Age-related effects of chlorpyrifos and parathion on acetylcholine synthesis in rat striatum

Examination of Concurrent Exposure to Repeated Stress and Chlorpyrifos on Cholinergic, Glutamatergic, and Monoamine Neurotransmitter Systems in Rat Forebrain Regions

Repeated stress has been reported to cause reversible impairment in the central nervous system (CNS). It was proposed that alterations in glutamatergic, cholinergic, and monoamine neurotransmitter systems after exposure to stress are initial CNS events contributing to this impairment and that exacerbation could occur with concurrent exposure to cholinesterase inhibitors. Effects of concurrent exposure to repeated stress and chlorpyrifos on activities of acetylcholinesterase (AChE), carboxylesterase, and choline acetyltransferase (ChAT); concentrations of excitatory amino acids, monoamines, and their metabolites; and maximum binding densities ( Bmax) and equilibrium dissociation rate constants ( Kd) of glutamatergic N-methyl-d-aspartate (NMDA) and total muscarinic cholinergic receptors were studied in the blood, hippocampus, cerebral cortex, or hypothalamus of adult Long-Evans rats. Stress treatments extended over 28 days included (1) control rats handled 5 days/week; (2) rats restrained 1 h/day for 5 days/week; (3) rats swum 30 min for 1 day/week; or (4) rats restrained 4 days/week and swum for 1 day/week. On day 24, each stress treatment group was randomly divided and injected either with corn oil or chlorpyrifos, 160 mg/kg subcutaneously (sc) (60% of the maximum tolerated dose), 4 h after restraint. Blood and brain tisssues were collected on day 28. Rats restrained and swum had a statistical trend toward increasing concentrations of glutamate in the hippocampus when compared to rats only swum ( p = .064). Chlorpyrifos administration decreased restraint-induced elevated aspartate in the hippocampus, and decreased Bmax of total muscarinic receptors in the cerebral cortex. In addition, chlorpyrifos decreased Bmax and Kd of total muscarinic receptors in the cerebral cortex of swum rats. Results demonstrated that chlorpyrifos inhibited AChE activity in blood, cerebral cortex, and hippocampus, but stress did not affect AChE activity. Carboxylesterase activity was inhibited by chlorpyrifos and by repeated restraint with swim. Swim stress decreased concentrations of norepinephrine in the hippocampus and hypothalamus, and increased concentrations of dopamine and its metabolite, DOPAC, in the hypothalamus. Both stress and chlorpyrifos altered serotonin concentrations, and the interactions of repeated stress and chlorpyrifos on serotonin approached significance in the hippocampus ( p = .06) and hypothalamus ( p = .08). Therefore, stress models were demonstrated to alter glutamatergic and monoamine responses, whereas chlorpyrifos alone had effects on cholinergic and monoamine systems in the rat CNS. However, the interactions between stress and chlorpyrifos significant at p < 0.05 were restricted to attenuation of elevated aspartate in the hippocampus of restrained with swim rats and decreased Kd of acetylcholine receptors in the cerebral cortex of swum rats and restrained rats.

Potential pharmacological strategies for the improved treatment of organophosphate-induced neurotoxicity

Organophosphates (OP) are highly toxic compounds that cause cholinergic neuronal excitotoxicity and dysfunction by irreversible inhibition of acetylcholinesterase, resulting in delayed brain damage. This delayed secondary neuronal destruction, which arises primarily in the cholinergic areas of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, could be largely responsible for persistent profound neuropsychiatric and neurological impairments such as memory, cognitive, mental, emotional, motor, and sensory deficits in the victims of OP poisoning. The therapeutic strategies for reducing neuronal brain damage must adopt a multifunctional approach to the various steps of brain deterioration: (i) standard treatment with atropine and related anticholinergic compounds; (ii) anti-excitotoxic therapies to prevent cerebral edema, blockage of calcium influx, inhibition of apoptosis, and allow for the control of seizure; (iii) neuroprotection by aid of antioxidants and N-methyl-d-aspartate (NMDA) antagonists (multifunctional drug therapy), to inhibit/limit the secondary neuronal damage; and (iv) therapies targeting chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may prevent secondary neuronal damage in both early and late stages of OP poisoning, and thus may be a beneficial approach to treating the neuropsychological and neuronal impairments resulting from OP toxicity.

