortex and hippocampus. However, only the high dose of AC260584, 10 mg/kg (s.c.), significantly increased acetylcholine release in these regions. Moreover, the increases in acetylcholine release produced by AC260584, 10 mg/kg, were attenuated by the muscarinic M1 receptor antagonist telenzepine (3 mg/kg, s.c.) but not by the 5-HT1A receptor antagonist N-[2-(4-2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridyl) cyclohexanecarboxamide (WAY100635, 0.2 mg/kg, s.c.). However, the increase in dopamine release produced by 10 mg/kg AC260584 was blocked by both telenzepine and WAY100635. In addition, pretreatment with the atypical antipsychotic drug risperidone (0.1 mg/kg, s.c.) potentiated AC260584 (1.0 mg/kg, s.c.)-induced acetylcholine and dopamine release in the medial prefrontal cortex. These findings suggest that the muscarinic M1 receptor agonist property of AC260584 contributes to its enhancement of cortical acetylcholine and dopamine efflux. Therefore, AC260584, as well as other muscarinic M1 receptor agonists, may be a valuable target for the development of drugs which can improve the cognitive deficits in schizophrenia and perhaps other neuropsychiatric disorders, as well.

Role of postsynaptic serotonin1A receptors in risperidone-induced increase in acetylcholine release in rat prefrontal cortex

Most atypical antipsychotic drugs increase acetylcholine release in the prefrontal cortex, but the detailed mechanism is still unknown. The present study examined the role of serotonin (5-HT)1A receptors in risperidone-induced increases in acetylcholine release in rat prefrontal cortex. Systemic administration of risperidone at doses of 1 and 2 mg/kg increased acetylcholine release in the prefrontal cortex in a dose-dependent manner. This increase was antagonized by systemic administration of high doses (1 and 3 mg/kg) of N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635), a 5-HT1A receptor antagonist/dopamine D4 receptor agonist, but not by a low dose (0.1 mg/kg) of the antagonist which antagonizes preferentially presynaptic 5-HT1A autoreceptors. Furthermore, local application of WAY100635 into the prefrontal cortex also attenuated risperidone-induced increases in acetylcholine release. WAY100635 alone did not affect acetylcholine release in the prefrontal cortex. On the other hand, local application of risperidone (3 and 10 microM), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (1 and 10 microM), and the dopamine D4 receptor antagonist 3-(4-(4-iodophenyl)piperazine-1-yl)methyl-1H-pyrrolo[2,3-b]pyridine (1 and 10 microM) into the cortex did not affect acetylcholine release in the prefrontal cortex. These results suggest that risperidone increases acetylcholine release in the prefrontal cortex through a complex mechanism which is enhanced by prefrontal 5-HT1A receptor activation.

Muscarinic agonists for the treatment of Alzheimer’s disease: progress and perspectives

Much interest has focused on the development of selective muscarinic agonists for the treatment of Alzheimer's disease (AD). Cholinergic replacement therapy is thought to be beneficial in alleviating some of the cognitive dysfunctions in this disorder. The cholinergic neuronal tracts are involved in memory and learning processes, and the extent of the degeneration of the cortical projections correlates with the severity of the dementia. An M1 selective muscarinic agonist may be effective in treating at least some of the cognitive symptoms in AD. Highly selective M1 agonists, producing cellular excitation, should be beneficial in AD, regardless of the extent of degeneration of presynaptic cholinergic projections to the frontal cortex or hippocampus. Functional abnormalities in AD may also occur along various signal transduction pathways mediated, in part, at least, by muscarinic receptors. In general, activities associated with mAChR subtypes and m1 receptors, in particular, indicate that M1 agonists may also be useful for this aspect of AD. Mismetabolism of amyloid precursor proteins (APPs) may induce AD. Recent studies indicate that the formation of the b-amyloid peptide (Abeta) and amyloid plaques is linked to the loss of cholinergic function in AD. New data on the activation of m1 mAChRs in conjunction with recent findings that the induction of such receptors stimulates neurotrophic-like activities, decreases tau phosphorylation and inhibits apoptosis indicate that restoring the cholinergic tone in AD may be useful both in improving memory function and in altering the onset and progression of AD dementia. This article focuses on the recent, promising developments in this field and assesses the value of muscarinic agonists currently under development for the treatment of AD.

