Muscarinic receptor antagonists (MRAs)
- The drug or substances bind to muscarinic receptors and produce no intrinsic activity or inhibit binding of agonist called as muscarinic receptor antagonists.For example: atropine, scopolamine, tropicamide etc.
They produce similar activity but some have specificity of action on the selective tissue or organ like pirenzepine inhibits gastric acid secretion due to its selectivity towards M1 receptors.
They are broadly classified in to: Naturally occurring alkaloids, Semi-synthetics and Synthetics
⇒ Naturally occurring alkaloids: these types of alkaloids are obtained from the plants and they are naturally occurring. Atropine is obtained from Atropa Belladona (solanaceae) and hyosciene is principally obtained from hyoscyamus niger (solanaceae).
⇒ Semi-synthetics have small changes in their chemical structure than parent compounds. Ex- atropine mononitrite. Homatropine produced by reaction between tropin & mandelic acid.
⇒ Synthetics: by lot of researches and development, these types of drugs are discovered. They have similar action like atropine but more selective to receptors. Ex- ipratropium, tiotropium, tropicamide etc. most of the synthetic derivatives like ipratropium(quaternary compounds) cannot cross BBB.