Like all 5-HT2 receptors, the 5-HT2A receptor is Gq/G11-protein coupled. This is the main excitatory receptor subtype among the GPCRs for serotonin, although 5-HT2A may also have an inhibitory effect[7] on certain areas such as the visual cortex and the orbitofrontal cortex.[8] This receptor was first noted for its importance as a target of serotonergic psychedelic drugs such as LSD and psilocybin mushrooms. Later it came back to prominence because it was also found to be mediating, at least partly, the action of many antipsychotic drugs, especially atypical antipsychotics.
Downregulation of post-synaptic 5-HT2A receptor is an adaptive process provoked by chronic administration of selective serotonin reuptake inhibitors (SSRIs) and atypical antipsychotics. Suicidal and otherwise depressed patients have had more 5-HT2A receptors than normal patients. These findings suggest that post-synaptic 5-HT2A overdensity is involved in the pathogenesis of depression.[9]
Paradoxical down-regulation of 5-HT2A receptors can be observed with several 5-HT2A antagonists.[10] Thus, instead of tolerance, reverse-tolerance would be expected from 5-HT2A antagonists. However, there is at least one antagonist at this site which has been shown to up-regulate 5-HT2A receptors.[10][11] Additionally, a couple of other antagonists may have no effect on 5-HT2A receptor number.[12] Nevertheless, upregulation is the exception rather than the rule. Neither tolerance nor rebound is observed in humans with regard to the slow-wave sleep (SWS) promoting effects of 5-HT2A antagonists.[13]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Cook EH, Fletcher KE, Wainwright M, Marks N, Yan SY, Leventhal BL (August 1994). "Primary structure of the human platelet serotonin 5-HT2A receptor: identify with frontal cortex serotonin 5-HT2A receptor". Journal of Neurochemistry. 63 (2): 465–469. doi:10.1046/j.1471-4159.1994.63020465.x. PMID8035173. S2CID40207336.
^Raote I (2007). "Serotonin 2A (5-HT2A) Receptor Function: Ligand-Dependent Mechanisms and Pathways". Ishier. Frontiers in Neuroscience. Press/Taylor & Francis. ISBN9780849339776. PMID21204452.
^Martin P, Waters N, Schmidt CJ, Carlsson A, Carlsson ML (1998). "Rodent data and general hypothesis: antipsychotic action exerted through 5-HT2A receptor antagonism is dependent on increased serotonergic tone". Journal of Neural Transmission. 105 (4–5): 365–396. doi:10.1007/s007020050064. PMID9720968. S2CID20944107.
^De Almeida RM, Rosa MM, Santos DM, Saft DM, Benini Q, Miczek KA (May 2006). "5-HT(1B) receptors, ventral orbitofrontal cortex, and aggressive behavior in mice". Psychopharmacology. 185 (4): 441–450. doi:10.1007/s00213-006-0333-3. PMID16550387. S2CID33274637.