Cholinergic modulation of basal and amphetamine-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Behavioral evidence suggests that muscarinic/cholinergic inhibition of brain dopaminergic activity may be a useful principle for developing novel antipsychotic drugs (APDs). Thus, oxotremorine, a muscarinic agonist, attenuates amphetamine-induced locomotor activity in rodents, an effect also produced by a wide variety of proven APDs, whereas scopolamine, a muscarinic antagonist, has the opposite effect. Since atypical APDs such as clozapine, olanzapine, risperidone, ziprasidone and quetiapine, increase brain acetylcholine as well as dopamine (DA) release in a region-specific manner, their effects on cholinergic and dopaminergic neurotransmission may also contribute to various actions of these drugs. Oxotremorine (0.5-1.5 mg/kg) dose-dependently and preferentially increased DA release in rat medial prefrontal cortex (mPFC), compared to the nucleus accumbens (NAC). However, S-(-)-scopolamine (0.5-1.5 mg/kg) produced similar increases in DA release in the mPFC, but the effect was much less than that of oxotremorine. Whereas a dose of S-(-)-scopolamine of 0.5 mg/kg comparably increased DA release in the mPFC and NAC, 1.5 mg/kg had no effect on DA release in the NAC. Oxotremorine-M (0.5 mg/kg), a M(1/4)-preferring agonist, also increased DA release in the mPFC, but not the NAC, an effect completely abolished by telenzepine (3 mg/kg), a M(1/4)-preferring antagonist, which by itself had no effect on DA release in either region. Oxotremorine (0.5, but not 1.5, mg/kg) attenuated amphetamine (1 mg/kg)-induced DA release in the NAC, whereas S-(-)-scopolamine did not. Oxotremorine (1.5 mg/kg) and S-(-)-scopolamine (0.5 mg/kg) modestly but significantly potentiated amphetamine (1 mg/kg)-induced DA release in the mPFC. These results suggest that stimulation of muscarinic receptors, in particular M(1/4), as indicated by the effect of oxotremorine-M and telenzepine, may preferentially increase cortical DA release and inhibit amphetamine-induced DA release in the NAC.

Arisugacins, selective acetylcholinesterase inhibitors of microbial origin

Synthetic inhibitors of acetylcholinesterase (AChE) recently have attracted particular attention for treatment of Alzheimer's disease. By systematic screening of microbial metabolites, we were able to discover the new AChE inhibitors, named arisugacins A and B, from the culture broth of Penicillium sp. FO-4259. The structures of arisugacins are members of the meroterpenoid compounds. Arisugacin A is a potent and highly selective inhibitor of AChE but does not inhibit butyrylcholinesterase in vitro. Arisugacin A is a good candidate as an excellent potential drug for treatment of Alzheimer's disease. Also reviewed is the current status of development of antidementia drugs.

M1 acetylcholine receptor stimulation increases the extracellular concentrations of glutamate and GABA in the medial prefrontal cortex of the rat

The present study was undertaken to examine the effects of different muscarinic receptor agonists on glutamate and GABA concentrations in the medial prefrontal cortex of the rat. In vivo perfusions were made in the conscious rat using a concentric push-pull cannulae system. Amino acid concentrations in samples were determined by HPLC with fluorometric detection. The intracortical perfusion of arecoline, a M1-M2 muscarinic receptor agonist, produced a significant increase in extracellular [GLU] and [GABA]. McN-A-343, a M1 muscarinic receptor agonist, but not the M2 muscarinic receptor agonist, oxotremorine, produced a significant increase in extracellular [GLU] and [GABA]. The effects of McN-A-343 on extracellular [GLU] and [GABA] were blocked by pirenzepine, a M1 muscarinic receptor antagonist. These results suggest that M1 muscarinic receptor stimulation increases the extracellular concentrations of GLU and GABA in the medial prefrontal cortex of the rat.

