ANTICHOLINERGIC

                           (Muscarinic receptor antagonists, Atropinic, Parasympatholytic) 



Conventionally, the term ‘anticholinergic drugs’ is restricted to those which block actions of ACh on autonomic effectors and in the CNS exerted through muscarinic receptors. Though nicotinic receptor antagonists also block certain actions of ACh, they are generally referred to as ‘ganglion blockers’ and ‘neuromuscular blockers’.

            Atropine, the prototype drug of this class, is highly selective for muscarinic receptors, but some of its synthetic substitutes do possess significant nicotinic blocking property in addition. The selective action of atropine can easily be demonstrated on a piece of guinea pig ileum where ACh induced contractions are blocked without affecting those evoked by histamine, 5-HT or other spasmogens. The selectivity is, however, lost at very high doses. All anticholinergics are competitive antagonists.

  CLASSIFICATION

 1. Natural alkaloids Atropine, Hyoscine (Scopolamine).

 2. Semisynthetic derivatives Homatropine, Atropine methonitrate, Hyoscine butyl bromide, Ipratropium bromide, Tiotropium bromide.

 3. Synthetic compounds

 (a) Mydriatics: Cyclopentolate, Tropicamide.

 (b) Antisecretory-antispasmodics:

  (i) Quaternary compounds: Propantheline, Oxyphenonium, Clidinium, Pipenzolate methyl bromide, Isopropamide, Glycopyrrolate.

 (ii) Tertiary amines: Dicyclomine, Valethamate, Pirenzepine.

 (c)Vasicoselective: Oxybutynin, Flavoxate, Tolterodine. 

 (d)Antiparkinsonian: Trihexyphenidyl (Benzhexol), Procyclidine, Biperiden. 

                In addition, many other classes of drugs, i.e. tricyclic antidepressants, phenothiazines, antihistamines and disopyramide possess significant antimuscarinic actions.

               The natural alkaloids are found in plants of the solanaceae family. The levo-isomers are much more active than the dextroisomers. Atropine is racemic while scopolamine is l-hyoscine.