Molecular mechanisms of regulation of neuronal muscarinic receptor sensitivity

Pharmacological evaluation of the long-term effects of xanomeline on the M(1) muscarinic acetylcholine receptor

Xanomeline is a unique agonist of muscarinic receptors that possesses functional selectivity at the M₁ and M₄ receptor subtypes. It also exhibits wash-resistant binding to and activation of the receptor. In the present work we investigated the consequences of this type of binding of xanomeline on the binding characteristics and function of the M₁ muscarinic receptor. Pretreatment of CHO cells that stably express the M₁ receptor for 1 hr with increasing concentrations of xanomeline followed by washing and waiting for an additional 23 hr in control culture media transformed xanomeline-induced inhibition of [³H]NMS binding from monophasic to biphasic. The high-affinity xanomeline binding site exhibited three orders of magnitude higher affinity than in the case of xanomeline added directly to the binding assay medium containing control cells. These effects were associated with a marked decrease in maximal radioligand binding and attenuation of agonist-induced increase in PI hydrolysis and were qualitatively similar to those caused by continuous incubation of cells with xanomeline for 24 hr. Attenuation of agonist-induced PI hydrolysis by persistently-bound xanomeline developed with a time course that parallels the return of receptor activation by prebound xanomeline towards basal levels. Additional data indicated that blockade of the receptor orthosteric site or the use of a non-functional receptor mutant reversed the long-term effects of xanomeline, but not its persistent binding at an allosteric site. Furthermore, the long-term effects of xanomeline on the receptor are mainly due to receptor down-regulation rather than internalization.

Mechanisms of M3 muscarinic receptor regulation by wash-resistant xanomeline binding

BACKGROUND/AIMS

Xanomeline has been shown to bind in a unique manner at M1 and M3 muscarinic receptors, with interactions at both the orthosteric site and an allosteric site. We have previously shown that brief exposure of Chinese hamster ovary cells that express the M3 receptor to xanomeline followed by removal of free agonist results in a delayed decrease in radioligand binding and receptor response to agonists. In the current study, we were interested in determining the mechanisms of this effect.

METHODS

Cells were treated with carbachol, pilocarpine or xanomeline for 1 h followed by washing and either used immediately or after waiting for 23 h. Control groups included cells that were not exposed to agonists and cells that were treated with agonists for 24 h. Radioligand binding and functional assays were conducted to determine the effects of agonist treatments.

RESULTS

The above treatment protocol with xanomeline resulted in similar effects of the binding of [(3)H]NMS and [(3)H]QNB. When receptor function is blocked using a variety of methods, the long-term effects of xanomeline binding were absent.

CONCLUSION

Our data indicate that xanomeline wash-resistant binding at the receptor allosteric site leads to receptor downregulation and that receptor activation is necessary for these effects.

Immediate and delayed consequences of xanomeline wash-resistant binding at the M3 muscarinic receptor

Xanomeline is thought to be a M1/M4 functionally selective agonist at muscarinic receptors. We have previously demonstrated that it binds in a unique manner at the M1 receptor. In the current study, we examined the ability of xanomeline to bind to the M3 receptor and determined the long-term consequences of this mode of binding in Chinese hamster ovary cells expressing M3 receptors. Xanomeline binds in a reversible and wash-resistant manner at the M3 receptor and elicits a functional response under both conditions. Long-term exposure to xanomeline resulted in changes in the binding profile of [(3)H]NMS and a decrease in cell-surface receptor density. Additionally, pretreatment with xanomeline was associated with antagonism of the functional response to subsequent stimulation by conventional agonists. Our results indicate that xanomeline binds to and activates the M3 muscarinic receptor in a wash-resistant manner, and that this type of binding results in time-dependent receptor regulation.

