Melatonin inhibits nicotinic acetylcholine receptor functions in bovine chromaffin cells

Melatonin is a neurotransmitter that modulates various physiological phenomena including regulation and maintenance of the circadian rhythm. Nicotinic acetylcholine receptors (nAChRs) play an important role in oral functions including orofacial muscle contraction, salivary secretion, and tooth development. However, knowledge regarding physiological crosstalk between melatonin and nAChRs is limited. In the present study, the melatonin-mediated modulation of nAChR functions using bovine adrenal chromaffin cells, a representative model for the study of nAChRs, was investigated. Melatonin inhibited the nicotinic agonist dimethylphenylpiperazinium (DMPP) iodide-induced cytosolic free Ca²⁺ concentration ([Ca²⁺](i)) increase and norepinephrine secretion in a concentration-dependent manner. The inhibitory effect of melatonin on the DMPP-induced [Ca²⁺](i) increase was observed when the melatonin treatment was performed simultaneously with DMPP. The results indicate that melatonin inhibits nAChR functions in both peripheral and central nervous systems.

Preliminary pharmacological investigation of the ischuretic property and safety of a hydro-ethanolic extract of Amaranthus spinosis (Fam: Amaranthaceae).

Background: Ischuria is a health and social problem, having a negative impact on sufferers. This study therefore was a preliminary investigation of the ischuretic property and safety for use of a hydro-ethanolic extract of Amaranthus spinosus used traditionally in managing ischuria. Methods: Phytochemical screening, thin layer chromatography and high performance liquid chromatography were performed on the extract to establish fingerprints for identification. Acetylcholine, Nicotine, and the extract were applied to an isolated rat urinary bladder to ascertain contractile response. The possible receptor site(s) of action was also investigated using isolated rabbit jejunum, and guinea-pig ileum preparations. In-house observation, hematological analysis, and liver and kidney function tests were performed on Sprague-Dawley rats, in acute and sub-acute toxicity studies. Results: The extract had contractile effects on the rat urinary bladder (similar to acetylcholine and nicotine) and rabbit jejunum. Its contractile effect of the guinea-pig ileum was significantly inhibited by hexamethonium (77.50 ± 8.50 %; P ≤ 0.001) and to a lesser extent by mepyramine (49.2 ± 6.80 %; P ≤ 0.001) and Atropine (22.45 ± 5.22 %; P ≤ 0.01). The extract (80-800 mg kg-1) was not lethal and a 160 and 240 mg kg-1 dose had no adverse effect on blood, liver, kidney metabolic function. Conclusions: The hydro-ethanolic extract of Amaranthus spinosus has ischuretic activity possibly mediated via nicotinic, histaminic and muscarinic receptor stimulation and is safety to use in ischuria.

Augmentation of catecholamine release elicited by an Eugenia punicifolia extract in chromaffin cells

Plant extracts of Eugenia punicifolia (Kunth) DC., Myrtaceae, are used in Amazon region of Brazil to treat diarrhea and stomach disturbances, and as hypoglycemic medicine. We have recently shown that an aqueous extract of E. punicifolia augmented cholinergic neurotransmission in a rat phrenic nerve-diaphragm preparation. In this study, we investigated the effects of an E. punicifolia dichloromethane extract (EPEX) in a neuronal model of cholinergic neurotransmission, the bovine adrenal chromaffin cell. EPEX augmented the release of catecholamine triggered by acetylcholine (ACh) pulses but did not enhance ACh-evoked inward currents, which were inhibited by 30 percent. Since EPEX did not cause a blockade of acetylcholinesterase or butyrylcholinesterase, it seems that EPEX is not directly activating the cholinergic system. EPEX also augmented K+-elicited secretion without enhancing the whole-cell inward calcium current. This novel and potent effect of EPEX in enhancing exocytosis might help to identify the active component responsible for augmenting exocytosis. When elucidated, the molecular structure of this active principle could serve as a template to synthesise novel compounds to regulate the exocytotic release of neurotransmitters.

Cellular and molecular mechanisms of dichlorvos neurotoxicity: Cholinergic, nonchlolinergic, cell signaling, gene expression and therapeutic aspects.

Inappropriate use of toxic chemicals is common in developing countries, where it leads to excessive exposure and high risks of unintentional poisoning. Risks are particularly high with the pesticides used in agriculture, poor rural populations live and work in close proximity to these compounds and often store these compounds in and around their homes. It is estimated that most of the death from pesticide poisoning occur in developing countries. Organophosphate insecticides have been extensively used in agriculture in developing countries. Dichlorvos is a synthetic insecticide and belongs to a family of chemically related organophosphate pesticides (OP). Toxicity of dichlorvos has been documented in accidental human poisoning, epidemiological studies, and animal models. In this review, molecular mechanisms of dichlorvos neurotoxicity have been described. Usage, biotransformation, environmental levels, general population and occupational exposure, effects on cell signaling receptors, mitochondrial metabolism, oxidative stress and gene expression of dichlorvos have been reviewed. Assessment of acute and chronic exposures as well as neurotoxicity risk for lifetime exposures to dichlorvos have also been considered. In addition special emphasis has been given to describe, the role of dichlorvos in the chronic neurotoxicity and its molecular targets that ultimately lead to neurodegeneration.

