Cholinergic Drugs: Definition, Types, & Applications

Any chemical substance that mimics the action and nature of another chemical substance known as acetylcholine or butyrylcholine can be called cholinergic. In general, “choline” is the group in various quaternary ammonium salts which have the N,N,N-trimethylammonium cation. It is generally found in animal tissues as a chemical substance that transmits signals from one neuron to another or from a neuron to a muscle cell. Thus, cholinergic meaning will be a chemical substance that mimics the action of the neurotransmitter acetylcholine in the animal body. 

Hence, any cholinergic drug will be a medication with the purpose of mimicking the function of acetylcholine or butyrylcholine in the animal body with the purpose of transmitting nerve impulses in case there is any problem with the normal functioning of acetylcholine. 

Cholinergic Drugs and their Characteristics

Cholinergic drugs are any of the groups of drugs that inhibit, enhance, mimic the action and function of the neurotransmitter acetylcholine which is the main transmitter of nerve impulses through the parasympathetic nervous system. This is because the parasympathetic nervous system is entirely cholinergic in nature and exclusively uses acetylcholine to pass the signals. Other examples include neuromuscular junctions, preganglionic neurons, the basal forebrain, and the brain stem. Merocrine sweat glands are examples of cholinergic receptors. In the field of neuroscience and related ones, the cholinergic definition is used in the following contexts:

• Any chemical substance that can release, alter or produce acetylcholine or butyrylcholine or mimics the action of acetylcholine on one or more of the body’s acetylcholine receptors, then it can be said to be cholinergic. A cholinergic drug is a medication that can function as a ligand in all the mentioned scenarios. 

• Cholinergic meaning is only fulfilled by the receptors if they exclusively use acetylcholine for transmission.

• According to the cholinergic definition, a synapse can be said to be cholinergic if it uses acetylcholine for neural transmission.  

Thus, in the above-given context, cholinergic drugs are used as replacements for acetylcholine in the parasympathetic nervous system which is a part of the autonomic nervous system responsible for the contraction of smooth muscles, dilation of the blood vessels, increase in the bodily secretions, and slowing of the heart rate.

Many of the cholinergic and anticholinergic drugs interact with and affect acetylcholine receptors. The mechanism of action of cholinergic drugs is by stimulating the nicotinic or muscarinic acetylcholine receptors. For example, one of the cholinergic drugs uses the mimicking action of acetylcholine, known as pilocarpine - a muscarinic agonist, is used in eye-drops to constrict the pupil of the eye, and decrease the intraocular pressure which is increased in the disease glaucoma. The uses of anticholinergic drugs include inhibition of acetylcholine receptors in turn inhibiting the resulting effect of signal transmission. One of the examples of anticholinergic drug uses is the use of antagonists that act on the muscarinic acetylcholine receptors. These drugs inhibit all the actions of the parasympathetic nervous system resulting in the drying up of the secretions of the body. 

Cholinergic Drugs Classification

As is now well-known from the information provided above that cholinergic drugs are pharmaceuticals or supplements that are used widely for targeting the parasympathetic nervous system. Not only the cholinergic effects of the cholinergic drugs but also the anticholinergic drug uses the same target systems of the parasympathetic nervous system.

The cholinergic drugs classification comprises direct-acting and indirect-acting. All the types of cholinergic drugs are included under these two categories only. They are briefly described below.

• Direct Acting Cholinergic Drugs: 

These are the drugs that carry out the cholinergic effects by stimulating the nicotinic or muscarinic cholinergic receptors. This classification falls directly within the context of cholinergic definition. They are further classified into choline esters and plant alkaloids. Examples of choline esters include acetylcholine, carbachol, bethanechol, and methacholine. Examples of plant alkaloids include the already mentioned pilocarpine, muscarine, nicotine, etc. 

• Indirect Acting Cholinergic Drugs: 

The mechanism of action of cholinergic drugs falling under this classification include the reversible and irreversible cholinesterase inhibitors or the medications that promote the release of acetylcholine. A class of anti-adrenergic drugs that inhibit the sympathetic nervous system is also included in this kind of classification of drugs. Examples of each of the types of cholinergic drugs in this classification are donepezil, malathion, paliperidone, and propranolol respectively. 

One of the important facts of cholinergic drugs is that since they mimic the action of acetylcholine they also become prone to be destroyed in the blood. The acetylcholinesterase enzyme is responsible for the degradation of acetylcholine and molecules that mimic its structure. But when the enzyme becomes inactive it leads to continuous stimulation of the cholinergic receptors by acetylcholine and similar compounds. This leads to their over simulation and continuous signalling of a cascade of secondary messengers resulting in the multiplication of the general effects such as continuous secretion of bodily fluids. This over-stimulation of the cholinergic receptors of the parasympathetic nervous system is known as the cholinergic crisis.

Conclusion on Cholinergic Drugs

From the given article it is clear that both the cholinergic and anticholinergic drugs are target-based drugs with a primary target for their effect being the parasympathetic nervous system. Both the drugs underline the cholinergic definition and are responsible for either mimicking and activating the release of acetylcholine which is a neurotransmitter and is extensively used by the parasympathetic nervous system of the autonomic nervous system. These drugs also affect the cholinergic receptors. The cholinergic drugs stimulate or activate the nicotinic or muscarinic receptors of the parasympathetic system whereas an anticholinergic drug uses the inhibitory technique of drug affection.