Fundamentals II: 11:00 - 12:00Scribe: Christopher Bannon

Tuesday, November 24, 2009Proof: David Davis

Dr. PillionCholinergic DrugsPage1 of 7

N – Nicotinic, M – Muscarinic , ACh – Acetylcholine, SMC – Smooth Muscle Cell, HR – Heart Rate, PKC – Protein Kinase C

S1]: Cholinergic Drugs
[S2] Acetyl Choline
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[S3] Figure 6-1
– The drugs are on the test. Were going to get a list of drugs, what they do, their side affects and the receptors they bind to is what’s on the test. All of this information helps give an overview so that understand some background materials and not just memorizing the drug facts.

Exam question are which drugs increase heart rate, which drugs cause dilation of the eye or constriction of the eye, which cause the patient to salivate or go to the bathroom. This is the underlying concept behind the autonomic nervous system. Terms nicotinic, muscarinic, parasympathetic, sympathetic, adrenergic, these are all important terms that need to understand for context of these drugs. To help understand drugs, start with “Fight or Flight” response

Not going to give a drug list to memorize, I will show the drugs on the slides, but the Textbook for Pharmacology has all these drugs in it and it is available online for free through access to Lister Hill Library if don’t want to buy it; it is a good resource to have though, especially for Systems next semester (renal, cardiac, respiratory). All these systems respond to adrenergic and cholinergic stimulation. This first part of the course is meant to be a platform/ foundation for the Systems material later on, so understand what the response is from a cholinergic agonist on the bladder. As learn overall scheme of the cholinergic system, able to tease out fact that ACh has different effects in different organs.

Drugs that impact the action of ACh, either by looking like Ach or by preventing the synthesis or release of ACh or by changing the rate of which ACh is broken down. As we get to those drugs, the system will be in your head as to how the system works.

Don’t be afraid to stop him if there is a problem with him going to fast or too slow through the material

[S4] Acetyl Choline
Like other molecules, ACh can be affected: where it’s made, where it’s stored, where it’s bound, how its removed and how it is taken back up again. How it is taken up again is not immediately obvious, but much of the ACh in the body, when it gets release and is broken down, the by products of ACh break down are Acetyl & Choline.

The acetyl group we use for metabolism, but the choline group (it is not excreted in feces or urine) but rather, the body recycles it because it is much easier to recycle it than to resynthesize it. There is a reuptake system, and there is a system of drugs that interact with the reuptake system.
If one were to block the uptake of choline back into a nerve, the effect would be to delay/ diminish the ability of the nerve to release ACh later on. If take a drug that blocks the reuptake of choline, the body has to synthesize new ACh from scratch which is very time consuming.  blocking the reuptake of choline, slows down the synthesis of ACh and less ACh is released.
A drug that slows down/ impedes the uptake of choline would affect the bodies heart rate. To determine the affect  ACh slows down the heart rate, therefore a drug that blocks choline uptake  less ACh is release and therefore there will be less of a “break” on the heart and therefore the heart rate Increases. Don’t memorize the drug but rather understand the system and one can get to the answer. (what he wants us to be able to do)
Now think about blocking the synthesis of ACh, this will decrease ACh that is release and effects of ACh on the body will be diminished.
The Storage of ACh is subtle but there is a system in the nerves cells which stores it
There is a set of drugs that blocks that storage of ACh. If don’t’ take ACh into a vesicle the nerve cannot release it upon stimulation.
For ACh to be released must form thousands of vesicles full of ACh and then upon the proper nerve impulse all the vesicles are puffed out at once. If one blocks the formation of the vesicles, which contain ACh, there isn’t that much of a response when nerve stimulation comes along.
There are disease processes where this happens
The release itself can be blocked by botulinum toxin, which is in Botox.
The binding to a receptor can be blocked if a receptor antagonist is used
A drug that competes with ACh for binding
The removal of ACh is done mostly by the enzymatic pathway that utilizes acetylcholinesterase. This enzyme will be discussed in detail as many drugs are used to treat diseases and insecticide poisonings where we go and go after that particular enzyme

