Biomedical subjects
Publications and source records attributed to M E Eldefrawi.
The total synthesis of pyrrolizidines 223H', 239K', 265H', and 267H' has been achieved starting from 1,5-hexadiene via a common synthetic intermediate 5. The affinity of 1-4 for nicotinic acetylcholine receptor was evaluated.
An automated flow fluorometer designed for kinetic binding analysis was adapted to develop a solid-phase competitive fluoroimmunoassay for urinalysis of opiates. The solid phase consisted of polymer beads coated with commercial monoclonal antibodies (MAbs) raised against morphine. Fluorescein-conjugated morphine (FL-MOR) was used as the fluorescein-labeled hapten. The dissociation equilibrium constant (K(D)) for the binding of FL-MOR to the anti-MOR MAb was 0.23 nM. The binding of FL-MOR to the anti-MOR MAb reached steady state within minutes and was displaced effectively by morphine and other opiates. Morphine-3-glucuronide (M3G), the major urinary metabolite of heroin and morphine, competed effectively with FL-MOR in a concentration-dependent manner for binding to the antimorphine MAb and was therefore used to construct the calibration curve. The sensitivity of the assay was 0.2 ng/mL for M3G. The assay was effective at concentrations of M3G from 0.2 to 50 ng/mL, with an IC50 of 2 ng/mL. Other opiates and heroin metabolites that showed >50% crossreactivity when present at 1 microg/mL included codeine, morphine-6-glucuronide, and oxycodone. Methadone showed very low crossreactivity (<5%), which is a benefit for testing in patients being treated for opiate addictions. The high sensitivity of the assay and the relatively high cutoff value for positive opiate tests allows very small sample volumes (e.g., in saliva or sweat) to be analyzed. A double-blind comparison using 205 clinical urine samples showed good agreement between this single-step competitive assay and a commercially performed enzyme multiplied immunoassay technique for the detection of opiates and benzoylecgonine (a metabolite of cocaine).
A new solid phase fluoroimmunoassay using a fully automated flow fluorometer adapted for urinalysis of drug metabolites is described. Fluorescein-conjugated benzoylecgonine (FL-BE) and monoclonal antibodies (mAb) against benzoylecgonine (BE) were the reagents used for demonstration. The solid phase consisted of anti-BE mAbs immobilized on the surface of polymethyl methacrylate (PMMA) beads. Free BE in solution competed with FL-BE and reduced bead-bound fluorescence in a concentration-dependent manner. The binding of FL-BE to the anti-BE mAb reached steady-state within minutes. FL-BE was not bound by uncoated beads nor beads coated with non-specific proteins or IgG. The signal-to-noise ratio was 33, and the sensitivity of the assay was 2 ng ml(-1) for BE. The effective concentration of BE was 1 to 100 ng ml(-1), with an IC50 value of 12 ng ml(-1). The mAb showed equal affinities for BE, cocaine, and cocaethylene, but a five order-of-magnitude lower affinity for ecgonine and ecgonine methylester. In a double-blind comparison using clinical urine samples, the data from this single-step competitive assay had excellent agreement with results obtained using a fiber-optic biosensor (FOB), and the EMIT assay performed commercially. The assay provided kinetic data rapidly and can be used to detect small analytes for which antibodies and fluorescein conjugates are available. The affinity of the mAb for FL-BE, calculated from kinetic analysis of the time course of the on and off reaction, was 2.25 x 10(-9) M.
