Regional differences in brain-soluble acetylcholinesterase and its molecular forms after acute poisoning by isoflurophate in rats

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.

The neurotoxicity of subchronic acetylcholinesterase (AChE) inhibition in rat hippocampus

The neurotoxic effects of long-term, low-level exposure to the commercially available insecticide, Fenthion, were examined in the present study. Young (2 month) adult, male Long-Evans rats were dermally exposed to Fenthion (25 mg/kg, 3X week) and sampled after 2 and 10 months exposure to assess neurotoxic damage in the hippocampus using morphological and biochemical endpoints. Histopathology, consisting of gliosis, swollen and necrotic neurons, and cell dropout, occurred in the dentate gyrus (DG), CA4 (hilus), and CA3 sectors as early as 2 months postexposure. Acetylcholinesterase (AChE) staining of brain tissues taken at this time was severely reduced in the septal nuclei, the DG molecular layer, the CA4, and the hippocampus proper. After 10 months exposure to Fenthion, cellular necrosis and gliosis intensified in the CA4 and CA3 regions and occasionally involved the CA2. Radiometric assays of AChE activity in the hippocampus indicated a 65 and 85% depression after 2 and 10 months exposure, respectively. Quinuclidinyl benzilate binding for the hippocampal muscarinic receptor was reduced by 6 and 15%, after 2 and 10 months exposure, respectively. A separate group of older (12 month) rats was exposed to the same dosing regimen of Fenthion and examined for neuropathological damage after 2 and 10 months exposure. Aged animals exposed for only 2 months expressed severe hippocampal degeneration in a pattern similar to that seen in the young adult after 10 months exposure (viz., DG, CA4, CA3). Aged animals exposed for 10 months showed more extensive histopathology of the CA4-2 and occasionally CA1. These observations indicate that in both young adult and aged animals, subchronic, low-level exposure to anticholinesterase compounds can result in serious neurotoxic consequences to the mammalian hippocampus.

Rat brain acetylcholinesterase turnover in vivo: use of a radioactive methylphosphonothiate irreversible inhibitor

A new organophosphorus compound was used in its non radioactive and tritiated forms in order to study rat brain acetylcholinesterase. We measured the activity recovery of the total enzyme and of its two main molecular forms (4 S and 10 S) as a function of time following the inhibition. The radioactive compound allowed us to study the disappearance of the inhibitor irreversibly bound to the enzyme in the main cholinergic areas. Both approaches gave similar results: acetylcholinesterase turn-over proceeds in two steps, a rapid one of about 30 mn and a slow one of about 2 days. Our results suggest an in vivo reactivation process concerning a fraction of the bound inhibitor.

Changes in the levels and forms of rat plasma cholinesterases during chronic diisopropylphosphorofluoridate intoxication

The effect of chronic administration of diisopropylphosphorofluoridate (DFP) on the levels and forms of plasma cholinesterase (ChE), were studied in male Wistar albino rats sacrificed at different time intervals after various schedules of treatment. In particular the inhibition and recovery rate of the enzymatic activity was evaluated for butyrylcholinesterase (BuChE), determined using butyrylthiocholine (BuThCh) as substrate and for acetylcholinesterase (AChE), measured using acetylthiocholine (AcThCh) in the presence of iso-OMPA 0.1 mM. At 1 1/2 and 24 hr after the DFP treatments, BuChE was considerably more depressed than was the case for AChE. Moreover, the recovery of BuChE proceeded more slowly, its activity being restored only seven days after the last treatment, while the recovery of AChE was completed 72 hr after the end of the treatments. Plasma molecular forms were separated by polyacrylamide gel electrophoresis and were revealed by enzymatic reaction with BuThCh or AcThCh as substrates. By using selective inhibitors, five main molecular forms of BuChE and two of AChE were found to exist in control plasma samples. A differential inhibition and recovery rate was observed among these forms after DFP intoxication. At 1 1/2 hr after the treatments, the BuChE activity was too low to be detected on the gels, but 24 hr thereafter, the quantitative determination of the different forms, performed by scanning densitometry, showed a significant increase of the two faster migrating ones. At the following time intervals, the electrophoretic pattern returned progressively towards normality. The faster migrating forms are therefore probably the first synthesized in the process of recovery of BuChE activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholinesterase molecular forms from rat and human erythrocyte membrane

1. Some of the biochemical characteristics of acetylcholinesterase from rat and human erythrocytes were studied. 2. Both for rat and man two different acetylcholinesterase molecular forms were identified by gel electrophoresis. The faster moving form is less conspicuous and is not present in all individuals, therefore single-banded and double-banded preparations of red cell acetylcholinesterase can be obtained. The two components appear to be isomers of different molecular size (approximately Mr 150 000 and Mr 245 000) as estimated by gel electrophoresis at different polyacrylamide concentrations. 3. A single band, with a molecular weight of approximately 135 000, was obtained by sodium dodecyl sulphate gel electrophoresis. These results suggest that the faster moving form is a protomer and the slower a dimer. 4. The different sedimentation values obtained by density gradient centrifugation in the presence of Triton X-100 of double-banded (5.3S) and single-banded (6.3S) rat and human acetylcholinesterase preparations, are consistent with a protomer-dimer hypothesis. 5. The isoelectric pattern observed for both double- and single-banded preparations was similar for rat and man acetylcholinesterase and showed a considerable microheterogeneity (thirteen activity bands for rat and eleven for man with isoelectric values from 4.6 to 5.9).

Mechanisms of recovery of brain acetylcholinesterase in rats during chronic intoxication by isoflurophate

During chronic intoxication by diisopropyl fluorophosphate (Isoflurophate, DFP; s.c. treatment on alternate days - first dose of 1.1 mg/kg, subsequent doses of 0.7 mg/kg each until the 23rd day) a partial recovery of enzymatic activity was found at 24 h after each DFP administration. Relative to maximal AChE depression at 90 min, these rises were more pronounced in the soluble portion of the enzyme than in total enzyme preparation, i.e., that containing mainly membrane-bound AChE. Moreover, from the 2nd DFP administration on, there was a persistent increase of medium-molecular-weight forms both in soluble and in total AChE. The results suggest an important role of the soluble portion of AChE and of medium forms in the process of recovery of enzymatic activity.