Development of an adverse outcome pathway for acetylcholinesterase inhibition leading to acute mortality

Adverse outcome pathways (AOPs) are designed to describe linkages of key events within a biological pathway that result in an adverse outcome associated with chemical perturbation of a well-defined molecular initiating event. Risk assessors have traditionally relied on data from apical endpoints (e.g., mortality, growth, reproduction) to derive benchmark values for use in determining the potential adverse impacts of chemicals. One goal in building reliable and well-characterized AOPs is to identify relevant in vitro assays and/or in vivo biomarkers that could be used in screening the potential hazard of substances, thereby reducing costs and increasing the number of chemicals that can be evaluated in a timely fashion. The purpose of this review article is to build an AOP for substances with a molecular initiating event of acetylcholinesterase inhibition leading to acute mortality following guidance developed by the Organisation for Economic Cooperation and Development. In contrast to most other AOPs developed to date, in which coverage is for a relatively limited taxonomic group or life stage, this AOP is applicable to a wide range of species at multiple life stages. Furthermore, while development of most AOPs has relied on data for a few model chemicals, the AOP described in the present review captures information from a large number of studies with a diversity of organophosphate and carbamate insecticides.

Time course of serum S100B protein and neuron-specific enolase levels of a single dose of chlorpyrifos in rats

Organophosphate (OP) compounds are a large class of chemicals, many of which are used as pesticides. It is suggested that OPs specifically affect glia and neurons. Effects of acute exposure to chlorpyrifos (CPF), which is a common organophosphorus pesticide used worldwide, on neuron-specific enolase (NSE) and S100B levels in rat blood during 7 days were assessed. Rats were evaluated either before (0 hr) or 2, 12, 24, 48 and 168 hr (7 days) after injection of CPF (279 mg/kg, s.c.) or vehicle (peanut oil, 2 ml/kg, s.c.) for clinical signs of toxicity. Immediately after the evaluation of toxicity, blood samples were taken for biochemical assays. CPF administration produced decreases in body-weight and temperature, which were observed for first time at 12 hr after CPF administration and continued for 168 hr (p < 0.05-0.001). Serum S100B and NSE levels were acutely increased 2 hr after CPF administration and remained high at 12 hr (p < 0.01-0.001). NSE and S100B levels were not different in either CPF or vehicle groups at following time points. Serum butyrylcholinesterase (EC 3.1.1.8; BuChE) activity was dramatically reduced at 2 hr after CPF and remained low at each time points during 7 days (p < 0.01-0.001). Our results suggest that the usefulness of serum levels of these glia- and neuron-specific marker proteins in assessing OP toxicity, specifically CPF-induced toxicity.

CHLORPYRIFOS INCREASES THE LEVELS OF HIPPOCAMPAL NMDA RECEPTOR SUBUNITS NR2A AND NR2B IN JUVENILE AND ADULT RATS

The present study investigated the effect of chlorpyrifos on NMDA receptor subunits NR2A and NR2B in juvenile and adult rats. Chlorpyrifos was administered with the dose of 40 and 70 mg/kg to juvenile and adult rats, respectively. Chlorpyrifos significantly inhibited the AChE activity in juvenile and adult rats (p < .05). NR2A and NR2B levels significantly increased in juvenile and adult rats by chlorpyrifos application (p < .05). Increased NR2A and NR2B levels may reflect increased glutaminergic activity, consequently neuronal damage. In the case of neuronal damage, learning and memory could be affected negatively even though NR2A and NR2B increased.

