Download Cholinergic Agonists and Antagonists

CLASS: 11:00 – 12:00
DATE: December 2, 2010
PROFESSOR: Pillion
I.
II.
III.
CHOLINERGIC AGONISTS AND ANTAGONISTS I
Scribe: Adam Baird
Proof:
Page 1 of 5
CHOLINERGIC DRUGS AND ANTI-CHOLINERGIC DRUGS [S1]
a. Today, we’ll take a look at acetylcholine (ACh) and how clinicians can use acetylcholine derivatives, mimics,
or cholinesterase inhibitors (drugs that increase the amount of acetylcholine by blocking its inhibition).
ACETYLCHOLINE [S2]
a. Specifically, we’ll discuss ways that we can intervene with ACh interactions in ways that will modify that
amount of ACh that is readily available.
b. Exam material: Know what ACh normally does, how ACh is normally regulated, what diseases involve
a disruption of ACh function, and what processes normally oppose the actions of ACh.
c. It’s important, when talking about this, to remember the dynamics of the sympathetic and parasympathetic
pathways.
d. The first family of ACh receptors that will be discussed are the nicotinic receptors. Why is called “nicotinic”?
The compounds in nature (in nicotine) were found to interact with ACh receptors. The receptors that ACh
bound to in common with nicotine became known as “nicotinic” receptors.
e. Likewise, if a receptor binds ACh, it is called a “cholinergic” receptor.
f. There are two kinds of cholinergic receptors:
1. Nicotinic – Bind ACh and nicotine
2. Muscarinic – Bind ACh and muscarine
g. There are chemicals that are clinically used that mimic nicotine or muscarine. There are also chemicals that
are clinically used that are antagonists of nicotine or muscarine. These drugs are used because they select
one or the other of the two cholinergic receptors.
h.
NO TITLE [S3]
a. Think about a situation where a neurotransmitter is going to transmit a signal to a second cell, which will then
fire off another neuron, and ultimately cause an effect. One would think that this process happens very
rapidly. A change in ion channels (where you can use it, recover, use it again, recover, etc.) will allow this
process to occur quickly. There must be very rapid response rates and recoveries in nerve-nerve
transmission.
b. Whenever a nerve interacts with another nerve, a nicotinic receptor is assumed (because a nicotinic
receptors has an ion channel). Think: nerve and ions (both have the letter “n”). On the other hand, think:
muscles and muscarine (both have the letter “m”).
c. Parasympathetic pathway: ACh is released from pre-ganglionic region and hits the nicotinic receptor, firing
the cell to interact with an end organ (like smooth muscle, for example). This is an example of a muscarinic
receptor.
d. Essentially, they work by changing the biochemistry inside the target cell. Some of them work by changing
the amount of N-acetaltriphosphate and diacylglycerol (which are involved in getting calcium to enter the cell
from the outside through calcium channels to the inside and also to release stored calcium from vesicles
inside the cell). Calcium is released as a result of stimulation of muscarinic receptors in some tissues.
Additionally, there are many types of receptors (making the study of this even more complex). In some
tissues, as a result of ACh binding to a muscarinic receptor, the amount of calcium is changed within the cells
(affecting muscles cells). M1, M3, and M5 muscarinic receptors are associated with muscle contraction (as a
result of calcium release). Thus, you wouldn’t expect to find this occurring in nerve-nerve connections. You
would expect to find this occurring among other cells (like glands, cardiac muscle, smooth muscle, etc.).
e. One exception: voluntary motor nerve connecting to skeletal muscle. If you stab a pen through someone’s
hand, there is an immediate response (which would be both voluntary and rapid).
f. There’s a difference between a muscle cell (that’s part of a voluntary motor nerve pathway) and one that is a
smooth muscle cell (that’s part of the control of blood vessels and blood flow, for example). These smooth
muscles cells are actually the major targets of ACh, and they have very profound effects on the “fight or flight”
response, heart rate, digestive tract, blood pressure, etc. They are actually regulating your blood flow and
blood pressure right now by a complex and coordinated regulation of which vascular beds are dilated and
which vascular beds are contracted. In order for the blood to flow to your fingers (required for you to write or
type, for example), the blood flow has to be controlled. To do this, vascular beds (and smooth muscle cells
surrounding it) are either contracted or relaxed.
g. The sympathetic pathway: there is a short, pre-ganglionic neuron, which releases ACh, which interacts with
post-ganglionic neuron, which then interacts with cardiac cells, smooth muscle cells, gland cells, nerve
terminals etc.
h. Notice that end organs are either under sympathetic or parasympathetic control. A lot of the cells have
interactions with both systems at the same time. As a result, a balance occurs between the two systems.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 2, 2010
Proof:
PROFESSOR: Pillion
CHOLINERGIC AGONISTS AND ANTAGONISTS I
Page 2 of 5
i. Whenever you think about the sympathetic pathway, think of the “fight or flight” response. Most of the time,
when you’re in your “fight or flight” response, you basically have minimized the parasympathetic pathway.
