Oral Pretreatment with Galantamine Effectively Mitigates the Acute Toxicity of a Supralethal Dose of Soman in Cynomolgus Monkeys Posttreated with Conventional Antidotes.

Earlier reports suggested that galantamine, a drug approved to treat mild-to-moderate Alzheimer's disease (AD), and other centrally acting reversible acetylcholinesterase (AChE) inhibitors can serve as adjunct pretreatments against poisoning by organophosphorus compounds, including the nerve agent soman. The present study was designed to determine whether pretreatment with a clinically relevant oral dose of galantamine HBr mitigates the acute toxicity of 4.0×LD50 soman (15.08 µg/kg) in Macaca fascicularis posttreated intramuscularly with the conventional antidotes atropine (0.4 mg/kg), 2-pyridine aldoxime methyl chloride (30 mg/kg), and midazolam (0.32 mg/kg). The pharmacokinetic profile and maximal degree of blood AChE inhibition (∼25%-40%) revealed that the oral doses of 1.5 and 3.0 mg/kg galantamine HBr in these nonhuman primates (NHPs) translate to human-equivalent doses that are within the range used for AD treatment. Subsequent experiments demonstrated that 100% of NHPs pretreated with either dose of galantamine, challenged with soman, and posttreated with conventional antidotes survived 24 hours. By contrast, given the same posttreatments, 0% and 40% of the NHPs pretreated, respectively, with vehicle and pyridostigmine bromide (1.2 mg/kg, oral), a peripherally acting reversible AChE inhibitor approved as pretreatment for military personnel at risk of exposure to soman, survived 24 hours after the challenge. In addition, soman caused extensive neurodegeneration in the hippocampi of saline- or pyridostigmine-pretreated NHPs, but not in the hippocampi of galantamine-pretreated animals. To our knowledge, this is the first study to demonstrate the effectiveness of clinically relevant oral doses of galantamine to prevent the acute toxicity of supralethal doses of soman in NHPs. SIGNIFICANCE STATEMENT: This is the first study to demonstrate that a clinically relevant oral dose of galantamine effectively prevents lethality and neuropathology induced by a supralethal dose of the nerve agent soman in Cynomolgus monkeys posttreated with conventional antidotes. These findings are of major significance for the continued development of galantamine as an adjunct pretreatment against nerve agent poisoning.

Learning and memory retention deficits in prepubertal guinea pigs prenatally exposed to low levels of the organophosphorus insecticide malathion.

High doses of malathion, an organophosphorus (OP) insecticide ubiquitously used in agriculture, residential settings, and public health programs worldwide, induce a well-defined toxidrome that results from the inhibition of acetylcholinesterase (AChE). However, prenatal exposures to malathion levels that are below the threshold for AChE inhibition have been associated with increased risks of neurodevelopmental disorders, including autism spectrum disorder with intellectual disability comorbidity. The present study tested the hypothesis that prenatal exposures to a non-AChE-inhibiting dose of malathion are causally related to sex-biased cognitive deficits later in life in a precocial species. To this end, pregnant guinea pigs were injected subcutaneously with malathion (20 mg/kg) or vehicle (peanut oil, 0.5 ml/kg) once daily between approximate gestational days 53 and 63. This malathion dose regimen caused no significant AChE inhibition in the brain or blood of dams and offspring and had no significant effect on the postnatal growth of the offspring. Around postnatal day 30, locomotor activity and habituation, a form of non-associative learning, were comparable between malathion- and peanut oil-exposed offspring. However, in the Morris water maze, malathion-exposed offspring presented significant sex-dependent spatial learning deficits in addition to memory impairments. These results are far-reaching as they indicate that: (i) malathion is a developmental neurotoxicant and (ii) AChE inhibition is not an adequate biomarker to derive safety limits of malathion exposures during gestation. Continued studies are necessary to identify the time and dose dependence of the developmental neurotoxicity of malathion and the mechanisms underlying the detrimental effects of this insecticide in the developing brain.