Comparative chlorpyrifos pharmacokinetics via multiple routes of exposure and vehicles of administration in the adult rat

Chlorpyrifos (CPF) is a commonly used organophosphorus pesticide. A number of toxicity and mechanistic studies have been conducted in animals, where CPF has been administered via a variety of different exposure routes and dosing vehicles. This study compared chlorpyrifos (CPF) pharmacokinetics using oral, intravenous (IV), and subcutaneous (SC) exposure routes and corn oil, saline/Tween 20, and dimethyl sulfoxide (DMSO) as dosing vehicles. Two groups of rats were co-administered target doses (5 mg/kg) of CPF and isotopically labeled CPF (L-CPF). One group was exposed by both oral (CPF) and IV (L-CPF) routes using saline/Tween 20 vehicle; whereas, the second group was exposed by the SC route using two vehicles, corn oil (CPF) and DMSO (L-CPF). A third group was only administered CPF by the oral route in corn oil. For all treatments, blood and urine time course samples were collected and analyzed for 3,5,6-trichloro-2-pyridinol (TCPy), and isotopically labeled 3,5,6-trichloro-2-pyridinol (L-TCPy). Peak TCPy/L-TCPy concentrations in blood (20.2 micromol/l), TCPy/L-TCPy blood AUC (94.9 micromol/lh), and percent of dose excreted in urine (100%) were all highest in rats dosed orally with CPF in saline/Tween 20 and second highest in rats dosed orally with CPF in corn oil. Peak TCPy concentrations in blood were more rapidly obtained after oral administration of CPF in saline/Tween 20 compared to all other dosing scenarios (>1.5 h). These results indicate that orally administered CPF is more extensively metabolized than systemic exposures of CPF (SC and IV), and vehicle of administration also has an effect on absorption rates. Thus, equivalent doses via different routes and/or vehicles of administration could potentially lead to different body burdens of CPF, different rates of bioactivation to CPF-oxon, and different toxic responses. Simulations using a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model for CPF are consistent with these possibilities. These results suggest that exposure route and dosing vehicle can substantially impact target tissue dosimetry. This is of particular importance when comparing studies that use varying exposure paradigms, which are then used for extrapolation of risk to humans.

Pyrethroid and organophosphate insecticide exposure in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson’s disease: an immunohistochemical analysis of tyrosine hydroxylase and glial fibrillary acidic protein in dorsolateral striatum

The pyrethroid insecticide permethrin and the organophosphate insecticide chlorpyrifos can experimentally produce Parkinson’s disease (PD)-associated changes in the dopaminergic nigrostriatal pathway, short of frank degeneration, although at doses considerably higher than from a likely environmental exposure. The ability of permethrin (200 mg/kg), chlorpyrifos (50 mg/kg), or combined permethrin + chlorpyrifos to facilitate nigrostriatal damage in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (30 mg/kg) C57BL/6 mouse model of PD was investigated in three separate experiments. Tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP) immunohistochemistry assessed nigrostriatal degeneration or nigrostriatal damage more subtle than frank degeneration. Four fields in the dorsolateral caudate-putamen were examined at two rostrocaudal locations. The dopaminergic neurotoxin MPTP decreased striatal TH immunopositive neuropil and increased GFAP immunopositive neuropil. Neither permethrin nor chlorpyrifos, alone or in combination, altered the effects of MPTP upon TH or GFAP immunostaining. Permethrin alone increased striatal GFAP immunopositive neuropil but not when combined with chlorpyrifos treatment. Therefore, combined administration of the two insecticides appeared to protect against an increase in a neuropathological indicator of striatal damage seen with permethrin treatment alone. Differences compared with analysis of entire striatum emphasize the value of varying the topographic focus used to assess nigrostriatal degeneration in studies of insecticides in PD.