Organophosphate (OP) anticholinesterases were found to modulate metabolic activities of human neuroblastoma cells and hepatocytes, which was detectable by the Cytosensor microphysiometer. The nerve gas ethyl-S-2-diisopropylaminoethyl methylphosphorothiolate (VX), at 10 microM, produced significant reduction in cell metabolism within 2 min, as measured by changes in the acidification rate of the medium. The reduction was dose- and time-dependent and irreversible after 4 h of exposure. Two alkaline degradation products of VX produced no cytotoxicity. Exposure for 24 h to 3 microM VX caused 36% and 94% irreversible loss of metabolism in hepatocytes and neuroblastoma cells, respectively. The insecticides parathion and chlorpyrifos stimulated hepatocyte metabolism but inhibited neuroblastoma cells. Their oxons were more active. Exposure of neuroblastoma cells for 4 h to VX, parathion, paraoxon, diisopropylfluorophosphate or chlorpyrifos gave an LC50 of 65, 775, 640, 340, or 672 microM, respectively, whereas 24 h gave an LC50 of 0.7, 3.7, 2.5, 29, and 31 microM, respectively. Preincubation of hepatocytes with phenobarbital enhanced their response to parathion and VX due to metabolic bioactivation. Atropine partially blocked the effects of VX and paraoxon on both cell types, which suggests the involvement of a muscarinic receptor as the target for cytotoxicity. There was no correlation between OP in vivo neurotoxicity and in vitro cytotoxicity. It is suggested that the former results from their cholinesterase inhibition, while the latter results from action on different targets and requires much higher concentrations.
The nicotinic acetylcholine receptor (AChR) controls signal transmission between cells in the nervous system. Abused drugs such as cocaine inhibit this receptor. Transient kinetic investigations indicate that inhibitors decrease the channel-opening equilibrium constant [Hess, G. P. & Grewer, C. (1998) Methods Enzymol. 291, 443-473]. Can compounds be found that compete with inhibitors for their binding site but do not change the channel-opening equilibrium? The systematic evolution of RNA ligands by exponential enrichment methodology and the AChR in Torpedo californica electroplax membranes were used to find RNAs that can displace inhibitors from the receptor. The selection of RNA ligands was carried out in two consecutive steps: (i) a gel-shift selection of high-affinity ligands bound to the AChR in the electroplax membrane, and (ii) subsequent use of nitrocellulose filters to which both the membrane-bound receptor and RNAs bind strongly, but from which the desired RNA can be displaced from the receptor by a high-affinity AChR inhibitor, phencyclidine. After nine selection rounds, two classes of RNA molecules that bind to the AChR with nanomolar affinities were isolated and sequenced. Both classes of RNA molecules are displaced by phencyclidine and cocaine from their binding site on the AChR. Class I molecules are potent inhibitors of AChR activity in BC3H1 muscle cells, as determined by using the whole-cell current-recording technique. Class II molecules, although competing with AChR inhibitors, do not affect receptor activity in this assay; such compounds or derivatives may be useful for alleviating the toxicity experienced by millions of addicts.
Ten 3 beta-ecgonine analogues were synthesized and characterized by 1H and 13C NMR, MS, and elemental analysis. The compounds were synthesized as (-)-stereoisomers from (-)-cocaine. These compounds were assessed for their ability to inhibit [3H]cocaine binding to rat striatal tissue and to inhibit [3H]DA uptake into rat striatal synaptosomes. In this series of compounds, the length of the spacer between the aryl group and the tropane skeleton ranged from 1 to 4 bond distances, and conformational flexibility of the linkage and orientation of the aryl ring system were controlled by various types of linkages. The most potent of the analogues was methyl-(1R-2-exo-3-exo)-8-methyl-3-(beta-styrenyl)-8-azabicyclo[3. 2.1] octane-2-carboxylate. One of the less potent compounds was found to inhibit [3H]cocaine binding and [3H]DA uptake with significantly different IC50 values, in contrast to 14 other 3 beta-substituted analogues. Molecular modeling and CoMFA analysis were used to obtain a rigorous structure-function relationship for the studied compounds. The results showed that the potencies of these 3 beta-substituted ecgonine methyl esters were dominated by steric effects and were acutely sensitive to the distance between the aryl ring and the tropane skeleton and to the orientation of the aryl ring system relative to the tropane skeleton. The current study provides a clearer picture of the shape and size of the putative hydrophobic binding pocket for the 3 beta substituent at the cocaine receptor as well as emphasizing the importance of a drug's free energy of solvation in obtaining structure-activity relationships.
