Stimulation Of Nicotinic Cholinergic Receptors Research Articles

Adrenal adrenoceptors in heart failure.

Heart failure (HF) is a chronic clinical syndrome characterized by the reduction in left ventricular (LV) function and it represents one of the most important causes of morbidity and mortality worldwide. Despite considerable advances in pharmacological treatment, HF represents a severe clinical and social burden. Sympathetic outflow, characterized by increased circulating catecholamines (CA) biosynthesis and secretion, is peculiar in HF and sympatholytic treatments (as β-blockers) are presently being used for the treatment of this disease. Adrenal gland secretes Epinephrine (80%) and Norepinephrine (20%) in response to acetylcholine stimulation of nicotinic cholinergic receptors on the chromaffin cell membranes. This process is regulated by adrenergic receptors (ARs): α2ARs inhibit CA release through coupling to inhibitory Gi-proteins, and β ARs (mainly β2ARs) stimulate CA release through coupling to stimulatory Gs-proteins. All ARs are G-protein-coupled receptors (GPCRs) and GPCR kinases (GRKs) regulate their signaling and function. Adrenal GRK2-mediated α2AR desensitization and downregulation are increased in HF and seem to be a fundamental regulator of CA secretion from the adrenal gland. Consequently, restoration of adrenal α2AR signaling through the inhibition of GRK2 is a fascinating sympatholytic therapeutic strategy for chronic HF. This strategy could have several significant advantages over existing HF pharmacotherapies minimizing side-effects on extra-cardiac tissues and reducing the chronic activation of the renin–angiotensin–aldosterone and endothelin systems. The role of adrenal ARs in regulation of sympathetic hyperactivity opens interesting perspectives in understanding HF pathophysiology and in the identification of new therapeutic targets.

Neurobiology of Addiction

Neurobiology of Addiction

An Invertebrate Model of the Developmental Neurotoxicity of Insecticides: Effects of Chlorpyrifos and Dieldrin in Sea Urchin Embryos and Larvae

Chlorpyrifos targets mammalian brain development through a combination of effects directed at cholinergic receptors and intracellular signaling cascades that are involved in cell differentiation. We used sea urchin embryos as an invertebrate model system to explore the cellular mechanisms underlying the actions of chlorpyrifos and to delineate the critical period of developmental vulnerability. Sea urchin embryos and larvae were exposed to chlorpyrifos at different stages of development ranging from early cell cleavages through the prism stage. Although early cleavages were unaffected even at high chlorpyrifos concentrations, micromolar concentrations added at the mid-blastula stage evoked a prominent change in cell phenotype and overall larval structure, with appearance of pigmented cells followed by their accumulation in an extralarval cap that was extruded from the animal pole. At higher concentrations (20-40 microM), these abnormal cells constituted over 90% of the total cell number. Studies with cholinergic receptor blocking agents and protein kinase C inhibitors indicated two distinct types of effects, one mediated through stimulation of nicotinic cholinergic receptors and the other targeting intracellular signaling. The effects of chlorpyrifos were not mimicked by chlorpyrifos oxon, the active metabolite that inhibits cholinesterase, nor by nonorganophosphate cholinesterase inhibitors. Dieldrin, an organochlorine that targets GABA(A )receptors, was similarly ineffective. The effects of chlorpyrifos and its underlying cholinergic and signaling-related mechanisms parallel prior findings in mammalian embryonic central nervous system. Invertebrate test systems may thus provide both a screening procedure for potential neuroteratogenesis by organophosphate-related compounds, as well as a system with which to uncover novel mechanisms underlying developmental vulnerability.

An invertebrate model of the developmental neurotoxicity of insecticides: effects of chlorpyrifos and dieldrin in sea urchin embryos and larvae.

