Effect of repeated organophosphate administration on carbachol-stimulated phosphoinositide hydrolysis in the rat brain

Neurotoxicity in Preclinical Models of Occupational Exposure to Organophosphorus Compounds

Organophosphorus (OPs) compounds are widely used as insecticides, plasticizers, and fuel additives. These compounds potently inhibit acetylcholinesterase (AChE), the enzyme that inactivates acetylcholine at neuronal synapses, and acute exposure to high OP levels can cause cholinergic crisis in humans and animals. Evidence further suggests that repeated exposure to lower OP levels insufficient to cause cholinergic crisis, frequently encountered in the occupational setting, also pose serious risks to people. For example, multiple epidemiological studies have identified associations between occupational OP exposure and neurodegenerative disease, psychiatric illness, and sensorimotor deficits. Rigorous scientific investigation of the basic science mechanisms underlying these epidemiological findings requires valid preclinical models in which tightly-regulated exposure paradigms can be correlated with neurotoxicity. Here, we review the experimental models of occupational OP exposure currently used in the field. We found that animal studies simulating occupational OP exposures do indeed show evidence of neurotoxicity, and that utilization of these models is helping illuminate the mechanisms underlying OP-induced neurological sequelae. Still, further work is necessary to evaluate exposure levels, protection methods, and treatment strategies, which taken together could serve to modify guidelines for improving workplace conditions globally.

The cholinesterase inhibitor DFP facilitates the expression of paradoxical sleep (PS) propensity in rats subjected to short-term PS deprivation

Short-term paradoxical sleep (PS) deprivation was used to examine the effects of chronic exposure to subtoxic doses of the cholinesterase inhibitor diisopropylfluorophosphate (DFP) on PS regulation. Rats were injected once daily with DFP (0.2 mg/kg per day; s.c.) for 11 consecutive days; control rats received a daily injection of oil vehicle. The experiment was conducted on the 10th and 11th days of treatment, when brain cholinesterase inhibition induced by DFP exposure was maximal. On the 10th day, an 8-h baseline recording was carried out. On the 11th day, a 6-h PS deprivation was carried out by manually awaking rats each time they showed polygraphic signs of PS; recordings were then continued for another 2 h to examine recovery sleep. During deprivation, though they slept less than controls, DFP-treated rats made more attempts to enter PS. After deprivation, their PS rebound had an overall amount comparable to that of the controls, but its time course was shortened: whereas PS elevation was manifested through the 2 h of recovery in the control group, it occurred only during the first hour in the DFP group. These results demonstrate that chronic, low-level DFP exposure facilitated the expression of the PS propensity that accumulated as a result of PS deprivation: it enhanced the tendency for PS during deprivation; it accelerated the rate of compensatory PS expression after deprivation. They support the hypothesis that DFP promotes PS initiation by increasing cholinergic transmission.

Gestational exposure to methylmercury alters neurotrophin- and carbachol-stimulated phosphatidylinositide hydrolysis in cerebral cortex of neonatal rats

Neurotrophin-stimulated signal transduction through the Trk receptors has been implicated in the development and survival of the nervous system. Phospholipase Cgamma (PLCgamma) is an early downstream effector for the Trk receptors, and catalyzes the hydrolysis of phosphatidylinositides (PI) to inositol phosphates (IPs) and diacylglycerol. The current study demonstrated that PI hydrolysis can be used as a measure of Trk stimulation in slices from neonatal rat brain, and examined changes in the ontogeny of neurotrophin-stimulated PI hydrolysis in animals exposed to MeHg during gestation. Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) stimulated PI hydrolysis in neocortical and cerebellar slices from neonatal rats in a concentration-dependent manner (30-1000 ng/ml). The neurotrophin-stimulated PI hydrolysis was completely blocked by K-252a, a compound known to inhibit Trk autophosphorylation. To examine the effects of MeHg on PI hydrolysis, Long-Evans dams were dosed p.o. on gestational days 6-15 with 0 or 2 mg/kg/day MeHg dissolved in saline. Pups were sacrificed on postnatal days (PND) 1, 4, 10, 14, and 21 and brain slices prepared from the neocortex and cerebellum. Neurotrophin-stimulated PI hydrolysis was highest on PND 1-4 and decreased with age in slices from both regions. Prior exposure to MeHg had no effect on NT-3 or BDNF-stimulated PI hydrolysis in the cerebellum; however, in the neocortex carbachol-stimulated PI hydrolysis and NT-3-stimulated PI hydrolysis were decreased on PND 1. In addition, NT-3-stimulated PI hydrolysis was increased on PND 14 compared to controls. Nerve growth factor (NGF), which had no effect in controls, increased PI hydrolysis in MeHg exposed animals. Acute exposure to 10 micro M MeHg increased basal PI hydrolysis in cortical slices and increased NT-3- and BDNF-stimulated PI hydrolysis in slices from the cerebellum. These data indicate that gestational exposure to MeHg can alter neurotrophin signaling in the neocortex at early postnatal times.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. II. Time course of behavioral state changes in rats

