Cholinergic system of brain tissue in rats poisoned with the organophosphate, 0,0-dimethyl 0-(2,2-dichlorovinyl) phosphate

The cholinergic system of the brain was investigated in rats acutely poisoned with the organophosphate, 0,0-dimethyl 0-(2,2-dichlorovinyl) phosphate (DDVP), (6 mg/kg, sc, with saline as a control). The amounts of three fractions of acetylcholine (ACh)--free (extraterminal), labile-bound (intraterminal/cytoplasmic), and stable-bound (intraterminal/vesicular)--increased in the rats over a period of 5 to 60 min after injection of DDVP, showing peaks which were 2.45, 1.82, and 1.4 times as high as the respective control amounts. No difference was seen in the amount of any fraction of ACh between treated and control rats killed 3 and 24 hr after injection. Acetylcholinesterase (AChE) activity decreased to between 12 and 43% of the control over a period of 5 to 180 min and recovered almost completely within 24 hr after injection. No appreciable changes were seen in either spontaneous or potassium-induced ACh release in brain tissue slices obtained from rats treated with DDVP. ACh synthesis in slices was suppressed significantly 20 min, but not 24 hr, after injection of DDVP. In the brain crude synaptosomal preparation, high-affinity choline uptake, which is generally thought to be a rate-limiting step for ACh synthesis, was suppressed 20 min after DDVP. No appreciable changes were seen in high-affinity choline uptake at 24 hr low-affinity choline uptake, and choline acetyltransferase activity after injection of DDVP. These results suggest that ACh synthesis and high-affinity choline uptake may be in a suppressed state when ACh concentration, especially intraterminal ACh, is increased and AChE activity is decreased in the brain cholinergic system of rats poisoned with DDVP. The increase in the intraterminal ACh may be due to an inhibition of AChE activity at this site and/or a re-uptake of ACh in the synaptic cleft, not to an inhibition of ACh release or an increase in ACh synthesis.     

Delayed neurotoxicity of O-ethyl O-4-cyanophenyl phenylphosphonothioate (cyanofenphos) in hens

Oral administration of a single dose of cyanofenphos (O-ethyl O-4-cyano-phenyl phenylphosphonothioate) between 10 and 500 mg/kg, caused delayed neurotoxocity in hens similar to that reported for other organophosphorus esters. The severity of the clinical condition depended upon the size of the administered dose. Hens given small doses showed only ataxia, while some of those treated with large doses developed paralysis and died. Hens given 500 mg/kg oral dose of tri-o-cresyl phosphate (TOCP) became paralysed, while those given parathion showed early weakness but subsequently recovered without developing delayed neurotoxicity. Control hens receiving atropine sulfate or gelatin capsules remained normal.     

Selective inhibition of rat pulmonary monooxygenase by O, O, S-trimethyl phosphorothioate treatment

The effects of oral administration of O,O,S-trimethyl phosphorothioate (OOS), an impurity present in widely used organophosphorus insecticides, were studied using pulmonary and hepatic microsomal enzymes of rats. The animals were treated with OOS at 10, 20 and 40 mg/kg, and were killed on day 3 after treatment. Their relative lung weights increased markedly at 20 and 40 mg/kg, increasing 94% at the highest dose, whereas the weight of liver decreased. At 20 mg/kg OOS, the cytochrome P-450 content of the lung and liver decreased to 83 and 80% of the control levels respectively. Pulmonary microsomal 7-ethoxycoumarin (7-Ec) O-deethylase decreased in a dose-dependent manner; activities were less than 10% of control at the 40 mg/kg dose. The activity of pulmonary coumarin hydroxylase also decreased following OOS treatment, but the decrease was not dose-dependent since no activity was detectable at doses over 10 mg/kg. In contrast, the effect of OOS treatment on hepatic monooxygenase activity was moderate. 7-Ec deethylase activity was not affected by OOS treatment at any dose level, while p-nitroanisole (p-NA) demethylase activity was decreased only at the 40 mg/kg dose of OOS. Pulmonary malathion carboxylesterase activity was not affected by OOS treatment. In contrast, a dose-dependent decrease was observed in the liver carboxylesterase. Time course effects of OOS treatment on these parameters were examined by treating rats at 20 mg/kg. The animals were killed 0.5, 1, 3 and 7 days after the treatment. The 7-Ec deethylase activity of pulmonary microsomes was decreased on days 0.5, 1 and 3 after treatment, the maximum decrease being observed on day 1. Significant decreases were not observed in hepatic microsomal activities of 7-Ec deethylase or p-nitroanisole demethylase throughout the experimental period; rather, these activities were higher on day 7. Hepatic microsomal malathion carboxylesterase was lower on days 0.5, 1 and 3 after OOS treatment.     