[S5] Schematic of Presynaptic Neuron
This slide has everything that we need to know (it is from the older version of the text, and the newer text has and even more complex diagram, but he likes the older one and it is “simple enough” for what we want to do).
When think about this slide, start with A and work way through B and then come down to bottom.
At the top this is meant to be a nerve cell and at the business end of the nerve cell there is a long axon at the top (goes above off of slide) but slide shows the axon terminal.
The Nerve Terminal contains not one but rather thousands of vesicles (but just 1 is shown); we create lots of these little vesicles.
These vesicles contain ACh.
ACh is packaged to be released upon stimulus
The chemistry of ACh is that it has a positive charge and therefore for it to be packaged it must be teamed up with negative charges to cancel out charges (otherwise charge repulsion occurs and can’t package the ACh tightly in the vesicle).
Are we having fun yet? Pharmacology can be FUN!!!!
For the negative charge, the body uses ATP (one of best thing that can be used). Body can also use proteins with a phosphate group, but we haven’t really discovered all proteins that body makes, in particular those that the body uses to allow ACh to be packaged but know ATP can be used ATP is contained within the vesicle to balance the charge from the ACh.
This is important because a drug will be developed that can interfere with this and it might even be that the drug, that’s up here, vesamical blocks the packaging of ACh inside the vesicle, but don’t know why (if it’s the transport or the packaging and 1 theory is that it blocks the ability of the negative and positive chargers to form a tight association, but whatever the reason is, the vesamicol and its cousins block the incorporation of ACh into the vesicle.
Before ACh can be taken up it must be produced, from Acetyl-CoA and choline. Acetyl-CoA is in cell and choline is hard to come by, so the body reuses.
The choline is taken up, by the transporter when the ACh is broken down, from the cleft.
The drug Hemicholiniums (means the family of drugs that are related to Hemicholinium).
Hemi = “half of” and choline “looks like choline”.
Competes with choline for the uptake process
Always have choline all the time and it is taken up.  if take a pill with Hemicholinium, Hemicholinium will compete with choline and not as much choline will be taken back up by the cell.
Net result is that less ACh is made  less ACh is produced and less ACh secreted  less of a cholinergic effect.
On Test: What effect does Hemicholinium have  Answer = less cholinergic response because produce less ACh at the nerve terminal
Hemicholiniums have the opposite effect of ACh
If ACh causes heart rate to decrease and if give person Hemicholinium most likely their heart rate will increase because the effect o ACh will be damped (any time this happens the Adrenergic effect is ready to take over.). Anything that lessens ACh allows the adrenergic effect to increase its action.
Hemicholiniums block choline uptake
The enzyme choline acetyl transferase = enzyme that makes the reaction happen
No specific inhibitors for this enzyme but probably in the future will have drugs to do this
Vesamicol blocks the uptake of ACH into the vesicle. The vesicle then (we have 1000 or so of these) sits around and wait.
Ca ion is an important ion for discharge of vesicle
[Ca] concentration inside the cell changes (inside the cell it is extremely low) and therefore we have a barrier from Ca to enter the cell and if it does enter the cell it is placed in vesicles of its own
These packets of Ca retain the Ca until the cell depolarizes
The ion that causes depolarization of the cell = Na+
When open up a sodium channel, sodium rushes in and therefore what’s inside the cell goes “kaplooey” and the Ca that was stored inside vesicles is release
This release of Ca triggers the merger of the ACh vesicle with the membrane of the axon terminal and this vesicle eventually gets pried open
This opening is kind of like velcro there are proteins on the surface of the vesicle and also those on the cell surfaces
SNAPs are on the cell and VAMPs are on the ACh vesicle.
SNAPs will have VAMPs will bind like velcro only when nerve cell is depolarized and Ca is released
Ca has 2 positive charges and it probably changes the orientation of these VAMPs to make them available and can then bind to the SNAPs and literally get ripped apart and what is inside the vesicle gets dumped out into the synapse
This explanation is kind of magic and hand-waiving.
This is done for 1000’s of vesicles over a millisecond
The release of ACh out into nerve cleft allows it to interact with about 3-4 different receptors (it will react with more than 1)
Ca channels (if take a Ca blocker preventing opening of Ca channels = affects this process
If nerve terminal didn’t allow Na to enter = affect this process
Local anesthetic agents block Na channels Na can’t come into cell and the cell can’t depolarized  CA can’t be released  ACh can’t be released
Local anesthetic have affects on this system as well.
Botulinum toxin, besides removal of wrinkles, is used here for removal of migraine headaches this is still in clinical trials
Cartoon on slide  Botox interferes between interaction between VAMPs and SNAPs
Botox prevents “Velcro attachment’ and the opening of the vesicle to allow ACh to release
Giving Botox  ACh is NOT release  muscles in the face become paralyzed and the Botox has the unusual pharmacokinetic property of having an extremely long half life  effects last weeks or even months.
This could be very bad for the host where botulinum takes up residence  weird that this happens
At present one company has monopoly but in future other companies will come up with botulinum toxin derivatives which will have different pharmacokinetics and pharmacochemistry which will allow them to go on to different organs in the body in a selective manner
Have organ specific effects
Class 10 yrs from now probably botulinum toxin derivatives will become new drugs
Botulinum toxin blocks the normal release of ACh at the terminal thereby causing paralysis
When ACh leaves the axon terminal it could go to: axon terminal receptors (presynaptic) choline receptors (post synaptic cell receptors), other receptors (post synaptic), or to acetylcholinesterase
Cell releasing ACH has receptors for ACh itself  ACh can come back and bind to the same cell and have an effect
ACh from inside the cell can NOT react with these receptors on the outside of the PM
ONLY upon release can affect the cells reactions
ACh released can influence the cell that released ACh itself
The Presynaptic membrane can have ACh receptors on it
The same holds true of the Adrenergic Receptors
These presynaptic receptors allow for feedback
In nature much feed back is negative but in this situation, we think it may stimulate release of more ACh (positive feedback)
ACH release is generally done to allow large amounts & in a rapid fashion. One way to do this is to have a receptor that binds ACh and spurs on more ACh release.
If this is true something else must coming and turn off the release. We think that this task is performed by (he didn’t finish the sentence). ACh must be continually made and packaged in vesicles but it can’t be made fast enough to allow for continuous firing of the nerve cell. If it did continuously fire, the muscle that it reacts with will cause continuous restriction and eventually lock up and become paralyzed. Like a lot of other system there must be a release of ACh, an action, and then a time for it to recover to allow for the next wave of action. Can get paralysis if constantly release ACh.
Presynaptic receptors have not been characterized well and we still don’t’ know exactly what they do. We don’t’ have a drug that specifically binds to these receptors yet.
If we had an agonist or antagonist and bound to these presynaptic ACh receptors only, then wed know for sure what these receptors do.
As we go through we will talk about sub types of receptors and certain types are found on the presynaptic membrane but in the world of not being easy to explain if this were the only pace they were found then this task would be simple but these type of receptors (for ACh) are found both on presynaptic cells and on target cells (post synaptic) making the determination of the role difficult.
Most of the targets for the ACh are found on the postsynaptic membrane (target cell) and care called Cholinoceptors because it is supposed to be both M and N. Both can be there. If this is one kind of cell may only have 1 kind of receptors.
If this is a skeletal muscle cell may have only N receptors
If it is a cardiac cell it may be only M receptors
If it is a smooth muscle cell in cardiac tissue or vascular SMC, it may have both kinds of receptors M and N and they are in a certain proportion
Every different cell in the body has a slightly different composition of these receptors (nerve, heart muscle, and smc have a different compliment of these receptors) and if give ACh to these 3 difference cells  3 different responses
ACh can bind to a receptor and illicit a response
This balloon structure is meant to be acetylcholinesterase
The name describes function = breaks ACh very quickly
When ACH is dumped in the synapse there is a lot of ACh present to degrade the ACh in a matter of milliseconds  ACh needs to get to its receptors or acetylcholinesterase will break it down
There is a balance between receptors and acetylcholinesterase molecules
ACh can also bind to Other Receptors, called heteroreceptors = ACh is a very small molecule and it can cross talk with other receptors (further complicating matters)
At presynaptic membrane, it could have an impact on other “stuff” going on the presynaptic cell to help auto-regulate its active
Life of ACh is very brief but it illicit a lot of responses
Number that is released is critical and the reuptake after degradation has a LOT to do with geography of the nerve terminus
If release ACh into the terminus it will be broken by acetylcholinesterase and if there is a reuptake mechanism right there this process will be down more expediently to allow for faster synthesis inside the presynaptic cell