Exposure of neonatal rats to parathion elicits sex-selective impairment of acetylcholine systems in brain regions during adolescence and adulthood

BACKGROUND

Organophosphates elicit developmental neurotoxicity through multiple mechanisms other than their shared property as cholinesterase inhibitors. Accordingly, these agents may differ in their effects on specific brain circuits.

OBJECTIVES

We gave parathion to neonatal rats [postnatal days (PNDs) 1-4], at daily doses of 0.1 or 0.2 mg/kg, spanning the threshold for barely detectable cholinesterase inhibition and systemic effects.

METHODS

We assessed neurochemical indices related to the function of acetylcholine (ACh) synapses (choline acetyltransferase, presynaptic high-affinity choline transporter, nicotinic cholinergic receptors) in brain regions comprising all the major ACh projections, with determinations carried out from adolescence to adulthood (PNDs 30, 60, and 100).

RESULTS

Parathion exposure elicited lasting alterations in ACh markers in the frontal/parietal cortex, temporal/occipital cortex, midbrain, hippocampus, and striatum. In cerebrocortical areas, midbrain, and hippocampus, effects in males were generally greater than in females, whereas in the striatum, females were targeted preferentially. Superimposed on this general pattern, the cerebrocortical effects showed a nonmonotonic dose-response relationship, with regression of the defects at the higher parathion dose; this relationship has been seen also after comparable treatments with chlorpyrifos and diazinon and likely represents the involvement of cholinesterase-related actions that mask or offset the effects of lower doses.

CONCLUSIONS

Neonatal exposure to parathion, at doses straddling the threshold for cholinesterase inhibition, compromises indices of ACh synaptic function in adolescence and adulthood. Differences between the effects of parathion compared with chlorpyrifos or diazinon and the non-monotonic dose-effect relationships reinforce the conclusion that various organophosphates diverge in their effects on neurodevelopment, unrelated to their anticholinesterase actions.

Effect of different administration paradigms on cholinesterase inhibition following repeated chlorpyrifos exposure in late preweanling rats

Chlorpyrifos (CPS) is widely used in agricultural settings and residue analysis has suggested that children in agricultural communities are at risk of exposure. This has resulted in a large amount of literature investigating the potential for CPS-induced developmental neurotoxic effects. Two developmental routes of administration of CPS are orally in corn oil at a rate of 0.5 ml/kg and subcutaneously in dimethyl sulfoxide (DMSO) at a rate of 1.0 ml/kg. For comparison between these methods, rat pups were exposed daily from days 10 to 16 to CPS (5 mg/kg) either orally dissolved in corn oil or subcutaneously dissolved in DMSO, both at rates of either 0.5 or 1.0 ml/kg. A representative vehicle/route group was present for each treatment. Both the low and high volume CPS in DMSO subcutaneous groups were lower than that of the low and high volume CPS in oil oral groups. At 4 h following the final administration, serum carboxylesterase was inhibited > 90% with all treatments. For cholinesterase activity in the cerebellum, medulla-pons, forebrain, and hindbrain, and serum, inhibition in the CPS-oil groups was similar and inhibition in the CPS-DMSO groups was similar. However, significantly greater inhibition was present in the high volume CPS-DMSO group as compared to the CPS-oil groups. Inhibition in the low volume CPS-DMSO group was generally between that in the CPS-oil groups and the high volume CPS-DMSO group. These data suggest that using DMSO as a vehicle for CPS may alter the level of brain ChE inhibition.