Effect of single or chronic injection with a carbamate, propoxur, on the brain cholinergic system and behavior of mice

Propoxur, an anticholinesterase carbamate, was injected singly (10 mg/kg, s.c.) or chronically (5 mg/kg/day, s.c., for 10 days) into mice. Animals were examined for effects on the cholinergic system in brain tissue and on behavior. Single injection caused an increase in brain acetylcholine (ACh) content at 10 and 60 min; and it caused decreases in acetylcholinesterase (AChE) activity at 10, 60 and 180 min, high-affinity choline uptake into synaptosomes at 10 and 60 min, and [3H]quinuclidinyl benzilate (QNB) binding at 10 min without causing any change in choline acetyltransferase (ChAT) activity. Open-field behavior and rotarod performance were depressed at 10 min and rectal temperature was decreased at 10, 60 and 120 min after a single injection. Chronic treatment caused decreases in high-affinity choline uptake and [3H]QNB binding 24 hr after the final injection without any changes in ACh content, AChE activity or ChAT activity. In behavioral tests conducted 10 min prior to daily administration, rotarod performance was slightly suppressed only during the period of injection. Although propoxur is considered to have effects similar to those of organophosphorus compounds, the changes observed in the present experiments seemed to be reversible.     

Chronic toxicity of technical fenitrothion in male rats

Weanling male Wistar rats were given 0.5, 1.0, 5.0 or 10.0 mg/kg body wt of technical fenitrothion (O,O-dimethyl O-(4-nitro-m-tolyl) phosphorothioate in peanut oil by daily oral gavage for 12 months. Each month, 10 vehicle-and 5 insecticide-treated rats from each group were killed, tissues (liver, kidney, brain, spinal cord, and sciatic nerve) were preserved for morphological examination and the effects of treatment were assessed by measuring changes in activities of blood plasma pseudocholinesterase, erythrocytic and brain acetylcholinesterase (AChE), and hepatic and renal nonspecific carboxylesterases (CE) as well as routine hematology and serum biochemistry. At 12 months, administration was stopped and the survivors were held for an additional 2 months to monitor the recovery from the biological effects of the treatment. No changes were observed in the routine hematological or serum biochemical parameters measured during the treatment period at any of the doses administered. The morphological changes observed could not be related either to the vehicle or to a dose-dependent effect of fenitrothion. A dose-dependent inhibition of the tissue esterases was observed. Little change was measured at 0.5 and 1.0 mg/kg/day. Marked reductions in hepatic CE, brain, and erythrocytic AChE activities were observed at doses of 5.0 and 10.0 mg/kg/day within 1 month of treatment and remained relatively constant for the duration of treatment. Within 1 month after treatment was stopped, tissue esterase levels were within normal limits with the exception of the brain AChE in rats given the two larger doses. Within 2 months after treatment was terminated, tissue esterase activities of all treated rats were comparable to control activities.     

Inhibition of mammalian brain acetylcholinesterase by ketamine

Ketamine caused a reversible inhibition of both membrane-bound and purified forms of acetylcholinesterase (AChE; EC 3.1.1.7.) prepared from beef brain caudate nucleus tissue. Apparent K_i values (× 10^?4 M) ranged between 4·9 and 6·9 for the different enzyme forms. Inhibition was of the mixed kinetic type, which suggests interactions of the drug with both active site(s) and other anionic sites on the enzyme. Rat brain AChE was inhibited by ketamine in vitro at concentrations commensurate with brain levels of the drug determined after i.v. administration to rats. Gas Chromatographic analyses demonstrated a 26 per cent increase in rat brain acetylcholine (ACh) 30 sec after the administration of ketamine (20 mg/kg, i.v.). ACh accumulation via inhibition of AChE may underlie certain pharmacological effects of ketamine which resemble cholinergic stimulation.     