Evidence for positive allosteric modulation of cognitive-enhancing effects of nicotine in healthy human subjects.

RATIONALE: Cognitive benefits of nicotinic acetylcholine receptor (nAChR) agonists are well established but have generally been of small magnitude and uncertain clinical significance. A way of raising the effect size may be to facilitate agonist-induced responses by co-administering a nAChR positive allosteric modulator (PAM). OBJECTIVE: The aim was to test whether galantamine, a PAM at several nAChR subtypes, can potentiate the cognitive-enhancing effects of nicotine. METHODS: Twenty-six adult never-smokers were treated, in a double-blind counterbalanced sequence, with nicotine (7 mg/24 h, transdermally) and galantamine (4 mg, p.o.) combined, nicotine alone, galantamine alone, and double placebo. A low dose of galantamine was chosen to minimize acetylcholinesterase inhibition, which was verified in blood assays. In each condition, participants were tested with three cognitive tasks. RESULTS: Nicotine significantly improved reaction time (RT) and signal detection in a visuospatial attention task and the Rapid Visual Information Processing Task. Galantamine did not modulate these effects. A trend toward RT reduction by galantamine correlated with acetylcholinesterase inhibition. In a change detection task, there were no effects of nicotine or galantamine alone on accuracy or RT. However, both drugs combined acted synergistically to reduce RT. This effect was not associated with acetylcholinesterase inhibition. CONCLUSIONS: A pattern consistent with allosteric potentiation of nicotine effects by galantamine was observed on one of six performance measures. This may reflect specific nAChR subtype involvement, or additional pharmacological actions of galantamine may have overshadowed similar interactions on other measures. The finding suggests that allosteric potentiation of nAChR agonist-induced cognitive benefits is possible in principle.

Developmental neurotoxicity of the organophosphorus insecticide chlorpyrifos: from clinical findings to preclinical models and potential mechanisms.

Organophosphorus (OP) insecticides are pest-control agents heavily used worldwide. Unfortunately, they are also well known for the toxic effects that they can trigger in humans. Clinical manifestations of an acute exposure of humans to OP insecticides include a well-defined cholinergic crisis that develops as a result of the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter acetylcholine (ACh). Prolonged exposures to levels of OP insecticides that are insufficient to trigger signs of acute intoxication, which are hereafter referred to as subacute exposures, have also been associated with neurological deficits. In particular, epidemiological studies have reported statistically significant correlations between prenatal subacute exposures to OP insecticides, including chlorpyrifos, and neurological deficits that range from cognitive impairments to tremors in childhood. The primary objectives of this article are: (i) to address the short- and long-term neurological issues that have been associated with acute and subacute exposures of humans to OP insecticides, especially early in life (ii) to discuss the translational relevance of animal models of developmental exposure to OP insecticides, and (iii) to review mechanisms that are likely to contribute to the developmental neurotoxicity of OP insecticides. Most of the discussion will be focused on chlorpyrifos, the top-selling OP insecticide in the United States and throughout the world. These points are critical for the identification and development of safe and effective interventions to counter and/or prevent the neurotoxic effects of these chemicals in the developing brain. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Spatial learning impairment in prepubertal guinea pigs prenatally exposed to the organophosphorus pesticide chlorpyrifos: Toxicological implications.

