Metrifonate and dichlorvos: effects of a single oral administration on cholinesterase activity in rat brain and blood

Correlation between acute brain injury and brain metabonomics in dichlorvos-poisoned broilers

Dichlorvos (DDVP) is an insecticide with neurotoxicity that is widely used in agricultural production and life. However, the effects of acute DDVP poisoning on brain tissue remain underinvestigated. The purpose of this study was to evaluate the differences within 15 min-6 h in plasma biochemical indexes, brain histology and metabolites among three groups of commercial broilers orally administered different dosages of DDVP one time: (1) high-dose group (11.3 mg/kg), (2) low-dose group (2.48 mg/kg) and (3) control group (0 mg/kg). The results of biochemical indexes showed that acute DDVP poisoning could cause hyperglycemia and oxidative stress in poisoned broilers. Histological examination showed that DDVP could induce brain edema, abnormal expression of glial fibrillary acidic protein (GFAP) and neuronal mitochondrial damage in broilers. Whole-brain metabolism showed that DDVP could significantly change the secretion of neurotransmitters, energy metabolism, amino acid metabolism and nucleotide metabolism. Correlation analysis showed that metabolites such as hypoxanthine, acetylcarnitine and glucose 6-phosphate were significantly correlated with blood glucose, biomarkers of oxidative stress and brain injury pathology. The results of this study provide new insights into the molecular mechanism of brain tissue responses to acute DDVP exposure in broilers and deliver important information for clinical research on neurodegenerative diseases caused by acute DDVP poisoning.

Non-linear model analysis of the relationship between cholinesterase activity in rats exposed to 2, 2-dichlorovinyl dimethylphosphate (dichlorvos) and its metabolite concentrations in urine

Urinary dialkylphosphates (DAPs) are measured to assess exposure to organophosphorus pesticides (OPs), but they are common metabolites of OPs and not specific indices for individual agents. Biomonitoring (BM) of urinary DAPs has been widely adopted as an assessment of individual exposure in general environments, however, guidance values for DAPs based on health effects have yet to be established. The present study aimed to clarify the relationship between the amount of urinary dimethylphosphate (DMP), a metabolite of dichlorvos (DDVP), and the inhibition of cholinesterase (ChE) activity in rats exposed to DDVP. The relationship was analyzed using a nonlinear model analysis, and the excretion level of urinary DMP equivalent to ChE 20 % inhibition (EL20) and the lower limit of the 95 % confidence interval of EL20 (ELL20) were estimated. EL20 and ELL20 (mg/24 h urine) of brain, erythrocyte, and plasma ChE activities after 10-day administration of DDVP were 0.21 and 0.15, 0.11 and 0.06, and 0.23 and 0.09, respectively. Extrapolating ELL20 of the brain ChE to humans, the range of 24 h urinary DMP concentration according to the 20 % inhibition of cholinesterase activity was estimated to be 20.5-30.8 mg/l. In conclusion, the amount of urinary DMP as ELL20 for DDVP exposure was identified and could probably be used as a novel index for the assessment of risk from OP exposure. Further studies are needed to clarify the ELL20 s derived from OPs other than DDVP, for informing efforts to establish guidance values of urinary OP metabolites that should prevent neurotoxicity.

Polypyridylruthenium(II) complexes exert anti-schistosome activity and inhibit parasite acetylcholinesterases

BACKGROUND

Schistosomiasis affects over 200 million people and there are concerns whether the current chemotherapeutic control strategy (periodic mass drug administration with praziquantel (PZQ)-the only licenced anti-schistosome compound) is sustainable, necessitating the development of new drugs.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the anti-schistosome efficacy of polypyridylruthenium(II) complexes and showed they were active against all intra-mammalian stages of S. mansoni. Two compounds, Rubb12-tri and Rubb7-tnl, which were among the most potent in their ability to kill schistosomula and adult worms and inhibit egg hatching in vitro, were assessed for their efficacy in a mouse model of schistosomiasis using 5 consecutive daily i.v. doses of 2 mg/kg (Rubb12-tri) and 10 mg/kg (Rubb7-tnl). Mice treated with Rubb12-tri showed an average 42% reduction (P = 0.009), over two independent trials, in adult worm burden. Liver egg burdens were not significantly decreased in either drug-treated group but ova from both of these groups showed significant decreases in hatching ability (Rubb12-tri-68%, Rubb7-tnl-56%) and were significantly morphologically altered (Rubb12-tri-62% abnormal, Rubb7-tnl-35% abnormal). We hypothesize that the drugs exerted their activity, at least partially, through inhibition of both neuronal and tegumental acetylcholinesterases (AChEs), as worms treated in vitro showed significant decreases in activity of these enzymes. Further, treated parasites exhibited a significantly decreased ability to uptake glucose, significantly depleted glycogen stores and withered tubercules (a site of glycogen storage), implying drug-mediated interference in this nutrient acquisition pathway.