The sea nettle jellyfish toxin (SNTX), which contains several polypeptides, was highly toxic to human hepatocytes. The Cytosensor microphysiometer was used continuously to monitor cell media acidification rate as an index of cellular metabolic activity. Cells exposed to > 1 microg SNTX protein/ml media exhibited a transient increase in metabolic activity, followed by a sharp decrease and cell death within minutes. The kinetics of the transient increase and subsequent decline increased with higher concentrations of SNTX. The biphasic and time-dependent response of hepatocytes to SNTX suggests that more than one mechanism may be involved in the toxicity of its different polypeptides. SNTX-induced cytotoxicity of hepatocytes was reduced by the presence of high titer antibodies against a heterologous jellyfish. Phenobarbital-induced cells became more vulnerable to SNTX, suggesting that some toxin component(s) require(s) bioactivation. Short-term exposure (1-2 h) to 10 microg/ml of the calcium ionophore calcimycin, or the non-selective monovalent cation ionophore gramicidin, had no effect on metabolic activity. However, 165 microg/ml gramicidin or 53 microg/ml calcimycin produced slight transient activation followed by steady decline in metabolic activity, while 20 h exposure to either ionophore produced total cell death. Exposure to even a 10-fold lower concentration of either ionophore killed 88% and 75%, respectively. This contrasts with the toxicity of SNTX which is detectable in minutes with as little as 3 microg/ml. Since pre-exposure to the organophosphate anticholinesterases VX and paraoxon, or the chemotherapeutic alkylating agents cyclophosphamide and mechlorethamine reduced the cytotoxic effects of SNTX, it suggests that phosphorylation or alkylation of cell protein(s) interferes with SNTX toxicity.
The nicotinic acetylcholine receptor (nAChR) of the electric organ of the electric ray. Torpedo sp., the richest source of nAChR, with similar structure and pharmacology to the mammalian skeletal muscle nAChR, carries several binding sites for different ligands. Incubation of Torpedo membrane-bound nAChRs with the agonist carbamylcholine (Carb) stimulated the binding of [3H]thienyl-cyclohexylpiperidine ([3H]TCP), which binds to the receptor's noncompetitive antagonist binding site in its ionic channel, with high affinity (Kd of 196 nM). The agonist-stimulated binding of [3H]TCP (i.e., binding to activated nAChRs) was inhibited in a concentration-dependent manner by four organophosphate (OP) anticholinesterases, chlorpyrifos oxon (CPO), chlorpyrifos (CPS), parathion (PS), and paraoxon (PO) with IC50 (concentration that inhibits 50% of the effect) values of 5, 150, 200, and 300 microM, respectively. The binding of CPO was totally reversible. The OPs had no effect on equilibrium binding of [alpha-125I]bungarotoxin ([alpha-125I]BGT) to the receptor's acetylcholine (ACh)-binding site, but preincubation of the membranes with the OPs increased this site's affinity for Carb. In absence of agonist, 100 microM of the OPs increased the binding of [3H]TCP by two- to fivefold with the following order of decreasing potency: PS > CPO > CPS > PO. The data suggest that in addition to inhibition of acetylcholinesterase, these OPs bind to a site on the nAChR that is different from the sites that bind ACh or TCP and that this binding induces nAChR desensitization. The relevance of this direct action of OPs on nAChRs on their acute toxicities is discussed.
A Protein C (PC) biosensor can be used to diagnose PC deficiency, to monitor the PC level in the blood of PC deficient patients, and to measure the PC concentration in other PC-containing samples, such as PC producing animal cell culture broth or transgenic animal milk. A fully functional biosensor requires extremely high sensitivity and specificity, and real-time measurement. To satisfy these requirements, it is proposed to develop an immuno-optical fiber biosensor that utilizes PC-specific biomolecules (PC probes) tagged with fluorophores. The method involves immobilizing monoclonal antibody against PC (anti-PC) on the surface of an optical fiber. When PC in a sample is adsorbed to the anti-PC on the fiber, it can be reached with the fluorophore tagged PC-probe. The intensity of light transported through the optical fiber, therefore, can be correlated with the concentration of PC in the sample. The sensor will be designed so it can be reused, following a simple elution step, thus reducing diagnostic expense. The preliminary study shows encouraging future for the real-time optical PC biosensor.