Current neuro-ophthalmic therapies

The clinical characteristics, differential diagnosis, and treatment options are presented for five different categories of neuro-ophthalmic disease. Nystagmus, optic neuritis, diplopia, pseudotumor cerebri, and temporal arteritis, are frequently encountered in neuro-ophthalmic practice. This article focuses on current therapies for these neuro-ophthalmic disorders. Potential differences in approach to pediatric versus adult patients are emphasized.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Regulation of m2 muscarinic receptor gene expression by platelet-derived growth factor: involvement of extracellular signal-regulated protein kinases in the down-regulation process

To study the role of mitogen-activated protein kinase in the regulation of M2 receptors, we studied the effect of platelet-derived growth factor (PDGF) on M2 receptor gene expression. PDGF (4 ng/ml) caused a time-dependent decrease in M2 receptor number and in m2 receptor mRNA levels in HEL 299 cells. The PDGF-induced loss in m2 mRNA required de novo protein synthesis and occurred through a decrease in the rate of transcription of the m2 receptor gene. The down-regulation of M2 receptors was not accompanied by an uncoupling of the remaining receptors, indicating a large receptor reserve in these cells. Preincubations with the phosphatidylinositol 3-kinase inhibitor wortmannin, the protein kinase C inhibitor GF 109203X and the cAMP-dependent protein kinase inhibitor H-8 did not attenuate PDGF-induced down-regulation, indicating a lack of involvement of these enzymes in the down-regulation process. Activation of the extracellular signal-regulated protein kinase (ERK) 1 and 2 proteins was measured by an "in gel" phosphorylation assay. Carbachol did not activate ERK1 or 2, whereas PDGF and 4 beta-phorbol 13,14-dibutyrate resulted in a large increase in ERK1 and 2 activity along with a decrease in m2 mRNA. Preincubation with PD 098059, an inhibitor of mitogen-activated protein kinase kinase, inhibited PDGF- and 4 beta-phorbol 13,14-dibutyrate-mediated activation of ERK 1 and 2 in a concentration-dependent manner. The inhibitory action of PD 098059 was reflected at the mRNA level attenuating both PDGF- and 4 beta-phorbol 13,14-dibutyrate-mediated decreases in m2 mRNA. These results suggest a role of ERK1 and 2 in the regulation of muscarinic m2 receptor gene expression.

Long-term carbachol treatment-induced down-regulation of muscarinic M2-receptors but not m2 receptor mRNA in a human lung cell line

1. The molecular mechanisms involved in the regulation of muscarinic receptor gene expression are poorly understood. We have investigated the effect of homologous stimulation on the regulation of M2 muscarinic receptor protein and gene in human embryonic lung fibroblasts (HEL 299 cells). 2. Saturation studies performed with the non-selective hydrophilic ([3H]-N-methyl-scopolamine, [3H]-NMS) and lipophilic (3H]-quinuclidinyl benzilate, [3H]-QNB) muscarinic antagonists revealed a single class of high affinity binding sites. 3. Carbachol (1 mM) induced a rapid down-regulation of [3H]-NMS binding sites. Within 12 h, the process had approached steady state with 40 to 60% loss of receptors at 12 and 24 h. 4. The loss of [3h]-QNB binding sites (40% reduction at 24 h) occurred at a slower rate than did loss of [3H]-NMS binding sites as a result of receptor sequestration. 5. Carbachol treatment was accompanied by a functional desensitization of the receptor after 24 h of agonist treatment. In untreated cells, forskolin induced a large increase in cyclic AMP accumulation which was inhibited significantly by carbachol. The inhibitory effect of carbachol on forskolin-induced cyclic AMP accumulation was lost following 24 h carbachol stimulation. 6. The steady state level of muscarinic m2 mRNA measured by Northern blot analysis was not affected by carbachol had no effect on the stability of m2 mRNA. 7. The rate of transcription of m2 muscarinic receptor gene as measured by nuclear RNA run-on assay was unaltered by carbachol stimulation. 8. These results suggest that homologous sequestration, desensitization, and down-regulation of M2 modifications of m2 muscarinic receptor mRNAs.