Modulation of rhythmical slow activity, long-term potentiation and memory by muscarinic receptor agonists

We investigated the cholinergic modulation of hippocampal rhythmical slow activity (or theta activity), long-term potentiation and a behavioral memory task. The intravenous administration of the muscarinic receptor agonists, AF102B ((+/-)-cis-2-methyl-spiro(1,3-oxathiolane-5,3') quinuclidine hydrochloride hemihidrate) and oxotremorine, induced rhythmical slow activity at doses of 1.0 mg/kg and 0.01 mg/kg, respectively. Long-term potentiation of population spike amplitude in the hippocampal CA1, which was induced by tetanic stimulation to the Schaffer collateral/commissural fiber, was increased by AF102B (1.0 mg/kg i.v.) and oxotremorine (0.01 mg/kg i.v.). Oral administration of AF102B and oxotremorine improved scopolamine-induced memory deficits in a passive avoidance task in mice at doses of 1.0 mg/kg and 0.2 mg/kg, respectively. The correspondence of the effective doses of muscarinic receptor agonists in these three experiments suggested the cholinergic correlation of rhythmical slow activity, long-term potentiation and memory.

Effect of muscarinic receptor modulators in the hypothalamic supraoptic nucleus of the rat

Muscarinic antagonists were injected into the hypothalamic supraoptic nucleus (SON) and their effects on the acetylcholine (ACh) release of this nucleus were studied by in vivo microdialysis techniques. Atropine, AF-DX116 (a M2-receptor antagonist), 4-DAMP (a M3-receptor antagonist) and pirenzepine (a M1-receptor antagonist) concentration-dependently increased the ACh release. The EC50 values for these antagonists were 15 nM for atropine, 7.8 microM for pirenzepine, 0.39 microM for AF-DX116 and 59 nM for 4-DAMP, suggesting the autoregulation of the ACh release through an activation of M2 and M3 subtypes of muscarinic receptors in the SON. The postsynaptic effect of muscarinic receptors on urine outflow was studied by microinjection of selective muscarinic receptor agonists and antagonists into the SON. McN-A-343 (a M1-receptor agonist) had no significant effect on urine outflow. Pre-microinjection of atropine, 4-DAMP, p-F-HHSiD (a M3-receptor antagonist) or pirenzepine into the SON concentration-dependently attenuated the oxotremorine-induced antidiuresis. In contrast, AF-DX116 and methoctramine had no effect on this oxotremorine-induced action. These results suggest that the M3-subtype may contribute to the antidiuretic actions. Nicotine produced an increase in ACh release in the SON and also induced potent antidiuretic effects, both of which were inhibited by hexamethonium. Thus, in the SON, the ACh release may be autoregulated by M2- and M3-subtypes of muscarinic receptors and the antidiuretic effects of ACh produced through an activation of the M3-subtype.

Acetylcholine measurement of cerebrospinal fluid by in vivo microdialysis in freely moving rats

Acetylcholine (ACh) and choline (Ch) levels in rat cerebrospinal fluid (CSF) were determined by in vivo microdialysis (CSF microdialysis) in both halothane-anesthetized and freely-moving rats. The Ch/ACh ratio in CSF perfused with Ringer's solution (30 microliters/30 min) containing 10(-5) M physostigmine, a centrally active cholinesterase inhibitor, was significantly lower than that in unprocessed CSF due to significantly higher ACh levels in the former. The successive measurement on the 2nd and 7th day after the guide cannula implantation demonstrated the feasibility of the CSF microdialysis method for repetitive monitoring of CSF ACh and Ch levels in freely moving rats without extensive tissue damage. Intraperitoneal administration of physostigmine caused an increase in CSF ACh levels, whereas administration of neostigmine, which cannot penetrate into the blood brain barrier, did not. Furthermore, a centrally active acetylcholinergic M1-receptor agonist, AF102B, produced an increase in CSF ACh and Ch levels. Thus, the present study demonstrates that CSF microdialysis is a useful method for evaluating overall central cholinergic activity and investigating the pharmacological effects of various drugs that act via the central cholinergic system.