Chlorpyrifos targets mammalian brain development through a combination of effects directed at cholinergic receptors and intracellular signaling cascades that are involved in cell differentiation. We used sea urchin embryos as an invertebrate model system to explore the cellular mechanisms underlying the actions of chlorpyrifos and to delineate the critical period of developmental vulnerability. Sea urchin embryos and larvae were exposed to chlorpyrifos at different stages of development ranging from early cell cleavages through the prism stage. Although early cleavages were unaffected even at high chlorpyrifos concentrations, micromolar concentrations added at the mid-blastula stage evoked a prominent change in cell phenotype and overall larval structure, with appearance of pigmented cells followed by their accumulation in an extralarval cap that was extruded from the animal pole. At higher concentrations (20-40 microM), these abnormal cells constituted over 90% of the total cell number. Studies with cholinergic receptor blocking agents and protein kinase C inhibitors indicated two distinct types of effects, one mediated through stimulation of nicotinic cholinergic receptors and the other targeting intracellular signaling. The effects of chlorpyrifos were not mimicked by chlorpyrifos oxon, the active metabolite that inhibits cholinesterase, nor by nonorganophosphate cholinesterase inhibitors. Dieldrin, an organochlorine that targets GABA(A )receptors, was similarly ineffective. The effects of chlorpyrifos and its underlying cholinergic and signaling-related mechanisms parallel prior findings in mammalian embryonic central nervous system. Invertebrate test systems may thus provide both a screening procedure for potential neuroteratogenesis by organophosphate-related compounds, as well as a system with which to uncover novel mechanisms underlying developmental vulnerability.

Selective inhibition of nicotinic cholinergic receptors by proadrenomedullin N-terminal 12 peptide in bovine adrenal chromaffin cells

We studied whether a novel proadrenomedullin derived peptide was present and what was its physiological function in cultured bovine adrenal chromaffin cells. We found a high level of proadrenomedullin N-terminal 12 peptide (PAMP-12) which consists of a peptide from 9th amino acid to 20th amino acid of proadrenomedullin N-terminal 20 peptide (PAMP-20). PAMP-12 was released from the cells along with catecholamine upon stimulation of nicotinic cholinergic receptors. When PAMP-12 was added in the incubation medium, this peptide inhibited nicotinic receptor-mediated catecholamine release and influx of Na + and Ca 2+ into the cells. PAMP-12 did not affect catecholamine release evoked by histamine or by depolarization by high concentration of potassium. PAMP-12 also inhibited synthesis of catecholamines as well as the activation of tyrosine hydroxylase by nicotinic stimulation. Thus, PAMP-12 is an endogenous peptide that regulates release and synthesis of catecholamines by acting on nicotinic cholinergic receptors in an autocrine manner in adrenal chromaffin cells.

Effect of stimulation of nicotinic cholinergic receptors on cortical cerebral blood flow and changes in the effect during aging in anesthetized rats

The effect of intravenous injection of nicotine on cortical cerebral blood flow (CBF) was examined in urethane anesthetized rats. Nicotine (3–30 μg/kg) increased cortical CBF, independent of mean arterial pressure. This response was attenuated to about a half of the control one after lesioning the nucleus basalis of Meynert (NBM) bilaterally. The response was not significantly influenced after blocking the muscarinic receptors, but was abolished after blocking the nicotinic receptors in the parenchyma of the brain. It is concluded that the nicotine-induced cortical vasodilation was mediated by activation of the nicotinic receptors in the NBM and also in the cortex of the brain. The threshold dose of nicotine for increasing cortical CBF was shifted in aged rats of 23–26 months, and the nicotine-induced increase in cortical CBF was much reduced in aged rats of 32–36 months. Activation of nicotinic receptors in the brain may be of therapeutic value in aged subjects in facilitating the cholinergic neural vasodilative system.

Beneficial effects of nicotine and cigarette smoking: the real, the possible and the spurious.