Muscarinic M2 receptor synthesis: study of receptor turnover with propylbenzilylcholine mustard

We have investigated the rate and the functional responsiveness of the newly synthesised M2 muscarinic receptors in HEL 299 cells following propylbenzilylcholine mustard treatment at 37 degrees C. Propylbenzilylcholine mustard induced a dose-dependent loss of the hydrophilic ligand [3H]N-methylscopolamine binding sites with 80% inactivation at 0.1 microM. The rate of muscarinic receptor synthesis in these cells, estimated from wash-out experiments following propylbenzilylcholine mustard treatment, was very slow and returned to control values after 36 h of propylbenzilylcholine mustard removal. The recovery of muscarinic receptors was blocked by the cycloheximide pre-treatment, indicating the synthetic pathway for the new receptors. In control cells as well as in cells treated with propylbenzilylcholine mustard and allowed to recover for 12 h, carbachol still inhibited forskolin-induced cAMP accumulation. These results show that (i) the rate of M2 muscarinic receptor synthesis is slow (ii) the recovery of receptors is mainly through increased synthesis and (iii) the newly synthesised receptors retain their full functional activity.

Selective inactivation of muscarinic receptor subtypes

Muscarinic receptors exist in multiple subtypes, denoted as M1, M2, M3 and M4, encoded by four distinct but related genes. A fifth gene product, m5, has also been predicted although this sequence awaits a pharmacological equivalent. Many tissues express more than one muscarinic receptor subtype, which may couple to different intracellular effectors and thus have different physiological roles. One way to characterize the role of each receptor is to selectively inactivate one receptor population, thus pharmacologically 'isolating' the muscarinic receptor subtype of interest. Selective receptor inactivation can be achieved using either a selective, irreversible antagonist, or protection using a selective, reversible antagonist against a non-selective irreversible antagonist. Therefore, combination of these two approaches may provide optimal selective inactivation. Several muscarinic alkylating agents have been identified, including phenoxybenzamine, EEDQ (N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) and propylbenzilylcholine mustard. These irreversible antagonists do not, in general, discriminate between muscarinic receptor subtypes and are frequently used to estimate the affinity and relative efficacy of muscarinic agonists. Consequently, use of these irreversible antagonists provides estimations of the 'receptor reserve' associated with a response mediated by muscarinic receptor activation. In contrast, 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)piperidine) selectively inactivates M3 receptors, but will not discriminate between M1, M2 or M4 receptors. In the absence of highly selective alkylating agents, receptor protection by reversible antagonists may be used. Thus, reversible antagonists, such as pirenzepine, methoctramine or para-fluorohexahydrosiladifenidol, at appropriate fractional receptor occupancies, may protect M1, M2 or M3 receptors against alkylation by phenoxybenzamine. Selective alkylation of M3 receptors by 4-DAMP mustard is enhanced with concurrent M2 protection. This approach has been applied to defining the role of these muscarinic receptor subtypes in the control of ileal smooth muscle tone. These data suggest that, in ileum, M2 receptors may act to inhibit beta-adrenoceptor activation, thereby offsetting relaxation, while M3 receptors directly mediate contraction.

Down-regulation and desensitization of the muscarinic M1 and M2 receptors in transfected fibroblast B82 cells

Murine fibroblast cell lines stably transfected with the rat muscarinic m1 or m2 receptor genes were used to study the homologous regulation of the muscarinic M1 or M2 receptors. The cells were pretreated with the muscarinic receptor agonists, (+)-cismethyl-dioxolane, carbachol, 2,8-dimethyl-3-methylene-1-oxa-8-aza-spiro- [4,5]decane ((+) or (-)-YM796) or the muscarinic receptor antagonist atropine for up to 24 h. Our study has demonstrated that the muscarinic receptor nonselective full agonist, (+)-cismethyl-dioxolane, induced the down-regulation of both the muscarinic M1 and the M2 receptors in association with desensitization of the receptor-mediated functions. The muscarinic M1 receptors are down-regulated without significant receptor internalization while the muscarinic M2 receptors are more sensitive to down-regulation than the muscarinic M1 receptors because of significant internalization of the muscarinic M2 receptors in our system. The muscarinic M1 receptor partial agonist, (-)-YM796 induced less down-regulation and no significant desensitization of the muscarinic M1 receptors with no substantial effect on the muscarinic M2 receptor density or function.

