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Sintesi nei Mitocondri
1) acetilCoA sintetasi
2) colin acetiltransferasi
Acetilseco 3’emilcolinio
(-)
(-)
COLINA:
substrato fondamentale, trasportata dal plasma al neurone tramite
un trasportatore con elevata affinità, non <saturato>
precursori: fosfolipidi e fosforilcolina
(lecitina e demenza)
60%
40
%
2benzoiletiltetramonio
ATP-colin-transferasi
+
Acetilcolina
fosfolipidi
1914: isolata
1921: Otto Loewi, significato funzionale, liberata a seguito della stimolazione
vagale, provoca nella rana, riduzione della attività cardiaca
Sintesi nei Mitocondri
1) acetilCoA sintetasi
2) colin acetiltransferasi
Acetilseco 3’emilcolinio
(-)
(-)
2benzoiletiltetramonio
Composti organici del merucurio
COLINA:
40%
substrato fondamentale, trasportata dal plasma al neurone tramite
un trasportatore con elevata affinità, non <saturato>
precursori: fosfolipidi e fosforilcolina
(lecitina e demenza)
Acetilcolina
60%
ATP-colin-transferasi
+
fosfolipidi
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
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).
Legame alla
superficie delle
cellule bersaglio
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
2abde
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.
2 molecole di Ach legano il recettore
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.
Nicotinic Receptors
Proprieta funzionali cambiano durante lo sviluppo
Adulto abde
conduttanza singolo canale più elevata (59 ps),
tempi apertura più brevi
Feto abdg
conduttanza singolo canale più bassa (39ps) tempi di
apertura più lunghi - Ridotta densità
Recettori Nicotinici centrali
Pentamerici omomerici (solo subunità a 7, 8 ) o eteromerici (subunità a e b
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 a,
be d
2) Epilessia notturna del lobo frontale: sonno, bambino, mutazioni di
a4
Aumentata desensitizzazione del recettore, ridotta conduttanza e tempo medio di
apertura.
MUSCARINIC RECEPTORs
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.
M1
p
PKC
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.
Correnti
inibitrici
K
M2 legati a Gi
inibiscono
la del
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.
Recettori muscarinici
Tipo
Localizzazione
Agonisti
M1
SNC; gangli autonomi
acetilcolina,
carbacolo, McNA-343
M2
Cuore (t. di conduzione,
atri); gangli auonomi
acetilcolina,
carbacolo
M3
Ghiandole esocrine;
muscolatura liscia;
endotelio
acetilcolina,
carbacolo
M4
SNC
M5
SNC
Antagonisti
Gproteine
Meccanismo di
trasduzione
Gq/11
 fosfolipasi C
AF DX 117
Gi/o
conduttanza K+; 
adenilato ciclasi
esaidrosiladifenolo
Gq/11
come M1
acetilcolina
Gi/o
come M2
acetilcolina
Gq/11
come M1
pirenzepina
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 a1 subunit
*
Hereditary
*
Subunits: a & b
*
Subunit: e
*
Neuronal
*
Immune neuropathies: Isaac's; Subacute autonomic
*
IgG antibody vs a3 subunit
*
Paraneoplastic syndrome: Associated with small cell lung carcinoma
*
Epilepsy
*
Benign neonatal & Nocturnal frontal lobe, Type 1
l Neural nicotinic, a4 subunit ; Chromosome 20q13.2-q13.3; Dominant
*
Nocturnal frontal lobe, Type 3
l Neural nicotinic, b2 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 a-7-nicotinic receptor
*
Mouse knockouts
*
Lethal: e-AChR subunit loss
*
CNS neuronal loss with subunit knockout
*
Neural nicotinic, b2 subunit of AChR (CHRNB2)
*
Defects localized in CA1 and CA3 fields in hippocampus & neocortex
*
a7 subunit: Minimal phenotype
*
a9 subunit: Altered innervation of cochlear hair cells
*
Autonomic dysfunction
*
Knockouts of neural nicotinic AChR subunits
*
a3 : Bladder enlargement; Dilated, unresponsive pupils
*
b2
*
Nicotine-elicited anti-nociception: Reduced
*
Neurons in hippocampus & neocortex: Reduced
*
*
a4
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: a-snake toxins; a-conotoxins
Synaptic and Post-synaptic molecules at the NMJ
MYASTHENIC & NEUROMUSCULAR JUNCTION
(NMJ) DISORDERS
BASIC CONCEPTS
Acetylcholine receptors (AChRs)
AChR structure
AChR subunit mutations: a; b; e; d
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
------------------------------------------------------------------------
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 D; AChR e subunit
Fast channel syndrome: Gating abnormality; AChR a or e subunit
Fast channel syndrome: Arthrogryposis; AChR d subunit
Also see: e subunit disorders
Normal numbers of AChRs at NMJs: Reduced Response to ACh
Fast-channel syndrome: Low ACh-affinity of AChR; AChR e subunit
Fast-channel syndrome: Reduced channel openings; AChR a subunit
High conductance & Fast closure of AChRs
Increased Numbers of AChRs at NMJs
Slow AChR channel syndrome: AChR subunit bL262M
No kinetic abnormalities in AChR function
Reduced Numbers of AChRs at NMJs
AChR mutations
Usually: e subunit: 17
Rarely: a (2q24), b (17p12), d 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.