The parasympathetic pathway can be thought of as “rest and digest”. It increases secretions (saliva, tear
formation, GI fluids, urinary output, etc.). The sympathetic pathway, when turned on, heart, lungs, eyes, etc.
are affected as they respond to the “fight or flight” response. Sweat glands, which secrete sweat, are not part
of the parasympathetic pathway though. Instead, sweat glands are part of the sympathetic pathway. In this
case, the sympathetic pathway does have muscarinic receptors. Stimulation of the cells in the sweat glands
(from the sympathetic pathway) causes the excretion of sweat.
j. In the kidney, there is an even more specific response. There are nerve tracks that release dopamine.
Dopaminergic receptors in the kidney help control the rate at which blood flow to the kidney occurs. If you
slow down blood flow to the kidney, more blood is available for other parts of the body. In a “fight or flight”
response, you don’t want to completely shut down the kidney, but you do want to diminish kidney blood flow
so that blood flow to your heart, skeletal muscles, etc. is increased.
k. The adrenal gland is somewhat unusual. It is stimulated by cholinergic signals through the binding of nicotinic
receptors. A major difference: it secretes epinephrine and norepinephrine into the blood stream. In the
sympathetic pathway, norepinephrine is released at nerve terminals. Here, however, norepinephrine (and
epinephrine) is released into the blood stream.
l. Endothelial cells line the blood vessels (which don’t have innervation from sympathetic or parasympathetic
systems). So what are the nerves innervating? The smooth muscles cells, which help control how tightly the
contractions are. These smooth muscle cells will encounter norepinephrine or acetylcholine (or more likely, a
balance between the two). When a “fight or flight” response takes place, there will be a greater response of
the norepinephrine (sympathetic response will cause vascular constriction). A blood vessel in the heart will
dilate (allowing more blood flow to cardiac muscle). A blood vessel in the gut will contract (reducing blood
flow to the digestive tract).
m. How does the body accomplish these two things at once (constriction of some blood vessels, but opening of
other blood vessels)? It’s very complicated. So, one single system won’t work (and indeed, it doesn’t). Two
systems are needed. In the “fight or flight” response, there is a large increase in norepinephrine release (in
the smooth muscle), which causes vasoconstriction in some areas, but there is also a dumping of
norepinephrine and epinephrine from the adrenal gland. The endothelial cells respond to this in process
called “endothelial derived relaxing factor” (which has now been identified as nitric oxide, which causes
vasodilation). Norepinephrine or epinephrine can cause vasoconstriction or vasodilation (depending on the
location). By altering the number of receptors in the particular location, an accommodating change can be
made in two different areas, in two different ways, but at the same time.
n. This occurs by having both a systemic response (through the blood stream) and a neural response. Because
blood vessels contain two types of cells (smooth muscles that have neural input and endothelial cells that
respond to things within the blood stream), there can be a dual-response. This dual-response can vary from
one part of the body to another though. The bottom line: this is a complex system! It is an “interplay” of
various, overlapping systems.
o. So when a drug blocks these systems, it can have major effects. Example: patient given a drug that blocks
the binding of norepinephrine to its receptor, then a resulting effect to blood pressure will occur (especially on
the heart rate, for example).
IV.
ACETYLCHOLINE [S4]
V.
COMPARE AND CONTRAST… [S5]
VI.
NO TITLE [S6]
a. Here is a picture of the cholinergic system. This is a cyclic system, so we’ll start at the top and work our way
around.
b. The first step in making ACh: to take up choline from the outside of the cell and move it into the cell.
c. The nerve terminals here have a specific transport system for choline – and this system fires every
millisecond. For this reason, it’s better to “reuse” the neurotransmitter (as opposed to making it brand new
each time). The choline is used over and over and over again. Some is lost along the way, so some new
choline must be made at some point. In order to reduce the amount of choline released, you’ll need to block
choline uptake (which can be accomplished by a drug that looks like choline, hemicholinium, a competitive
inhibitor). With a drug that looks like choline, and competes with choline for uptake, the nerve terminal will
have less choline available, and less ACh will be made.