A fiber optic evanescent fluoroimmunosensor was used to rapidly detect and quantitate coca alkaloids as cocaine equivalents in leaf extracts of five Erythroxylum species. A monoclonal antibody (mAb) made against benzoylecgonine (BE), a major metabolite of cocaine, was immobilized covalently on quartz fibers and used as the biological sensing element in the portable fluorometer. Benzoylecgonine-fluorescein (BE-FL) was used as the optical signal generator when it bound to the fiber. If present, cocaine competed for the mAb and interfered with the binding of BE-FL, thereby reducing the fluorescence transmitted by the fiber. Calibration curves were prepared by measuring (over 30 s) the rates of fluorescence increase in the absence, or presence of cocaine. Ethanol or acid extracts of dry coca leaves were assayed by this fiber optic biosensor, gas chromatography and a fluorescent polarization immune assay. Biosensor values of cocaine content of leaves from five Erythroxylum species were not significantly different from gas chromatography values, but had higher variance. The biosensor assay was rapid and did not require cleanup of the crude leaf extracts. Cocaine in acid extracts was reduced significantly after 4 weeks at 23 degrees C and after 3 weeks at 37 degrees C. Fibers (mAb-coated), stored at 37 degrees C in phosphate-buffered solution (0.02% NaN3), gave stable responses for 14 days.
A fiber-optic biosensor was developed for detection of cocaine, its metabolites, and other coca alkaloids, using a monoclonal antibody (mAb) against a derivatized benzoylecgonine (BE). The mAb was immobilized noncovalently on quartz fibers and a flow fluorometer was used to detect changes in evanescent wave fluorescence. A fluorescein (FL) conjugate of BE bound to the mAb specifically in a saturable manner and with high affinity (Kd = 7.6 nM). Cocaine or other test compounds competed with FL-BE for binding to the mAb in a concentration-dependent manner, thereby reducing the initial rate or steady-state fluorescence. Addition of cocaine to the flow buffer after reaching steady-state fluorescence enhanced the dissociation of bound FL-BE, and cocaine removal allowed fiber regeneration for multiple measurements. The detection limits for cocaine, cocaethylene, norcocaine, and BE were 5, 5, 29, and 30 ng/ml, respectively, but for ecgonine it was 4600 ng/ml and for methylecgonine it was 2000 ng/ml. Tropacocaine was detected at 10 ng/ml, but atropine was detected at 2900 ng/ml. The biosensor discriminated by 833-fold between cocaine and its stereoisomer pseudococaine. Structural features necessary for high-affinity recognition by this mAb are benzoate and 3 beta configuration, both of which are found in BE, cocaine, norcocaine, and cocaethylene.
Photoaffinity ligands are useful tools for the isolation, purification, and characterization of proteins. As a step toward the goal of producing a photoaffinity probe for the dopamine transporter, isocyanato and azido derivatives of 3-[(phenylcarbamoyl)oxy]ecgonine methyl ester were synthesized and tested for their ability to interact with the cocaine receptor of mammalian brain via two different assays. The ability of two isothiocyanato (N=C=S) (para and meta) and two azido (N3) (para and meta) derivatives, as well as (-)-cocaine, to inhibit [3H]cocaine binding and [3H]dopamine uptake and to covalently interact with the cocaine-binding site was tested. The p-N=C=S was the most potent, with IC50 values of 0.23 and 0.49 microM for [3H]cocaine binding and [3H]dopamine uptake. The m-N3 and p-N3 inhibited [3H]cocaine binding with IC50 values of 0.63 and 1.00 microM and inhibited [3H]dopamine uptake with IC50 values of 5.08 and 1.32 microM, respectively. Preincubation of synaptosomal membranes with the m- or p-N=C=S isomer either in reduced lighting or under ultraviolet light followed by two washes resulted in inhibition of 70% and 85% of [3H]cocaine binding, respectively, indicating the highly reactive properties of these compounds. After preincubation in reduced lighting, m-N3 and p-N3 inhibited 0% and 13% of [3H]cocaine binding, while following preincubation under ultraviolet light, the inhibition increased to 61% and 68%, respectively. Thus, the isothiocyanato derivatives appear to bind irreversibly to the cocaine receptor in the presence or absence of ultraviolet light, whereas the azido derivatives are photoreactive compounds which may prove useful in the purification of the receptor.