Exposure of the developing brain to chlorpyrifos (CPF), an organophosphorus (OP) pesticide used extensively in agriculture worldwide, has been associated with increased prevalence of cognitive deficits in children, particularly boys. The present study was designed to test the hypothesis that cognitive deficits induced by prenatal exposure to sub-acute doses of CPF can be reproduced in precocial small species. To address this hypothesis, pregnant guinea pigs were injected daily with CPF (25mg/kg,s.c.) or vehicle (peanut oil) for 10days starting on presumed gestation day (GD) 53-55. Offspring were born around GD 65, weaned on postnatal day (PND) 20, and subjected to behavioral tests starting around PND 30. On the day of birth, butyrylcholinesterase (BuChE), an OP bioscavenger used as a biomarker of OP exposures, and acetylcholinesterase (AChE), a major molecular target of OP compounds, were significantly inhibited in the blood of CPF-exposed offspring. In their brains, BuChE, but not AChE, was significantly inhibited. Prenatal CPF exposure had no significant effect on locomotor activity or on locomotor habituation, a form of non-associative memory assessed in open fields. Spatial navigation in the Morris water maze (MWM) was found to be sexually dimorphic among guinea pigs, with males outperforming females. Prenatal CPF exposure impaired spatial learning more significantly among male than female guinea pigs and, consequently, reduced the sexual dimorphism of the task. The results presented here, which strongly support the test hypothesis, reveal that the guinea pig is a valuable animal model for preclinical assessment of the developmental neurotoxicity of OP pesticides. These findings are far reaching as they lay the groundwork for future studies aimed at identifying therapeutic interventions to treat and/or prevent the neurotoxic effects of CPF in the developing brain.

Acute toxicity of organophosphorus compounds in guinea pigs is sex- and age-dependent and cannot be solely accounted for by acetylcholinesterase inhibition.

This study was designed to test the hypothesis that the acute toxicity of the nerve agents S-[2-(diisopropylamino)ethyl]-O-ethyl methylphosphonothioate (VX), O-pinacolyl methylphosphonofluoridate (soman), and O-isopropyl methylphosphonofluoridate (sarin) in guinea pigs is age- and sex-dependent and cannot be fully accounted for by the irreversible inhibition of acetylcholinesterase (AChE). The subcutaneous doses of nerve agents needed to decrease 24-h survival of guinea pigs by 50% (LD(50) values) were estimated by probit analysis. In all animal groups, the rank order of LD(50) values was sarin > soman > VX. The LD(50) value of soman was not influenced by sex or age of the animals. In contrast, the LD(50) values of VX and sarin were lower in adult male than in age-matched female or younger guinea pigs. A colorimetric assay was used to determine the concentrations of nerve agents that inhibit in vitro 50% of AChE activity (IC(50) values) in guinea pig brain extracts, plasma, red blood cells, and whole blood. A positive correlation between LD(50) values and IC(50) values for AChE inhibition would support the hypothesis that AChE inhibition is a major determinant of the acute toxicity of the nerve agents. However, such a positive correlation was found only between LD(50) values and IC(50) values for AChE inhibition in brain extracts from neonatal and prepubertal guinea pigs. These results demonstrate for the first time that the lethal potencies of some nerve agents in guinea pigs are age- and sex-dependent. They also support the contention that mechanisms other than AChE inhibition contribute to the lethality of nerve agents.

Effective countermeasure against poisoning by organophosphorus insecticides and nerve agents.

The nerve agents soman, sarin, VX, and tabun are deadly organophosphorus (OP) compounds chemically related to OP insecticides. Most of their acute toxicity results from the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that inactivates the neurotransmitter acetylcholine. The limitations of available therapies against OP poisoning are well recognized, and more effective antidotes are needed. Here, we demonstrate that galantamine, a reversible and centrally acting AChE inhibitor approved for treatment of mild to moderate Alzheimer's disease, protects guinea pigs from the acute toxicity of lethal doses of the nerve agents soman and sarin, and of paraoxon, the active metabolite of the insecticide parathion. In combination with atropine, a single dose of galantamine administered before or soon after acute exposure to lethal doses of soman, sarin, or paraoxon effectively and safely counteracted their toxicity. Doses of galantamine needed to protect guinea pigs fully against the lethality of OPs were well tolerated. In preventing the lethality of nerve agents, galantamine was far more effective than pyridostigmine, a peripherally acting AChE inhibitor, and it was less toxic than huperzine, a centrally acting AChE inhibitor. Thus, a galantamine-based therapy emerges as an effective and safe countermeasure against OP poisoning.