Comparative in vivo effects of parathion on striatal acetylcholine accumulation in adult and aged rats

Aged rats are more sensitive to the acute toxicity of the prototype organophosphate insecticide, parathion. We compared the acute effects of parathion on diaphragm and brain regional cholinesterase activity, muscarinic receptor binding and striatal acetylcholine levels in 3- and 18-month-old male Sprague-Dawley rats. Adult and aged rats were surgically implanted with a microdialysis cannula into the right striatum 5-7 days prior to parathion treatment. Rats were given either vehicle (peanut oil, 2 ml/kg) or one of a range of dosages of parathion (adult: 1.8, 3.4, 6.0, 9.0, 18 and 27 mg/kg, s.c.; aged: 1.8, 3.4, 6 and 9 mg/kg, s.c.) and body weight, functional signs of toxicity, and nocturnal motor activity were recorded for seven days. Three and seven days after parathion treatment, microdialysis samples were collected and rats were subsequently sacrificed for biochemical measurements. Higher dosages of parathion led to significant time-dependent reductions in body weight in both age groups. Rats in both age groups treated with lower dosages showed few overt signs of cholinergic toxicity while equitoxic high dosages (adult, 27 mg/kg; aged, 9 mg/kg) elicited marked signs of cholinergic toxicity (involuntary movements and SLUD [i.e., acronym for Salivation, Lacrimation, Urination and Defecation] signs) with peak effects being noted 3-4 days after treatment. Nocturnal activity (ambulation and rearing) was reduced in both age groups following parathion dosing, with more prominent effects in adults and rearing being more consistently affected. Dose- and time-dependent inhibition of cholinesterase activity was noted in both diaphragm and striatum. Total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) binding was significantly lower in aged rats, and both total binding and muscarinic agonist ([(3)H]oxotremorine methiodide] binding was significantly reduced in both age-groups treated with the highest dosages of parathion (adult, 27 mg/kg; aged, 9 mg/kg). In contrast to relatively similar levels of cholinesterase inhibition, striatal extracellular acetylcholine levels were significantly lower (2.2- to 2.9-fold) in aged rats at both 3 and 7 day time-points compared to adult rats treated with equitoxic dosages (i.e., 9 and 27 mg/kg, respectively). No age-related differences in in vitro striatal acetylcholine synthesis or in vivo acetylcholine accumulation following direct infusion of the cholinesterase inhibitor neostigmine (1 microM) were noted. While aged rats are more sensitive than adults to the acute toxicity of parathion, lesser acetylcholine accumulation was noted in the striatum of aged rats exhibiting similar levels of cholinesterase inhibition. These findings suggest that lesser acetylcholine accumulation may be required to elicit cholinergic signs in the aged rat, possibly based on aging-associated changes in muscarinic receptor density.

Screening for developmental neurotoxicity using PC12 cells: comparisons of organophosphates with a carbamate, an organochlorine, and divalent nickel

BACKGROUND

In light of the large number of chemicals that are potential developmental neurotoxicants, there is a need to develop rapid screening techniques.

OBJECTIVES

We exposed undifferentiated and differentiating neuronotypic PC12 cells to different organophosphates (chlorpyrifos, diazinon, parathion), a carbamate (physostigmine), an organochlorine (dieldrin), and a metal (divalent nickel; Ni2+) and examined indices of cell replication and differentiation for both short- and long-term exposures.

RESULTS

In undifferentiated cells, all the agents inhibited DNA synthesis, with the greatest effect for diazinon, but physostigmine eventually produced the largest deficits in the total number of cells after prolonged exposure. The onset of differentiation intensified the adverse effects on DNA synthesis and changed the rank order in keeping with a shift away from noncholinergic mechanisms and toward cholinergic mechanisms. Differentiation also worsened the effects of each agent on cell number after prolonged exposure, whereas cell growth was not suppressed, nor were there any effects on viability as assessed with trypan blue. Nevertheless, differentiating cells displayed signs of oxidative stress from all of the test compounds except Ni2+, as evidenced by measurements of lipid peroxidation. Finally, all of the toxicants shifted the transmitter fate of the cells away from the cholinergic phenotype and toward the catecholaminergic phenotype.

CONCLUSIONS

These studies point out the feasibility of developing cell-based screening methods that enable the detection of multiple end points that may relate to mechanisms associated with developmental neurotoxicity, revealing some common targets for disparate agents.

