CHOLINESTERASES, A TARGET OF PHARMACOLOGY AND TOXICOLOGY

HI-6 modulates immunization efficacy in a BALB/c mouse model

HI-6 is used as an antidote to nerve agents. It can also act as an antagonist to acetylcholine receptors (AChRs) including the nicotinic receptor, α 7 nAChR which is involved in regulating the immune response through macrophages. This experiment investigated the efficacy of HI-6 to regulate the immune response. Laboratory BALB/c mice received HI-6 and/or keyhole limpet hemocyanin (KLH) as an antigen. Antibody production was investigated after either 21 or 65 days when either single or repeated dose of antigen was applied. We confirmed that HI-6 significantly improved vaccination efficacy when KLH was given in a dose of 1mg/kg. The effect was dose dependent. A combination of HI-6 and KLH produced a vaccination of almost the same efficacy as that for Freund's complete adjuvant. The findings point at the suitability of HI-6 for improving vaccination efficacy at the level of immunity regulation by the nervous system.

An acetylcholinesterase-based chronoamperometric biosensor for fast and reliable assay of nerve agents

The enzyme acetylcholinesterase (AChE) is an important part of cholinergic nervous system, where it stops neurotransmission by hydrolysis of the neurotransmitter acetylcholine. It is sensitive to inhibition by organophosphate and carbamate insecticides, some Alzheimer disease drugs, secondary metabolites such as aflatoxins and nerve agents used in chemical warfare. When immobilized on a sensor (physico-chemical transducer), it can be used for assay of these inhibitors. In the experiments described herein, an AChE- based electrochemical biosensor using screen printed electrode systems was prepared. The biosensor was used for assay of nerve agents such as sarin, soman, tabun and VX. The limits of detection achieved in a measuring protocol lasting ten minutes were 7.41 × 10⁻¹² mol/L for sarin, 6.31 × 10⁻¹² mol/L for soman, 6.17 × 10⁻¹¹ mol/L for tabun, and 2.19 × 10⁻¹¹ mol/L for VX, respectively. The assay was reliable, with minor interferences caused by the organic solvents ethanol, methanol, isopropanol and acetonitrile. Isopropanol was chosen as suitable medium for processing lipophilic samples.

Impact of melatonin on immunity: a review

Melatonin is a hormone produced by the pineal gland. In addition to its hormonal effect, it has strong antioxidant properties. Melatonin is probably best known for its ability to control circadian rhythm; it is sold in many countries as a supplement or drug for improving of sleep quality. However, melatonin’s effect is not limited to control of circadian rhythm:. it is involved in other effects, including cell cycle control and regulation of several important enzymes, including inhibition of inducible nitric oxide synthase. Melatonin affects immunity as well. It can modulate the immune response on disparate levels with a significant effect on inflammation. The role of melatonin in body regulatory process is not well understood; only limited conclusions can be drawn from known data. The current review attempts to summarize both basic facts about melatonin’s effects and propose research on the lesser known issues in the future.

Syzygium jambos and Solanum guaraniticum show similar antioxidant properties but induce different enzymatic activities in the brain of rats

Syzygium jambos and Solanum guaraniticum are both employed in Brazil as medicinal plants, even though their potential toxicity is not well established and they are frequently misused. The aim of this study was investigate the effect of the aqueous leaf extracts of both plants on δ-aminolevulinate dehydratase (δ-ALA-D) and acetylcholinesterase (AChE) activities and the antioxidant action against oxidative damage induced by sodium nitroprusside in rats, using in vitro assays. In addition, the presence of gallic, caffeic and chlorogenic acids, as well as rutin, quercetin and kaempferol as bioactive compounds in the extracts was identified by HPLC and their levels quantified. The antioxidant activities of both extracts were assessed by their capabilities to scavenge nitric oxide and to inhibit lipid peroxidation. Only Syzygium jambos presented thiol-peroxidase-like activity. Although neither extract affected the AChE activity, the aqueous extract of Solanum guaraniticum inhibited brain δ-ALA-D activity, suggesting a possible impairment effect on the central nervous system. Our results showed that both extracts exhibited efficient free radical scavenger activity and are an interesting source of bioactive compounds, justifying their use in folk medicine, although Solanum guaraniticum extract could have neurotoxicity properties and we therefore suggest that its use should be restricted to ensure the health of the population.

