lesson 3 Flashcards
Speak about the nicotinic receptors, where are they? What is their structure?
We have seen how the cholinergic transmission works for the autonomous system, let’s now talk about the somatic system. Here we have motor neurons going from the spinal chord to the skeletal muscles. here we find the neuromuscular junction where we find the nicotinic ACh receptors, those, as we know, are ligand gated ion channels. Which means that they are permeable to Na+ and once stimulated can depolarize the postsynaptic cell. Each receptor is composed by 5 subunits, at the neuro muscular junction (NMJ), 2 of those subunits are alpha subunits, 1 is beta, one is delta and the fifth one is an epsilon subunit in adults while in the fetus it is a gamma subunit. In other areas, different from the skeletal muscles, the receptor is only formed by alfa and beta subunits. The alfa subunit is the only one that can bind to the receptor. The Nicotinic receptor is present at high density in the junctional areas, so when we have the release of ACh thanks to the action potential who travelled through the highly myelinated axon, it immediately interacts with the receptor and this leads to post synaptic excitation, so the muscle contracts.
Speak abount the modulation of the cholinergic transmission at the level of neuromuscular junctions. What are the two classes of compounds that can be used?
When we want to modulate the contraction, so we want to make the skeletal muscles relax, we can use two classes of compounds: the depolarizing agents and the competitive nondepolarizing agents
the competitive nondepolarizing agents are competitive antagonists meaning that they bind to the same site as ACh and block its action. Since the binding is reversible to stop their effect we can increase the amount of ACh available by using anti-AChE agents. In America the native populations used to prepare a mixture in which they dipped their arrows and used them to hunt because this mixture would relax the muscles of the animals and prevent them from escaping.
The second group, the depolarizing agents have the same effect but work differently, they depolarize the membrane by opening the channels in the same manner as Ach, acting like agonists. Compounds like succinylcholine bound to the receptor and stimulate it, but but how come that these agonists have the same effect as the competitive antagonists? To answer we should know that succinylcholine is not easily degraded by AChE so it will have a longer effect and create repetitive muscle contractions, also called fasciculations. Only after reaching these repetitive contractions we will have the blocking of the neuromuscular transmission with paralysis, an that happens because of desensitization. It is a useful compound because the paralysis happens later after the administration allowing the doctor to administer the medication and obtain the paralysis when it is required. Stopping the administration also stops the paralysis since the duration of the effect is very very short, which is of vital importance since, in this case, AChE does not help with the degradation.
The main clinical use of neuromuscular blocking agents in as an adjuvant in surgical anaesthesia, they are used when we want less side effects, infact general anaesthetics might lead to respiratory and cardiovascular depression and the patient also has a very long recovery period. with neuromuscular blocking agents the side effects are minimized. These compounds are administered parenterally, normally intravenously.
How can we block the cholinergic transmission at the level of the ganglia? Why is it so difficult?
the last application of cholinergic modulators transmission is in the autonomic ganglia. We know that in the ganglion we have nicotinic receptors. there are multiple nicotinic receptors subunits, but in the ganglia alfa3 and beta4 are the most abundant and the most important as well, while in most cases there are two alfa4 subunits and three beta2 subunits.
Modulating the transmission at the level of the ganglia is a mess, because the ganglia are involved in both the sympathetic and parasympathetic system. that means that the drugs are not going to be very specific and thus have a limited clinical use. If we use compounds similar to nicotine the major action will consists initially of a transient stimulation and then a more persistent depression of all autonomic ganglia. more specifically these compounds have a biphasic effect depending on the dosage: small dosages stimulate the ganglia and with larger doses we have an initial stimulation culminating in the blockage of transmission.
Among the agents that block ganglionic nicotinic receptors there are trimethaphan and hexamethonium that impair transmission. Hexamethonium appears to block the channel after it opens, so it is an antagonist, while Trimethaphan acts by competing with Ach.
As we said they have limited clinical use, infact they were the first effective therapy for the treatment of hypertension, however they had a lot of side effects including tachycardia, constipation, reduced motility of the GI tract etc… this is due to the role of ganglionic transmission in both sympathetic and parasympathetic neurotransmission
Speak about the adrenergic transmission and about the cathecolamines. How are they obtained?