A light addressable potentiometric sensor was used to measure acetylcholinesterase (AChE) activity in order to evaluate the protective effects of quaternary compounds and NaF against enzyme phosphorylation and aging by two organophosphates. The use of the immobilized AChE made possible the quick removal of reagents (i.e., organophosphate, 2-pralidoxime, and protectant), thereby permitting accurate determination of AChE activity before and after phosphorylation and aging. Paraoxon was 15-fold more potent in inhibiting AChE than DFP, while the percent aging following phosphorylation by diisopropylfluorophosphate (DFP) was much higher. Sodium fluoride (NaF), the most effective protectant against phosphorylation and aging, and the quaternary ammonium compounds reduced significantly AChE inhibition by DFP and paraoxon, to similar degrees. Even though the percent AChE activity that was lost to aging was reduced by these agents, aging as a percent of phosphorylated AChE was not reduced. Thus, their major effect was in reducing the percent AChE phosphorylation, which consequently resulted in reduction of total aged AChE. The finding that quaternary ammonium compounds protect against phosphorylation is consonant with the proposed presence of the active site of AChE in an aromatic gorge.
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.
We previously reported (J. Med. Chem. 1993, 36, 1188-1193) that changes to the ring size of the piperidine and cyclohexyl rings of the high-affinity and selective dopamine (DA)-uptake inhibitor 1-[1-(2-benzo[b]thienyl)cyclohexyl]piperidine (BTCP, 2) caused different, and in some cases opposite, changes in affinity for sites on the DA transporter labeled by [3H]BTCP and [3H]-cocaine. These results suggested that the radioligands label different sites on the transporter. In the present study, we extend the structure-activity relationships (SAR) of BTCP by studying the binding characteristics of a series of N,N-disubstituted 1-(2-benzo[b]thienyl)cyclohexylamines 7-32 at the DA transporter. Cyclohexyl was selected as opposed to other ring sizes since it corresponds to BTCP. The binding results indicate that a considerable degree of structural variation is permitted for the N-substituents, while still retaining nanomolar affinity for sites on the transporter (studied in rat forebrain homogenates). As observed in our earlier study, the differential effects of structural change on binding to sites on the DA transporter labeled by these radioligands suggests that they are different and distinct binding sites. In general, and up to a point, increasing the size and lipophilicity of the N substituents resulted in improvements in binding but appeared to have less predictable effects on DA-uptake inhibition (as measured in rat brain synaptosomes). The binding of these compounds to sites labeled by [3H]BTCP appeared to correlate best with IC50 for DA-uptake inhibition. To our surprise, the monoalkyl N-substituted BTCP derivatives displayed the highest affinity for the DA transporter of all the compounds in this series. For example, the N-(cyclopropylmethyl) derivative 14 displayed IC50's = 23 nM ([3H]cocaine) and 1 nM ([3H]-BTCP), and the N-butyl derivative 10 showed IC50's = 60 nM ([3H]cocaine) and 0.3 nM ([3H]-BTCP). BTCP exhibited IC50's of 39 nM ([3H]cocaine) and 5 nM ([3H]BTCP) in this assay. The observation that N,N-dibutyl derivative 31 exhibited low ratios of IC50 [3H]cocaine/IC50 DA reuptake and IC50 [3H]BTCP/IC50 DA reuptake suggests that it may be a potential candidate for cocaine antagonism studies. The effect of additional amino, amide, and aromatic groups on the N-substituents was examined, and the results are discussed. The failure of all of the compounds in this series to bind phenycyclidine receptors coupled with their high affinity and range of selectivities at the DA transporter identifies many of them as useful tools for probing the mode of action of BTCP at this site.
