Could histamine receptors be thought of as part of the cholinergic system?

[SpunkySkunk347]

A few years ago, one of my psychiatrists and I were discussing the side effects of quetiapine, and when I mentioned the drowsiness caused by the H1 antagonism, he said something along the lines of "the histamine receptors could be thought of as being intertwined with the cholinergic system". How true is this?

Then several months later, after I had taken my psychiatrist's statement as absolute truth (he had a PHD after all), I got into an argument with another psychiatrist while I was being a resident at a psychiatric hospital -- because I said that quetiapine doubles as an anti-histamine due to its anti-cholinergic effects, and with that smug "I'm a doctor, you're not" look on his face told me "you mean anti-histamine effects". So I name-dropped the H1 receptopr, and said that the histamine receptors were part of the cholinergic system. Although I was sort of just being a lorazepam-induced douche bag, how right/wrong was I?

 

[sekio]

The two are "intertwined" in the sense that many H1 antagonists are also muscarinic acetylcholine antagonists. The reverse does not neccesarily hold true, and it is erroneous to claim that the histaminergic H1 receptor are involved with the acetylcholine system in any way.

For instance, atropine is an excellent antimuscarinic agent but don't expect it to relieve your hayfever or put you to sleep. On the other hand the adage of the two systems being intertwined is demonstrated quite nicely by e.g. diphenhydramine being a good antimuscarinic as well as an antihistamine.

 

[SpunkySkunk347]

how about neuro-physiologically?
are the histamine receptor system and the cholinergic system somewhat dependent on each other?

 

[Dondante]

^They're distinct, independent systems AFAIK.

For instance, atropine is an excellent antimuscarinic agent but don't expect it to relieve your hayfever or put you to sleep.

The converse is true for hydroxyzine, a fairly selective H1 inverse agonist...no anticholinergic side effects.

 

[SpunkySkunk347]

yeah but, inverse agonist

 

[Dondante]

^Most "antihistamines" are actually H1 inverse agonists, not antagonists.

 

[SpunkySkunk347]

^Most "antihistamines" are actually H1 inverse agonists, not antagonists.

really? what about like diphenhydramine?