In vitro and in vivo study of effects of fermented soybean product (chungkookjang) on NGF secretion ability and NGF receptor signaling pathway

In order to investigate the effects of a fermented soybean product (Chungkookjang, CKJ) on nerve growth factor (NGF) metabolism, NGF secretion ability and its related signaling pathway were analyzed in B35 neuronal cells and the Tg2576 mouse model of Alzheimer's disease (AD). In B35 cells, the concentration of NGF significantly increased upon treatment with Taegwang (TG)-CKJ and Shinhwa (SH)-CKJ extracts compared with vehicle. Further, a significant increase in PC12 cell length as well as the phsophorylation levels of TrkA and Akt, which are members of a high affinity NGF receptor signaling pathway, were observed after treatment with TG-CKJ and SH-CKJ conditional medium (CM). On the other hand, there was no difference in activation of the NGF receptor p75^NTR signaling pathway between vehicle and all CKJ treated groups. In Tg2576 mice showing early stage of AD, the concentrations of NGF in the serum and brain were reduced compared with those in Non-Tg mice. Treatment of Tg2576 mice with SH-CKJ, which contains high concentrations of total flavonoids and phenolic compounds, for 8 weeks dramatically recovered the NGF level to that of Non-Tg mice. Furthermore, the low phosphorylation levels of TrkA and Erk in the NGF receptor TrkA signaling pathway were rapidly recovered to those of Non-Tg mice after SH-CKJ treatment in vehicle treated Tg2576 mice, whereas the phosphorylation level of Akt was maintained at a constant level. These results suggest that CKJ may stimulate NGF secretion ability as well as the NGF receptor TrkA signaling pathway in PC12 cells and Tg2576 mice.

Caffeine inhibits acetylcholinesterase, but not butyrylcholinesterase

Caffeine is an alkaloid with a stimulant effect in the body. It can interfere in transmissions based on acetylcholine, epinephrine, norepinephrine, serotonin, dopamine and glutamate. Clinical studies indicate that it can be involved in the slowing of Alzheimer disease pathology and some other effects. The effects are not well understood. In the present work, we focused on the question whether caffeine can inhibit acetylcholinesterase (AChE) and/or, butyrylcholinesterase (BChE), the two enzymes participating in cholinergic neurotransmission. A standard Ellman test with human AChE and BChE was done for altering concentrations of caffeine. The test was supported by an in silico examination as well. Donepezil and tacrine were used as standards. In compliance with Dixon’s plot, caffeine was proved to be a non-competitive inhibitor of AChE and BChE. However, inhibition of BChE was quite weak, as the inhibition constant, K_i, was 13.9 ± 7.4 mol/L. Inhibition of AChE was more relevant, as K_i was found to be 175 ± 9 μmol/L. The predicted free energy of binding was −6.7 kcal/mol. The proposed binding orientation of caffeine can interact with Trp86, and it can be stabilize by Tyr337 in comparison to the smaller Ala328 in the case of human BChE; thus, it can explain the lower binding affinity of caffeine for BChE with reference to AChE. The biological relevance of the findings is discussed.

