1914: isolata
1921: Otto Loewi, significato funzionale, liberata a seguito della stimolazione vagale, provoca nella rana, riduzione della attività cardiaca
COLINA
: substrato fondamentale, trasportata dal plasma al neurone tramite un trasportatore con elevata affinità, non <saturato>precursori: fosfolipidi e fosforilcolina (lecitina e demenza)
60% Acetilcolina
40
%
fosfolipidi +
Sintesi nei Mitocondri 1) acetilCoA sintetasi 2) colin acetiltransferasi
2benzoiletiltetramonio Acetilseco 3’emilcolinio
(-)
(-)
ATP-colin-transferasi
COLINA
: substrato fondamentale, trasportata dal plasma al neurone tramite un trasportatore con elevata affinità, non <saturato>precursori: fosfolipidi e fosforilcolina (lecitina e demenza)
60% Acetilcolina
40% fosfolipidi +
Sintesi nei Mitocondri 1) acetilCoA sintetasi 2) colin acetiltransferasi
2benzoiletiltetramonio Acetilseco 3’emilcolinio
(-)
(-)
ATP-colin-transferasi
Composti organici del merucurio
Storage and release
ACh in cholinergic nerve fibers is taken up into synaptic vesicles by an uptake process that is inhibited by the drug vesamicol. In the presence of vesamicol, cholinergic fibers soon have no ACh stored in vesicles for release. Transmission fails although other functions of the fiber are still intact
Vesicular release depends on depolarization of the nerve terminal and the influx of calcium ion. At the motor end-plate in the neuromuscular junction this results in a relatively massive release of ACh (hundreds of vesicles and thousands of ACh molecules per vesicle) and an end-plate potential that normally results in depolarization of the muscle cell and contraction. The release of ACh at various cholinergic junctions can be blocked by certain toxins, most notably those produced by Clostridium species.
Botulinum toxin A, from Clostridium botulinum binds to cholinergic nerve terminals and is internalized. Once internalized it acts on the vesicle release process and prevents exocytosis. All junctional release of ACh is inhibited by such toxins.
In patients poisoned by Clostridium botulinum the immediate clinical problem is flaccid paralysis and respiratory failure.
Botulino: blocco rilascio
Vedova nera: aumenta rilascio
Curaro: blocca i recettori post-sinaptici
Effetto
tossico traslocazione della
catena L all’interno delle cellule bersaglio
Legame alla superficie delle cellule bersaglio
paralisi progressiva della muscolatura che colpisce prima a livello del volto e poi scende verso la gola, il tronco, le estremita' fino a provocare una paralisi respiratoria e cardiaca nel caso in cui non si riesca a bloccare in tempo l'azione della tossina.3.
Altri sintomi visione doppia (diplopia), dilatazione della pupilla (midriasi), sensazione di formicolio al volto, problemi nell'eseguire de terminati movi menti con gli arti superiori, sensazione di mancanza di sensibilita' al volto e al collo, difficolta' a urinare (anuria) e a deglutire i cibi solidi e liquidi (disfagia).
ACETILCOLIN ESTERASI Assicura l’efficienza della neurotrasmissione colinergica
Ciclo del messaggio chimico : 2 msec nella trasmissione neuromuscolare 1 msec muscolo liscio
Sede: dendriti e nel pericarion dei neuroni, collocato nello spazio sinaptico legato ad una rete di collageno che forma la lamina basale che riempie lo spazio tra neurone e cellula muscolare striata
1) Acetilcolinesterasi (AchE) presente in alte concentrazioni nelle sinapsi colinergiche (INIBIZIONE DA SUBSTRATO)
2) Butyrylcholinesterasi o pseudocolinesterasi, , idrolizza la Ach in periferia (Fegato intestino cuore e polmoni). Ha bassa affinità per la Ach, è infatti attiva ad alte concentrazioni di Ach. Rappresenta una riverva di AchE quando questa è poca o assente, come durante lo sviluppo e differenziamento
AChE inhibitors, also designated AChEIs, include echothiophate, edrophonium, neostigmine, physostigmine. Other AChEIs include various so-called nerve gas agents such as sarin and soman.
RECETTORI della ACETILCOLINA
1)NICOTINICI, recettori canali:
a)Muscolari: placca neuromuscolare
b)Neuronali: gangli del SNA e in altre parti del SN
1)MUSCARINICI, recettori accoppiati a proteine G
The acetylcholine receptor is a pentaramic protein consisting of five subunits
2
each subunit encoded by a seperate gene. For all five subunits to assemble correctly the gene expression must be precisely coordinated. The five subunits are arranged in a barrel-like configuration around a central ion pore.