Muscarinic potentiation of excitatory amino acid-evoked dopamine release in mesencephalic cells: specificity for the NMDA response and role of intracellular messengers

Of the five cloned muscarinic receptor subtypes, dopamine (DA) neurons in the substantia nigra and ventral tegmental areas have been shown to be selectively enriched with the mRNA for the m5 subtype, suggesting that muscarinic modulation of DA neurons may have a distinct pharmacology. In the present study we have used dissociated cell cultures of fetal rat ventral mesencephalon to characterize muscarinic modulation of DA neurons. [3H]DA release stimulated by activation of N-methyl-D-aspartate (NMDA) receptors was potentiated by carbachol, a mixed muscarinic-nicotinic agonist, and by oxotremorine-M, a muscarinic agonist. Neither carbachol nor oxotremorine-M had an effect on [3H]DA release evoked by the non-NMDA agonists, kainate or quisqualate. A nicotinic agonist, DMPP, had no effect on NMDA-stimulated release. Potentiation of NMDA-stimulated [3H]DA release by oxotremorine-M was inhibited by the broad spectrum muscarinic antagonist, QNB, and by low concentrations of a putative M1 antagonist, pirenzepine, while much higher concentrations of a purported M2 antagonist, AF-DX 384, were required to reverse the oxotremorine-M effect. The muscarinic antagonist, 4-DAMP, was active in a concentration range between that required for pirenzepine and AF-DX 384. Further experiments examined intracellular messenger mechanisms coupled to the muscarinic receptors modulating NMDA-stimulated [3H]DA release. In contrast to oxotremorine-M, two muscarinic agents with only weak partial agonism with respect to phosphoinositide turnover, pilocarpine and arecoline, had no effect on NMDA-stimulated [3H]DA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of muscarinic receptor agonists and anticholinesterase drugs on high voltage spindles and slow waves

The effects of muscarinic agonists (AF102B, pilocarpine, oxotremorine) and anticholinesterases (physostigmine, tetrahydroaminoacridine) were investigated on the incidence of thalamically generated rhythmic high voltage spindles and on scopolamine (0.2 mg/kg)-induced neocortical slow wave activity (i.e. increased sum amplitude value of the 1-20 Hz band in a quantitative electroencephalography (qEEG) analysis). AF102B and pilocarpine decreased high voltage spindles and scopolamine increased sum amplitude values at 3 and 9 mg/kg, but not at 1 mg/kg. Oxotremorine was less potent than AF102B or pilocarpine in suppressing high voltage spindles. Oxotremorine had no effect on the scopolamine-induced qEEG changes. Tetrahydroaminoacridine decreased high voltage spindles at 1, 3 and 9 mg/kg and slow waves at 9 mg/kg. Physostigmine decreased high voltage spindles and slow waves at 0.12 and 0.36 mg/kg. Based on the present results we propose that agonists possessing muscarinic M1 receptor activity are effective in decreasing high voltage spindles and scopolamine-induced slow wave activity, but agonists showing predominant muscarinic M2 receptor activity may be less effective in decreasing high voltage spindles and slow waves. Furthermore, tetrahydroaminoacridine decreased high voltage spindles at doses lower than those required to decrease scopolamine-induced slow waves. Physostigmine decreased high voltage spindles and slow waves over the same dose range. This result may indicate that non-cholinergic mechanisms are involved in the tetrahydroaminoacridine-induced decrease in high voltage spindles.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Regulation of spontaneous acetylcholine release in the hypothalamic vasopressinergic supraoptic nucleus of a freely moving rat: a study by in vivo microdialysis