The effects of organophosphate-induced cholinergic stimulation on the antibody response to sheep erythrocytes in inbred mice

In a previous study, we demonstrated that parathion suppressed both the primary IgM and IgG response to sheep erythrocytes (SRC) in inbred and outbred mice (G. P. Casale, S. D. Cohen, and R. A. DiCapua, 1982, Toxicologist2, 94). Suppression occurred after a dosage which produced cholinergic effects but was absent after a lower dosage which did not produce cholinergic signs. This information suggested that immunosuppression might be mediated indirectly as a result of toxic chemical stress. The present study evaluated the relationship between the anticholinesterase action of parathion, malathion, and dichlorvos (DDVP) and their effects on the primary humoral response to SRC. Male $\text{C57B1}\text{6}$ mice were given a single dose of parathion (16 mg/kg, po), malathion (720 mg/kg, po), or DDVP (120 mg/kg, po) 2 days after immunization with SRC. Two days later, tissues were removed for cholinesterase (CHE) assay and enumeration of splenic antibody-forming cells (PFC). All three compounds produced moderate to severe cholinergic poisoning. DDVP produced cholinergic signs beginning $\text{1}\text{2}$ hr after dosing and lasting $\text{1}\text{2}$ to 1 hr. This profile was associated with a rapid but transient inhibition of brain CHE activity. In contrast, malathion and parathion produced prolonged cholinergic poisoning (4 to 7 hr) and prolonged suppression of brain CHE activity. All three compounds suppressed the primary IgM response. However, when they were given as multiple lower doses, none of the compounds suppressed the primary IgG response. These latter treatments produced no cholinergic signs. The cholinomimetic agent, arecoline (65 mg/kg, ip) produced a short-lived cholinergic crisis but no IgM suppression. Sustained-release arecoline produced prolonged cholinergic poisoning (3 to 5 hr) and reduced the number of IgM PFC to 50% of control. These results demonstrated that organophosphate-induced immunosuppression was associated with severe cholinergic stimulation. The immunosuppression may result from direct action of acetylcholine upon the immune system or it may be secondary to the toxic chemical stress associated with cholinergic poisoning.     

Subacute toxicity in rats of orally administered fenitrothion alone and in a selected formulation

Young adult male and female Sprague-Dawley rats received 1.0, 5.0, 10.0, or 20.0 mg/kg body wt of technical fenitrothion (O,O-dimethyl O-(4-nitro-m-tolyl)phosphorothioate) in ethanol:propylene glycol (25:75) in the absence or presence of 1.5% ($\text{v}\text{v}$) of each of Atlox 3409F (emulsifier) and Dowanol TPM (cosolvent) by daily gavage for 30 consecutive days. At 7, 14, 21, and 30 days of treatment, five randomly selected rats from each treatment group were killed; tissues (liver, kidney, brain, spinal cord, and distal sciatic nerve) were preserved for morphological examination. The effects of treatment were assessed by measuring changes in blood plasma pseudocholinesterase (ψChE), erythrocytic and brain acetylcholinesterase (AChE), and hepatic and renal nonspecific carboxylesterase (CE) as well as routine hematology and serum biochemistry. At 8, 30, 60, and 90 days after treatment, five animals of each group were killed to assess the reversibility of any observed biochemical, physiological, or histopathological effects. No changes in the routine hematological and serum biochemical parameters or in tissue morphology were observed which could be related either to the vehicle, emulsifier(s)-cosolvent, or to a dose-dependent effect of fenitrothion. A dose-dependent inhibition of the tissue esterases was observed at doses above 1.0 mg fenitrothion/kg body wt. Recovery of esteratic activity from the treatment was rapid except for those rats receiving the largest dose; however, within 2 months after treatment, the activities of all treated rats were comparable to control activities. The emulsifier (Atlox 3409F) and the cosolvent (Dowanol TPM), at concentrations routinely employed in aerial spraying formulations, did not substantially enhance or reduce the toxicity of fenitrothion.     

