Acetylcholine (ACH) is a very important mediator. The activity of cholinergic neurons of the central nervous system (CNS), which travel from the basal structures of the forebrain to the hippocampus, provides the possibility of learning and memorization. Damage to these neurons leads to Alzheimer's disease.
In the peripheral nervous system, cholinergic are all motor neurons of skeletal muscles, preganglionic neurons that innervate sympathetic and parasympathetic ganglia, as well as postganglionic nerve fibers that carry out parasympathetic innervation of the heart muscle, smooth muscles of the intestine and bladder, as well as smooth muscles of the eye responsible for accommodation processes. and close vision.
Acetylcholine (ACh) is synthesized by the transfer of an acetyl group from acetyl coenzyme A (acetyl-CoA) to choline by the enzyme choline acetyl transferase. Choline acetyltransferase is present exclusively in cholinergic neurons. Choline enters the neuron from the intercellular space by active transport. Acetyl-CoA is synthesized in mitochondria, which synthesize choline acetyltransferase and are located in large quantities in nerve endings.
After the release of acetylcholine (ACh) into the synaptic cleft, it is destroyed by acetylcholinesterase (AChE) with the formation of choline and acetic acid, which are recaptured and reused for the synthesis of new mediator molecules.
The stages of synthesis, decay and reuptake of acetylcholine (ACh) are shown in the figure below.
(A) Scheme for the synthesis of acetylcholine (ACh) from acetylcoenzyme A (Acetyl-CoA) and choline by choline acetyltransferase (ChAT).(B) Degradation of the acetylcholine molecule by acetylcholinesterase (AChE).
Dashed arrows indicate the reuse of acetic acid and choline.
There are mediator-dependent acetylcholine (ACh) receptors and receptors associated with G-proteins. Ionotropic acetylcholine (ACh) receptors are called nicotinic receptors because the first substance that caused their activation was nicotine isolated from the tobacco plant. Metabotropic ACh receptors are called muscarinic, since their activator is muscarine, a substance isolated from poisonous fly agaric mushrooms.
1. Nicotinic receptors. Nicotinic receptors are concentrated in the neuromuscular synapses of skeletal muscles, in all autonomic nerve ganglia, and also in the central nervous system. Under the action of ACh, the ion channel opens and Ca 2+ and Na + ions rapidly enter the cell, which leads to depolarization of the target neuron.
Nicotinic receptors are discussed in more detail when describing the process of skeletal muscle innervation in a separate article on the site.
2. Muscarinic receptors. G-protein-dependent muscarinic receptors are concentrated (a) in the temporal lobe of the brain, where they are involved in the process of memory formation; (b) in autonomic ganglia; (c) in cardiac muscle fibers, including conductive fibers; (d) in the smooth muscles of the intestine and bladder; (e) in the secretory cells of the sweat glands.
There are five subtypes of muscarinic receptors - M 1 -M 5 M 1, M 3 - and M 5 receptors - excitatory: through enzyme cascades, phospholipase C is activated and intracellular Ca 2+ levels increase. M 2 - and M 4 receptors are inhibitory autoreceptors that reduce the intracellular level of cAMP and/or increase the release of K + from the cell during hyperpolarization.
Cholinergic processes in the heart and other internal organs are described in a separate article on the site.
3. Reuptake of acetylcholine. The products of hydrolysis of acetylcholine in the synaptic cleft - choline and the acetyl group - are captured by molecules of specific carriers back into the cell.
4. Strychnine poisoning. Strychnine blocks glycine receptors. Excruciating convulsions in strychnine poisoning are due to disinhibition of α-motor neurons caused by a violation of the inhibitory influences of Renshaw cells. Clinical manifestations are similar to those of tetanus toxin poisoning, which is known to interfere with the release of glycine from Renshaw cells.
In the course of pathoanatomical studies of the unchanged brain using labeled strychnine molecules, it was shown that glycine receptors are present in large numbers on the associative neurons of the trigeminal nucleus, which innervates the masticatory muscles, as well as the nucleus of the facial nerve, which innervates the mimic muscles. It is these two muscle groups that are more prone to convulsions during poisoning.
(A) Synthesis and reuptake of acetylcholine (ACh) molecules in the CNS. Nicotinic receptors (n-ACh receptors) are located on the postsynaptic membrane. (1) Choline molecules are taken up from the extracellular fluid and transferred to the nerve ending.
(2) Under the action of the mitochondrial enzyme choline acetyltransferase (CAT), acetylation of choline by acetyl coenzyme A (acetyl-CoA) occurs with the formation of acetylcholine (ACh).
(3) ACh molecules are placed in synaptic vesicles.