Acetylcholine binds to the alpha subunit, which consists of 457 amino acids. The main binding site for acetylcholine is on the alpha subunit within a pocket of the external part of the peptide chain. Intracellular ions are collected within the folds of the receptor and attracted to charged residues within the walls of the folds. Residues are located at the ends of the pores to help determine the ionic selectivity of the channel: oppositely charged residues attract, therefore the negative receptors of an acetylcholine receptor attract cations. Acetylcholine reacts with the residues to form weak bonds which cause an alosteric change in the subunit configurations and allows ions to enter the channel. The channel is nonselective between cations, producing an inward flow of positive charges. These positive charges initiate the action potential which causes the muscle to contract.
2 molecole di Ach legano il recettore
Nicotinic Receptors
Proprieta funzionali cambiano durante lo sviluppo
Adulto
conduttanza singolo canale più elevata (59 ps), tempi apertura più breviFeto
conduttanza singolo canale più bassa (39ps) tempi di apertura più lunghi - Ridotta densità
Recettori Nicotinici centrali
Pentamerici omomerici (solo subunità ) o eteromerici (subunità e
Presentano una elevata permeabilità al CALCIO (>> omomerici)
Glutamato 237 dell’anello intermedio del poro regola la permeabilità al calcio
1) Sindrome del canale lento della Ach: miastenia AD, per singole mutazioni in
e
2) Epilessia notturna del lobo frontale: sonno, bambino, mutazioni di
Aumentata desensitizzazione del recettore, ridotta conduttanza e tempo medio di apertura.
Transduction of the ACh message is more complex in the muscarinic family of receptors. And the family of muscarinic receptors is more complex than the nicotinic family. There are at least 5 muscarinic receptor subtypes expressed in humans. For most purposes it is sufficient to concentrate on M1, M2 and M3 receptors.
1) M1 receptors : autonomic ganglia central nervous system.
2) M2 receptors : > the supraventricular parts of heart the heart.
3) M3 receptors, smooth muscles and glands,
endothelial cells in the
vasculature.
M2 legati a Gi inibiscono la adenolato ciclasi e aprono i canali K
The bottom line is that M1 and M3 receptors generally mediate excitatory responses in effector cells. Thus, M1 receptors promote depolarization of postganglionic autonomic nerves, and M3 receptors mediate contraction of all smooth muscles (an apparent exception to be noted below) and increased secretion in glands. It is useful to remember that excess ACh levels in the body (for example caused by inhibition of AChE) are associated with GI cramping, salivation, lacrimation, urination, etc.
Correnti inibitrici del K PKC
M1 p
MUSCARINIC RECEPTORs
Recettori muscarinici
come M1 Gq/11
acetilcolina M5 SNC
come M2 Gi/o
acetilcolina M4 SNC
come M1 Gq/11
esaidrosil- adifenolo acetilcolina,
carbacolo Ghiandole esocrine;
muscolatura liscia;
endotelio M3
conduttanza K+; adenilato ciclasi Gi/o
AF DX 117 acetilcolina,
carbacolo Cuore (t. di conduzione,
atri); gangli auonomi M2
fosfolipasi C Gq/11
pirenzepina acetilcolina,
carbacolo, McN- A-343
SNC; gangli autonomi M1
Meccanismo di trasduzione G-
proteine Antagonisti
Agonisti Localizzazione
Tipo
M1, M3, M5 = Gq/11 M2, M4 = Gi/o
EFFETTI DEL BLOCCO DEI RECETTORI MUSCARINICI
Atropina alcaloide naturale estratto dalle
foglie di alcune Solanacee (Atropa belladonna, ...
Scopolamina, nota anche come ioscina, è un farmaco alcaloide allucinogeno
ottenuto da piante della famiglia delle Solanaceae, come l'Hyoscyamus nige
ACETYLCHOLINE RECEPTORS: Disorders
* Muscle
* Myasthenia Gravis
* Autoimmune: IgG vs 1 subunit
* Hereditary
* Subunits: &
* Subunit:
* Neuronal
* Immune neuropathies: Isaac's; Subacute autonomic
* IgG antibody vs 3 subunit
* Paraneoplastic syndrome: Associated with small cell lung carcinoma
* Epilepsy
* Benign neonatal & Nocturnal frontal lobe, Type 1
Neural nicotinic, 4 subunit ; Chromosome 20q13.2-q13.3; Dominant
* Nocturnal frontal lobe, Type 3
Neural nicotinic, 2 subunit (CHRNB2) ; Chromosome 1p21; Dominant
* Schizophrenia: Attention disorder
* Lack of inhibition of P50 response to auditory stimulus
* Linked to dinucleotide polymorphism at 15q13-q14: Site of -7-nicotinic receptor
* Mouse knockouts
* Lethal: -AChR subunit loss
* CNS neuronal loss with subunit knockout
* Neural nicotinic, 2 subunit of AChR (CHRNB2)
* Defects localized in CA1 and CA3 fields in hippocampus & neocortex
* 7 subunit: Minimal phenotype