Cigarette smoking is an established risk factor for cancer and cardiovascular disease, and is the leading cause of avoidable disease in most industrialized countries. Less well-known are possible beneficial effects, which are briefly considered in this survey. Preliminary data suggest that there may be inverse associations of smoking with uterine fibroids and endometriosis, and protective effects on hypertensive disorders and vomiting of pregnancy are likely. Smoking has consistently been found to be inversely related to the risk of endometrial cancer, but cancers of the breast and colon seem unrelated to smoking. Inverse associations with venous thrombosis and fatality after myocardial infarction are probably not causal, but indications of benefits with regard to recurrent aphthous ulcers, ulcerative colitis, and control of body weight may well reflect a genuine benefit. Evidence is growing that cigarette smoking and nicotine may prevent or ameliorate Parkinson's disease, and could do so in Alzheimer's dementia. A variety of mechanisms for potentially beneficial effects of smoking have been proposed, but three predominate: the 'anti-estrogenic effect' of smoking; alterations in prostaglandin production; and stimulation of nicotinic cholinergic receptors in the central nervous system. Even established inverse associations cannot be used as a rationale for cigarette smoking. These data can be used, however, to clarify mechanisms of disease, and point to productive treatment or preventive options with more narrowly-acting interventions.

In vivo regulation of [3H]acetylcholine recognition sites in brain by nicotinic cholinergic drugs.

The in vivo regulation of [3H]acetylcholine [( 3H]ACh) recognition sites on nicotinic receptors in rat brain was examined by administering drugs that increase stimulation of nicotinic cholinergic receptors, either directly or indirectly. After 10 days of treatment with the cholinesterase inhibitor diisopropyl fluorophosphate, [3H]ACh binding in the cortex, thalamus, striatum, and hypothalamus was decreased. Scatchard analyses indicated that the decrease in binding in the cortex was due to a reduction in the apparent density of [3H]ACh recognition sites. In contrast, after repeated administration of nicotine (5-21 days), the number of [3H]ACh recognition sites was increased in the cortex, thalamus, striatum, and hypothalamus. Similar effects were observed in the cortex and thalamus following repeated administration of the nicotinic agonist cytisin. The nicotinic antagonists mecamylamine and dihydro-beta-erythroidine did not alter [3H]ACh binding following 10-14 days of administration. Further, concurrent treatment with these antagonists and nicotine did not prevent the nicotine-induced increase in these binding sites. The data indicate that [3H]ACh recognition sites on nicotinic receptors are subject to up- and down-regulation, and that repeated administration of nicotine results in a signal for up-regulation, probably through protracted desensitization at the recognition site.

The effects of membrane fatty acid modification of clonal pheochromocytoma cells on depolarization-dependent exocytosis.

We have begun studying the role of membrane lipids in the exocytotic release process using the pheochromocytoma clone, PC12. The phospholipid fatty acid composition of the cells was modified by growth in the presence of specific fatty acids. None of the fatty acid modifications affected K+-stimulated release of [3H] norepinephrine. This observation indicates that the individual steps of the secretion process, including the extent of depolarization produced by K+, the response of the voltage-dependent Ca2+ channels to depolarization, and the subsequent steps in Ca2+-dependent exocytosis were unaffected by the fatty acid changes. In contrast, exocytosis evoked by stimulation of nicotinic cholinergic receptors with carbamylcholine or direct activation of action potential Na+ channels with veratridine was diminished in cells enriched with unsaturated fatty acids. The diminished output of the release systems was observed at all concentrations of carbamylcholine and veratridine tested. Since the events of exocytosis subsequent to Ca2+ influx were unaffected by unsaturated fatty acids, it appears likely that the magnitude of the depolarization produced by carbamylcholine and veratridine was reduced. The loss of carbamylcholine-stimulated release did not correlate with the simple presence of the fatty acids, but paralleled closely the time and concentration-dependent changes in the phospholipid fatty acid composition. However, when oleate and arachidonate were simultaneously added to the culture medium, the inhibitory effects on carbamylcholine-stimulated release were additive, whereas the changes in fatty acid composition were antagonistic. Thus, exposure of PC12 cells to unsaturated fatty acids causes specific, reversible decreases in the activities of at least 2 stimulus/secretion systems. However, the mechanistic explanation for these changes is not readily apparent from a simple analysis of total phospholipid fatty acid composition.