Irreversible suppression of calcium entry into nerve terminals by methylmercury.

Measurements of uptake of 45Ca into rat forebrain synaptosomes depolarized with high K+ and EPP amplitudes at the rat neuromuscular junction were used to assess the effects of methylmercury (MeHg) on voltage-dependent Ca++ uptake and subsequent transmitter release at model central and peripheral synapses. The objectives were to: determine whether MeHg altered uptake of 45Ca into purified synaptosomes depolarized by high K+; compare its effects with those produced by HgCl2; ascertain whether the "fast" or "slow" components of Ca++ uptake were affected preferentially by MeHg; and determine whether a functional correlate to the effects on 45Ca uptake could be observed electrophysiologically at the mammalian neuromuscular junction. HgCl2 (10-500 microM) produced a concentration-dependent decrease of total depolarization-induced 45Ca uptake. Peak inhibition occurred at 200 microM Hg++ which suppressed nerve terminal Ca++ uptake to approximately 5% of Hg-free control values, a result similar to that obtained previously by others. Similarly, MeHg also suppressed total 45Ca uptake although the maximal inhibition produced (70% at 200 microM MeHg) was less than that produced by HgCl2. The effect of MeHg was apparent both in nonpreviously depolarized synaptosomes after a 1-sec depolarization ("fast uptake") and after 10-sec incubation in synaptosomes predepolarized with 41 mM K+ in Ca-free solutions before addition of MeHg and 45Ca ("slow uptake"). A significant decrease in the slow phase of 45Ca uptake occurred with 200 and 500 microM MeHg.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between cimetidine, methylparathion, and parathion.

The purpose of this study was to determine whether cimetidine exerts a protective effect against toxicity of organophosphate pesticides requiring metabolic activation in mice and rats. In both species, pretreatment with cimetidine (75 mg/kg/ip) 30 min prior to methylparathion (MP) reduced lethality and delayed the onset of cholinergic signs of toxicity. In contrast when cimetidine was administered simultaneously with MP to mice, neither MP-induced lethality nor cholinergic signs of toxicity were altered. Moreover, cimetidine pretreatment failed to decrease parathion-induced toxicity in mice. In conclusion, pretreatment with cimetidine decreases the toxicity of MP but not of parathion in mice and rats.

Pharmacokinetics of methyprylon following a single oral dose.

Single oral doses of 300 mg of methyprylon were administered to 10 healthy volunteers. Plasma concentrations of methyprylon and its dehydro metabolite were measured using a recently developed HPLC assay. Plasma concentration-time data were fitted to a two-compartment model with either first-order absorption, zero-order absorption, or two consecutive, discontinuous, first-order absorption rate constants. Based on the criteria of visual inspection, the correlation coefficient, standard deviations of the parameter estimates, and the residual sum of squares, it was concluded that the zero-order absorption model fit the data best. Mean (+/- SD) values for the half-life (9.2 +/- 2.2 h), apparent clearance, (11.91 +/- 4.42 mL/h/kg) and apparent steady-state volume of distribution, (0.97 +/- 0.33 L/kg) were found.

Brain cholinergic, behavioral, and morphological development in rats exposed in utero to methylparathion.

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.

Effect of subchronic administration of methyl parathion on in vivo protein synthesis in pregnant rats and their conceptuses.

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.

Effect of chloramphenicol pretreatment on malathion-induced acute toxicity in the rat.

Pretreatment of rats with chloramphenicol (CAP) (100 mg/kg, ip) 30 min prior to a single oral LD50 dose of malathion (MTH) at 340 mg/kg completely protected against MTH-induced signs of cholinergic toxicity. Pretreatment with CAP also decreased the extent and duration of MTH-induced inhibition of cholinesterase (ChE). It was previously established that CAP inhibits (1) the cytochrome-P-450-catalyzed oxidative desulfuration of methylparathion to the much more toxic oxygen analog methylparaoxon (MOX) and (2) the carboxyesterase in rat liver. Since carboxyesterases account for 60% or more of the catabolism of MTH in the rat, the present results were surprising. Thus it appears that the inhibition of MTH toxicity by CAP pretreatment is attributable to inhibition by CAP of the metabolic activation of MTH to MOX.

Differential actions of dopamine agonists and antagonists on the gamma-butyrolactone-induced in mouse brain dopamine.

gamma-Butyrolactone (GBL) increased the dopamine concentration in the forebrain of the mouse. Apomorphine dose-dependently antagonized the GBL effect, while piribedil was less effective. Haloperidol prevented the antagonism of GBL by apomorphine but pimozide was ineffective in blocking apomorphine. After chronic treatment with haloperidol or pimozide, there was no alteration of the maximum GBL-induced increase in dopamine nor was there any significant change in the antagonism by apomorphine, although a trend toward increased sensitivity to apomorphine was noted in the group withdrawn from haloperidol. These results suggest that in the mouse, haloperidol is a more effective antagonist of presynaptic dopamine autoreceptors than pimozide, while apomorphine is a better presynaptic agonist than piribedil.

Supersensitivity to dopaminergic agonists induced by haloperidol.

Haloperidol caused a significant reduction in the spontaneous locomotor activity of mice when added to their diet for 11 days. Upon removal of the drug from their diet these mice exhibited withdrawal hyperactivity for several days that was characterized by an increase in activity over control or pre-halopericol values. Similar results were obtained when mice were fed a diet containing pimozide. Withdrawal hyperactivity was not detected after 1 or 3 days of haloperidol containing diet, but was maximal after 6 days of this diet. Dose-response curves of apomorphine-stimulated motor activity and rearing behavior were shifted to the left when determined in mice during the period of withdrawal hyperactivity. Dopaminergic agonists (apomorphine, piribedil, L-DOPA and d-amphetamine) induced gnawing at lower doses in mice removed from a chronic haloperidol-containing diet for 2 days than in mice maintained on a control diet. These results support the hypothesis that prolonged blockade of central dopaminergic receptors by neuroleptics causes subsequent behavioral effects that may be due to the development of enhanced receptor sensitivity.

Supersensitivity to dopamine agonists following unilateral, 6-hydroxydopamine-induced striatal lesions in mice.

Following an injection of 6-hydroxydopamine into the left striatum, mice initially circled spontaneously in the direction of the lesion and gradually developed an ability to circle in the opposite direction when challenged with apomorphine or L-dopa. Dose-response curves for apomorphine- and L-dopa-induced circling rates shifted to the left as the duration between the time of the 6-hydroxydopamine injection and the time of testing increased from 2 to 30 days. There was a positive correlation between the rate of apomorphine-induced circling and the reduction in the dopamine concentration in the lesioned forebrain. Doses of apomorphine and L-dopa required to elicit circling responses were 1/6 to 1/10 of those required to increase locomotor activity in normal mice. These data support the concept of development of supersensitivity of postsynaptic dopaminergic receptors in the lesioned striatum.