Acetylcholinesterase inhibitors: pharmacology and toxicology

Acetylcholinesterase is involved in the termination of impulse transmission by rapid hydrolysis of the neurotransmitter acetylcholine in numerous cholinergic pathways in the central and peripheral nervous systems. The enzyme inactivation, induced by various inhibitors, leads to acetylcholine accumulation, hyperstimulation of nicotinic and muscarinic receptors, and disrupted neurotransmission. Hence, acetylcholinesterase inhibitors, interacting with the enzyme as their primary target, are applied as relevant drugs and toxins. This review presents an overview of toxicology and pharmacology of reversible and irreversible acetylcholinesterase inactivating compounds. In the case of reversible inhibitors being commonly applied in neurodegenerative disorders treatment, special attention is paid to currently approved drugs (donepezil, rivastigmine and galantamine) in the pharmacotherapy of Alzheimer's disease, and toxic carbamates used as pesticides. Subsequently, mechanism of irreversible acetylcholinesterase inhibition induced by organophosphorus compounds (insecticides and nerve agents), and their specific and nonspecific toxic effects are described, as well as irreversible inhibitors having pharmacological implementation. In addition, the pharmacological treatment of intoxication caused by organophosphates is presented, with emphasis on oxime reactivators of the inhibited enzyme activity administering as causal drugs after the poisoning. Besides, organophosphorus and carbamate insecticides can be detoxified in mammals through enzymatic hydrolysis before they reach targets in the nervous system. Carboxylesterases most effectively decompose carbamates, whereas the most successful route of organophosphates detoxification is their degradation by corresponding phosphotriesterases.

Cholinesterase enzymes inhibitors from the leaves of Rauvolfia reflexa and their molecular docking study

Plants of the Apocynaceae family have been traditionally used in the treatment of age-related brain disorders. Rauvolfia reflexa, a member of the family, has been used as an antidote for poisons and to treat malaria. The dichloromethane, ethanol and methanol extracts from the leaves of Rauvolfia reflexa showed potential acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitory activities, with IC50 values in the 8.49 to 52.23 g/mL range. Further cholinesterase inhibitory-guided isolation of these extracts afforded four bioactive compounds, namely: (E)-3-(3,4,5-trimethoxyphenyl)acrylic acid (1), (E)-methyl 3-(4-hydroxy-3,5-dimethoxyphenyl) acrylate (2), 17-methoxycarbonyl-14-heptadecaenyl-4-hydroxy-3-methoxycinnamate (3) and 1,2,3,4-tetrahydro-1-oxo-β-carboline (4). The isolated compounds showed moderate cholinesterase inhibitory activity compared to the reference standard, physostigmine. Compounds 1 and 2 showed the highest inhibitory activity against AChE (IC50 = 60.17 µM) and BChE (IC50 = 61.72 µM), respectively. Despite having similar molecular weight, compounds 1 and 2 were structurally different according to their chemical substitution patterns, leading to their different enzyme inhibition selectivity. Compound 2 was more selective against BChE, whereas compound 1 was a selective inhibitor of AChE. Molecular docking revealed that both compounds 1 and 2 were inserted, but not deeply into the active site of the cholinesterase enzymes.

Linking pesticide exposure and dementia: what is the evidence?

There has been a steep increase in the prevalence of dementia in recent decades, which has roughly followed an increase in pesticide use some decades earlier, a time when it is probable that current dementia patients could have been exposed to pesticides. This raises the question whether pesticides contribute to dementia pathogenesis. Indeed, many studies have found increased prevalence of cognitive, behavioral and psychomotor dysfunction in individuals chronically exposed to pesticides. Furthermore, evidence from recent studies shows a possible association between chronic pesticide exposure and an increased prevalence of dementia, including Alzheimer's disease (AD) dementia. At the cellular and molecular level, the mechanism of action of many classes of pesticides suggests that these compounds could be, at least partly, accountable for the neurodegeneration accompanying AD and other dementias. For example, organophosphates, which inhibit acetylcholinesterase as do the drugs used in treating AD symptoms, have also been shown to lead to microtubule derangements and tau hyperphosphorylation, a hallmark of AD. This emerging association is of considerable public health importance, given the increasing dementia prevalence and pesticide use. Here we review the epidemiological links between dementia and pesticide exposure and discuss the possible pathophysiological mechanisms and clinical implications of this association.