(4) ACh is released and binds to the appropriate receptors.
(5) Hydrolysis of mediator molecules occurs under the action of acetylcholinesterase (AChE).
(6) Choline fragments of molecules are transported back to the cytosol.
(7) Under the action of transferases, new acetylcholine molecules are synthesized, which are again placed in synaptic vesicles.
(8) The acetate fragment of the molecule moves into the cytosol.
(9) In mitochondria, new acetyl-CoA molecules are synthesized from acetic acid.
(B) Mediator-dependent nicotinic receptor. The addition of ACh causes the entry of a large amount of Na + ions into the cell and the exit of a small amount of K + ions from the cell.
Acetylcholinesterase an enzyme that breaks down a neurotransmitter acetylcholine.
Acetylcholine is released from the presynapse into the synaptic cleft and binds to a receptor on the postsynapse, thus effecting signal transmission between nerve cells. To transmit a new signal, it is necessary to remove the "spent" acetylcholine from the synaptic cleft. Acetylcholinesterase catalyzes the hydrolysis of acetylcholine to choline and acetic acid. From choline, a new acetylcholine is subsequently synthesized.
Disruption of the cholinergic systems is associated with various neurodegenerative diseases. Blocking acetylcholinesterase leads to the accumulation of acetylcholine and, consequently, increased transmission of excitation, which makes this enzyme a promising therapeutic target in drug development. Acetylcholinesterase inhibitor donepezil, used in the treatment of Alzheimer's disease, helps to reduce the symptoms of the disease.
Irreversible blocking of acetylcholinesterase underlies the mechanism of action of deadly poisonous substances: sarin, some snake venoms, organophosphate insecticides, V-gases.
Molecule models of acetylcholinesterase and its inhibitor donepezil
According to existing concepts, the mechanism of action of FOS is based on their selective inhibition of the enzyme acetylcholinesterase, or simply cholinesterase, which catalyzes the hydrolysis of acetylcholine, a chemical transmitter (mediator) of nervous excitation. There are 2 types of cholinesterase: true, "contained mainly in the tissues of the nervous system, in skeletal muscles, as well as in erythrocytes, and false, contained mainly in blood plasma, liver and some other organs. Acetylcholinesterase itself is true, or specific, cholinesterase, as soon as it hydrolyzes the named mediator. And it is precisely this that we will designate in the future by the term "cholinesterase". Since the enzyme and mediator are the necessary chemical components of the transmission of nerve impulses in synapses - contacts between two neurons or the endings of a neuron and a receptor cell, we should dwell in more detail on their biochemical role.
Acetylcholine is synthesized from choline alcohol and acetyl coenzyme A * under the influence of the choline acetylase enzyme in the mitochondria of nerve cells and accumulates in the ends of their processes in the form of bubbles with a diameter of about 50 nm. It is assumed that each such vial contains several thousand molecules of acetylcholine. At the same time, it is now customary to distinguish between acetylcholine, which is ready for secretion and located in the immediate vicinity of the active zone, and acetylcholine outside the active zone, which is in equilibrium with the former and is not ready for release into the sypaptic gap. In addition, there is also the so-called stable fund of acetylcholine (up to 15%), which is not released even under conditions of blockade of its synthesis. ** Under the influence of nervous excitation and Ca 2+ ions, acetylcholine molecules pass into the synaptic cleft - a space 20-50 nm wide that separates the end of the nerve fiber (presynaptic membrane) from the innervated cell. On the surface of the latter there is a postsynaptic membrane with cholinergic receptors - specific protein structures that can interact with acetylcholine. The effect of the mediator on the cholinergic receptor leads to depolarization (reduction of charge), a temporary change in the permeability of the postsynaptic membrane for positively charged Na + ions and their penetration into the cell, which in turn equalizes the voltage potential on its surface (shell). *** This gives rise to a new impulse in the neuron of the next stage or causes the activity of the cells of one or another organ: muscles, glands, etc. (Fig. 5). Pharmacological studies have revealed a significant difference in the properties of cholinergic receptors of various synapses. Receptors of one group, showing selective sensitivity to muscarine (fly agaric poison), are called muscarinic-sensitive, or M-cholinergic receptors; they are present mainly in the smooth muscles of the eyes, bronchi, gastrointestinal tract, in the cells of the sweat and digestive glands, in the heart muscle. Cholinergic receptors of the second group are excited by small doses of nicotine and are therefore called nicotine-sensitive, or H-cholinergic receptors. These include receptors of the autonomic ganglia, skeletal muscles, the medulla of the adrenal glands, and the central nervous system.