Differential regulation of muscarinic receptor subtypes in rat brain regions by repeated injections of parathion

Repeated injections with increasing moderate doses of parathion into adult male rats for 21 days resulted in 84-90% inhibition of acetylcholinesterase in the brain without overt signs of toxicity. Muscarinic acetylcholine receptor (mAChR) affinities for ligands were unaffected, but there was significant down-regulation of the m4 receptor subtype gene product, m1 mRNA and m3 mRNA in the frontal cortex as well as the m4 subtype and m4 mRNA in the striatum. However, in the hippocampus, there were no significant reductions in either the m1 receptor subtype nor its mRNA. The data suggest that the receptor subtype down-regulations in the cortex and striatum are due to reductions in mRNA expression. Since the degrees of inhibition of acetylcholinesterase were similar in the 3 brain regions, it is suggested that the in situ concentrations of paraoxon were also similar. Accordingly, the absence of down-regulation of the m1 receptor in the hippocampus is not due to a lower concentration of paraoxon than in the cortex or striatum. It is possible that injections of higher parathion doses would produce down-regulation of mAChRs in the hippocampus, and that the hippocampus may have differences in the feed-back mechanisms for receptor regulation from those in the frontal cortex and the striatum.

Muscarinic cholinergic receptor binding in rat brain following traumatic brain injury

Recent evidence suggests that excessive activation of muscarinic cholinergic receptors (mAChRs) contributes significantly to the pathophysiological consequences of traumatic brain injury (TBI). To examine possible alterations in mAChRs after TBI, the affinity (Kd) and maximum number of binding sites (Bmax) of mAChRs in hippocampus, neocortex, brain stem and cerebellum were determined by [3H]QNB binding. Three groups of rats were examined: 1 h post-TBI (n = 21), 24 h post-TBI (n = 21) and sham-injured rats (n = 21). Kd values were significantly higher in hippocampus and brain stem at 1 but not 24 h post-TBI compared with sham-injured controls (P < 0.05). Kd values did not significantly differ in neocortex and cerebellum at 1 or 24 h post-TBI compared with sham-injured controls. Bmax values did not significantly differ in any brain areas at 1 or 24 h post-TBI compared with sham-injured controls. These results show that TBI significantly decreases the affinity of mAChRs in hippocampus and brain stem at an early stage post-TBI, which may contribute to desensitization of mAChRs after TBI. The findings of no change in Bmax values are consistent with a transient elevation in ACh concentrations after TBI.

Muscarinic antagonists in the treatment of acquired pendular and downbeat nystagmus: a double-blind, randomized trial of three intravenous drugs

We performed a double-blind, randomized trial of intravenous scopolamine, benztropine, and glycopyrrolate in 7 patients with acquired nystagmus and oscillopsia. Five patients had pendular nystagmus and 2, downbeat nystagmus. We recorded eye movements with a magnetic search coil technique and tested visual acuity and motion perception before and after administration of each drug. Scopolamine reduced nystagmus in all patients. Benztropine was moderately effective and glycopyrrolate had a negligible impact. Visual acuity improved only with scopolamine; motion discrimination and oscillopsia improved significantly with scopolamine and benztropine. Pendular and downbeat nystagmus respond to intravenous antagonists of central muscarinic receptors.