Exposure to multiple cholinergic pesticides impairs olfactory learning and memory in honeybees

Pesticides are important agricultural tools often used in combination to avoid resistance in target pest species, but there is growing concern that their widespread use contributes to the decline of pollinator populations. Pollinators perform sophisticated behaviours while foraging that require them to learn and remember floral traits associated with food, but we know relatively little about the way that combined exposure to multiple pesticides affects neural function and behaviour. The experiments reported here show that prolonged exposure to field-realistic concentrations of the neonicotinoid imidacloprid and the organophosphate acetylcholinesterase inhibitor coumaphos and their combination impairs olfactory learning and memory formation in the honeybee. Using a method for classical conditioning of proboscis extension, honeybees were trained in either a massed or spaced conditioning protocol to examine how these pesticides affected performance during learning and short- and long-term memory tasks. We found that bees exposed to imidacloprid, coumaphos, or a combination of these compounds, were less likely to express conditioned proboscis extension towards an odor associated with reward. Bees exposed to imidacloprid were less likely to form a long-term memory, whereas bees exposed to coumaphos were only less likely to respond during the short-term memory test after massed conditioning. Imidacloprid, coumaphos and a combination of the two compounds impaired the bees' ability to differentiate the conditioned odour from a novel odour during the memory test. Our results demonstrate that exposure to sublethal doses of combined cholinergic pesticides significantly impairs important behaviours involved in foraging, implying that pollinator population decline could be the result of a failure of neural function of bees exposed to pesticides in agricultural landscapes.

Exposure to acetylcholinesterase inhibitors alters the physiology and motor function of honeybees

Cholinergic signaling is fundamental to neuromuscular function in most organisms. Sub-lethal doses of neurotoxic pesticides that target cholinergic signaling can alter the behavior of insects in subtle ways; their influence on non-target organisms may not be readily apparent in simple mortality studies. Beneficial arthropods such as honeybees perform sophisticated behavioral sequences during foraging that, if influenced by pesticides, could impair foraging success and reduce colony health. Here, we investigate the behavioral effects on honeybees of exposure to a selection of pesticides that target cholinergic signaling by inhibiting acetylcholinesterase (AChE). To examine how continued exposure to AChE inhibitors affected motor function, we fed adult foraging worker honeybees sub-lethal concentrations of these compounds in sucrose solution for 24 h. Using an assay for locomotion in bees, we scored walking, stopped, grooming, and upside down behavior continuously for 15 min. At a 10 nM concentration, all the AChE inhibitors caused similar effects on behavior, notably increased grooming activity and changes in the frequency of bouts of behavior such as head grooming. Coumaphos caused dose-dependent effects on locomotion as well as grooming behavior, and a 1 μM concentration of coumaphos induced symptoms of malaise such as abdomen grooming and defecation. Biochemical assays confirmed that the four compounds we assayed (coumaphos, aldicarb, chlorpyrifos, and donepezil) or their metabolites acted as AChE inhibitors in bees. Furthermore, we show that transcript expression levels of two honeybee AChE inhibitors were selectively upregulated in the brain and in gut tissues in response to AChE inhibitor exposure. The results of our study imply that the effects of pesticides that rely on this mode of action have subtle yet profound effects on physiological effects on behavior that could lead to reduced survival.

Pharmacokinetics and dopamine/acetylcholine releasing effects of ginsenoside Re in hippocampus and mPFC of freely moving rats

AIM

To investigate the pharmacokinetics and dopamine/acetylcholine-releasing effects of ginsenoside Re (Re) in brain regions related to learning and memory, and to clarify the neurochemical mechanisms underlying its anti-dementia activity.

METHODS

Microdialysis was conducted on awake, freely moving adult male SD rats with dialysis probes implanted into the hippocampus, medial prefrontal cortex (mPFC) or the third ventricle. The concentrations of Re, dopamine (DA) and acetylcholine (ACh) in dialysates were determined using LC-MS/MS.

RESULTS

Subcutaneous administration of a single dose of Re (12.5, 25 or 50 mg/kg) rapidly distributed to the cerebrospinal fluid and exhibited linear pharmacokinetics. The peak concentration (C(max)) occurred at 60 min for all doses. Re was not detectable after 240 min in the dialysates for the low dose of 12.5 mg/kg. At the same time, Re dose-dependently increased extracellular levels of DA and ACh in the hippocampus and mPFC, and more prominent effects were observed in the hippocampus.

