A partial agonist activates a receptor only part of the way, even when it fills every available receptor. A 2016 review describes psychedelics as agonists or partial agonists at the 5-HT2A receptor, with particular importance for those on large layer V neurons of the neocortex (Nichols, 2016).
DMT is a measured example. A 2009 study confirmed it as a potent partial agonist at 5-HT2A, and found it caused the head-twitch response in normal mice but not in mice bred without that receptor (Keiser et al., 2009).
“Partial” describes how far the receptor is switched on, not how strong a drug feels or how safe it is. How strongly a drug binds is a separate measure: a 2016 lab comparison found LSD bound 5-HT2A more tightly than psilocin or DMT (Rickli et al., 2016). The same drug can also act fully at one receptor and partially at another, and results differ between cell systems, which is one reason findings from dishes and mice are labelled preliminary until tested in people.
Sources
- Nichols, 2016, Pharmacological Reviews
- Keiser et al., 2009, Nature
- Rickli et al., 2016, European Neuropsychopharmacology
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