* 9 subunit: Altered innervation of cochlear hair cells
* Autonomic dysfunction
* Knockouts of neural nicotinic AChR subunits
* 3 : Bladder enlargement; Dilated, unresponsive pupils
* 2
* Nicotine-elicited anti-nociception: Reduced
* Neurons in hippocampus & neocortex: Reduced
* 4
* Nicotine-elicited anti-nociception: Reduced
* Muscarinic
* IgG vs M3-muscarinic AChRs: Occur in both 1° & 2° Sjögren's
* Toxins
* Nicotinic agonists: Nicotine; Anatoxin A
* Nicotinic antagonists
* Peptides: -snake toxins; -conotoxins
* Other: d-tubocurarine; Histrionicotoxin; Lophotoxin; Epibatidine
* Muscarinic agonists: Muscarine; Arecoline; Pilocarpine; Green mamba snake
* Muscarinic antagonists: Scopolamine; Atropine
MYASTHENIC & NEUROMUSCULAR JUNCTION (NMJ) DISORDERS
BASIC CONCEPTS
Acetylcholine receptors (AChRs) AChR structure
AChR subunit mutations: ; ; ; Neuromuscular junction (NMJ) Presynaptic
Postsynaptic
ACQUIRED NMJ DISORDERS Botulism
Myasthenia gravis
Autoimmune myasthenia gravis Childhood MG
Drug-induced MG Neonatal: Transient MG Ocular
Anti-AChR-Antibody-Negative Thymoma
Domestic animals
Myasthenic syndrome (Lambert-Eaton) Snake venom toxins
---
Synaptic and Post-synaptic molecules at the NMJ CONGENITAL & FAMILIAL NMJ DISORDERS2 General features
AChRs: Kinetic abnormalities Presynaptic defects
Congenital MG + Episodic apnea (Familial infantile): ChAT; 10q11 Paucity of synaptic vesicles & Reduced quantal release
Congenital Lambert-Eaton-like Episodic ataxia 2: CACNA1A; 19p13 Synaptic defects
Acetylcholinesterase (AChE) deficiency at NMJs: ColQ; 3p25 Postsynaptic defects: AChR disorders
Kinetic abnormalities in AChR function Reduced Numbers of AChRs at NMJs
Increased Response to ACh: Slow AChR channel syndromes Delayed channel closure: AChR mutations
Repeated channel reopenings: AChR mutations Reduced Response to ACh
Fast-channel syndrome: Mode-switching kinetics ; AChR subunit Fast channel syndrome: Gating abnormality; AChR or subunit Fast channel syndrome: Arthrogryposis; AChR subunit Also see: subunit disorders
Normal numbers of AChRs at NMJs: Reduced Response to ACh Fast-channel syndrome: Low ACh-affinity of AChR; AChR subunit Fast-channel syndrome: Reduced channel openings; AChR subunit High conductance & Fast closure of AChRs
Increased Numbers of AChRs at NMJs
Slow AChR channel syndrome: AChR subunit L262M No kinetic abnormalities in AChR function
Reduced Numbers of AChRs at NMJs AChR mutations
Usually: subunit: 17
Rarely: (2q24), (17p12), subunit (2q33) Rapsyn: 11p11
Other hereditary MG syndromes
Benign congenital MG & Facial malformations Congenital MG: Other
Familial immune Limb-girdle MG: Familial Plectin deficiency: Plectin; 8q24
The muscular weakness and fatigability associated with myasthenia gravis are caused by an autoimmune attack on the acetylcholine receptor at the neuromuscular junction. Antibodies have been shown to decrease the usefulness of acetylcholine receptors through accelerated endocytosis and blockade of the receptor.
Endocytosis is the process of extracellular substances being incorporated into the cell by vesicles forming inward through budding of the plasma membrane.
Researchers have been able to demonstrate the effect of antibodies on acetylcholine receptor degradation by using radioactively labeled alpha bungaroo toxin, a snake poison, to follow the rate of degradation. Antibodies from patients with MG cause a two to three fold increase in the rate of degradation of acetylcholine receptors. The myasthenic antibodies cause a cross linking between the acetylcholine receptors, causing the linked receptors to be drawn together into clusters and rapidly endocytosed.
In myasthenic patients the neuromuscular junction has decreased numbers of acetylcholine receptors, a wider synaptic cleft, and simplified synaptic folds.
These changes account for the clinical features of myasthenia gravis. Decreased numbers of acetylcholine receptors result in fewer interactions between acetylcholine and it's receptors, leading to decreased activation of action potentials. When the transmission of action potentials decreases, the power of the muscle's contraction is reduced, causing weakness. During repeated nerve stimulation the amount of acetylcholine normally declines, or runs down. In myasthenia gravis, this run down occurs more rapidly due to a decrease of receptors in myasthenic junctions, causing muscular fatigability. The wider synaptic cleft and simplified synaptic folds also work to decrease the number of interactions between acetylcholine and acetylcholine receptors.