We employed a brain microdialysis method to examine the possible regulation of spontaneous acetylcholine (ACh) release in the hypothalamic vasopressinergic supraoptic nucleus (SON) of rats. We monitored the basal ACh release in the SON-microdialysate. The addition of tetrodotoxin (10(-6) M) to the perfusate (saline containing 10(-4) M physostigmine) decreased the basal ACh release. A muscarinic receptor antagonist, atropine (non-selective) or pirenzepine (M1-selective), increased the basal ACh release in a concentration-dependent manner. The maximal increase occurred at 20-40 min after the start of the infusion of antagonists. The ED50 values for the stimulatory effects of atropine and pirenzepine were 9.4 x 10(-8) and approx. 10(-4) M, respectively. The effect of atropine (10(-6) M) was inhibited by simultaneous addition of the muscarinic agonist oxotremorine (10(-5) M). The results showed a negative feedback regulation of the spontaneous ACh release through the activation of muscarinic receptors in the SON. The weak effect of pirenzepine in increasing the ACh release, compared with atropine, suggests that ACh release in the nucleus is mainly regulated by the non-M1-muscarinic receptor subtype.

Muscarinic autoreceptors of Torpedo electric organ are of the M1 subtype: evidence by radioligand binding using selective antagonists

The presynaptic muscarinic autoreceptor of Torpedo marmorata electric organ has been characterised by radioligand binding studies using the subtype-selective antagonists pirenzepine, (+)-telenzepine, methoctramine, and AF-DX 116. The presynaptic receptor had relatively high affinity for the M1 antagonists pirenzepine and (+)-telenzepine (Ki = 35 and 7 nM, respectively) and lower affinities for the M2 antagonists AF-DX 116 and methoctramine (Ki = 311 and 277 nM, respectively). Comparison of these binding data with those from an M2 receptor (rat heart membranes) assayed under identical conditions and with data in the recent literature suggests that the Torpedo muscarinic autoreceptor has a pharmacology most similar to the M1 pharmacological subtype of muscarinic acetylcholine receptor.

Role of muscarinic receptor subtypes in central antinociception

1. The ability to modify the pain threshold by the two M1-muscarinic agonists: McN-A-343 and AF-102B and by the specific M2-agonist arecaidine was examined in mice and rats by using three different noxious stimuli: chemical (writhing test), thermic (hot-plate test) and mechanical (paw pressure test). 2. In the mouse hot-plate test McN-A-343 (20-50 micrograms per mouse i.c.v.) and AF-102B (1-10 mg kg-1 i.p.) produced significant antinociception which was prevented by atropine (1 microgram per mouse i.c.v.) and by the two selective M1 antagonists: pirenzepine (0.01 micrograms per mouse i.c.v.) and dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by the specific M2-antagonist AFDX-116 (0.1 micrograms per mouse i.c.v.), naloxone (1 mg kg-1 i.p.) or by the acetylcholine (ACh) depletor hemicholinium-3 (HC-3) (1 micrograms per mouse i.c.v.). McN-A-343 and AF-102B were able to increase the pain threshold also in the mouse acetic acid writhing test and in rat paw pressure test. These antinociceptive effects were completely prevented by dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by AFDX-116 (0.1 microgram per mouse or rat i.c.v.). 3. In contrast with the M1-agonists, the M2-agonist arecaidine (0.1-2 micrograms per mouse or rat i.c.v.) did not induce antinociception in all three analgesic tests. However, arecaidine, at the same i.c.v. doses, was able to reduce the pain threshold in the hot-plate and paw pressure tests.4. The site of muscarinic control of the pain threshold is localized in the CNS since drugs which do not cross the blood-brain barrier such as McN-A-343, pirenzepine and arecaidine exerted their effects only if injected i.c.v.5. On the basis of the above findings and existing literature we suggest that the postsynaptic muscarinic receptors involved in antinociception belong to the M1 subtype. Nevertheless, presynaptic autoreceptors (M2 subtype) may play a role in pain regulation since they are involved in modulation of endogenous ACh release.