CONCLUSION

The combined study of the pharmacokinetics and pharmacodynamics of Re demonstrate that increase of extracellular levels of DA and ACh, particularly in the hippocampus, may contribute, at least in part, to the anti-dementia activity of Re.

The effects of huperzine A on gastrointestinal acetylcholinesterase activity and motility after single and multiple dosing in mice

The acetylcholinesterase inhibitor (AChEI), huperzine A has been used in the treatment of the cognitive deterioration associated with Alzheimer’s disease (AD). However, the side-effects of huperzine A associated with increased cholinergic activity, particularly in the gastrointestinal system, are evident. It is not yet known how quickly these side-effects become tolerated; this information would provide guidance to doctors on how to use huperzine A so as to attenuate the adverse events. The present study aimed to observe the effects of huperzine A on gastrointestinal motility and acetylcholinesterase (AChE) activity in mice. After oral administration of huperzine A with single and multiple dosing, the gastrointestinal motility and AChE activity of the mice were examined. The results revealed that, following a single dose of huperzine A, the AChE activity in the stomach and duodenum were significantly inhibited and the gastrointestinal motility was significantly increased. However, following multiple doses (7 or 28 doses, one dose per day), no significant changes in the AChE activity and gastrointestinal motility were identified. These findings indicate that the gastrointestinal adverse effects of huperzine A may be well-tolerated relatively quickly and do not recur. Additionally, it suggests that patients with AD are likely to have minimal gastrointestinal side-effects after taking multiple doses of huperzine A.

Laboratory examination in nerve agent intoxication

Diagnosis of nerve agent intoxication is based on anamnestic data, clinical signs and laboratory examination. For acute poisoning, cholinesterase activity in the blood (erythrocyte AChE, plasma/serum BuChE) is sensitive, simple and most frequent laboratory examination performed in biochemical laboratories. Specialized examinations to precise treatment (reactivation test) or to make retrospective diagnosis (fluoride induced reactivation etc.) can be conducted. Other sophisticated methods are available, too.

Acute exposure to a sublethal dose of imidacloprid and coumaphos enhances olfactory learning and memory in the honeybee Apis mellifera

The decline of honeybees and other pollinating insects is a current cause for concern. A major factor implicated in their decline is exposure to agricultural chemicals, in particular the neonicotinoid insecticides such as imidacloprid. Honeybees are also subjected to additional chemical exposure when beekeepers treat hives with acaricides to combat the mite Varroa destructor. Here, we assess the effects of acute sublethal doses of the neonicotinoid imidacloprid, and the organophosphate acaricide coumaphos, on honey bee learning and memory. Imidacloprid had little effect on performance in a six-trial olfactory conditioning assay, while coumaphos caused a modest impairment. We report a surprising lack of additive adverse effects when both compounds were administered simultaneously, which instead produced a modest improvement in learning and memory.

Alpha7 nicotinic acetylcholine receptor is a target in pharmacology and toxicology

Alpha7 nicotinic acetylcholine receptor (α7 nAChR) is an important part of the cholinergic nerve system in the brain. Moreover, it is associated with a cholinergic anti-inflammatory pathway in the termination of the parasympathetic nervous system. Antagonists of α7 nAChR are a wide group represented by conotoxin and bungarotoxin. Even Alzheimer's disease drug memantine acting as an antagonist in its side pathway belongs in this group. Agonists of α7 nAChR are suitable for treatment of multiple cognitive dysfunctions such as Alzheimer's disease or schizophrenia. Inflammation or even sepsis can be ameliorated by the agonistic acting compounds. Preparations RG3487, SEN34625/WYE-103914, SEN12333, ABT-107, Clozapine, GTS-21, CNI-1493, and AR-R17779 are representative examples of the novel compounds with affinity toward the α7 nAChR. Pharmacological, toxicological, and medicinal significance of α7 nAChR are discussed throughout this paper.