Age-Related Brain Cholinesterase Inhibition Kinetics following In Vitro Incubation with Chlorpyrifos-Oxon and Diazinon-Oxon

Chlorpyrifos and diazinon are two commonly used organophosphorus insecticides (OPs), and their primary mechanism of action involves the inhibition of acetylcholinesterase by their metabolites chlorpyrifos-oxon (CPO) and diazinon-oxon (DZO), respectively. The study objectives were to assess the in vitro age-related inhibition kinetics of neonatal rat brain cholinesterase (ChE) for CPO and DZO by estimating the bimolecular inhibitory rate constant (k(i)) values. Brain ChE inhibition and k(i) values following CPO and DZO incubation with neonatal Sprague-Dawley rat brain homogenates were determined at postnatal day (PND) 5, 12, and 17 and compared with the corresponding inhibition and k(i) values obtained in the adult rat. A modified Ellman method was utilized for measuring the ChE activity. CPO caused a greater ChE inhibition than DZO as evidenced from the estimated k(i) values of both compounds. Neonatal brain ChE inhibition kinetics exhibited a marked age-related sensitivity to CPO, with the order of ChE inhibition being PND 5 > PND 7 > PND 17 with k(i) values of 0.95, 0.50, and 0.22 nM(-1)hr(-1), respectively. In contrast, DZO ChE inhibition was not age related in the neonatal brain, and the estimated k(i) value at all PND ages was 0.02 nM(-1)hr(-1). These results demonstrated an age- and OP-selective inhibition of rat brain ChE, which may be critically important in understanding the potential sensitivity of juveniles to specific OPs exposures.

Cerebral acetylcholine and choline contents and turnover following low-dose acetylcholinesterase inhibitors treatment in rats

Male Sprague-Dawley rats were treated for 3 weeks with (1) regular tap drinking water plus subcutaneous (s.c.) saline (0.5 ml/kg) injections three times/week, (2) pyridostigmine bromide (PB) in drinking water (80 mg/L) plus s.c. saline injections three times/week, (3) regular tap drinking water plus s.c. sarin (0.5 x LD(50)) injections three times/week, or (4) PB in drinking water plus s.c. sarin injections three times/week. Repeated doses of sarin, in the presence or absence of PB, were devoid of acute toxicity during the three-week treatment period. Two, 4, and 16 weeks post-treatment, animals were given an intravenous pulse injection of choline labeled with 4 deuterium atoms (D4Ch) followed, after 1 min, by microwave fixation of the brain in vivo. Tissue levels of endogenous acetylcholine (D0ACh), endogenous choline (D0Ch), D4Ch, and ACh synthesized from D4Ch (D4ACh) were measured by gas-chromatography mass-spectrometry in hippocampus, infundibulum, mesencephalon, neocortex, piriform cortex, and striatum. Ch uptake from blood and ACh turnover were estimated from D4Ch and D4ACh concentrations in brain tissue, respectively. Statistically significant differences among brain regions were found for D0Ch, D4Ch, D0ACh and D4ACh at 2, 4 and 16 weeks post-treatment. However, differences in the values of these parameters between control and drug treatments were found only for D0ACh and D0Ch at 2 and 4 weeks, but not at 16 weeks post-treatment. In conclusion, the results from these experiments do not support a delayed or persistent alteration in cholinergic function after exposure to low doses of PB and/or sarin.

Age-dependent pharmacokinetic and pharmacodynamic response in preweanling rats following oral exposure to the organophosphorus insecticide chlorpyrifos