CONCLUSIONS/SIGNIFICANCE

Our data provide compelling evidence that ruthenium complexes are effective against all intra-mammalian stages of schistosomes, including schistosomula (refractory to PZQ) and eggs (agents of disease transmissibility). Further, the results of this study suggest that schistosome AChE is a target of ruthenium drugs, a finding that can inform modification of current compounds to identify analogues which are even more effective and selective against schistosomes.

Acute toxic effects of inhaled dichlorvos vapor on respiratory mechanics and blood cholinesterase activity in guinea pigs

Using a modified noninvasive volume-displacement plethysmography system, we investigated the effects of inhaled dichlorvos (2,2-dimethyl-dichlorovinyl phosphate, or DDVP) vapor on the respiratory mechanics and blood cholinesterase activity of guinea pigs. Data revealed significant dose-dependent changes in several pulmonary parameters. Animals exposed to a DDVP concentration of 35 mg/m(3) did not show any significant changes in frequency, tidal volume, or minute ventilation. However, animals exposed to 55 mg/m(3) DDVP showed significantly decreased respiratory frequency and significantly increased tidal volume with no significant changes in minute ventilation. Similarly, animals exposed to 75 mg/m(3) DDVP showed significantly decreased respiratory frequency along with significantly increased tidal volume. The decreased respiratory frequency was large enough in the high exposure group to offset the increased tidal volume. This effect resulted in significantly decreased minute ventilation by the end of exposure, which remained attenuated 10 min after exposure. An analysis of whole-blood cholinesterase activity revealed significantly decreased activity for both acetylcholinesterase (AChE) and butyl-cholinesterase (BChE). Peak inhibition occurred for both enzymes at the end of exposure for all three concentrations and rapidly recovered within several minutes of exposure. Analysis of blood samples using gas chromatography-mass spectroscopy (GC-MS) revealed that minute ventilation may only play a minimal role in the dosimetry of inhaled DDVP vapor.

Spatial discrimination deficits by excitotoxic lesions in the Morris water escape task

The effects of the cholinesterase inhibitors (ChEI) metrifonate and donepezil were assessed on spatial performance of rats with bilateral lesions of the entorhinal cortex (EC), which is thought to model early changes in the brains of patients suffering from Alzheimer's disease. In the present study, we found that spatial discrimination deficits in rats, induced by bilateral ibotenic acid (IBO) lesions of the EC region can partially be antagonised by treatment with the cholinesterase inhibitors metrifonate (30 mg kg(-1)) and donepezil (0.3 and 3 mg kg(-1)). Performance was improved in the spatial discrimination task compared with that of the EC-lesioned control group. It is concluded that the rat with bilateral EC lesions is a suitable deficit model for the assessment of effects of putative Alzheimer therapeutics.

Dissociable effects of acetylcholinesterase inhibitors and phosphodiesterase type 5 inhibitors on object recognition memory: acquisition versus consolidation

RATIONALE

Phosphodiesterase enzyme type 5 (PDE5) inhibitors and acetylcholinesterase (AChE) inhibitors have cognition-enhancing properties. However, it is not known whether these drug classes affect the same memory processes.

OBJECTIVE

We investigated the memory-enhancing effects of the PDE5 inhibitor sildenafil and AChE inhibitors metrifonate and donepezil in the object recognition task to find out whether acquisition or consolidation processes were affected by these drugs.

METHODS

The object recognition task measures whether rats remembered an object they have explored in a previous learning trial. All drugs were given orally 30 min before or immediately after learning to study acquisition and consolidation, respectively.

