IntroNT-Colinérgico2016.pdf
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Sistemas de Neurotransmisores Introducción Neurotransmisión colinérgica Síntesis y degradación de acetilcolina (Ach), Colina acetil transferasa (ChaT) y aceticolinesterasa (AChE) Recaptación por el terminal sináptico: transportador de colina (ChT) Llenado de las Vesículas: Transportador Vesicular de ACh (VAT) Receptores Nicotínicos y Muscarínicos FSN 2016 Eleonora Katz Neurotransmisión Eléctrica Química Purves 3rd edition Neurotransmisión Química Pequeñas moléculas NT= ACh y las Aminas biogenicas (dopamina, NE, Epinefrina, serotonina, histamina) Aminoácidos: gaba, glutamato, aspartato, glicina Purinas: ATP, ADP Neuropéptidos: CGRP, ACTH, Enkefalinas, etc.. Tres criterios básicos para determinar si una molécula es un neurotransmisor en una dada sinapsis Purves 3rd Edition Purves 3rd Edition Cargado de las vesículas de NT Receptores ionotrópicos y metabotrópicos 2015 Sistema colinérgico [1] Langley JN. On the reaction of cells and nerve-endings to certain poisons, chiefly as regards the reaction of striated muscle to nicotine and to curari. J Physiol 1905;33:374–413. (“sustancias receptivas”) [2] Loewi O. On the background of the discovery of neurochemical transmission. J Mt Sinai Hosp N Y 1957;24:1014–6. 1921 [3] Dale H. Transmission of nervous effects by acetylcholine: Harvey Lecture, May 20, 1937. Bull N Y Acad Med 1937;13:379–96. 1930 [4] Fatt P, Katz B. An analysis of the endplate potential recorded with an intracellular electrode. J Physiol (Lond) 1951;115:320–70. In 1936 Dale shared the Nobel Prize in Physiology or Medicine with Otto Loewi “for their discoveries relating to the chemical transmission of nerve impulses”. Experimento de Otto Loewi en 1921 Otto Loewi llamó a esa sustancia “Vagusstoff” luego Henry dale demostró que esa sustancia era Aceticolina. Fatt P, Katz B 1951 MEPPs Liberación espontánea de ACh : potenciales miniatura MEPP = 1 cuanto o paquete = 1 vesícula de ACh conteniendo ~ 6000-10000 moléculas de ACh Se activan ~ 3000 a 5000 canales Síntesis de Acetilcolina ? Recaptación de Colina Se expresa específicamente en neuronas colinérgicas Fig. 2. Electrophysiological characterization of ACh release at CHT NMJs. (A) Examples of evoked EPP traces (arrows) from CHT (left) and CHT (right) sternomastoid NMJs 1 or 4 h after birth. Cargado de las vesículas de ACh Terminación del efecto de la acetilcolina: Degradación por las colinesterasas La ACh actúa sobre dos tipos de receptores nicotínicos ionotrópicos canales iónicos muscarínicos metabotrópicos acoplados a proteínas G Estructura cuaternaria de los receptores nicotínicos Esquema basado en el receptor del órgano eléctrico de Torpedo Californica Los receptores nicotínicos son proteínas oligoméricas compuestas por 5 subunidades glicosiladas que atraviesan la membrana y conforman un canal catiónico 290 KDa 40-65 KDa TM2 poro del canal Pertencen a la superfamilia de receptores activados por ligando con asa de cisteina formada por los receptores nicotínicos, GABAA, GabaC , glicina y 5HT3 (el receptor de glutamato tiene una estructura diferente) S S Topología de plegamiento de las subunidades de receptores con ‘asa de cisteína’ Sitio de unión a la acetilcolina Agonistas y antagonistas competitivos en las subunidades α de todos los nAChR: 2 cisteínas contiguas, las Tyr 93, 190 y 198 y el Trp 149 (excepto en α5 Asp 190) En músculo: γTrp 53; δΤrp55 -Τyr 111; δArg113 y γTyr 117 Selectividad iónica Na+, K+ y Ca2+ PNa/PK ∼ 1 PCa/PNa 0.2-20 TM2 3 anillos de aa cargados negativamente orientados hacia el poro del canal, determinantes de la selectividad iónica K+ K+ + + K K Permeabilidades relativas PK/PNa y PCa/PNa Corriente neta y potencial de membrana (Eq de los iones permeantes) A Vm ̴ -60 a Entrada masiva de Na+ Despolarización Entrada de Ca2+ Segundo Mensajero Activación de otras corrientes Hiperpolarización Ca2+ Ca2+ Ca2+ + Na+ 2+ Na Na+ + + Ca2+ Ca2+ Ca Na Na Subunidades Colinérgicas Nicotínicas nAChR Musculares Alpha1 (α1) Beta1 (β1) Gamma (γ) Delta (δ) Epsilon (ε) nAChR Neuronales Alpha2 (α2) Alpha3 (α3) Alpha4 (α4) Alpha5 (α5) Alpha6 (α6) Beta2 (β2) Beta3 (β3) Beta4 (β4) Alpha9 (α9) Alpha10 (α10) Alpha7 (α7) Alpha8 (α8) Los receptores que difieren en la composición de sus subunidades presentan propiedades biofísicas, funcionales y farmacológicas diferentes Receptores Nicotínicos: Diversidad funcional y farmacológica Participan en la neurotransmisión rápida excitatoria (Placa neuromuscular y SN autónomo) En la modulación de la liberación de otros NT (SN central) En la inhibición rápida en la sinapsis eferente olivococlear (SNC a las CCE del órgano de Corti en el oído interno) Antagonistas Agonistas α-Bungarotoxina ACh nicotina citisina DMPP epibatidina anatoxina tubocurarina n-Bungarotoxina dihidro-β-eritroidina mecamilamina Los tipos de subunidades ensambladas determinan las propiedades biofísicas, farmacológicas y fisiológicas del canal. Músculo fetal vs adulto (canales nativos) δ Fetal ε Adulto Oocitos αβγδ vs. αβγε (canales recombinantes) Farmacología de los receptores recombinantes expresados en sistemas heterólogos y su correlato con los receptores nativos Respuestas obtenidas con el nAChR α7 homomérico expresado en ovocitos de Xenopus laevis Respuestas obtenidas en neuronas de hipocampo de Clarke, 1992 Otras subunidades? Diferente procesamiento posttraduccional? Receptor α β γδ 1 1 orden de potencia de agonistas suberildicolina > ACh > epibatidina > α β 3 4 α β 4 2 α 7 orden de potencia de antagonistas α -bungarotoxina > d-tubocurarina > β mecamilamina > DH E dimetilfenilpiperazonio ≈ (DMPP) > citisina nicotina epibatidina > DMPP > nicotina ≈ ≈ citisina suberildicolina > ACh mecamilamina > d-tubocurarina > β DH E β epibatidina > nicotina > citisina mecamilamina > DH E > MLA > d> DMPP > ACh > carbacol epibatidina > DMPP > citisina > nicotina > ACh > colina tubocurarina α -bungarotoxina > MLA > dβ tubocurarina > DH E Farmacología de las corrientes sensibles a α-Bungarotoxina en neuronas ganglionares intracardíacas de Cuevas y Berg, 1998 Caracterización de distintos tipos de corrientes en neuronas de hipocampo de rata Corrientes evocadas por ACh de Alkondon y Albuquerque, 1993 K+ K+ + + K K Permeabilidades relativas PK/PNa y PCa/PNa Corriente neta y potencial de membrana (Eq de los iones permeantes) A Vm ̴ -60 a Entrada masiva de Na+ Despolarización Entrada de Ca2+ Segundo Mensajero Activación de otras corrientes Hiperpolarización Ca2+ Ca2+ Ca2+ + Na+ 2+ Na Na+ + + Ca2+ Ca2+ Ca Na Na Compuesto Células ciliadas ACh a-j agonista µ EC50 ( M): α α α 9 y 9 10 agonistak, n µ EC50 : 11.4 M 7; 13.5, 17 (cobayo) 4,8,9 22 (pollo)10 b, d, i, j agonistan µ EC50: 63.7 M carbacol agonista µ EC50: 87 M DMPP agonista parcial suberildicolina agonista parcial nicotina antagonista µ IC 50: rango M citisina sin actividad agonista muscarina sin actividad agonista α -bungarotoxina agonista parcialk b, d, i, j agonista parcialn j antagonistaµk,n, competitivon IC 50: 31.5 M b, d, j d, e, g, h c, d, g, h antagonista IC 50: rango nM d-tubocurarina antagonista IC 50: rango nM atropina antagonista µ IC 50: rango M antagonistaµk,n competitivon IC 50: 43.1 M k, n antagonista µ IC 50: 83 M b, d, j antagonista ,reversible nM estricnina bicuculina b, d, i, j a, c, d, e, g, h d, g d ≈ µ antagonista IC 50: 1 M IC50: rango antagonista reversible IC 50: 4 nM k,l k, m antagonista IC 50: 17.9 nM k antagonista IC 50: 300 nM k, n antagonista competitivon µ IC 50: 1 M m µ antagonista IC 50: 0.8 M nAChR mAChR Transmisión sináptica colinérgica rápida en la sinapsis neuromuscular Transmisión colinérgica en el SNC Schematic representation of a hypothetical cholinergic synapse illustrating general synaptic localization and function of cholinergic receptors in the CNS En el SNC los nACHRs se encuentran principalmente fuera de la brecha sináptica modulando la liberación de otros NT o modulando la excitabilidad de otras neuronas UNMuscular Sistema Nervioso Autonomo Adicción a la nicotina 2008 Nicotine-Induced Upregulation of Native Neuronal Nicotinic Receptors Is Caused by Multiple Mechanisms Anitha P. Govind, Heather Walsh, and William N. Green Department of Neurobiology, University of Chicago, Chicago, Illinois 60637 J. Neurosci. 