lesson 2 Flashcards
Describe the cholinergic transmission in the CNS and peripheral nervous system
Cholinergic transmission is the name associated with all the processes of neural communication associated with the neurotransmitter acetylcholine. Acetylcholine (ACh) is a molecule that can be found in many districts of our nervous system:
- In the Central Nervous System (CNS): and in this case it’s deregulation is associated with diseases like Alzheimer’s disease and Parkinson’s disease. for the time being we will not focus on this district.
- In the Somatic Nervous System: at the level of neuromuscular junctions. Here the nervous message travels through the motor neuron and releases the neurotransmitter ACh within the neuromuscular junction communicating with the skeletal muscles. these muscles have specific receptors for ACh called nicotinic receptors. In the somatic nervous system we have extremely long fibers so the axons will be heavily myelinated to make it possible for the information to travel faster. Here at the level of the effectors, which are skeletal muscles, ACh has always an excitatory effect.
- in the Autonomous Nervous System: both in the parasympathetic and sympathetic systems. at the level of the parasympathetic system ACh is released by cranial and spinal neurons synaptic with a ganglion that also happens to have nicotinic receptors. from here a new fiber takes the nervous message to the smooth muscles were, once again ACh is released, but in this case we find another type of receptors called muscarinic receptors. For the sympathetic nervous system the message goes from the thoracic and lumbar neurons to the ganglia where ACh is released and recognised by N receptors (nicotinic) or ACh could be released at the level of adrenal medulla (midollo del surrene) where the interaction with N receptors causes the release of Epinephrine and norepinephrine. From the ganglion the sympathetic fibers take the message to the smooth muscles were norepinephrine is released and recognized by alpha and beta adrenergic receptors. it might also happen that the message travels from the ganglia to the sweat glands were ACh is released and recognised by M (muscarinic) receptors. In the parasympathetic system we have a longer fiber compared to the sympathetic one which is lightly myelinated and a shorter unmyelinated postganglionic axon. While In the sympathetic NS it is the opposite: we have shorter fibers which are lightly myelinated while the postganglionic fiber is longer and unmyelinated. It is also important to remember that the effector of sympathetic and parasympathetic systems are the same and this is the reason why there must always be a balance between them. In these cases the effect ca be either excitatory or inhibitory depending on the neurotransmitter and receptors found on the target organs.
Descrobe the cholinergic receptors
There are two types of cholinergic receptors:
- Nicotinic receptors are ligand-gated ion channels whose activation always causes a rapid (millisecond) increase in cellular permeability to Na+ and Ca2+, depolarization, and excitation
- Muscarinic receptors are G proteins coupled receptors (GPCRs). there are 5 subtypes: M1,M2, M3,M4 and M5, sometimes leading to inhibition and sometimes leading to excitation and they are not necessarily linked to changes in ion permeability. M type receptors followed by odd numbers (1,3,5) are coupled to Gq proteins, a G protein that increases calcium in the cell inducing the MAP kinases, while those followed by even number (2,4) are coupled to Gi/0 proteins which decrease cAMP levels and decrease the voltage of calcium channels, When acetylcholine binds to M1, 3 or 5 it leads to contraction while when it binds to M2 and M4 it stimulates relaxation of the muscles, for example M2 receptors stimulate the relaxation of the smooth muscles of the ventricle and atrium in the heart while M3 receptors stimulate the contraction of these muscles and also secretion in the glands. The most studied muscarinic receptors are M3 and M2.
What are the effect of the cholinergic transmission on smooth muscles?
What happens when ACh is released in the heart? because of the M2 receptors the effects are going to be a slower heartbeat and a loss of force a loss of contraction, also a loss of conduction velocity so if the release of ach we have a slow down of the heart, since this organ is controlled by the parasympathetic system it is active when we are at rest so the heart slows down when we are sleeping or resting.
The blood vessels have no ACh receptors so there is no effect for the ach release except for the erectile tissue where the main effect is dilation of the vessels causing erection.
At the level of bronchi the main effect is the restriction of the smooth muscles of the bronchi because of M3 receptors.
Glands also have M3 receptors and that cause secretion
In the GI (gastrointestinal tract) the main effect is an increased motility of the smooth muscles, sphincter dilatation and gastric acid secretion in the stomach
The effect in bladder is contraction preventing urine from being released always because of M3 receptors
In the eye we also have M3 receptors that cause the constriction of the pupil and the contraction of the ciliary muscle
In the salivary glands and lachrymary glands we have an increased secretion because of M3 receptors
Explain the mechanism of function of a normal cholinergic synapse. How does AChE work?
In a typical cholinergic synapse choline enters the cell through a secondary active transport, in the cytoplasm choline meets CoA (acetyl-Coenzyme A) and ACh is formed thanks to an enzyme called choline-acetyltransferase (ChAT). Next Vesicular acetylcholine transporter (VAChT) allows the storage of ACh in a vesicle. When the action potential arrives, it activates the fusion of the vesicle with the endoplasmic membrane and the release of ACh in the synaptic gap. The vesicle has a protein complex called SNARE useful to promote the fusion of the vesicle with the membrane. Now ACh can be recognized by the receptors (M and N) on the postsynaptic membrane.
