Antihistamines and Antimuscarinics

[seminoma]

KLN talks about diphenhydramine and other "strong antihistamines" being atropine-like. Can someone remind me how the antihistamines have antimuscarinic effects and thus how diphenhydramine IV can be used to treat the extrapyramidal/parkinsonian symptoms?

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[thehundredthone]

The first generation antihistaminics are simply not selective for H1 receptors. They are potent antagonists at muscarinic (and sometimes alpha and 5HT) receptors too. As an aside, H1 antihistaminics are actually inverse agonists rather than antagonists.

http://www.biomedcentral.com/1471-2210/5/8

 

[seminoma]

The first generation antihistaminics are simply not selective for H1 receptors. They are potent antagonists at muscarinic (and sometimes alpha and 5HT) receptors too. As an aside, H1 antihistaminics are actually inverse agonists rather than antagonists.

http://www.biomedcentral.com/1471-2210/5/8

Thank you!

 

[Phloston]

TCAs, anti-psychotics and first gen H1 antagonists all have anti-muscarinic effects.

The mechanism is that they bind directly to the receptors. As thehundredthone has put it, this is due to non-selectivity.