Naltrexone Inhibits Catecholamine Secretion Evoked by Nicotinic Receptor Stimulation in the Perfused Rat Adrenal Medulla

The purpose of the present study was to examine the effect of naltrexone, an opioid antagonist, on secretion of catecholamines (CA) evoked by cholinergic nicotinic stimulation and membrane-depolarization from the isolated perfused rat adrenal gland and to establish the mechanism of its action. Naltrexone (3x10 (-6) M) perfused into an adrenal vein for 60 min produced time-dependent inhibition in CA secretory responses evoked by ACh (5.32x10 (-3) M), high K+ (5.6x10 (-2) M), DMPP (10 (-4) M) and McN-A-343 (10 (-4) M). Naltrexone itself did also fail to affect basal CA output. In adrenal glands loaded with naltrexone (3x10 (-6) M), the CA secretory responses evoked by Bay-K-8644, an activator of L-type Ca2+ channels and cyclopiazonic acid, an inhibitor of cytoplasmic Ca2+-ATPase, were also inhibited. However, in the presence of met-enkephalin (5x10 (-6) M), a well-known opioid agonist, the CA secretory responses evoked by ACh, high K+, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly inhibited. Collectively, these experimental results demonstrate that naltrexone inhibits greatly CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors as well as that by membrane depolarization. It seems that this inhibitory effect of naltrexone does not involve opioid receptors, but might be mediated by blocking both the calcium influx into the rat adrenal medullary chromaffin cells and the uptake of Ca2+ into the cytoplasmic calcium store, which are at least partly relevant to the direct interaction with the nicotinic receptor itself.

D-Amphetamine Causes Dual Actions on Catecholamine Release from the Rat Adrenal Medulla

The present study was designed to examine the effect of d-amphetamine on CA release from the isolated perfused model of the rat adrenal gland, and to establish its mechanism of action. D- amphetamine (10~100microM), when perfused into an adrenal vein of the rat adrenal gland for 60 min, enhanced the CA secretory responses evoked by ACh (5.32x10-3 M), excess K+ (5.6x10-2 M, a membrane depolarizer), DMPP (10-4 M, a selective neuronal nicotinic Nn-receptor agonist) and McN-A-343 (10-4 M, a selective M1-muscarinic agonist) only for the first period (4 min), although it alone has weak effect on CA secretion. Moreover, d-amphetamine (30microM) in to an adrenal vein for 60 min also augmented the CA release evoked by BAY-K-8644, an activator of the dihydropyridine L-type Ca2+ channels, and cyclopiazonic acid, an inhibitor of cytoplasmic Ca2+ ATPase only for the first period (4 min). However, in the presence of high concentration (500microM), d-amphetamine rather inhibited the CA secretory responses evoked by the above all of secretagogues. Collectively, these experimental results suggest that d-amphetamine at low concentrations enhances the CA secretion from the rat adrenal medulla evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization, but at high concentration it rather inhibits them. It seems that d-amphetamine has dual effects as both agonist and antagonist at nicotinic receptors of the isolated perfused rat adrenal medulla, which might be dependent on the concentration. It is also thought that these actions of d-amphetamine are probably relevant to the Ca2+ mobilization through the dihydropyridine L-type Ca2+ channels located on the rat adrenomedullary chromaffin cell membrane and the release of Ca2+ from the cytoplasmic store.

Neuronal Nicotinic Acetylcholine Receptor Expression, Aging and Mouse Strain-Specific Hippocampal Cytoarchitecture

The differing response by individuals to nicotine reflects the biological outcome of a combination of genetic and environmental factors. Because nicotine imparts its effects through interacting with neuronal nicotinic acetylcholine receptors (nAChR), and mice of different inbred strains differ in their responses to nicotine; mice afford an excellent model for experimentally dissecting the biology of these varied responses. To begin this investigation, we compared the expression of nAChR subunits α3, α4, α5, α7, β2 and β4 in the dorsal hippocampus between 8 mouse strains that differ in their response to nicotine in defined ways. In terms of neuronal distribution, all nAChR subunits co-localized with glutamic acid decarboxylase (GAD) positive interneurons, and heterogeneity in nAChR subunit expression defines four interneuron subgroups. An unexpected finding was that nAChRs are also expressed by astrocytes in a mouse strain¬specific manner and their occurrence varies inversely with nAChR+ interneurons. This relationship is dynamic during the animal’s life span where aged animals exhibit increased nAChR+ astrocyte/interneuron ratios. These findings reveal a complex interplay between genetic and developmental factors that individualize the expression of this modulatory neurotransmitter system in the mammalian nervous system, and would likely customize the response to nicotine.

Pharmacological characteristics of homologous upregulation of nicotinic ?_7 receptors in brain / 中国药理学通报

10 ?mol?L -1) increased the binding sites of [ 125I] ?-Bgt significantly (P0.05).Conclusion Chronic treatment of choline, nicotine or methyllycaconitine can upregulate the nicotinic ? 7 receptors at certain doses. The effect of choline on upregulation of ? 7 receptors is different from that of nicotine.

Interactions of nicotinic and muscarinic acetylcholine receptors / 中国药理学通报

Nicotinic and muscarinic acetylcholine receptors have the same endogenous ligand ACh and are distributed together in many tissues, so it is reasonable to believe that there must be some interactions existed between them. The functions of muscarinic receptors in the tissues innervated by the parasympathetic cholinergic postfibers, can be modulated by the ganglionic nicotinic receptors through stimulating ACh release. In ganglia, the postsynaptic nicotinic receptor activities can be modulated by the presynaptic muscarinic and nicotinic autoreceptors through regulating ACh release. Moreover, The functions of muscarinic receptors can be changed by nicotinic receptor desensitization or blockade. The two types of receptor act on each other and keep in a varied homeostasis of cholinergic nervous system.