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An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to study the maturation-dependent sensitivity of brain cells to two organophosphorus pesticides (OPs), chlorpyrifos and parathion, and to their oxon derivatives. Immature (DIV 5-15) or differentiated (DIV 25-35) brain cells were treated continuously for 10 days. Acetylcholinesterase (AChE) inhibitory potency for the OPs was compared to that of eserine (physostigmine), a reversible AChE inhibitor. Oxon derivatives were more potent AChE inhibitors than the parent compounds, and parathion was more potent than chlorpyrifos. No maturation-dependent differences for AChE inhibition were found for chlorpyrifos and eserine, whereas for parathion and paraoxon there was a tendency to be more effective in immature cultures, while the opposite was true for chlorpyrifos-oxon. Toxic effects, assessed by measuring protein content as an index of general cytotoxicity, and various enzyme activities as cell-type-specific neuronal and glial markers (ChAT and GAD, for cholinergic and GABAergic neurons, respectively, and GS and CNP, for astrocytes and oligodendrocytes, respectively) were only found at more than 70% of AChE inhibition. Immature compared to differentiated cholinergic neurons appeared to be more sensitive to OP treatments. The oxon derivates were found to be more toxic on neurons than the parent compounds, and chlorpyrifos was more toxic than parathion. Eserine was not neurotoxic. These results indicate that inhibition of AChE remains the most sensitive macromolecular target of OP exposure, since toxic effects were found at concentrations in which AChE was inhibited. Furthermore, the compound-specific reactions, the differential pattern of toxicity of OPs compared to eserine, and the higher sensitivity of immature brain cells suggest that the toxic effects and inhibition of AChE are unrelated.

Physiologically based pharmacokinetic (PBPK) modeling of foreign chemicals is dependent on the accurate determination of their tissue/blood distribution coefficients, Kp (partition coefficients). The present study was undertaken to evaluate the validity of the in vitro estimation of the Kp values of the organothiophosphate insecticides parathion and methyl parathion by equilibrium dialysis. Data derived from previously published studies that utilized single-pass perfusions of mouse livers in situ with parathion or methyl parathion were analyzed to determine liver/perfusate Kp values from the equation Kp = (t 1/2ss) (Q)/(0.693) (VH), where Kp is the liver/perfusate distribution ratio, t 1/2ss is the half-life for approach to steady state of the chemical, VH is the liver volume, and Q is the perfusate flow rate. Kp values for methyl parathion were calculated to be 16.4 +/- 7.5 and 9.5 +/- 2.7 (mean +/- SD) for perfused livers and equilibrium dialysis, respectively, while estimates of Kp for parathion were 15.6 +/- 6.3 and 19.5 +/- 5.5 for perfused livers and equilibrium dialysis, respectively. These results indicate that equilibrium dialysis can be utilized to give an accurate estimate of tissue partitioning of parathion and methyl parathion from perfusate into perfused mouse livers.

The dimethyl-substituted organothiophosphate insecticides methyl parathion and azinphos-methyl are thought to undergo glutathione-mediated detoxification in mammals. In the present study, depletion of hepatic glutathione in the mouse by pretreatment with diethyl maleate potentiated the acute toxicities of methyl parathion and azinphos-methyl, whereas depletion of hepatic glutathione by pretreatment with buthionine sulfoximine did not. Furthermore incubation of 50 microM methyl parathion with mouse hepatic microsomes for 5 min in the presence of 1 mM diethyl maleate led to significantly greater (p less than 0.05) production of methyl paraoxon, compared to incubations in the absence of diethyl maleate. Conversely, 1 mM diethyl maleate had no effect on metabolic activation of azinphos-methyl by mouse hepatic microsomes, while 10 mM inhibited slightly production of azinphos-methyl oxon from azinphos-methyl. These results suggest normal levels of hepatic glutathione are not required for detoxification of methyl parathion or azinphos-methyl in the mouse. Moreover the potentiation of the acute toxicity of methyl parathion following diethyl maleate pretreatment could result, at least in part, from enhanced production of methyl paraoxon. However, diethyl maleate likely acts through another mechanism(s) as well since it did not enhance the metabolic activation of azinphos-methyl in vitro. These data raise serious doubts about the participation of glutathione in the detoxification of methyl parathion and azinphos-methyl in vivo in the mouse.

Bobwhite quail (Colinus virginianus) that received either 0, 2, 4, or 8 mg/kg methyl parathion (O,O-dimethyl o-p-nitrophenyl phosphorothioate) treatment were investigated as to their susceptibility to predation by a cat (Felis domesticus) predator. Four hours after receiving methyl parathion (MP), physical activity levels were monitored in quail and included the number of seconds spent still, walking, running, or flying before and after a cat was introduced into an experimental arena. The cholinesterase (ChE) activity for each quail on experiment was determined. Quail that were captured exhibited significantly greater inhibition of brain ChE activity and spent significantly more time being still than noncaptured birds. Birds receiving MP at 8 mg/kg spent more seconds being still than those in other treatment groups and had ChE activity reduced to 42.8% of normal activity. There was a tendency for quail at increasing treatment levels to be more susceptible to capture by the cat predator. The neurological and behavioral effects of methyl parathion may have important ecological ramifications.

Pregnant rats received daily po doses of the organophosphate methyl parathion (MPTH) from Day 6 through Day 15 or 19 of gestation at doses causing no (1.0 mg/kg) or minimal (1.5 mg/kg) signs of maternal toxicity. Following the dose of MPTH on Day 15 or 19, in vivo protein synthesis was measured 0.5, 1.0, and 2.0 hr after sc injection of L-[1-14C]valine at a dose of 5 microCi/mmol/100 g body wt. The specific activity of [14C]valine in the free amino acid pool and protein bound pool was significantly reduced in various regions of maternal brain and in maternal viscera, placenta, and whole embryos (Day 15), and in fetal brain and viscera (Day 19). The inhibitory effect of MPTH on net protein synthesis was dose dependent, greater on Day 19 than 15 of gestation and more pronounced in fetal than in maternal tissues.

The purpose of this study was to determine the effects of subchronic administration of the organophosphate methylparathion (MPTH) during gestation on behavior and development of brain cholinergic neurons in the offspring. Pregnant rats received daily po doses of MPTH from Day 6 through Day 20 of gestation at doses causing no (1.0 mg/kg) or minimal (1.5 mg/kg) visible signs of maternal toxicity. Acetylcholinesterase (AChE) and choline acetyltransferase (CAT) activities, and [3H]quinuclidinyl benzilate (QNB) binding to muscarinic receptors, were determined in several brain regions at 1, 7, 14, 21, and 28 days postnatal age and in maternal brain at Day 19 of gestation. Prenatal exposure to 1.5 mg MPTH/kg reduced AChE and increased CAT activity in all brain regions at each developmental period and in maternal brain. Similar exposure to 1.0 mg MPTH/kg caused a significant but smaller and less persistent reduction in AChE activity but no change in brain CAT activity of the offspring. Both doses of MPTH decreased the Bmax of 3H-QNB binding in maternal frontal cortex but did not alter the postnatal pattern of 3H-QNB binding. In parallel studies, prenatal exposure to MPTH did not affect a variety of behaviors. However, cage emergence, accommodated locomotor activity, and operant behavior in a mixed paradigm were impaired in rats exposed to 1.0 but not to 1.5 mg/kg MPTH. No morphological changes were observed in hippocampal or cerebellar tissue. Thus, subchronic prenatal exposure to MPTH altered postnatal development of cholinergic neurons and caused subtle alterations in selected behaviors of the offspring.