CHOLINERGIC STIMULATION
In the ANS acetylcholine is the major autonomic nerve transmitter in all the preganglionic fibers and the parasympathetic postganglionic fibers and the sympathetic postganglionic fibers supplied to the sweat glands.
Acetylcholine is acting on the nicotinic receptors in the preganglionic level of both sympathetic and parasympathetic systems, and muscarinic receptors at the postganglionic level.
This can be detailed as follows:-
1.Preganglionic fibers of autonomic ganglia (Nn)
2.Preganglionic fibers that supplied to the adrenal medulla (Nn)
3.Postganglionic fibers of the parasympathetic system (M)
4.Postganglionic fibers of the sympathetic system that supplied to the sweat glands(M)
Nn means the nicotinic receptor at the neuronal level the name earned because it was first identified by nicotine.
M means the muscarinic receptor because it was first identified by using muscarine.
Here we deal with drugs that mimic acetylcholine's cholinergic stimulant actions.
These drugs are known as cholinomimetics and are categorized as follows:-
1.Direct Acting Stimulants (Agonists)
These are acting by chemically binding with the acetylcholine's nicotinic and muscarinic receptors in the body.
a.Acetylcholine
This is the natural endogenous prototype drug secreted within our body and affects almost every system in our body. This can be summarised as follows:-
Cardiovascular system In this system if acetylcholine dominates by acting on its muscarinic receptors in the parasympathetic postganglionic level as in rest and sleep, it decreases heart rate, B.P., and contractility.
Digestive system Increases intestinal motility
Urinary system Increases contractility of the bladder
Pulmonary system It increases pulmonary secretions.
The eye In the eyes it causes pupillary constriction (miosis)
Periphery It causes contraction of the muscles by its somatic nicotinic receptors(Nm) at the neuromuscular junction
Endocrine system It causes the release of adrenaline from the adrenal medulla by its nicotinic action.
As a medicine, this drug is very rarely used to produce miosis in the eyes and its use is very limited as it has widespread unwanted actions and is rapidly destroyed by acetylcholinesterase and eliminated by the body.
Side effects
Diarrhea and Decreased BP
Urination
Miosis
Bronchial secretions and Bradycardia
Excitation of the bone muscles
Lacrimation
Salivation
The side effects can be easily memorized by the anagram 'DUMBELS'
b.Bethanechol
It is chemically a carbamic acid ester.
It acts mainly on muscarinic receptors
Therapeutically it can be used to relieve constipation and urinary retention.
Side effects
Similar to that of acetylcholine
c.Carbachol
It is also similar to bethanechol a carbamic ester but because of more side effects due to nicotinic effects the drug is very rarely used to produce miosis during eye surgery and in glaucoma
Acting on both muscarinic and nicotinic receptors
Side effects similar to that of acetylcholine
d.Pilocarpine
Pilocarpine is an alkaloid from the leaves of typical south American shrubs from the genus Piocarpus
Causes,
miosis
decreased heart rate
bronchial contractions
increase salivary, lacrimal and sweat secretions
The drug is unaffected by the enzyme acetylcholinesterase and hence yield prolonged action.
This is the drug of choice for the treatment of glaucoma
It has the advantage of acting primarily on muscarinic receptors only.
Side effects
Similar to acetylcholine but in addition to that it can enter the CNS to give effects like migraine,delusions, dizziness, etc.
e.Methacholine
Because of its short duration of action, it can be used for diagnosing asthma and bronchial hyperactivity.
Acting mainly on muscarinic receptors.
Side effects are due to generalized cholinergic stimulation.
-Continued (Indirect Acting Agonists)next post.
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