Cortical and Striatal in Vivo Uptake and Metabolism of Plasma Choline in the Rat: Effects of Haloperidol and Apomorphine

Abstract Following intravenous injection of a tracer dose of radioactive choline (³H–Ch), blood ³H–Ch was incorporated into parietal cortex at 1.2% per min. while the corresponding rate in the striatum was 6.4. It is suggested that this linear relationship in the time–interval 10 sec. to 20 min. following ³H–Ch injection demonstrates the in vivo existence of active uptake mechanisms of plasma Ch into brain. Furthermore, 36 % per min. of cortical ³H–Ch was converted to Ch metabolites other than acetylcholine (ACh) while the striatal percentage was only 6. This is probably due to the distribution of ³H–Ch into an “ACh–available” Ch pool within the cholinergic neurones which show the highest density in the striatum. Based on the present results and data in the literature a hypothetical scheme for the fate of brain Ch is discussed. The above mentioned parameters were also examined in cataleptic rats following haloperidol treatment (2 mg/kg intraperitoneally) as well as in rats slightly hyperactive following combined treatment with haloperidol (2 mg/kg intraperitoneally) and apomorphine (10 mg/kg intravenously). The results can be interpreted in the light of a dopaminergic inhibition on intrastriatal cholinergic neurones.     

Disposition,elimination and metabolism of O-ethylO-4-nitrophenyl phenylphosphonothioate after subchronic dermal application in male cats

The metabolism, distribution, and excretion of the insecticide O-ethylO-4-nitrophenyl phenylphosphonothioate (EPN) were studied in the male cat. Each cat was given a daily dermal dose of 0.5 mg/kg [¹⁴C] EPN for 10 consecutive days. Fifteen days after the last dose, the cats had excreted 62% of the cumulative dose in the urine and 10% in the faces. No ¹⁴CO₂ was detected in the expired air. O-Ethyl phenylphosphonic acid (EPPA) was identified as the major urinary and fecal metabolite. Phenylphosphonic acid (PPA) was the second highest metabolite. Only traces of the intact EPN were recovered in the urine and feces. The disposition studies performed 1, 5, 10 and 15 days after the administration of the last dose showed that EPN was the major compound identified in the brain, spinal cord, sciatic nerve, adipose tissue, plasma and kidney. Most of the radioactivity in the liver was identified as EPPA followed by PPA. The time course of plasma EPN, determined after the 10th daily dose was biphasic. The slower process had a half-life of 17.0 days. After tissue distribution was completed, tissue elimination was adequately represented as a single first-order process.     

Cholinesterase inhibition by phenothiazine and nonphenothiazine antihistaminics: analysis of its postulated role in synergizing organophosphate toxicity.

Several antihistaminic compounds inhibit in vitro pseudocholinesterase (ChE) from several sources (human, rat, or horse plasma), pI50 ranging from 5.0 to 6.0. This inhibition is competitive with acetylcholine (ACh) and is reversible. Acetylcholinesterase (AChE) from rat brain was nonsensitive to antihistaminics even at a concentration of 4 mm.Antihistaminics do not compete with either organophosphates or quaternary ammonium compounds in inhibiting ChE. Inhibition of ChE by antihistaminics is more pronounced at alkaline pH values than at acidic or neutral pH. Administration of promethazine ip to rats at a dose of 50 mg/kg resulted in a 90% inhibition of ChE 1 hr later, while the AChE was totally nonsensitive to the inhibitor even when the dose of the antihistamine was doubled. Results were analyzed in relation to the postulated role of ChE inhibition in antihistaminic potentiation of organophosphorus intoxication.     

Neuropathy target esterase in mouse whole blood as a biomarker of exposure to neuropathic organophosphorus compounds

The adult hen is the standard animal model for testing organophosphorus (OP) compounds for organophosphorus compound‐induced delayed neurotoxicity (OPIDN). Recently, we developed a mouse model for biochemical assessment of the neuropathic potential of OP compounds based on brain neuropathy target esterase (NTE) and acetylcholinesterase (AChE) inhibition. We carried out the present work to further develop the mouse model by testing the hypothesis that whole blood NTE inhibition could be used as a biochemical marker for exposure to neuropathic OP compounds. Because brain NTE and AChE inhibition are biomarkers of OPIDN and acute cholinergic toxicity, respectively, we compared NTE and AChE 20‐min IC₅₀ values as well as ED₅₀ values 1 h after single intraperitoneal (i.p.) injections of increasing doses of two neuropathic OP compounds that differed in acute toxicity potency. We found good agreement between the brain and blood for in vitro sensitivity of each enzyme as well for the ratios IC₅₀(AChE)/IC₅₀(NTE). Both OP compounds inhibited AChE and NTE in the mouse brain and blood dose‐dependently, and brain and blood inhibitions in vivo were well correlated for each enzyme. For both OP compounds, the ratio ED₅₀(AChE)/ED₅₀(NTE) in blood corresponded to that in the brain despite the somewhat higher sensitivity of blood enzymes. Thus, our results indicate that mouse blood NTE could serve as a biomarker of exposure to neuropathic OP compounds. Moreover, the data suggest that relative inhibition of blood NTE and AChE provide a way to assess the likelihood that OP compound exposure in a susceptible species would produce cholinergic and/or delayed neuropathic effects. Copyright © 2016 John Wiley & Sons, Ltd.     