Muscarinic receptor characteristics and regulation in rat cerebral cortex: changes during development, aging and the oestrous cycle

The effects of postnatal development, aging and the oestrous cycle on muscarinic acetylcholine receptor (mAChR) properties were examined in in vitro living slices of rat neocortex. Using the hydrophilic antagonist ([3H]NMS) to label cell surface mAChRs, an increase in both Bmax and Kd was found during the first postnatal weeks. These values peaked at between 20-40 days postnatally and then declined to adult levels. After 3 months of age, a steady decline in receptor number started: it was 10.1% lower at 10 months and 38.7% lower at 17 months of age. In contrast, Kd values increased, being 31.7 and 20% higher respectively at these ages. Carbachol-induced (4 h at 37 degrees C) down-regulation of receptor number was approximately 22.2% in newborn and 26.1% in adult (3-month-old) rats, but only 16.3% at 20-40 days of age. The degree of carbachol-induced down-regulation of mAChR was not affected in the older animals. Veratridine, which increases neural activity, also induced a significant reduction in [3H]NMS binding sites of 11.4% in rats aged 0-20 days and 22.4% in 3-month-old rats, but at 20-40 and 40-60 days of age no significant down-regulation of receptor number was observed. Furthermore, down-regulation was absent in the 10-month-old rats as well. Since a great variation in Bmax and Kd values was seen in 3-month-old females but not in male rats, we investigated mAChR characteristics during the oestrous cycle of female rats. In pro-oestrus, mACh receptor number was increased and affinity decreased in comparison with di-oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Down-regulation of muscarinic receptors and the m3 subtype in white-footed mice by dietary exposure to parathion

The effect of ad libitum dietary exposure (as occurs in the field) to parathion for 14 d was investigated on the muscarinic acetylcholine receptor (mAChR) in brains and submaxillary glands of adults of a field species, the white-footed mouse Peromyscus leucopus. Immunoprecipitation using subtype selective antibodies revealed that the relative ratios of the m1-m5 mAChR subtypes in Peromyscus brain were similar to those in rat brain. There was little variability in acetylcholinesterase (AChE) activity in control mice brains but large variability in 39 exposed mice, resulting from differences in food ingestion and parathion metabolism. Accordingly, data on radioligand binding to mAChRs in each mouse brain were correlated with brain AChE activity in the same mouse, and AChE inhibition served as a biomarker of exposure reflecting in situ paraoxon concentrations. Exposure to parathion for 14 d reduced maximal binding (Bmax) of [3H]quinuclidinyl benzilate ([3H]QNB), [3H]-N-methylscopolamine ([3H]NMS), and [3H]-4-diphenylacetoxy-N-methylpiperidine methiodide ([3H]-4-DAMP) by up to approximately 58% without affecting receptor affinities for these ligands. Maximal reduction in Bmax of [3H]QNB and [3H]-4-DAMP binding occurred in mice with highest AChE inhibition, while equivalent maximal reduction in Bmax of [3H]NMS occurred in mice with only approximately 10% AChE inhibition, without further change at higher parathion doses. This is believed to be due to the hydrophilicity of [3H]NMS, which limits its accessibility to internalized desensitized receptors. In submaxillary glands (mAChRs are predominantly m3 subtype), there were significant dose-dependent reductions in [3H]QNB binding and m3 mRNA levels in exposed mice, revealed by Northern blot analyses. The reduction in m3 receptors is suggested to result mostly from reduced synthesis at the transcription level, rather than from translational or posttranslational events. The data suggest that down-regulation of mAChRs occurs after dietary exposure for 14 d to sublethal concentrations of parathion in a field rodent species, and that significant though incomplete recovery in AChE and mAChRs occurs in 7 d following termination of exposure.

Developmentally regulated secreted factors control expression of muscarinic receptor subtypes in embryonic chick retina

Two molecular mass subtypes of muscarinic receptor are expressed by the chick retina (72 and 86 kDa). During development, the ratio of subtypes changes, with the 72-kDa form becoming predominant. We have found that subtypes switch can occur in retina cell culture, and have investigated factors that influence this in vitro increase in the 72-kDa receptor. Increases similar to those in vivo occurred when cells were cultured at 10(5) cells/cm2, but not at 10-fold lower density. High-density cultures, maintained on coverslips, showed no receptor development when transferred to large volumes of fresh medium, indicating that cell-cell contact alone was not responsible for induction. However, replacement of fresh medium with conditioned medium (from high-density cultures) resulted in normal induction. There were no morphological differences between cultures with high and low levels of the 72-kDa receptor. Conditioned medium also induced 72-kDa receptors in low-density cultures, consistent with a minimal role for cell-cell contact. Efficacy of conditioned medium was markedly dependent on age. Media from cells cultured 1-4 days had no effect, but media from cells cultured 5-8 and 1-8 days elicited 1.6-fold and fourfold increases in the 72-kDa subtype, respectively. The data indicate that maturing retina cells secrete developmentally regulated factors that are necessary for abundant expression of the 72-kDa muscarinic receptor subtype.