d. How would hemicholinium affect heart rate? Recall: ACh slows down the heart rate. So, hemicholinium drug
diminishes choline uptake (less choline into the cell, less ACh made), meaning that the heart rate will go up
(because less cholinergic action and more adrenergic action). Any drug that diminishes ACh released with
increase heart rate.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 2, 2010
Proof:
PROFESSOR: Pillion
CHOLINERGIC AGONISTS AND ANTAGONISTS I
Page 3 of 5
e. Once choline is in the cell, there is an enzyme that causes the production of ACh. There may be about 5,000
vesicles for this. ATP and other proteins are also found in these vesicles (along with ACh). The ACh, then, is
just waiting to be expressed. How is ACh expressed at the synaptic membrane? What stimulates a nerve cell
to release ACh? The nerve cell is activated, there is a depolarization event, which open sodium channels
open, sodium and potassium enter the cell, and a calcium channel (that is normally closed) is now opened.
What is the concentration of calcium in the cytoplasm of the cell before it is depolarized? It is very low. As the
calcium rushes into the cells, vesicle associated proteins interact with membrane associated proteins
(vesicles on those are called VAMPs and the synaptic membrane on those are called SNAPs). There is a
drug called botulinum toxin that can be injected to cause paralysis of muscle (similar to botox, for example).
f. Once ACh is dumped out, it encounters an enzyme called acetytalcholeresterase that will break down into
acetyl (which will be reused) and acetate (which will drift off and be used as a fuel). The action of a drug that
is an ACh receptor? The cholinergic effect will be increased. So, the choleresterase inhibitor drugs will
increase the amount of ACh, and in turn, slow down the heart rate.
g. The effect of the sympathetic system on the lungs? It causes bronchodilation (resulting in more oxygen). So
the effect of cholinergic stimulation is bronchoconstriction.
h. All of the effects of choleresterase inhibitors will increase ACh (which will be the opposite effect of the “fight or
flight” response).
i. Remember: the sympathetic effect (“fight or flight”) is the opposite of the cholinergic effect.
j. One membrane (that gives off the ACh) has receptors for ACh as well. These receptors, upon seeing ACh,
turn off the process of ACh release (so the cell can recover for the next line of firing).
k. When giving a cholinomimetic drug, you must be careful (because the receptors shown here are distinct).
Some drugs will bind to particular receptors; other drugs will bind to other receptors (and have opposite
effects). The receptors at the “bottom” are the muscarinic and nicotinic receptors and the receptors at the
“top” are involved in diminishing the effect of ACh.
l. Notice the pre-synaptic receptors. Notice the post-synaptic receptors (which could be muscarinic or nicotinic).
VII.
NO TITLE [S7]
a. This is the same picture as before, only it’s looking at the adrenergic system (as discussed a few lectures
ago).
b. The point: this is a mirror image of the last slide.
c. Here, you have tyrosine taken up, which is then converted to dopamine, which is then incorporated as
norepinephrine in vesicles. When stimulation occurs, calcium rushes into the cell. The vesicles are merged.
Norepinephrine is dumped. This process of release can be inhibited by guanethidine or bretylium. The
norepinephrine, then, binds to adrenergic receptors (beta-1, beta-2, or alpha-1) or a pre-synaptic receptor
(alpha-2, which is a release inhibitor, as it diminishes the release of norepinephrine).
d. This system will block the re-uptake of norepinephrine, which allows the norepinephrine effect to last longer
and there is a pronounced increase in the adrenergic effect.
e. Many of the same pathways and mechanisms are the same here.
f. One major difference: the cholinergic system has an enzyme to breakdown ACh, but the adrenergic system
does not. The norepinephrine, then, results in a slower releasing, longer lasting ACh effect.
VIII.
SUBTYPES OF Ach RECEPTORS [S8]
a. Muscarinic
1. M1
2. M2
3. M3
4. M4
5. M5
b. Nicotinic
1. NM
2. NN
c. The nicotinic receptors are ion channels. They allow the entry of sodium and potassium.
d. The muscarinic receptors are involved with G-protein signaling pathways.
e. The M1, M3, and M5 receptors are involved with diacylglycerol that stimulate the release of calcium, in turn
causing muscle contraction.
f. The M2 and M4 receptors are involved in inhibiting cAMP production.
g. What role does cAMP play in “fight or flight” response? When a cell is given norepinephrine, it stimulates
adenylyl cyclase, which stimulates the formation of cAMP, which triggers an increase in heart rate.
h. To oppose the action of norepinephrine, cAMP levels would do what?
i. Norepinephrine increases cAMP in the cell, so ACh decreases the cAMP in the cell.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 2, 2010
Proof:
PROFESSOR: Pillion
CHOLINERGIC AGONISTS AND ANTAGONISTS I
Page 4 of 5
j. The balance between the two systems (right now, for example, in your heart) allows for a quick response
change when needed (when signaled).