Adrenergic transmission uses different neurotransmitters compared to the cholinergic transmission but the effector organs are the same, the two transmissions are infact, always balanced. The Adrenergic transmission is only present in the autonomous sympathetic nervous system, here we have thoracic and lumbar neurons that arrive to the ganglion, here ACh is synthetized, on the ganglion we have nicotinic receptors and then the postganglionic fiber. This fiber to the smooth muscles where the catecholamines are released and find their adrenergic alfa and beta receptors. Also, some nervous fibers arrive to the adrenal medulla where ACh is released and recognized by nicotinic receptors. this linkage stimulates the releases of catecholamines, mostly epinephrine.
Noradrenergic neurons in the periphery have very long axons (contrary to the parasympathetic system where there are longer fibers before the ganglion). These axons end in a series of varicosities strung along the branching terminal network. These varicosities are rich in vesicles containing neurotransmitters
Adrenergic transmission depends on the synthesis of three catecholamines:
- norepinephrine NE: it is the principal transmitter of most sympathetic postganglionic fibers and of certain tracts in the CNS (we will not care about this for now)
- dopamine DA: it is the predominant transmitter of the mammalian extrapyramidal system and of several mesocortical and mesolimbic neuronal pathways; it is also precursor of the other two molecules
- epinephrine EPI: is the major hormone of the adrenal medulla
How do we obtain these molecules? Everything starts from tyrosine. this amino acid gets converted by tyrosine monooxygenase into dopa which is in turn converted, by a less specific enzyme, decarboxylase in dopamine. this enzyme can also act on other aromatic amino acid. Then dopamine is converted by dopamine beta-hydroxylases in norepinephrine. Next the enzyme S-adenosylmethionine transforms NP in epinephrine. This entire process happens in the adrenal medulla.
What happens in the adrenergic transmission synapses? What are the effects of this transmission?
In the synapsis of the neurons involved in adrenergic transmission there is always a tyrosine transporter, so tyrosine enters in the cytoplasm of the neuron and here we have all the enzyme previously seen, they transform tyrosine into dopamine, and all of these events happen in the cytoplasm. Next dopamine is transferred from the cytoplasm in a vesicle thanks to a specific transporter, called VMAT, used by DA but also which is also usable by norepinephrine. In the vesicle we find the hydroxylase necessary to produce NE.
There are also other ways to obtain norepinephrine in the vesicles though ATP, Infact in other vesicles NE can be found together with ATP and Neuropeptide Y (NPY), the latter is a sympathetic neurotransmitter and neuromodulator released during sympathetic nerve stimulation, so when the nerve is stimulate and receives and action potential we have the release of the vesicles containing NE and also NPY and ATP are released.
after the release in the synaptic gap norepinephrine will interact with the receptor of the post synaptic cell membrane, these are G coupled alfa or beta adrenergic receptors. ATP can also interact with receptors called P2X which are ligand gated ion channels or P2Y which are G coupled receptors, meaning that ATP works as a neurotransmitter, causing a rapid contraction in smooth muscles. ATP can also help the transportation of adenosine, this is a presynaptic neurotransmitter which causes a negative feedback on the release of the neurotransmitters in order to regulate them and block adrenergic transmission.
There are important enzymes that can destroy NE and the other catecholamines, among those there is MAO: a class of enzymes located in the outer mitochondrial membrane, deputed to the oxidative breakdown of norepinephrine, epinephrine and dopamine.
Noradrenalin is also subject to a reuptake, 90% of the molecule found in the presynaptic cytoplasm is infact recycled while a 10% is synthetized de novo. Altough it is important to notice that not all the reuptaken noradrenaline can be recycled. The reuptake happens thanks to a transporter found on the pre-synaptic membrane called NET.
To remember the actions of catecholamines we need to remember what is useful for the “fight” in the “fight or flight” reactions. catecholamines have:
- A peripheral excitatory action on certain types of smooth muscle, such as those in blood vessels supplying skin, kidney, and mucous membranes; and on gland cells, such as those in salivary and sweat glands
- A peripheral inhibitory action on certain other types of smooth muscle, such as those in the wall of the gut, in the bronchial tree, and in blood vessels supplying skeletal muscle
- A cardiac excitatory action that increases heart rate and force of contraction
- Metabolic actions, such as an increase in the rate of glycogenolysis in liver and muscle and liberation of free fatty acids from adipose tissue
- Endocrine actions, such as modulation (increasing or decreasing) of the secretion of insulin, renin, and pituitary hormones
- Actions in the CNS, such as respiratory stimulation, an increase in wakefulness and psychomotor activity, and a reduction in appetite.
