CHOLINERGIC DRUGS

The transmission of a message along the nerves occurs through electric impulses. At the ends of the nerve fibres there is no direct contact between the ends and effector nerves. At the junction, known as synapse, there is a very minute gap which does not allow the transmission of the electric impulse. This transmission is accomplished by chemical mediators, known as  neurohumoral agents, which are stored in a vesicle cell. When . an electric impulse reaches the vesicle, it releases these chemical mediators which are transmitted through the synapse. The impulses then react with the receptor at the effector cell membrane leading initiations of the impulse at that cell.

Many tissues or organs have been connected with two sets of nerve fibres. One set gets one of the major chemical transmission while the other is utilizing the second one. The effect produced by one agent is opposed to that produced by the other, i.e., if one increases activity, the other decreases it. (Those transmitters which mimic the stimulation of the sympathetic nerves are known as adrenergic agents, whereas those which mimic the parasympathetic system are called as cholinergic agents. Thus cholinergic agents are drugs which stimulate the effect of cells inverted by postganglionic parasympathetic cholinergic nerves. The effects of stimulation of cholinergic system by acetylcholine include the stimulation of gastrointestinal smooth muscles, contraction of muscles of urinary bladder, cardiac inhibition, peripheral vasodilation, contraction of pupil of eye, etc.

The enzyme acetylcholinesterase hydrolyses acetylcholine to choline and acetic acid at or near the site of liberation. Drugs that inhibit the action of acetylcholinesterase are known as anticholinesterase agents. They prolong the life of acetylcholine, accumulate it at the sites of cholinergic receptor and give rise to effects which are similar to excitation of cholinergic system.

The cholinergic agents are the esters of choline (e.g. acetylcholine, methacholine and carbachol), cholinominetic alkaloids (eg pilocarpine, muscarine and arecholine) and cholinesterase inhibitors (e.g. neostigmine, organophosphorus compounds). Cholinergic drugs and anticholinesterases are used to treat cardiac arrhythmias, for the relief of atony of gut and urinary bladder, for decreasing the intraocular pressure in glaucoma and to relieve muscular weakness in myasthenia gravis.