The potential for muscarinic receptor subtype-specific pharmacotherapy for Alzheimer's disease

In several neurodegenerative disorders, including Alzheimer's disease, a loss of the cholinergic projections of the basal forebrain to the cerebral cortex and hippocampus occurs. Studies of the anatomic and physiologic characteristics of these ascending cholinergic systems suggest that they are important in processing information and in memory function. Muscarinic receptors are situated at various critical control points in these pathways. Activation of postsynaptic muscarinic receptors often increases the excitability of neurons; thus, the signal-to-noise ratio for sensory processing is enhanced. In addition, muscarinic receptors negatively control cholinergic tone at presynaptic sites. Molecular biologic methods have disclosed the existence of five muscarinic receptors, which are coupled to different second messenger systems. The evidence reviewed suggests that at least four of the five muscarinic receptor genes are expressed as functional receptor proteins in the neocortex and hippocampal formation. On the basis of the current information about their pharmacologic properties and coupling mechanisms in nervous tissue, drugs that selectively affect subtypes of muscarinic receptors could enhance cortical cholinergic function and thereby ameliorate certain cognitive impairments in Alzheimer's disease.

Cholinergic strategies in the treatment of Alzheimer's disease

Since the identification of the cholinergic deficit, strategies aimed at enhancing cholinergic neurotransmission have dominated the field of pharmacology in Alzheimer's disease (AD). These strategies include increasing acetylcholine precursor availability, delaying synaptic degradation and stimulating muscarinic receptors. Although most clinical trials report mild symptomatic improvements in some patients, support for large-scale clinical use of cholinomimetics in AD is not yet available. This article presents the most representative clinical trials, discusses the limitations of the cholinergic strategies and suggests future directions in the treatment of AD.

In vivo probing of the brain cholinergic system in the aged rat. Effects of long-term treatment with acetyl-L-carnitine

The regulation of acetylcholine (ACh) release by the different subtypes of muscarinic (M) receptors in the hippocampus of freely-moving Fischer and Sprague-Dawley rats, was investigated. Atropine (10 mumol/kg i.p.) induced a pronounced increase of ACh release (+400% over basal values) in the hippocampus of young rats (3 months) while the effect was drastically reduced (+100% over basal values) in old rats (24 months). The preferential M2 antagonist AF-DX 116 (50 mumol/kg i.p.) showed similar effects in young and old rats being, furthermore, 10 times less potent than atropine. The preferential M1 antagonist pirenzepine (50 mumol/kg i.p.) was even less potent than AF-DX 116 in enhancing ACh release in young rats, while the effect was more pronounced in the old ones. Therefore, the effect of the preferential M3 antagonist 4-DAMP was studied. 4-DAMP 10(-6) M, dissolved in the Ringer solution perfusing the hippocampus, induced an enhancement of ACh release (+200% and +70% over basal values, in young and old rats, respectively) which was comparable to that obtained after atropine at the same concentration. AF-DX 116 and pirenzepine, on the other hand, were by far less potent. Six months' pretreatment with acetyl-l-carnitine (ALCAR) reduced the significant differences between young and old rats in the release response after M1 and M3 receptor antagonists. Taken all together, these findings indicate that the regulation of ACh release, at least in the hippocampus, is mainly through the M3 receptors subtype of muscarinic receptors and that this subtype is the most involved in the aging process. Moreover, the ability of ALCAR to preserve the receptor-mediated functional ACh release response with respect to old animals suggests that ALCAR could be utilized in the amelioration of receptor functionality in the aging brain.

Diagnostic and pharmacological approaches in Alzheimer's disease

Alzheimer's disease is a chronic progressive disease affecting higher intellectual functioning. The clinical diagnosis is made when the onset of illness is insidious, the course slowly progressive and all the treatable causes of dementia have been ruled out. The use of more stringent criteria has improved clinical diagnosis, but at best only 80% of patients are accurately diagnosed. Ultimately the diagnosis depends upon pathological confirmation. The neuritic plaques and neurofibrillary tangles described by Alzheimer, although not pathognomonic for the disease, continue to be the basis for pathological diagnosis. The aetiology and pathophysiology of Alzheimer's disease are presently unknown. Epidemiological studies have suggested a genetic basis for the disorder, and many biochemical studies have linked it to degeneration of central cholinergic neurons, and possibly to abnormalities of other neurotransmitter systems. A marker which would permit accurate diagnosis early in the course of disease would be of major importance to researchers and clinicians alike. No marker has been found to date, although recent research results are promising. Various pharmacological strategies have been employed in the treatment of Alzheimer's disease. More recently attempts have focused on enhancing central cholinergic transmission. Despite the well-founded rationale for these studies, results have been modest.