* (Acetyl coenzyme A is a compound of acetic acid with a nucleotide containing several amino acids and an active SH group. Cleaving off acetate, which is used to build the acetylcholine molecule, it turns into coenzyme A)
** (Glebov R. N., Primakovskiy G. N. Functional biochemistry of synapses. M.: Medicine, 1978)
*** (According to the established point of view, the occurrence of a potential difference between the outer and inner sides of the surface layer of the cell is due to the uneven distribution of Na + and K + ions on both sides of the cell membrane. At the same time, the compensating flow of K + ions, directed in the opposite direction when the mediator acts on the postsynanthic membrane, is somewhat delayed, which leads for a short time to depletion of the outer surface of the cell in positive ions.)
The molecules of acetylcholine, which have fulfilled their mediator function, must be immediately inactivated, otherwise the discreteness in the conduction of the nerve impulse will be disturbed and an excessive function of the cholinergic receptor will appear. This is what cholinesterase does, which instantly hydrolyzes acetylcholine. The catalytic activity of cholinesterase exceeds almost all known enzymes: according to various sources, the splitting time of one molecule of acetylcholine is about one millisecond, which is commensurate with the speed of transmission of a nerve impulse. The implementation of such a powerful catalytic effect is ensured by the presence in the cholinesterase molecule of certain sites (active centers) that have an exceptionally well-pronounced reactivity with respect to acetylcholine. * Being a simple protein (protein), consisting of only one amino acids, the cholinesterase molecule, as now found out, based on its molecular weight, contains from 30 to 50 such active centers.
* (Rosengart V. I. Cholinesterase. Functional role and clinical significance. - In the book: Problems of Medical Chemistry. M.: Medicine, 1973, p. 66-104)
As can be seen from fig. 6, the area of the cholinesterase surface, which is in direct contact with each mediator molecule, includes 2 centers located at a distance of 0.4-0.5 mm: an anionic, carrying a negative charge, and an esterase. Each of these centers is formed by certain groups of amino acid atoms that make up the structure of the enzyme (hydroxyl, carboxyl, etc.). Acetylcholine, thanks to the positively charged nitrogen atom (the so-called cationic head), is oriented by electrostatic forces on the surface of the cholinesterase. In this case, the distance between the nitrogen atom and the acidic group of the mediator corresponds to the distance between the active centers of the enzyme. The anionic center attracts the cationic head of acetylcholine to itself and thereby contributes to the convergence of its ester group with the esterase center of the enzyme. Then the ether bond breaks, acetylcholine is divided into 2 parts: choline and acetic, the acetic acid residue is attached to the esterase center of the enzyme and the so-called acetylrosane cholinesterase is formed. This extremely fragile complex instantly undergoes spontaneous hydrolysis, which frees the enzyme from the rest of the mediator and leads to the formation of acetic acid. From this moment, cholinesterase is again able to perform a catalytic function, and choline and acetic acid become the starting products for the synthesis of new acetylcholine molecules.
Formed in the body (endogenous) acetylcholine plays an important role in life processes: it promotes the transmission of nervous excitation in the central nervous system, autonomic ganglia, and the endings of parasympathetic (motor) nerves. Acetylcholine is a chemical transmitter (mediator) of nervous excitation; the endings of the nerve fibers for which it serves as a mediator are called cholinergic, and the receptors that interact with it are called cholinergic receptors. Cholinergic receptors are complex protein molecules (nucleoproteins) of a tetrameric structure, localized on the outer side of the postsynaptic (plasma) membrane. By nature, they are heterogeneous. Cholinergic receptors located in the region of postganglionic cholinergic nerves (heart, smooth muscles, glands) are designated as m-cholinergic receptors (muscarinic-sensitive), and located in the area of ganglionic synapses and in somatic neuromuscular synapses - as n-cholinergic receptors (nicotine-sensitive) (S. V . Anichkov). This division is associated with the peculiarities of the reactions that occur during the interaction of acetylcholine with these biochemical systems, muscarine-like (lowering blood pressure, bradycardia, increased secretion of the salivary, lacrimal, gastric and other exogenous glands, constriction of the pupils, etc.) in the first case and nicotine-like ( contraction of skeletal muscles, etc.) in the second. M- and n-cholinergic receptors are localized in various organs and systems of the body, including the central nervous system. Muscarinic receptors have been divided in recent years into a number of subgroups (m1, m2, m3, m4, m5). The localization and role of m1 and m2 receptors is currently the most studied. Acetylcholine does not have a strictly selective effect on various cholinergic receptors. To one degree or another, it affects m- and n-cholinergic receptors and subgroups of m-cholinergic receptors. The peripheral muscarine-like action of acetylcholine