- Prejunctional actions that either inhibit or facilitate the release of neurotransmitters, the inhibitory action being physiologically more important
Speak about the different adrenergic receptors and about the 3 main differences between tha cholinergic and adrenergic transmission
We know that noradrenaline receptors, also called adrenoceptors, are G protein dependant, but coupled with different g proteins: we have alfa1 adrenergic receptors, alfa2 and beta. these different receptors are differently located, specifically alpha1 receptors are mainly present in smooth muscles and their activation induce contraction. alfa1 receptors are coupled with Gg proteins and are associated with the release of calcium into the cytoplasm leading to excitation of the cell, that is why they cause smooth muscle contraction.
alpha2 adrenoreceptors are manly presynaptically located and their activation reduces the neurotransmitter release. Anyway they can also be found in he postsyanptical membrane in some smooth muscle where they cause contraption. The alfa2 receptors are coupled with Gi/0 proteins, they are inhibitory towards adenylate cyclises pathway leading to less cAMP. Being this an inhibitory pathway they also inhibit the calcium channels involved of neurotransmitter release, in that is way they decrease the release of neurotransmitters.
There are different beta receptors, in the heart they cause contraction, while in the smooth muscle they cause relaxation. Beta receptors are coupled with Gs proteins which stimulate adenylate cyclase pathway and have an excitatory function most of the times, but not always because it depends on the type of tissue, since for different tissues the activation of PKA(protein kinase A) is different.
Alpha receptors are more sensitive to noradrenaline and less to epinephrine while betas are the opposite.
Each kind of receptor can be divided in 3 groups so we have: alfa1A, alfa1B, alfa1D.(there is no C) then Alfa2a, 2b and 2c. Beta receptors are divided in beta1, beta2 and beta3. Scientists thought that modulating different subtypes could end up in developing drugs with a very specific action, but unfortunately we do not have those specific compounds yet except for some acting on beta receptors.
Adrenoceptors are important in mediating vascular tone, smooth muscle tone, and cardiac contractility hypertension, asthma, ischemic heart disease, heart failure, and other conditions
DIFFERENCES BETWEEN CHOLINERGIC AND ADRENERGIC TRANSMISSION
- adrenergic transmission only happens in the sympathetic nervous system, cholinergic also happens in the parasympathetic and somatic nervous system
- In the adrenergic transmission we have the characteristic of varicosity in the synapses
- in the cholinergic transmission we have a massive release of ACh in a specific area, instead for the adrenergic one at the level of the varicosities we will have the release of only a few vesicles, releasing NE in a very spread way in a larger area.
What drugs can be used to control the adrenergic transmission? speak about cocain and agonists
How can we modulate the adrenergic transmission? for example how can we stop the effect of noradrenaline? There are compounds that block the activity of NET transporter like cocaine.
For the rest, our ability to modulate this transmission is mainly related to the use of agonists or antagonists at the level of the receptors. The effect that we have is dependant on the type of receptor.
Agonists can mimic the action of NE and they can either be real agonists which have a direct action or they can be indirect acting.
direct acting compounds can be nonselective or selective. The nonselective compounds do not recognize between beta and alfa receptors. While selective agonists only recognize one receptor like only alpha1 or beta1. The selective ones are preferred over the non selective adrenergic ones because they cause less side effects.
When we use agonists we do not have all the effects controlled by the catecholamines at once, actually we can only have some more specific effects depending on the receptor that the antagonist is binding to, which is better by the pharmacological point of view.