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.

Muscarinic receptor regulation and 2nd messenger responses in rat neocortex cultures

Primary cultures of dissociated cerebral cortex cells were used to characterize the muscarinic acetylcholinergic receptors (mAChR) present and to study receptor down-regulation and receptor mediated 2nd messenger responses induced by muscarinic agonists. Binding of the hydrophilic antagonist [3H]N-methyl scopolamine ([3H]NMS) to the cultured cells was saturated after one hour at 4 degrees C with a Kd of 93 pM and a Bmax of 958 fmol/mg protein. Competition binding studies with several antagonists and agonists indicated that the mAChR present in the culture were of a mixed M1/M3 subtype. The number of muscarinic receptors at the cell surface decreased by 60% after one hour pre-incubation of the cultures with 10 microM carbachol or oxotremorine. After down-regulation with carbachol affinity for pirenzepine was decreased, while low affinity sites for 4-DAMP were lost, indicating that especially M1 subtypes are sensitive to this type of regulation. Carbachol and oxotremorine-M induced a 2-3 fold increase in phosphatidyl inositide (PI) turnover, which was blocked with high affinity by both pirenzepine and 4-DAMP. Down-regulation of the mAChR and stimulation of PI-turnover by agonists with different potency and intrinsic activity appeared highly correlated. These data suggest that activation of the PI second-messenger system is involved in the desensitization and down-regulation of the muscarinic acetylcholine receptor.

DFP-induced regulation of cardiac muscarinic receptor mRNA in vivo measured by DNA-excess solution hybridization

Relationship between in vivo down-regulation of cardiac muscarinic receptors and changes in their encoding mRNA was investigated. Rats were treated either once or for ten days with an irreversible inhibitor of acetylcholinesterase, followed by measurements of cardiac acetylcholinesterase, the density and affinity of muscarinic receptors, and the concentration of mRNA coding for these receptors. mRNA was quantitated using the sensitive method of DNA-excess solution hybridization. Our data indicate that while short-term treatment resulted in a marked decrease in the density of cardiac muscarinic receptors by 34%, there was no accompanying significant change in the concentration of their mRNA. In contrast, long-term inhibition of acetylcholinesterase significantly decreased the concentration of both receptors and mRNA by 40% and 29%, respectively. These results are indicative of multiple mechanisms of down-regulation of cardiac muscarinic receptors, some of which might involve alterations at the transcriptional level.

Lack of desensitization of muscarinic receptor-mediated second messenger signals in rat brain upon acute and chronic inhibition of acetylcholinesterase

We studied the effects of acute and chronic in vivo inhibition of acetylcholinesterase on both the density and function of brain muscarinic cholinergic receptors. Adult male rats were treated either once or multiple times over a period of 10 days with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). The concentration and affinity of muscarinic receptors in various brain regions were determined using radioligand binding techniques. Acute DFP treatment resulted in a significant reduction in receptor number only in the brain stem, while chronic treatment caused receptor down-regulation in the brain stem, cerebral cortex, and striatum. There was no change in ligand affinity in any of the brain regions. In sharp contrast, muscarinic receptor function was fully preserved, in terms of coupling of the receptors to increased phosphoinositide hydrolysis in the cerebral cortex, hippocampus, and striatum, or inhibition of cyclic AMP formation in the cerebral cortex or striatum. Therefore, there is a marked lack or correlation between DFP-induced muscarinic receptor down-regulation and receptor desensitization.