To stop the release of ACh the only way is through a degrading enzyme called ACh esterase AChE, this enzyme is very active and effective especially at the neuromuscular junction, and is found on the membrane of the effector cell membrane. AChE separates choline and acetate, and that means that choline can reuse the secondary active transport to enter the cell once again, and produce a new molecule of ACh. AChE has two important sites: the anionic one which is complementary with the positive part of acetylcholine and leads the molecule to come closer to the receptor, then the esteric site binds the other part producing acetic acid and choline.
why are drugs that modulate cholinergic transmission difficult to synthitise? speak about experimental drugs in this field
In which way would it be possible to modulate the cholinergic transmission when there is too little or too much cholinergic transmission because of an illness? Many drugs have been studied with this purpose, but only a few of them are on the market, Pharmacologic manipulation of cholinergic transmission has met with only limited success because the complex actions of ACh which make it difficult to obtain selective effects:
- Hemicholinium is an experimental drug able to block the choline transporter, resulting in less production of ACh to tone down the transmission,
- Vesamicol too is only experimental and is able to block the vesicular transporter of ACh so, ACH would still be processed in the cytoplasm, but will not enter the vesicle, toning down the cholinergic transmission.
Speak about AChE inhibitors
Also we have AChE inhibitors that block the degradation of AChE increasing the cholinergic transmission. many compounds can act as AChE inhibitors, the first class is simple alcohols, they have a rapid but reversible effect that lasts only 2-10 minutes. Carbamic acid esters like Neostigmine, create a covalent bond with the enzyme which is still reversible but takes a lot longer. they have an half life of 15-30 minutes, with this drug the receptor is blocked for about 3 hours. Then we have organophosphates which are toxic. Because of the phosphate, they create with the enzyme a nearly irreversible bond so the half life of these drugs is hundreds of hours, making organophosphates toxic because they create an increase of cholinergic processes that cannot be relieved. Irreversible acetylcholinesterase inhibitors are unfortunatly used as chemical warfare. In these cases the compund pralidoxime which is a reactivator of AChE can be of help. this compound is positively charged and cannot cross barriers, that means that it has a local effect. In these cases early treatment is important to ensure that pralidoxime reaches the phosphorylated AChE while the latter still can be reactivated. Certain phosphorylated AChEs can undergo a fairly rapid process of “aging,” so that within the course of minutes or hours they become completely resistant to the reactivators. All of these compounds share a positive charge which makes them difficult to absorb, and prevents them from going through the brain barrier. That means that they have a local effect making them useful, for example, when we have to increase the parasympathetic tone of muscles. The first two compounds are used in myasthenia gravis, a condition that creates a weakness in our muscles because of the small number of acetylcholinesterase receptors. This happens because of an autoimmune problem that leads to less muscle contraction, if we decrease the degradation of ACh it is possible to increase the contraction. In this image we can see an experiment where the electrical activity of the muscles has been registered both in an healthy person and in someone who has this illness, in the second graph we can see the loss of contraction because of the weakness of the muscle. If we administer neostigmine there is an increase in the time in which acetylcholine is there to contract this muscle. Another use of these inhibitors is in case of glaucoma, to decrease intraocular pressure facilitating the outflow of aqueous humour. These drugs are also useful when there is a paralysis of smooth muscle like in paralytic ileus and atony of the urinary bladder. for this use the compounds are administered orally but they create some side effect because of the inevitable systemic effect. the side effect include: increase of gastric acid secretion and increased saliva secretion.
Speak about the use of agonists and antagonists for cholinergic muscarinic receptors
At the level of the receptors it is possible to use agonists to enhance the transmission and antagonists to block the transmission. Since the muscarinic receptors are specifically located would it be possible to find a drug that is able to identify only one receptor? In the future maybe yes, but this drug does not exist yet, for now we cannot have a specific compound for only one muscarinic receptor because all types are very similar. Muscarinic receptor agonists can be divided in two groups: choline esters like carbachol that have a positive charge so they do not get absorbed through barriers and membranes, allowing them to have a topic effect. These compounds also have an increased resistance to hydrolysis. Carbachol is a molecule used to cure glaucoma, it has a topic effect and causes the induction of miosis to reduce intraocular pression by opening the angle between the iris and the cornea. Bethanechol is also a choline ester that primarily affects the urinary and GI tracts: in the urinary tract it is useful in treating urinary retention and inadequate emptying of the bladder, In the GI tract it stimulates peristalsis and increases sphincter pressure. The second group is cholinomimetic alkaloids: among this there are many compounds coming from nature, for example muscarine was isolated from the mushroom Amanita muscaria, Pilocarpine, obtained from the leaflets of South American plants. This compound was discovered because the natives found out that the chewing of leaves of these plants caused salivation. Pilocarpine is used for the treatment of Sjogren Syndrome, an autoimmune disorder occurring primarily in women in whom secretions, particularly salivary and lacrimal, are compromised. Arecoline comes from betel nuts, which are consumed as a euphoretic masticatory mixture by the natives of the Indian subcontinent and East Indies, but since they don’t have a cationic head, they can pass the through the membranes and also reach the CNS. The we have Muscarinic receptors antagonists. These compounds can be obtained naturally, or can be either semisynthetic or fully synthetic and are used predominantly to inhibit effects of parasympathetic activity in the respiratory tract, urinary tract, GI tract, eye, and heart. Atropine comes form a batch called belladonna, it has some side effects that gets worse the more we increase the dosage. at a low dosage there are side effects like dryness of mouth and inhibition of sweating, but if we increase the dosage we have acceleration of heartbeat and dilation of pupils, increasing even more there are going be problems with the bladder and reduced peristalsis and the side effect can cause coma. Nonetheless at a low dosage atropine can be used to inhibit bronchoconstriction in order to treat asthma or obstructive pulmonary disease or even for for rhinorrhea (runny nose). To have less side effects it is better not to administer this drug orally, for example if the target are the bronchi it is possible to use a spray. In the past this drug was also used for peptic ulcer but since it was taken orally and had side effects, and nowadays there are better compounds for this condition.