Methyl parathion activation by a partially purified rat brain fraction

Organophosphorus pesticides are one of the most commonly used insecticide classes. They act through a potent inhibition of acetylcholinesterase (AChE). Many of them must undergo transformation into the corresponding oxon analogs to inhibit AChE. This study showed that a brain tissue subfraction transformed methyl parathion (O,O-dimethyl O-p-nitrophenyl phosphorothioate) in vitro. Methyl parathion activation was assayed by solvent extraction of the products followed by HPLC and GC-MS analyses and, indirectly, by the inhibition of AChE present in the incubation mixture. The lack of impairment of AChE after 2 h of incubation of the brain subfraction with methyl parathion and, alternatively, with NADPH, CO, SKF 525-A, piperonyl butoxide or nitrogen indicated that this brain subfraction transformed methyl parathion without the involvement of a mixed-function oxidative pathway.The results from HPLC analysis did not show a peak corresponding to methyl paraoxon (O,O-dimethyl O-p-nitrophenylphosphate), but showed the production of an unidentified peak which eluted nearby standard methyl parathion (retention times of 10.65 and 8.86 min, respectively). GC-MS analysis suggested that the unidentified product could be a methyl parathion isomer.     

Effects of multiple dosing of fenthion, fenitrothion, and desbromoleptophos in young chicks

The effects of multiple doses of desbromoleptophos, fenitrothion, and pure fenthion on brain acetylcholinesterase (AChE), brain neurotoxic esterase (NTE), and walking were investigated in immature chicks, below the age of sensitivity to organophosphorus ester-induced delayed neurotoxicity (OPIDN). Ten milligrams per kilogram per day of delayed neurotoxicant desbromoleptophos (DBL), 15 mg/kg.d of the non-neurotoxicant fenitrothion (FTR), and 3 mg/kg.d of the suspected neurotoxicant fenthion (FEN) were given orally for 7 d to 3-d-old chicks. Behavioral testing was performed for treated and control chicks on various days after treatment. Brain NTE and AChE assays were carried out for treated and control chicks on each day of behavioral testing. DBL altered gait and inhibited both NTE and AChE; FEN altered gait and inhibited AChE but not NTE; and FTR did not affect gait, while inhibiting AChE but not NTE. NTE and AChE inhibition were 70% and 55%, respectively, 24 h after the last treatment, for the chicks treated with DBL. NTE returned to normal levels by around d 25 and AChE by 20 d after the last treatment. FTR caused more than 50% AChE inhibition but no NTE inhibition, 24 h after last treatment. NTE inhibition for the FEN-treated chicks never exceeded 11% during the whole period of the experiment, whereas 54% inhibition of AChE was seen 1 d after last treatment. DBL and FEN significantly altered the gait of treated chicks, but the non-OPIDN-inducing FTR did not. This study confirms that alterations in the gait of young chicks are not direct consequences of either NTE or AChE inhibition, and that fenthion-induced functional deficits can be distinguished from classical OPIDN.     

Phenthoate-induced changes in the profiles of acetylcholinesterase and acetylcholine in the brain of Anabas testudineus (Bloch): Acute and delayed effect

A concomitant decrease in acetylcholinesterase (AChE) activity and increase in acetylcholine (ACh) concentration was observed in the brain of Anabas testudineus (Bloch) during a 48-h phenthoate treatment. On return to phenthoate-free fresh water, the decline in enzyme activity and rise in ACh concentration was found to persist for up to 20 days following exposure. Normal values for brain AChE activity and ACh concentration were restored only after 60 days following transfer to phenthoate-free fresh water. It is concluded that inhibition of AChE results in ACh accumulation which, in turn, initiates a faster rate of recovery of the enzyme in fish exposed to the highest concentration of phenthoate.     