Targeted deletion of the kynurenine aminotransferase ii gene reveals a critical role of endogenous kynurenic acid in the regulation of synaptic transmission via alpha7 nicotinic receptors in the hippocampus.

It has been postulated that endogenous kynurenic acid (KYNA) modulates alpha7* nicotinic acetylcholine receptor (nAChR) and NMDA receptor activities in the brain.a To test this hypothesis, alpha7* nAChR and NMDA receptor functions were studied in mice with a targeted null mutation in the gene encoding kynurenine aminotransferase II (mKat-2-/- mice), an enzyme responsible for brain KYNA synthesis. At 21 postnatal days, mKat-2-/- mice had lower hippocampal KYNA levels and higher spontaneous locomotor activity than wild-type (WT) mice. At this age, alpha7* nAChR activity induced by exogenous application of agonists to CA1 stratum radiatum interneurons was approximately 65% higher in mKat-2-/- than WT mice. Binding studies indicated that the enhanced receptor activity may not have resulted from an increase in alpha7* nAChR number. In 21-d-old mKat-2-/- mice, endogenous alpha7* nAChR activity in the hippocampus was also increased, leading to an enhancement of GABAergic activity impinging onto CA1 pyramidal neurons that could be reduced significantly by acute exposure to KYNA (100 nM). The activities of GABA(A) and NMDA receptors in the interneurons and of alpha3beta4* nAChRs regulating glutamate release onto these neurons were comparable between mKat-2-/- and WT mice. By 60 d of age, KYNA levels and GABAergic transmission in the hippocampus and locomotor activity were similar between mKat-2-/- and WT mice. Our findings that alpha7* nAChRs are major targets for KYNA in the brain may provide insights into the pathophysiology of schizophrenia and Alzheimer's disease, disorders in which brain KYNA levels are increased and alpha7* nAChR functions are impaired.

Low concentrations of pyridostigmine prevent soman-induced inhibition of GABAergic transmission in the central nervous system: involvement of muscarinic receptors.

This study was designed to investigate the effects of the cholinesterase inhibitors soman and pyridostigmine bromide (PB) on synaptic transmission in the CA1 field of rat hippocampal slices. Soman (1-100 nM, 10-15 min) decreased the amplitude of GABAergic postsynaptic currents (IPSCs) evoked by stimulation of Schaffer collaterals and recorded from CA1 pyramidal neurons. It also decreased the amplitude and frequency of spontaneous IPSCs recorded from pyramidal neurons. Whereas the maximal effect of soman on evoked GABAergic transmission was observed at 10 nM, full cholinesterase inhibition was induced by 1 nM soman. After 10-15-min exposure of hippocampal slices to 100 nM PB, GABAergic transmission was facilitated and cholinesterase activity was not significantly affected. At nanomolar concentrations, soman and PB have no direct effect on GABA(A) receptors. The effects of soman and PB on GABAergic transmission were inhibited by the m2 receptor antagonist 11-[[[2-diethylamino-O-methyl]-1-piperidinyl] acetyl]-5,11-dihydrol-6H-pyridol[2,3-b][1,4]benzodiazepine-6- one (1 nM) and the m3 receptor antagonist 4-diphenylacetoxy-N-methyl-piperidine (100 nM), respectively, and by the nonselective muscarinic receptor antagonist atropine (1 microM). Thus, changes in GABAergic transmission are likely to result from direct interactions of soman and PB with m2 and m3 receptors, respectively, located on GABAergic fibers/neurons synapsing onto the neurons under study. Although the effects of 1 nM soman and 100 nM PB were diametrically opposed, they only canceled one another when PB was applied to the neurons before soman. Therefore, PB, acting via m3 receptors, can effectively counteract effects arising from the interactions of soman with m2 receptors in the brain.