Lets compare Diphenhydramine and Quetiapine for instance.
Diphenhydramine has potent antagonization effects on Histamine H1 receptors, and Muscarinic acetylcholine receptors (all subtypes).
Just wondering if someone could supply some binding affinity charts to this thread. Histamine antagonism in the brain is inherently sedating, but apparently many of the 2nd-gen antihistamines are poor at crossing the blood brain barrier, and therefore less sedating, (it depends on which one though, some 2nd-gen ones are still sedating).
Adrenergic antagonism helps with sedation/anxiolysis too. A few antihistamines can have additional mechanisms of action that may add to anxiolysis/relaxation/sedation.
) Serotonin transporter (IC50 = 100n M) Norepinephrine transporter (IC50 = 260n M) Dopamine transporter (IC50 = 1,000n M)α1-adrenergic (Ki = 12n M) Alpha Adrenergic 2a (Ki = 1130n M) Alpha Adrenergic 2b (Ki = 480n M) Alpha Adrenergic 2c (Ki = 550n M) D1 (Ki = 10-15n M) D5 (Ki = 15-17n M) D2 (Ki = 1.55n M) D3 (Ki = 0.74n M) H1 (Ki = 1800n M) 5HT2A (Ki = 53n M) 5HT2C (Ki = 10,000n M) 5HT6 (Ki = 3666n M) 5HT7 (Ki = 377.2n M) M1 (Ki = 10,000n M) NR1\NR2B Subunit containing NMDA receptor antagonist Ifenprodil site (IC50 = 2m M) And also acts on the following: D4 inverse agonist (Ki = 5-9n M) 5HT1A agonist (Ki = 1927n M) Sigma 1 Irreversible inactivation by HPP (Ki = 3n M) Sigma 2 agonist (Ki = 54n M)α1-adrenergic (Kd = 39 n M) α2-adrenergic (Kd = 405 n M) H1 (Kd = 725 n M) 5-HT2A (Ki = 13 n M) 5-HT2B (Ki = 74 n M) 5-HT2C (Ki = 192 n M) Also has the following actions: 5-HT1A receptor partial agonist (Kd = 78 n M) SERT transporter inhibitor (Kd = 160 n M) So, if one is able to understand these, you can see from the numbers why some are more effective than others for sedation, likelihood of unwanted side effects, and why dose is so important. I don't doubt that Seroquel etc are just as good though.
Generally the broader action that these compounds have at blocking receptors, the more sedatibg they will be.
) α2C-adrenergic (Ki = 18 n M) α2A-adrenergic (Ki = 20 n M) α2B-adrenergic (unspecified, likely similar to α2A/α2B-adrenergic) D1 (Ki = 4,167n M) D2 (Ki = 1,460n M) D3 (Ki = 5,723n M) D4 (Ki = 25n M) H1 (Ki = 5.1 n M) 5-HT1A (Ki=18n M; IC50=1,000n M) 5-HT2A (Ki = 69 n M) 5-HT2B (unspecified, ~20-fold lower than for 5-HT2A/2C) 5-HT2C (Ki = 39 n M) 5-HT3 (unspecified, likely similar to 5-HT2A/2C) 5-HT7 (Ki = 265 n M) m ACh receptors (Ki = 794 n M? Diphenhydramine is a good and cheap option, but you gain a tolerance to it extremely fast, so everyday use is not really beneficial.
Tolerance to sedation from antihistamines can build very rapidly. Yep, as I said in my first post some antihistamines have additional mechanisms of action that may add to the sedation.
Ive taken many of the drugs mentioned (havent tried the anti-psychs) and mirtazapine has a much more relaxing enjoyable sedation then say, hydroxyzine.
Ive discussed this with a doctor and he said that the sedation has alot to do with antagonism of other receptors like the adrenergic receptors, which lends mirtazapine its hypnotic-like feeling.
Where as Quetiapine has potent antagonizing effects on Histamine H1, but also possesses antimuscarinic actions at M1 and M3.
On top of that, it antagonizes Dopamine, Serotonin and Epinephrine.
If it wasn't for its appetite enhancement, I'd call mirt the perfect AD for my needs.