Cholinergic agonists - Introduction
These are the drugs which increase cholinergic transmission and therefore produce all the similar effects shown by parasympathetic system.
Hence these drugs are also called as parasympathomimetics. Before going to directly discuss about these drugs, let's have few details about cholinergic receptors and how they work.
In the previous tutorial we have seen that acetylcholine acts as mediator at both autonomic ganglia as well as parasympathetic postganglionic fibers.
How a same mediator can produce different actions?
The answer is quite simple. Mediator is same but its receptors are different. Acetylcholine receptors at ganglia are nicotinic acetylcholine receptors (nAchR) whereas at parasympathetic post ganglionic fibers they are muscarinic acetylcholine receptors (mAChR).
How they are classified?
Ok, it's fine that acetylcholine acts on two different receptors to produce different actions. How they classified as muscarinic and nicotinic? Is there any evidence?
Yes, we have evidence. Animal experiments give more information about receptors and their selectivity.
For example, low dose of acetylcholine shows following effects
- Pupilary constriction
- Slowing of heart
- Fall in blood pressure
At the same time high dose of acetylcholine produces
- CNS stimulation
- Rise in blood pressure
- Increase in heart rate
You can easily observe that at low dose acetylcholine produces one set of actions and high dose a different set of actions. Even blood pressure is reversed by high dose of acetylcholine.
That means acetylcholine not acting on one receptor instead acts on two different receptors each activated at different doses. This can be further confirmed by action of muscarine and nicotine on these receptors.
The first set of actions that produced by low dose of acetylcholine are reproduced by muscarine hence receptors are named as muscarine receptors. Similarly, the second sets of effects produced by high dose of acetylcholine are reproduced by nicotine hence the receptor names as nicotinic receptors.
These two receptors can also be further differentiated by specific antagonist. Atropine is specific antagonist on muscarinic receptors blocking all muscarinic actions with no effect on nicotinic actions.
Points to remember
- Acetylcholine is a common mediator for both muscarinic and nicotinic receptors.
- Low dose muscarinic and high dose nicotinic actions
- Atropine can block muscarinic receptors but not nicotinic receptors.
So let's discuss these two types of receptors in detail in the next section.