Piperidine and cyclohexyl ring homologues of the high-affinity dopamine (DA) uptake inhibitor 1-[1-(2-benzo[b]thienyl)cyclohexyl]piperidine (BTCP, 3) were each prepared in four steps from the appropriate cycloalkanones. These compounds were tested for their ability to displace [3H]BTCP and [3H]cocaine and to inhibit [3H]DA uptake in rat striatal homogenates. The ratios IC50([3H]cocaine)/IC50([3H]BTCP) ranged from 62 for BTCP to 1.5 for 1-[2-(benzo[b]thienyl)-cyclopentylamine (17); cocaine gave a ratio of 0.6. This indicates that BTCP is the most selective of all the compounds tested for sites labeled by [3H]BTCP whereas cocaine is most selective for sites labeled by [3H]cocaine. The wide differences in the relative abilities of these compounds to displace [3H]BTCP and [3H]cocaine suggests that these two radioligands are labeling different sites on the transporter. In general, the compounds structurally related to BTCP exhibited greater selectivity for sites labeled by [3H]BTCP. However, several of the BTCP-related derivatives showed greater (compared with BTCP and cocaine) ability to displace [3H]cocaine. Most notably, 1-[1-(2-benzo[b]thienyl)cyclohexyl]pyrrolidine (7) exhibited a 3.4-fold greater affinity for these sites compared with BTCP and a 9-fold greater affinity at these sites than cocaine. Most of the BTCP homologues displayed greater ability to inhibit [3H]DA uptake in rat forebrain synaptosomes than cocaine. BTCP and 7 were the most potent of all the compounds tested in terms of their ability to inhibit uptake of [3H]DA. IC50 ratios for [3H]cocaine binding/[3H]DA uptake ranged from 0.47 for 1-[1-(2-benzo[b]thienyl)cyclopentyl]homopiperidine (11) to 8.8 for 1-(2-benzo[b]thienyl)cyclohexylamine (4). The importance of this ratio remains unclear in terms of identification of potential cocaine antagonists. As for BTCP, all of the compounds tested showed Ki values > 10,000 nM for displacement of [3H]TCP from rat brain homogenates. These compounds were able to displace the highly selective sigma receptor probe [3H]-(+)-pentazocine from guinea pig brain homogenates with Ki values ranging from 125 to 9170 nM. The significance of their sigma-binding activity in light of their dopaminergic properties is unclear. The diverse binding properties of these compounds at the DA-uptake site and their spectrum of inhibitory activities for [3H]DA uptake identifies them as a useful base for the development of subtype selective probes at this site. These compounds will allow further study of the structure and function of the "cocaine" receptor as well as the development of potential cocaine antagonists.
Philanthotoxin (PhTX) is a neurotoxic constituent of the paralytic venom of the digger wasp, Philanthus triangulum. PhTX inhibits glutamate receptors of insect muscles mostly as a channel blocker, thereby producing muscle paralysis. Since glutamate receptor blockers may be of value as selective insect control agents, numerous derivatives of PhTX were synthesized and tested for their potencies as inhibitors of insect skeletal muscle glutamate receptors. Structure-activity relationship studies revealed that shortening the polyamine chain length reduced potency, and quaternarization of the nitrogen destroyed it. The potency was increased by a bulky anchoring group with moderate hydrophobicity at the end of the polyamine chain. The conversion of the tryosyl moiety to 3,5-diiodo-tyrosyl also increased potency and so did lengthening the butyryl chain from 4 to 10 carbons. Not only did PhTXs inhibit different subtypes of glutamate receptors, including the mammalian N-methyl-D-aspartate receptor, but also nicotinic receptors of insects and vertebrates. Because of this low selectively, and the hydrophilicity of the derivatives tested, which interferes with their penetration to the target receptor, these compounds cannot be used as insecticides. Nevertheless, the insect skeletal muscle glutamate receptor is a viable target for selective insecticides and major changes in PhTX structure may possibly produce derivatives that can be potential insecticides.
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.