Rats were repeatedly administered with low doses of diisopropylfluorophosphate (DFP; 0.2 mg/kg/day, SC), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle or of saline. Recordings of the sleep-wake states were obtained in the 6 h following 1, 3, 6, 9, 13, 17, and 21 injections, as well as 2, 4, and 19 days after 9-day treatment. DFP administration increased waking at the expense of slow-wave sleep (SWS), but not of paradoxical sleep (PS); as a result, the PS/SWS ratio was strongly enhanced. These changes developed across days, were maximal after six to nine injections, and were then maintained at that level until cessation of treatment. This time course of behavioral state alterations paralleled the time course of ChE inhibition in the mesopontine cholinergic nuclei and the pontine reticular formation described in the companion article. In contrast, after DFP withdrawal, behavioral states returned to control values more rapidly (in 2-4 days) than did ChE activity. These results are discussed regarding the promoting role of cholinergic neurotransmission in brain-activated states.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. I. Time course of neurochemical changes in the rat pontomesencephalic tegmentum

Rats were repeatedly administered with a low dose of diisopropylfluorosphosphate (DFP; 0.2 mg/kg/day, SC, for 9 or 21 days), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle. Neurochemical changes occurring in the pontomesencephalic tegmentum (PMT), a brain stem region critically involved in behavioral state control, were evaluated at various times of treatment and after DFP withdrawal. First, enzyme assay revealed a profile of ChE inhibition in the whole PMT which looked like that observed in the striatum; both the inhibition and recovery proceeded more slowly than they did in the plasma. Second, quantitative histochemistry indicated that ChE activity in the mesopontine cholinergic nuclei and the pontine reticular formation progressively decreased across the first days of DFP exposure, to reach an asymptotic level of inhibition after 6 days (74-82% inhibition). The inhibition was less pronounced in the locus coeruleus (49%). Third, [3H]QNB autoradiography showed that muscarinic receptor density was unchanged in any of the PMT areas selected. These results are discussed regarding the question of regional variation in susceptibility to anti-ChE agents. To what extent behavioral state alterations occur concomitantly with ChE activity changes is assessed in the companion article.

Acetylcholinesterase regeneration in mouse CNS studied by a microphotometric method

Acetylcholinesterase (AChE) regeneration after single and repeated doses of diisopropylfluorophosphate (DFP) was investigated in neostriatum of central nervous system of Balb mice, by means of a microphotometric method. In the present study, three experimental groups received the following DFP treatments: group I was administered a single dose of 3 mg/kg; group II received weekly doses of 3 mg/kg for 9 weeks, and group III was exposed to 28 weekly treatments of 1 mg/kg, plus a last treatment of 3 mg/kg, for a total of 29 doses. The results of microphotometric analysis for all the experimental groups have shown a distinct reversibility pattern of DFP inhibition, consistent with the process of AChE regeneration. Nevertheless, one week following the last DFP treatment, a persisting difference (approximately 22%) in the AChE concentration between control and the experimental groups was observed. Based on our results and on the correlation reported in the literature between AChE reduction and muscarinic receptor density, the hazardous implications of organophosphate environmental contamination on human and animal health are pointed out.

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.