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
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
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
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
Muscarinic Receptors
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