Distinct muscarinic receptor subtypes differentially modulate acetylcholine release from corticocerebral synaptosomes

The effect of McN-A-343 and oxotremorine on acetylcholine (ACh) release and choline (Ch) transport was studied in corticocerebral synaptosomes of the guinea pig. The synaptosomes were preloaded with [3H]Ch after treatment with the irreversible cholinesterase inhibitor, diisopropyl fluorophosphate, and then tested for their ability to release isotope-labeled ACh and Ch in the presence and absence of these agents. The kinetics of release were determined at the resting state (basal release) and in the presence of 50 mM K+. Under either condition, McN-A-343 enhanced the release of isotope-labeled ACh, whereas oxotremorine inhibited the K(+)-evoked release but had no effect on the basal release. The enhancing effect of McN-A-343 on basal ACh release was fully blocked by the selective M1 muscarinic antagonist, pirenzepine (100 nM). In contrast to its enhancing effect on ACh release, McN-A-343 potently inhibited Ch efflux as well as Ch influx. These effects were not blocked by atropine, a nonselective muscarinic antagonist. Oxotremorine had no effect on Ch transport. Binding studies showed that McN-A-343 was 3.6-fold more potent in displacing radiolabeled quinuclidinyl benzilate from cerebral cortex muscarinic receptors (mostly M1 subtype) than from cerebellar receptors (mostly M2 subtype), whereas oxotremorine was 2.6-fold more potent in the cerebellum. The displacements of radio-labeled pirenzepine and cis-dioxolane confirmed the M1 subtype preference of McN-A-343 and the M2 subtype preference of oxotremorine.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathoexcitatory action of a novel muscarinic receptor agonist, AF102B, and its blockade by pirenzepine

The pharmacological effects of a novel, selective muscarinic (M1) receptor agonist, AF102B (cis-2-methyl-spiro-(1,3-oxathiolane-5,3')-quinuclidine hydrochloride hemihydrate), on sympathetic nerve activity are described. Intravenous administration of AF102B (1 and 10 mg/kg) produced a dose-dependent increase in cardiac sympathetic nerve activity accompanied by tachycardia in spinal-intact rats. In addition, AF102B (10 mg/kg) caused a marked increase in cardiac sympathetic nerve activity and heart rate in pithed rats. Pirenzepine (50 micrograms/kg) inhibited these sympathoexcitatory effects of AF102B (10 mg/kg) in pithed rats. These findings suggest that AF102B possesses a sympathoexcitatory action which is mediated by M1-receptors.

Effects of a M1 muscarinic receptor agonist on the central cholinergic system, evaluated by brain microdialysis

The effects of a novel M1-receptor agonist, AF102B (FKS-508; cis-2-methylspiro(1,3-oxathiolane-5,3')quinuclidine), on the central cholinergic system in vivo were evaluated by determination of acetylcholine (ACh) content in the rat brain after microwave irradiation and by measurement of ACh release with microdialysis perfusion in freely moving rats. Intraperitoneal administration of AF102B resulted in a significant decrease of ACh content in the brain, while AF102B produced an increase of in vivo ACh release. The present results suggest that ACh content in the brain after treatment with muscarinic agents may be related to the changes of ACh release, in which both M1 and M2 muscarinic receptors may be involved.

An M3-like muscarinic autoreceptor regulates the in vivo release of acetylcholine in rat striatum

Selective muscarinic antagonists were used in an attempt to characterize the muscarinic autoreceptor modulating the release of acetylcholine in the striatum of the rat. In vivo microdialysis was applied to infuse atropine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine), pirenzepine or AF-DX 116 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro[2,3-b][1,4]benzodiazepine-6-one), leading to a dose-dependent increase in the overflow of acetylcholine, the order of potency being: atropine greater than 4-DAMP greater than pirenzepine greater than AF-DX 116. We conclude from these data that the muscarinic receptor modulating release in the striatum is of the M3 type.