Therapeutic efficacy of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) against organophosphate intoxication

The objective of the present study was to investigate whether reduction of central acetylcholine (ACh) accumulation by adenosine receptor agonists could serve as a generic treatment against organophosphate (OP) poisoning. The OPs studied were tabun ( O-ethyl- N-dimethylphosphoramidocyanidate), sarin (isopropylmethylphosphonofluoridate), VX ( O-ethyl- S-2-diisopropylaminoethylmethylphosphonothiolate) and parathion ( O, O-diethyl- O-(4-nitrophenyl)phosphorothioate). The efficacy of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) against an OP intoxication was examined on the basis of the occurrence of clinical symptoms that are directly associated with such intoxication. CPA (1-2 mg/kg) effectively attenuated the cholinergic symptoms and prevented mortality in lethally tabun- or sarin-intoxicated rats. In contrast, CPA (2 mg/kg) proved to be ineffective against VX or parathion intoxication. Intracerebral microdialysis studies revealed that survival of sarin-poisoned and CPA-treated animals coincided with a minor elevation of extracellular ACh concentrations in the brain relative to the baseline value, whereas an 11-fold increase in transmitter levels was observed in animals not treated with CPA. In VX-intoxicated rats, however, the ACh amounts increased 18-fold, irrespective of treatment with CPA. The striatal acetylcholinesterase (AChE) activity following a lethal sarin intoxication was completely abolished in the vehicle-treated animals, whereas 10% and 60% AChE activity remained in animals treated with 2 mg/kg CPA 1 min after or 2 min prior to the poisoning, respectively. In VX-intoxicated animals the AChE activity in the brain was strongly reduced (striatum 10%, hippocampus 1%) regardless of the CPA treatment. These results demonstrate that CPA is highly effective against tabun or sarin poisoning, but fails to protect against VX or parathion. Survival and attenuation of clinical signs in tabun- or sarin-poisoned animals are associated with a reduction of ACh accumulation and with protection of AChE activity in the brain.     

Differential metabolism of O-ethyl O-4-nitrophenyl phenylphosphonothioate by rat and chicken hepatic microsomes

The in vitro hepatic metabolism of O-ethyl O-4-nitrophenyl phenylphosphonothioate (EPN) was investigated in the hen (a species that is sensitive to EPN delayed neurotoxicity) and the rat (an insensitive species). EPN, which produced a Type I binding spectrum on incubation with cytochrome P-450, was converted by liver microsomes from both species to its oxygen analog, O-ethyl O-4-nitrophenyl phenylphosphonate (EPNO), and to p-nitrophenol (PNP). The formation of EPNO and PNP was dependent on the presence of NADPH in the reaction mixture and could be inhibited by either SKF-525A or by anaerobic conditions. The rates of EPNO and PNP formation by rat liver microsomes were, however, 3- and 20-fold higher, respectively, than the rates of formation by chicken liver microsomes. There was also a 4-fold difference in the cytochrome P-450 contents of the liver microsomes. The EPNO-hydrolyzing activity of rat liver microsomes was much greater than that of chicken liver microsomes. EPNO metabolism, in contrast to EPN metabolism, did not require NAPDH nor was it inhibited by SKF-525A or by anaerobic conditions. Prior exposure of rats to phenobarbital (PB) or Arochlor 1254 resulted in an increase in hepatic microsomal EPN metabolism and cytochrome P-450 content. On the other hand, 3-methylcholanthrene (3-MC) treatment elevated microsomal cytochrome P-450 but did not increase EPNO or PNP formation. Pretreatment with EPN did not alter either microsomal EPN metabolism or cytochrome P-450 levels. In chickens, prior exposure to PB, 3-MC or 100 mg/kg EPN increased EPNO and PNP formation by liver microsomes as well as cytochrome P-450 levels; prior exposure of chickens to 15 mg/kg EPN did not alter these variables. The λ_max Soret bands of the reduced hepatic cytochrome P-450 complexes from these animals differed as follows (rat then chicken): untreated, 450 vs 452 nm; PB-treated, 450 vs 451 nm; and 3-MC-treated, 448 vs 449 nm. None of the above treatments had an effect on EPNO metabolism by liver microsomes.     