RESULTS

Sildenafil given immediately after the first trial improved the memory performance after 24 h and resulted in an inverted U-shaped dose-effect curve with the peak dose at 3 mg/kg. When given before the first trial, sildenafil also improved the memory performance. However, the dose needed for the best performance under this condition was 10 mg/kg, suggesting that the dose-effect curve shifted to the right. This can be explained by the metabolic clearance of the high dose of sildenafil. Donepezil had no memory improving effect when given after the first trial. However, when given before the first trial, a gradually increasing dose-effect curve was found which had its maximum effect at the highest dose tested (1 mg/kg). Likewise, only when metrifonate (30 mg/kg) was given before the first trial did rats show an improved memory performance.

CONCLUSION

Our data strongly suggest that PDE5 inhibitors improve processes of consolidation of object information, whereas AChE inhibitors improve processes of acquisition of object information.

Effects of acute and repeated administration of a cholinesterase inhibitor on timing behaviour

It has been hypothesised that a leftward shift in the response distribution obtained in the peak interval (PI) procedure is a characteristic of cognitive enhancement in which mental processes are speeded. Metrifonate, a cholinesterase inhibitor with reported cognitive enhancing properties in many animal models of learning and memory, was tested in the PI procedure. Acute administration of 3 and 60 mg/kg but not 1 and 30 mg/kg in fully trained rats shifted the response distribution to the right, whereas subchronic administration of 10, 30 or 50 mg/kg during task acquisition had no effect on timing behaviour. On the basis of the present data, it can be concluded that the effects of a cognition enhancer in the PI procedure cannot be predicted from the scalar expectancy theory (SET). Furthermore, SET does not appear to be an appropriate tool for analysing the acquisition of timing behaviour.

Rapid and sensitive neurotoxicity test based on the morphological changes of PC12 cells with simple computer-assisted image analysis

In order to develop a rapid and sensitive bioassay for the screening of chemicals with possible neurotoxicity, a computer-assisted simple image-analysis system was developed to quantify small changes in the specific morphology of the cultured pheochromocytoma cell line, PC12. This cell forms a neuron-like microfibril network (neurites) in response to a nerve growth factor (NGF) stimulation in vitro. Dichrolvos (DDVP) and methylmercury chloride (MMC) were employed as model neurotoxicants. In DDVP treatment, there was no large difference in the ED50s (effective dose that reduces the morphological index by 50%) among the toxicities determined from various morphological indices, but they were significantly lower than those observed by whole-cell-area-based toxicity assay using the hepatoblastoma cell line, Hep G2. In contrast, in MMC treatment, neurite-length-based toxicity was observed as early as 2 h, and at 48 h this was lower by over three orders of magnitude compared with whole-cell-area-based one (2.06 x 10(-7) mM vs. 6.42 x 10(-4) mM). These results demonstrate that the developed bioassay using image analysis of nerve-tissue-derived cell morphology allows us to screen possible neurotoxic chemicals very rapidly with highly enhanced sensitivity, particularly for some chemicals that preferentially act on nerve fibers.

Efficacy and safety of a loading-dose regimen versus a no-loading-dose regimen of metrifonate in the symptomatic treatment of Alzheimer's disease: a randomized, double-masked, placebo-controlled trial. Metrifonate Study Group