- if the Agonist in the drug is capable of binding to the aplha1 receptor, the most important effects obtained are: the contraction of vascular smooth muscle, the dilation of the pupil, the contraction of the heart, the erection of hair so goosebumps, and contraction of prostate. So alfa1 receptors all lead to contraction. Since they can stimulate the contraction of vascular smooth muscles they are able to increase blood pressure and are used in patients suffering form hypotension, also these compounds are used as nasal decongestant, for example Phenylephrine is an α1-selective agonist
- Agonists binging to alpha2 receptors have CNS effects, and stimulate platelets aggregation. Alpha2 are also present in the cholinergic nerve terminals and since they’re coupled to Gi/0, they inhibit the release of transmitter both in cholinergic and adrenergic nerve terminals, in fat cells it stimulates lipolysis and contraction of smooth muscles. this compounds were used in the past for hypertension because of their pre-synaptical action that inhibits the release of neurotransmitters and block contraction.
- for beta1 which are Gs coupled receptor, there are agonists that have mainly effect on the heart since beta1 is the most abundant of the adrenoreceptors in the heart tissue. Agonists increase the force and rate of contraction of the heart.
- beta2 adrenoreceptors are also Gs coupled, and the agonists promote smooth muscles relaxation in the respiratory, uterine, and vascular tissues. But isn’t it weird that Gs coupled receptors cause contraction in the heart but relaxation in the smooth muscles? That is because s PKA activity in the smooth muscles is linked to relaxation instead of contraction. Agonists at the level of skeletal muscles stimulate potassium uptake and glycogenesis in the liver. beta adrenergic agonists though mostly nonselective and thus not very specific, they are used for treatment of bronchoconstriction because of they cause relaxation in smooth muscles but they also affect the heart and bladder. Lately beta2 specific drugs have been developed to only act on bronchi and not the heart, while we can use compounds that bind to both and are nonselective for example in cardiac arrest cases, to make the heart contract, and they can still be used for bronchoconstriction but not in asthmatic patients where the heart side effects would be a problem. It is important to point out that even if we created beta2 selective agonists they are still not too chemically different from the non-specific ones, that means that they might still have little side effects. To be even safer these compounds are administered through aerosol so very little quantity can reach the heart. We have different types of these compounds: albuterol, which has a short half-life, is short acting but acts immediately this is the compound used when having an asthmatic attack. to control the illness continously it is better to use salmeterol which is long acting but has a delayed effect, normally 2 applications of this compound per day are enough to control asthma.
- for beta3 receptors, a few years ago it has been discovered that they are present in the bladder and thus agonists binding to them can stimulate the relaxation of the smooth muscles present in the bladder, while in the fat cells they can stimulate lipolysis. Beta3 selective agonists are quite new and are used to control the bladder, they were discovered in 1989 and are useful for people with urinary incontinence. This happens when the bladder is constantly contracted so there is always a stimulus to go to the toilet. These compounds cause the bladder to relax and alleviate the symptoms.
Would it be possible to use epinephrine and norepinephrine clinically?
Yes. Epinephrine is mainly used for its effect on the cardiovascular apparatus, since it raises blood pressure making it one of the most important vasopressors. The mechanism of the rise in blood pressure due to EPI is a triad of effects:
- a direct myocardial stimulation that increases the strength of ventricular contraction (positive inotropic action);
- an increased heart rate (positive chronotropic action);
- vasoconstriction in many vascular beds—especially in the precapillary resistance vessels of skin, mucosa, and kidney—along with marked constriction of the veins
At low concentrations, Epinephrine has predominantly b1 and 2 effects, while at higher concentrations, its a1 effects become more pronounced. the effect also variates based on the tissue it is acting on: injected EPI decreases blood flow where it is not needed in “fight” situation, like in the skin but it stimulates the heart rate and dilation of bronchi, for what concerns the smooth muscles it depends on what muscles they act on, since energy and thus sugar is needed during a fight it inhibits insulin and also elevates the concentrations of glucose and lactate in blood. For emergencies the injection of epinephrine is used to relieve hypersensitivity reactions thanks to its vasopressor effect and restoring cardiac rhythm in patients with cardiac arrest. Also it can prolong the effect of local anaesthetics by contracting the vases around said area.
Norepinephrine is less used then epinephrine and acts mainly on apha1 receptors and beta1 receptors. It is less potent and one of the few times it is used is on intensive care patients to support blood pressure.
Let’s remember that EPI and NE are approximately equipotent in stimulating β1 receptors. NE is a potent α1 agonist and has relatively little action on β2 receptors; however, it is somewhat less potent than EPI on the α receptors of most organs.