Donepezil‐ or rivastigmine‐induced acetylcholinesterase inactivation is not modulated by neramexane in rat brain

In Alzheimer's disease (AD), the involvement of cholinergic deficit and overstimulation of N‐methyl‐D‐aspartate (NMDA) receptors by excessively released glutamate has been proposed. Since no existing drug currently addresses both pathologies, a combination therapy may be used clinically to target both mechanisms. The objective of the current study was to determine whether the NMDA receptor antagonist, neramexane affects the acetylcholinesterase (AChE) inhibition produced by donepezil and rivastigmine, two drugs used in the treatment of AD, or a prototype organophosphate compound diisopropylphosphorofluoridate (DFP) in rat cortex, hippocampus, striatum, and brainstem. Neramexane, at therapeutically relevant or higher doses (3.1, 6.2, or 12.4 mg/kg, ip), did not produce any apparent behavioral toxicity. Donepezil (0.75 mg/kg, ip), rivastigmine (0.35 mg/kg, ip) or DFP (1.5 mg/kg, ip) were administered at doses that produced approximately 50% inhibition of AChE and were well tolerated. Also, neramexane in combination with either of the AChE inhibitors did not produce any behavioral toxicity. This magnitude of AChE inhibition has been shown to increase extracellular acetylcholine (ACh) concentrations to therapeutically relevant levels. Neramexane (12.4 mg/kg) did not inhibit AChE activity in any of the brain regions tested when evaluated over a 24‐h period. Rats receiving neramexane alone or in combination with an AChE inhibitor (donepezil, rivastigmine, or DFP) were sacrificed 60 min after neramexane or DFP, 15 min after donepezil, and 30 min after rivastigmine administration. The times at which drug (donepezil, rivastigmine, or DFP) effects were assessed were based on maximal AChE inhibition established from previously reported time course studies. The current findings indicate that neramexane alone did not affect AChE activity at any dose tested nor did it influence the AChE inhibition produced by donepezil or rivastigmine in any brain region examined. However, a low dose of neramexane (3.1 mg/kg) significantly attenuated the inhibition of AChE produced by DFP in the hippocampus, striatum and brainstem, while higher doses (6.2 or 12.4 mg/kg) attenuated DFP‐induced AChE inhibition in all four of the brain regions evaluated. These results suggest that AChE inhibition produced by donepezil or rivastigmine is not altered by the co‐administration of neramexane. Drug Dev Res 68:253–260, 2007. © 2007 Wiley‐Liss, Inc.     

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/m³ did not show any significant changes in frequency, tidal volume, or minute ventilation. However, animals exposed to 55 mg/m³ DDVP showed significantly decreased respiratory frequency and significantly increased tidal volume with no significant changes in minute ventilation. Similarly, animals exposed to 75 mg/m³ 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 butylcholinesterase (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.     

Effects of 4-dimethylaminophenol,Co2EDTA,or NaNO2 on cerebral blood flow and sinus blood homeostasis of dogs in connection with acute cyanide poisoning

The effects of the cyanide antidotes DMAP, Co₂EDTA, and NaNO₂ on cerebral blood flow (CBF) and cerebral blood gases were investigated in connection with acute poisoning of dogs by cyanide. The substances were injected intravenously. Local CBF as measured with thermocouples in the cingulum increased by 100–200% after a non-lethal dose of KCN (1 mg/kg) and by 50% after injection of NaNO₂ (15 mg/kg), that oxidized some 20% of the total hemoglobin to ferrihemoglobin. Co₂EDTA (10 mg/kg) induced a decrease in local CBF of 30% and in brain temperature of 0.5°C. The temperature diminished also after poisoning by KCN, but it rose by 0.15°C after the administration of NaNO₂. Local CBF and sinus sagittalis blood flow increased by 60–160% for about 15 min, and the brain temperature decreased by 0.4–0.5°C when DMAP (3.25 mg/kg) or Co₂EDTA (15 mg/kg) was injected 1 min after poisoning by cyanide (4 mg/kg), a dose that always caused respiratory arrest. Immediately after injection of DMAP the brain temperature rose transiently by 0.1–0.2°C. Co₂EDTA did not exert such an effect. In the sinus sagittalis blood of artificially ventilated animals pCO₂ decreased rapidly by 10–20 mmHg after poisoning and approached the initial level after treatment with DMAP or Co₂EDTA. The highest value of pO₂ was about 80 mmHg and 50 mmHg after injection of DMAP and Co₂EDTA, respectively; thereafter pO₂ declined to 20 mmHg or 40 mmHg at 20 min. The lactate concentration increased by 60–70% without tendency to return to normal.