Since anti-histamines double as sleeping pills because of their potential for drowsiness, I often wondered if that was because they were CNS depressants. What is the drowsiness caused by? I don’t recall reading that these anti-histamines were sedatives but only carried word drowsiness which seems rather vague. What’s the straight dope?
Yes, antihistamines are generally considered CNS depressants, as they do decrease CNS activity. Some of the newer ones like Claritin, Allegre, et al are felt to cause less depression, on the theory that they don’t cross the blood/brain barrier like earlier antihistamines. However, that theory is now in doubt, as there is evidence that they can cross said barrier and sedate folks to some extent.
It’s especially important to not mix antihistamines with other CNS depressants like alcohol, opiate pain killers, barbiturates, and benzodiazepines.
All the “original” or first-generation anti-histamines have what’s called anticholinergic properties. This means they oppose the action of a brain chemical (neurotransmitter) called acetylcholine. It is this anticholinergic effect that’s responsible for many of the side effects of anti-histamines including drowsiness, dry mouth, constipation (if used chronically), blurred vision, and, in elderly men in particular, the inability to pass urine.
The newer anti-histamines have much less of these anticholinergic side effects. Moreover, they tend to only affect tissues outside of the brain. In other words, they act peripherally with little “central effect”. Hence, no drowsiness (drowsiness is a “central” effect).
And yet another simulopost with Qad. Coincidence? I think not!
BTW, IIRC, the mechanism of their action on the CNS is generally thought to be due to their anticholinergic effects. Some antihistamines have fewer anticholingeric effects than others. These are also the ones that were thought to not cross the blood/brain barrier as readily.
Damn! Great minds do post alike!!
So then what about Ambien? It has its affect on GABA… Does that mean that it isn’t a CNS depressant?
I’m not sure I understand your question/point. Are you suggesting that because anti-histamines exert their CNS depressant effects by virtue of their anticholinergic properties, that all other drugs that cause CNS depression must also act in the same way (via anticholinergic mechanisms)? If so, obviously not.
There are many different ways to depress the CNS, running the gamut from the nonspecific (alcohol, barbiturates, or a good knock to the head), to anticholinergic activity (antihistamines, tricyclic antidepressants), to GABA agonism (benzodiazepines and “nonbenzodiazepine GABA agonists”), to adrenergic antagonism (beta blockers). That’s just off the top of my head. I’m sure there are more.
It wasn’t a point… just a question. I suppose I’m a little unclear.
I’m also using the term CNS depressant rather broadly which undoubtedly adds to the confusion. To rephrase…
How does Ambien “work”?
That should do it.
I am no expert, not even close, but my superficial “understanding” of such things leads to me to say that the mechanism of action of Ambien is not (well) understood.
Although it’s got a different structure than drugs like valium, lorazepam, etc. (i.e. benzodiazepines), and does not behave like a traditional ligand of the GABA receptor, Ambien’s effects still very closely resemble those of the benzodiazepines.
Using the old “lock and key” model, benzodiazepines act like keys for GABA locks, with benzo’s unlocking the GABA receptor thereby causing their pharmacologic effects. Conversely, Ambien does not work as a key for the GABA receptor.
On the other hand, Ambien does seem to modulate the GABA receptor (the lock). I think of this as Ambien loosening the locks, or, perhaps, augmenting the effect once the lock has been opened.
(I apologize if the “lock and key” business comes across as patronizing. But, it’s impossible on a message board to know who understands what about neurotransmitter receptors and I wanted to make sure the concept was made clear, if not the actual receptor physiology).