Central muscarinic activities of an M1-selective agonist: preferential effect on reversal of amnesia

Effects of FKS-508 (AF102B; cis-2-methylspiro (1,3-oxathiolane-5,3')-quinuclidine), a novel M1-selective agonist, on central muscarinic responses in mice were examined in comparison with oxotremorine. FKS-508 was slightly less potent (6 times) in reversal of scopolamine-induced amnesia (passive avoidance failure), but far less potent (260 and 55 times) in producing hypothermia and tremor than oxotremorine. These results show that the selective M1 agonist FKS-508 differentiates highly between the central muscarinic effects.

Differential effects of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism in freely moving rats

A dialysis loop cannula was implanted into rat striatum under anesthetized condition, and the area was perfused with Ringer's solution under freely moving condition after 3 days for surgical recovery. Dopamine (DA) and 3,4-dihydroxyphenylacetic acid recovered in the dialysate were measured by high-performance liquid chromatography with electrochemical detection. The effects of M1- and M2-muscarinic receptor agents, which were perfused continuously into the striatum through the dialysis membrane, were investigated. Continuous perfusion of AF102B, an M1-selective agonist, and oxotremorine, a non-selective agonist, resulted in a dose-dependent increase in the striatal DA release. Pirenzepine (10(-5) and 10(-7) M), an M1-selective antagonist, decreased the release of DA, and the stimulatory effect of AF102B (10(-5) M) was completely inhibited by 10(-5) and 10(-7) M pirenzepine, while the stimulatory effect of oxotremorine (10(-4) M) was only partly inhibited by 10(-5) M pirenzepine. AF-DX116 (10(-5) M), an M2-selective antagonist, increased the DA release, and showed an additive effect on the DA release evoked by AF102B (10(-5) M), whereas it produced no significant effect on oxotremorine (10(-5) M)-evoked DA release. These results suggest that in vivo DA release in the rat striatum is modulated by different subtypes of muscarinic receptors; i.e., the stimulatory effect is mainly mediated by M1-sites and inhibitory effect is mainly mediated by M2-sites. The changes in the DA release induced by the various drugs were prevented by pretreatment with tetrodotoxin (TTX). Since action potential-dependent DA release (exocytosis) is blocked by the pretreatment with TTX, those drugs affect DA release by means of action potential-dependent processes.

(+-)-cis-2-methyl-spiro(1,3-oxathiolane-5,3') quinuclidine (AF102B): a new M1 agonist attenuates cognitive dysfunctions in AF64A-treated rats

(+-)-cis-2-Methyl-spiro(1,3-oxathiolane-5,3')quinuclidine (AF102B), a new muscarinic agonist of utmost rigidity, exhibits a high selectivity for M1 muscarinic receptors. In rats having a cholinergic hypofunction induced by the intracerebroventricular administration of ethylcholine aziridinium (AF64A), AF102B reversed cognitive impairments in a step-through passive avoidance task and in an 8-arm radial maze. AF102B reversed cognitive impairments at significantly lower doses than those needed to induce side-effects. In addition, AF102B exhibited low toxicity. The results suggest that AF102B may prove useful for treatments of cholinergic deficiencies and cognitive impairments, like those reported in Alzheimer's disease.

Amelioration of experimental amnesia (passive avoidance failure) in rodents by the selective M1 agonist AF102B

Effect of AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine) on experimental amnesia was examined using a passive avoidance task in rodents. The amnesia was produced by anti-cholinergic agents, AF64A (intracerebroventricularly) and scopolamine (subcutaneously). AF102B ameliorated the memory deficits in AF64A-treated rats at 0.1-1 mg/kg, i.p. and at 1-5 mg/kg p.o. and in scopolamine-treated mice at 1-10 mg/kg, i.p. These results suggest that AF102B may compensate for central cholinergic defects and could be developed as a possible therapeutic drug for senile dementia of the Alzheimer type.