is manifested in slowing heart rate, dilating peripheral blood vessels and lowering blood pressure, activating the peristalsis of the stomach and intestines, contracting the muscles of the bronchi, uterus, gallbladder and bladder, increasing the secretion of the digestive, bronchial, sweat and lacrimal glands, constriction of the pupils ( miosis). The latter effect is associated with increased contraction of the circular muscle of the iris, which is innervated by postganglionic cholinergic fibers of the oculomotor nerve (n. oculomotorius). At the same time, as a result of the contraction of the ciliary muscle and the relaxation of the ligament of the ciliary girdle, a spasm of accommodation occurs. Pupil constriction due to the action of acetylcholine is usually accompanied by a decrease in intraocular pressure. This effect is partly explained by pupil dilation and flattening of the iris of the Schlemm's canal (scleral venous sinus) and fountain spaces (iriocorneal angle spaces), thereby improving the outflow of fluid from the internal media of the eye. It is possible, however, that other mechanisms are also involved in the reduction of intraocular pressure. Due to their ability to reduce intraocular pressure, substances that act like acetylcholine (cholinomimetics, anticholinesterase drugs) are widely used to treat glaucoma1. The peripheral nicotine-like effect of acetylcholine is associated with its participation in the transmission of nerve impulses from preganglionic fibers to postganglionic fibers in the autonomic nodes, as well as from motor nerves to striated muscles. In small doses, it is a physiological transmitter of nervous excitation, in large doses it can cause persistent depolarization in the synapse region and block the transmission of excitation. Acetylcholine also plays an important role as a mediator in the central nervous system. It is involved in the transmission of impulses in different parts of the brain, while in small concentrations it facilitates, and in large concentrations it inhibits synaptic transmission. Changes in the metabolism of acetylcholine can lead to impaired brain function. Some of its centrally acting antagonists are psychotropic drugs. An overdose of acetylcholine antagonists can cause disturbances in higher nervous activity (hallucinogenic effect, etc.). Acetylcholine chloride (Acetylcholini chloridum) is produced for use in medical practice and experimental studies.
Irreversible inhibition of cholinesterase causes death. Cholinesterase inhibitors are organophosphorus compounds (chlorophos, dichlorvos, tabun, sarin, soman, binary poisons). These substances bind covalently to serine at the active site of the enzyme. Some of them are synthesized as insecticides, and some as CWAs (nerve poisons). Death occurs as a result of respiratory arrest.
Reversible cholinesterase inhibitors are used as therapeutic drugs. For example, in the treatment of glaucoma and intestinal atony.
CATECHOLAMINES: norepinephrine and dopamine.
Adrenergic synapses are found in postganglionic fibers, in the fibers of the sympathetic nervous system, in various parts of the brain. Catecholamines in nervous tissue are synthesized by a common mechanism from tyrosine. The key enzyme in the synthesis is tyrosine hydroxylase, which is inhibited by end products.
NORADRENALIN is a mediator in the postganglionic fibers of the sympathetic and in various parts of the central nervous system.
DOPAMINE is a mediator of pathways, the bodies of neurons of which are located in the part of the brain that is responsible for controlling voluntary movements. Therefore, when dopaminergic transmission is disturbed, the disease parkinsonism occurs.
Catecholamines, like acetylcholine, accumulate in synaptic vesicles and are also released into the synaptic cleft when a nerve impulse arrives. But the regulation in the adrenergic receptor occurs differently. In the presynaptic membrane there is a special regulatory protein - alpha-achromogranin (Mm = 77 kDa), which, in response to an increase in the concentration of the mediator in the synaptic cleft, binds the already released mediator and stops its further exocytosis. There is no enzyme that destroys the neurotransmitter in adrenergic synapses. After the impulse is transmitted, the mediator molecules are pumped by a special transport system by active transport with the participation of ATP back through the presynaptic membrane and re-incorporated into the vesicles. In the presynaptic nerve ending, the excess of the mediator can be inactivated by MAO, as well as by catecholamine-O-methyltransferase by methylation at the hydroxy group. Cocaine inhibits the active transport of catecholamines.
Signal transmission in adrenergic synapses proceeds according to the mechanism known to you from the lectures on the topic “Biochemistry of hormones” with the participation of the adenylate cyclase system. The binding of the mediator to the postsynaptic receptor almost instantly causes an increase in the concentration of cAMP, which leads to rapid phosphorylation of the proteins of the postsynaptic membrane. As a result, the generation of nerve impulses by the postsynaptic membrane changes (it is inhibited). In some cases, the direct cause of this is an increase in the permeability of the postsynaptic membrane for potassium, or a decrease in conductivity for sodium (these events lead to hyperpolarization).