This prospective, randomized, double-masked, placebo-controlled, parallel-group study assessed the safety and efficacy of 2 dosage regimens of once-daily metrifonate in patients with probable Alzheimer's disease (AD) of mild-to-moderate severity. A total of 395 patients were randomized to receive placebo (n = 134) or metrifonate in 1 of 2 regimens. The loading-dose group (n = 133) received a daily loading dose of metrifonate 100 mg or 150 mg (by weight) for 2 weeks, followed by a daily maintenance dose of metrifonate 50 mg for 4 weeks; the no-loading-dose group (n = 128) received the daily maintenance dose of metrifonate 50 mg for 6 weeks. The primary measure of efficacy was the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog); secondary measures of efficacy included the Mini-Mental State Examination (MMSE), the Clinician's Interview Based Impression of Change with Caregiver Input (CIBIC-Plus), the Clinician's Interview Based Impression of Severity with Caregiver Input (CIBIS-Plus), and the ADAS-Noncognitive Subscale (ADAS-Noncog). Safety was assessed by the prevalence of premature study termination and treatment-emergent adverse events, as well as by changes in vital signs, findings on electrocardiographic and neurologic examinations, and abnormalities on laboratory tests. At 4 weeks of treatment, defined by the protocol as the target efficacy determination, the mean ADAS-Cog scores of the intent-to-treat population (last observation carried forward) favored the loading-dose group versus the placebo group, but the difference was not statistically significant. However, at week 6, the difference in mean ADAS-Cog scores was statistically significant compared with placebo. At neither week 4 nor week 6 was there a statistically significant difference in the mean ADAS-Cog scores of the no-loading-dose and placebo groups. For the CIBIC-Plus, the treatment difference between the placebo and loading-dose groups significantly favored metrifonate at week 6 but not at week 4, whereas the treatment difference between the placebo and no-loading-dose groups was statistically significant at both time points. For the MMSE, CIBIS-Plus, and ADAS-Noncog, treatment differences for both groups versus placebo did not reach statistical significance at either week 4 or 6. Assessment of the frequency of adverse events in metrifonate-treated patients revealed that the no-loading-dose regimen was better tolerated than the loading-dose regimen. Given the overall similar efficacy and more favorable safety profile associated with the no-loading-dose regimen versus the loading-dose regimen observed in this study, the no-loading-dose regimen appears to be the better strategy for initiating metrifonate treatment in patients with probable AD of mild-to-moderate severity.

Effect of subchronic treatment with metrifonate and tacrine on brain cholinergic function in aged F344 rats

The effects of 21-day treatment with the acetylcholinesterase inhibitors metrifonate (80 mg kg(-1) per os (p.o.)) and tacrine (3 mg kg(-1) p.o.), twice daily, on cortical and hippocampal cholinergic systems were investigated in aged rats (24-26 months). Extracellular acetylcholine levels were measured by transversal microdialysis in vivo; choline acetyltransferase and acetylcholinesterase activities were measured ex vivo by means of radiometric methods. Basal cortical and hippocampal extracellular acetylcholine levels, measured 18 h after the last metrifonate treatment, were about 15 and two folds higher, respectively, than in control and tacrine-treated rats. A challenge with metrifonate further increased cortical and hippocampal acetylcholine levels by about three and four times, respectively. Basal extracellular acetylcholine levels, measured 18 h after the last treatment with tacrine were not statistically different from those of the control rats. A challenge with tacrine increased cortical and hippocampal extracellular acetylcholine levels by about four and two times. A 75% inhibition of cholinesterase activity was found 18 h after the last metrifonate administration, while only a 15% inhibition was detectable 18 h after the last tacrine administration. The challenge with metrifonate or tacrine resulted in 90 and 80% cholinesterase inhibition, respectively. These results demonstrate that in aging rats a subchronic treatment with metrifonate results in a long-lasting, cholinesterase inhibition, and a persistent increase in acetylcholine extracellular levels which compensate for the age-associated cholinergic hypofunction. Metrifonate is therefore a potentially useful agent for the cholinergic deficit accompanying Alzheimer's disease.

Effects of subchronic administration of metrifonate on cholinergic neurotransmission in rats

The effects of subchronic oral administration of metrifonate, a long-acting cholinesterase (ChE) inhibitor, on cholinergic neurotransmission were assessed in young adult male Wistar rats. Animals were treated twice daily with metrifonate. In a pilot study testing a 100 mg/kg dose of metrifonate for up to 14 days, ChE activity was found to steadily decrease to reach maximum inhibition levels of about 55%, 80% and 35% in brain, erythrocytes and plasma. Steady-state inhibition levels were attained by the 10th day of treatment. When metrifonate-treatment was discontinued, ChE activity in plasma returned to control levels within another day, while erythrocyte and brain ChE activity took more than 2 weeks to recover. In subsequent dose-response studies, metrifonate treatment was given for 3 and 4.5 weeks at doses of 0, 12.5, 25, 50, and 100 mg/kg, to different groups of animals, respectively. Correlation analysis indicted that brain ChE inhibition was more accurately reflected by erythrocyte than by plasma ChE inhibition, although all effects were highly correlated. The changes in ChE activity were not paralleled by changes in other parameters of the cholinergic neurotransmission, such as acetylcholine synthesis rate or acetylcholine receptor binding. It is therefore concluded that repeated administration of metrifonate to rats induces a long-lasting inhibition of ChE activity in a dose-related and predictable manner, which is neither subject to desensitization nor paralleled by counterregulatory downregulation of muscarinic or nicotinic receptor binding sites in brain.