IX.
ACETYLCHOLINE RECEPTORS [S9]
a. Muscarinic: M1 – M5
1. Transmembrane receptors linked to a G-protein
2. M1, M3, and M5 are excitatory, increase phospholipase C
3. M2 and M4 are inhibitory, inhibit adenylyl cyclase activity
4. It is both excitatory and inhibitory
5. It is able to respond in various ways
b. Nicotinic: NM and NN
1. Muscle and Nerve
2. Ion channels (sodium and potassium)
3. Excitatory
4. 2 molecules of ACh bind, channels open
X.
TWO WAYS TO ACTIVATE CHOLINERGIC RESPONSES [S10]
a. Exam material…
b. Direct Cholinomimetic Drugs
1. Resembles ACh in structure
2. Binds to ACh receptor
3. Activates signaling pathways
c. Indirect Cholinomimetic Drugs
1. Resembles ACh in structure
2. False substrate for acetylcholinesterases
3. Causes ACh levels to increase above normal
4. The endogenous ACh lasts longer, has a bigger effect
5. Activates both nicotinic and muscarinic receptors
XI.
NO TITLE [S11]
a. The nicotinic receptors are found on neuromuscular endplates, skeletal muscles, and ganglionic cells.
b. The muscarinic receptors are found on end organs and some nerves.
c. There is some overlap.
d. A direct acting drug may be a natural alkyloid (like muscarine or nicotine, or a chemical derivative that could
create a similar response) might be able to recognize muscarinic receptors from nicotinic receptors.
e. An indirect acting drug (because it inhibits acetylcholinesterases) stimulates both systems. There are both
reversible and irreversible inhibitors that can be used.
XII.
ACETYLCHOLINE DERIVATIVES [S12]
a. Acetylcholine derivatives: All of these interact with muscarinic. One of them (carbachol) interacts with both
muscarinic and nicotinic, meaning it isn’t very useful.
b. They cause meiosis, lower IOP, lower heart rate
c. An anti-cholinergic would have opposite effects
XIII.
TABLE 7-3 [S13]
a. You are responsible for this. Study this in your book.
XIV.
TABLE 7-3 [S14]
a. You are responsible for this too. Study this in your book.
XV.
ACETYLCHOLINESTERASE [S15]
a. Normally reduces ACh in synaptic cleft
b. Responsible for termination of ACh action
c. Allows nerve terminal to recover after depolarization
d. Necessary for normal nerve function
e. Located on post-synaptic membrane cleft
f. Enzyme is transiently acetylated as ACh is hydrolyzed
XVI.
NO TITLE [S16]
a. Notice the acetyl group. Notice the choline group. Notice the positive charge.
b. It is water-soluble (because it is positively charged) but it is not very lipid-soluble.
c. If ACh is injected into the blood stream, it won’t get to the brain (because of the positive charge).
d. The enzyme acetylcholinesterase binds to it at two sites: the ester site (where the acetyl is) and the anionic
site (where the choline is). The choline is clipped and set free. An intermediate results, lasting for about a
millisecond, and then is also clipped and released. The enzyme, finally, is able to do this process again and
again.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 2, 2010
Proof:
PROFESSOR: Pillion
CHOLINERGIC AGONISTS AND ANTAGONISTS I
Page 5 of 5
e. An inhibitor: something that looks like ACh (namely insecticides and nerve gases, which have the highest
affinity for acetylcholinesterase and can in turn, cause the most damage).
f. Would an acetyalcholinesterase inhibitor gain access to the CNS? It could, but if it is positively charged, it will
have a harder time doing so.
XVII. ACETYLCHOLINESTERASE INHIBITORS [S17]
a. These are all used clinically.
b. One of them (neostigmine) is positively charged and cannot get into the CNS as readily as the others.
c. Edrophonium is very rapid acting (called “ultra-short” acting) drug. It is usually used to diagnose MG or to
determine how treatment is working. If a patient has muscle weakness, you can inject them with edrophonium
and test how they react. In this case, ACh would increase at the nerve terminals when you do this. A person
who has MG has antibodies against some of the cholinergic receptors, as a result, their cholinergic response
is diminished and they develop muscle weakness. (Their muscle function would increase when edrophonium
is given.)
[End 48:00 mins]