2012 Nicotine causes changes in brain nicotinic acetylcholine receptors (nAChRs) during smoking that initiate addiction. Nicotine-induced upregulation is the long-lasting increase in nAChR radioligand binding sites in brain resulting from exposure. The mechanisms causing upregulation are not established. Many different mechanisms have been reported with the assumption that there is a single underlying cause. Using live rat cortical neurons, we examined for the firsttime how exposure and withdrawal of nicotine shape the kinetics of native alpha4beta2-containing nAChR upregulation in real time. Upregulation kinetics demonstrates that at least two different mechanisms underlie this phenomenon. First, a transient upregulation occurs that rapidly reverses,faster than nAChR degradation, and correspondsto nAChR conformational changes as assayed by conformational-dependent, subunit-specific antibodies. Second, a long-lasting process occurs correlating with increases in nAChR numbers caused by decreased proteasomal subunit degradation. Previous radioligand binding measurements to brain tissue have measured the second process and largely missed the first. We conclude that nicotine-induced upregulation is composed of multiple processes occurring at different rates with different underlying causes. Cholinergic connectivity: it’s implications for psychiatric disorders Elizabeth Scarr 1,2*, Andrew S. Gibbons1,2, Jaclyn Neo1,2, Madhara Udawela 2,3 and Brian Dean1,2 2013 Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system Vesicular neurotransmitter transporters depend differentially on the chemical and electrical components of the H+ electrochemical gradientThe vacuolar-type H+-ATPase generates the H+ electrochemical gradient (ΔμH+) required for transport of all classical neurotransmitters into synaptic vesicles. However, different vesicular neurotransmitter transporters rely to differing extents on the two components of ΔμH+, the chemical gradient (ΔpH) and the electrical gradient (Δψ). The vesicular accumulation of monoamines and ACh (left) involves the exchange of protonated cytosolic transmitter for two lumenal H+. The resulting movement of more H+ than charge dictates a greater dependence on ΔpH than Δψ for both VAChT and VMAT. Vesicular glutamate transport (right) may not involve H+ translocation. In the absence of Δψ, however, disruption of ΔpH inhibits uptake, suggesting that the transport of anionic glutamate involves exchange for nH+, resulting in the movement of n + 1 charge and hence greater dependence on Δψ than ΔpH. Transport of the neutral zwitterion GABA (and glycine) involves the movement of an equal number of H+ and charge, consistent with the similar dependence of VGAT on ΔpH and Δψ. These differences suggest that vesicles storing monoamines or ACh may have mechanisms to favor the accumulation of ΔpH at the expense of Δψ, whereas those storing glutamate may promote a larger Δψ. The extent to which vesicles differ in their expression of these two components remains unknown, but intracellular chloride carriers such as the synaptic vesicleassociated ClC-3 promote vesicle acidification by dissipating the positive Δψ developed by the vacuolar H+ pump, thereby disinhibiting the pump to make larger ΔpH. The VGLUTs can also contribute to formation of ΔpH because as an anion, glutamate entry similarly dissipates Δψ to promote ΔpH. Interestingly, a Cl− conductance associated with the VGLUTs may also promote acidification by Cl−. Frederik De Smet, Arthur Christopoulos & Peter Carmeliet 2014 To discharge the electrocytes at the correct time, the electric eel uses its pacemaker nucleus, a nucleus of pacemaker neurons. When an electric eel spots its prey, the pacemaker neurons fire and acetylcholine is subsequently released from electromotor neurons to the electrocytes, resulting in an electric organ discharge.