Twenty years of cholinergic intervention in Alzheimer's disease: A tale of disappointment and ultimate confidence

The authors present an overview of drugs that operate on the basis of the cholinergic hypothesis, either currently available for the palliative treatment of Alzheimer's disease or in the later stages of clinical development. Of the many conceivable strategies that are being exploited, only inhibitors of acetylcholinesterase have been shown to have sufficient therapeutic potential and manageable side-effect profiles to be turned into broadly acknowledged drugs. These compounds, and others based on the modulation of cholinergic neurotransmission, are discussed and a differentiation of their clinical potential is attempted.

Subchronic treatment increases the duration of the cognitive enhancement induced by metrifonate

The study compared the efficacy of acute versus chronic metrifonate treatment to improve initial and reversal learning of the water maze spatial navigation task in medial septal-lesioned rats. Acute oral administration of 30 mg/kg metrifonate at 30 min, but not at 150 or 360 min, before training improved the initial acquisition of the water maze task. In contrast, improvement of initial learning performance of medial septal-lesioned rats pretreated for 21 days with metrifonate was observed irrespective of the timing of metrifonate treatment relative to behavioral testing. Reversal learning was assessed after a four-day wash-out period. No drug treatment was administered during this part of the study. All the medial septal-lesioned rats that had received only acute treatment with metrifonate during the initial learning stage were now as impaired as vehicle treated medial septal-lesioned rats. However, the group subchronically pretreated with metrifonate performed better than the vehicle-treated medial septal-lesioned controls. These results indicate that both acute and subchronic treatment with metrifonate can facilitate spatial learning in medial septal-lesioned rats and the transient nature of this beneficial effect after single acute administration is transformed into a long-lasting improvement by subchronic treatment.

Behavioral characterization of metrifonate-improved acquisition of spatial information in medial septum-lesioned rats

We investigated the effects of acute oral pretraining treatment with an indirect acetylcholinesterase inhibitor, metrifonate, on water maze spatial navigation in medial septum-lesioned rats. We observed that metrifonate (30 mg/kg, orally) (1) does not alter the pattern of exploration of lesioned rats at the water maze pool or retrieval of spatial memory, (2) effectively reverses the acquisition defect, (3) enhances reversal learning, and (4) improves acquisition of water maze navigation by facilitating the encoding of the spatial representation of a specific environment. These results indicate that metrifonate does not improve escape performance to the hidden platform by modulating exploration strategy, but that metrifonate enhances the speed and accuracy of development and durability of spatial memory engrams, and facilitates learning capacity that depends on activity of the septo-hippocampal projection.

Metrifonate improves spatial navigation and avoidance behavior in scopolamine-treated, medial septum-lesioned and aged rats

We investigated the effects of acute p.o. pretraining treatment with an indirect acetylcholinesterase inhibitor, metrifonate, on water maze spatial navigation and passive avoidance behavior. Metrifonate (10-100 mg/kg, orally, p.o.) did not improve the water maze or passive avoidance performance of young intact rats. However, in young rats metrifonate over a broad dosage range (10-100 mg/kg, p.o.) was able to alleviate the adverse effects of scopolamine (a muscarinic acetylcholine receptor antagonist; 0.4 and 2.0 mg/kg in water maze and passive avoidance study, respectively) and medial septum-lesioning on spatial reference and working memory and passive avoidance performance. In old (23-month-old) rats, a defect of water maze and passive avoidance behavior was observed. In old rats, metrifonate improved spatial reference memory function in the water maze and also passive avoidance at 10-30 mg/kg, but the 3 mg/kg dose was ineffective. Very old (27-month-old) rats had a more severe impairment of water maze performance than old rats, and metrifonate 3-30 mg/kg did not improve their spatial navigation. These results show that metrifonate may over a wide range of doses stimulate cognitive functioning, but during advanced aging neurobiological defects develop that may mask some of the therapeutic effects of metrifonate in rats.