Previous studies have indicated that juvenile rats are more susceptible than adults to the acute toxicity from exposure to the organophosphorus insecticide chlorpyrifos (CPF) and age-dependent differences in metabolism and sensitivity to cholinesterase (ChE) inhibition may be responsible. Metabolism involves CYP450 activation and detoxification of CPF to CPF-oxon and 3,5,6-trichloro-2-pyridinol (TCP), as well as cholinesterase (acetyl- and butyrylcholinesterase), carboxylesterase (CaE), and A-esterase (PON-1) detoxification of CPF-oxon to TCP. The pharmacokinetics of CPF, TCP, and the extent of blood (plasma/RBC), and brain ChE inhibition in rats were determined on postnatal days (PND)-5, -12, and -17 following oral gavage administration of 1 and 10mg CPF/kg of body weight. As has been seen in adult animals, for all preweanling ages the blood TCP exceeded the CPF concentration, and within each age group there was no evidence of non-linear kinetics over the dose range evaluated. Consistent with previous results, younger animals demonstrated a greater sensitivity to ChE inhibition as evident by the age-dependent inhibition of plasma, RBC, and brain ChE. The brain may be particularly sensitive in younger animals (i.e. PND-5) due to substantially lower levels of ChE activity relative to later preweanling stages and adults. Of particular importance was the observation that even in rats as young as PND-5, the CYP450 metabolic capacity was adequate to metabolize CPF to both TCP and CPF-oxon based on the detection of TCP in blood and extensive ChE inhibition (biomarker of CPF-oxon) at all ages. In addition, the increase in the blood TCP concentration ( approximately 3-fold) in PND-17 rats relative to the response in the younger rats, are consistent with an increase in CYP450 metabolic capacity with age. This is the first reported study that evaluated both the pharmacokinetics of the parent pesticide, the major metabolite, and the extent of ChE inhibition as a function of preweanling age. The results suggest that in the preweanling rat, CPF was rapidly absorbed and metabolized, and the extent of metabolism and ChE inhibition was age-dependent.

Interactive toxicity of the organophosphorus insecticides chlorpyrifos and methyl parathion in adult rats

The acute interactive toxicity following exposure to two common organophosphorus (OP) insecticides, chlorpyrifos (CPF) and methyl parathion (MPS), was investigated in adult male rats. Oral LD1 values were estimated by dose-response studies (CPF = 80 mg/kg; MPS = 4 mg/kg, in peanut oil, 1 ml/kg). Rats were treated with both toxicants (0.5 or 1 x LD1) either concurrently or sequentially, with 4-h intervals between dosing. Functional signs of toxicity (1-96 h) and cumulative lethality (96 h) were recorded. Rats treated with CPF (1 x LD1) did not show any signs of toxicity although MPS (1 x LD1) elicited slight to moderate signs (involuntary movements) within 1-2 h. Concurrent exposure (LD1 dosages of both CPF and MPS) caused slight signs of toxicity only apparent between 24 and 48 h after dosing. When rats were treated sequentially with MPS first followed by CPF 4 h later, slight signs of toxicity were noted between 6 and 24 h, whereas reversing the sequence resulted in 100% lethality within 1 h of the second dosage. Following exposure to lower dosages (0.5 x LD1), the CPF first group showed higher signs of cholinergic toxicity compared with MPS first or concurrent groups. Cholinesterase inhibition in plasma, diaphragm, and frontal cortex was generally higher in rats treated sequentially with CPF first than in those treated initially with MPS from 4 to 24 h after dosing. Plasma and liver carboxylesterase inhibition at 4 h was also significantly higher in the CPF first (62-90%) compared with MPS first (22-43%) group, while at 8 and 24 h, there was no significant difference between any of the treatment groups. ChE inhibition assays to evaluate in vitro hepatic detoxification of oxons indicated that carboxylesterase (CE)- and A-esterase-mediated pathways are markedly less important for methyl paraoxon (MPO) than chlorpyrifos oxon (CPO) detoxification. CPF pretreatment blocked hepatic detoxification of methyl paraoxon while MPS pretreatment had minimal effect on hepatic CPO detoxification ex vivo. These findings suggest that the sequence of exposure to two insecticides that elicit toxicity through a common mechanism can markedly influence the cumulative action at the target site (acetylcholinesterase, AChE) and consequent functional toxicity.