[S6] Subtypes of ACh Receptors
There used to be just 2, now there are 7 types of receptors (actually list is about 17, but 7 in text)
Trouble is that new molecules keep getting discovered because want someday to have specific drugs that bind to M1 ACh receptors that don’t bind to M2  if this turns out to be the case can use them for specific effects on certain organs.
IN book there is a list of organs which are the primary sites of these receptors
He will NOT ask a specific test about whether cardiac has more M1, M2, M3, etc. but the point that we do need to recognize is that different organs have different receptors
Later in course in GI tract  Gut has an M4 receptors  it is Muscarinic receptors and it binds ACh  Never really need to know more than this anyway.
Won’t ask on a test for a difference in the sub groups.
One thing to point out though is that M2 and M4 are the ones that are found on the Presynaptic membrane AS WELL AS on the post synaptic membrane
They are involved and inhibitory drugs and their regulation of those cells
What’s complicated is that can inhibit, in this case they work by inhibiting adenylyl cyclase activity
If remember biochemistry and g-protein coupled receptors
G-proteins are complicated (there are 22 of them) some of them work by having a stimulatory subunit that increases the action of adenylyl cyclase
When a drug binds to that receptor the net effect inside the cell is an INCREASE in camp.
We will talk about the Beta Adrenergic receptor and this is how it works.
Binds to the receptors, increases adenylyl cyclase, increases cAMP formation  INCREASING the heart rate
Some of the muscarinic receptors have a different subunit  so they have an inhibitory subunit that blocks the action of adenylyl cyclase.