Download ach new

ACh was first isolated around 1914; its functional significance was first
established in about 1921 by Otto Loewi, a German physiologist and later
(1936) Nobel laureate. Loewi demonstrated that ACh is the substance
liberated when the vagus nerve is stimulated, causing slowing of the
heartbeat. Subsequently he and others showed that ACh is also liberated as a
transmitter at the motor end plate of striated (voluntary) muscles of
vertebrates, and it has since been identified as a transmitter at many neural
synapses and in many invertebrate systems as well
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
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
Acetylcholine (ACh) is terminated by hydrolysis, which is greatly accelerated by one or more of the
cholinesterase enzymes:
1) Acetylcholinesterase (AChE) is present in high concentration in cholinergic synapses (SNC, muscolo
sch .
2) Butyrylcholinesterase, also known as pseudocholinesterase is important for hydrolyzing ACh in the
circulation. (Fegato intestino cuore e polmoni)
It is important to recognize that the neurotransmitter actions of acetylcholine are terminated by a chemical
reaction that forms two products (choline and acetate) which are essentially inactive. Diffusion of ACh from
the synaptic region plays a minor role because AChE is so active.
AChE inhibitors, also designated AChEIs, include echothiophate, edrophonium, neostigmine,
physostigmine. Other AChEIs include various so-called nerve gas agents such as sarin and soman.
Ach E: inibizione substrato, BchE attiva solo ad alte concentrazioni di substrato (rappresenta una riserva
di AchE se qs è poca o assente, come durante differenziamento e sviluppo cell.
Actions of acetylcholine
Acetylcholine (ACh) has diverse actions on a number of cell types mediated by
two major classes of receptors:
1) Nicotinic receptors are ligand-gated ion channels.
2) Muscarinic receptors are part of the transmembrane, G protein coupled
receptor family.
\
NICOTINIC RECEPTORS
1) nicotinic muscle (Nm): neuromuscular junction of skeletal muscle;
2) nicotinic neuronal (Nn): autonomic ganglia and other parts of
the nervous system
When ACh or other agonists occupy the receptor site on the external surface of
the cell membrane there is a conformational change in the ion channel and an
increase in conductance to the ion(s) for which that channel is selective. Thus,
when Nm receptors are activated, there is an influx of cations through the ion
channel and depolarization of the motor end plate. In short, nicotinic receptors
rather directly transduce the ACh external messenger into an action on the cell.
The acetylcholine receptor is a pentaramic protein
consisting of five subunits (2 alpha units, one beta unit,
one gamma unit, and one delta unit); 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.
Nicotinic Receptors
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.
Muscarinic Receptors
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
Other: d-tubocurarine; Histrionicotoxin; Lophotoxin; Epibatidine
Muscarinic agonists: Muscarine; Arecoline; Pilocarpine; Green mamba snake
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.