The receptor psychedelics target is less abundant in depression, imaging review finds

The serotonin 2A receptor sits at the centre of two separate arguments about depression. It is the receptor through which classic psychedelics act, and it has been studied for decades as a possible marker of depression itself. A systematic review and meta-analysis published in Molecular Psychiatry on October 6 pools what four decades of imaging has found, and the answer is more qualified than either argument usually allows [s1].

What was pooled

The authors searched databases from inception to August and September 2024 for positron emission tomography, single photon emission computed tomography, and post-mortem radioligand binding studies of cortical 5-HT2A receptors in major depressive disorder and in suicide [s1].

Thirty-one studies met the criteria, covering 556 patients with major depressive disorder or people who died by suicide, and 526 controls [s1]. Binding data were pooled in random-effects meta-analyses of mean difference using Hedges' g, alongside meta-analyses of variance [s1]. Simple linear regression tested the relationship between receptor binding and baseline depression severity in the PET and SPECT studies [s1].

The in vivo result

Cortical 5-HT2A receptor binding was significantly lower in living patients with major depressive disorder than in controls, in frontal, prefrontal, cingulate and anterior cingulate cortex, and — on sensitivity analysis — in temporal cortex [s1]. Effect sizes ran from Hedges' g = -0.40 to -0.57 [s1].

The patients in question had not taken antidepressants for periods ranging from one week to never [s1]. In frontal and cingulate regions, the size of the binding effect correlated with baseline depression severity [s1].

There was study-level evidence of lower regional binding in never-medicated patients than controls, which on exploratory meta-analysis reached significance in anterior cingulate cortex [s1].

Most PET and SPECT studies were rated good or fair quality, and there was limited evidence of publication bias [s1].

The post-mortem result, which points the other way

Most post-mortem analyses were negative, and the included studies were of variable quality [s1].

That divergence is not a footnote. In vivo imaging measures binding in a living brain under physiological conditions; post-mortem radioligand binding measures receptor density in tissue. If receptor number were straightforwardly reduced in depression, one would expect both to show it.

Several explanations are compatible with the split, and the review does not adjudicate between them. Living brains contain endogenous serotonin competing with the tracer, so lower measured binding can reflect higher serotonin occupancy rather than fewer receptors. Post-mortem samples come from people whose clinical state at death, medication history and cause of death vary. And post-mortem study quality was itself variable [s1].

The caveat the authors put in their own conclusion

Their stated conclusion is that in vivo 5-HT2A receptor binding is reduced in major depressive disorder in frontal, cingulate and temporal cortex — and that this finding is based mainly on studies using antagonist or inverse agonist radiotracers [s1].

That qualification matters more than it sounds. Agonist and antagonist tracers do not label the same receptor population in the same way: agonist tracers preferentially bind receptors in a high-affinity, G protein-coupled state, while antagonist tracers bind more of the total receptor pool. A finding that rests mainly on antagonist tracers describes total binding, and cannot distinguish a change in receptor number from a change in receptor state.

Effect sizes of -0.40 to -0.57 [s1] are moderate. They describe a group-level shift with substantial overlap between patients and controls, which is why nothing in this literature functions as a diagnostic test.

Why the psychedelic literature makes this receptor interesting

The receptor is the mechanism of action for psilocybin. A 2019 PET study in eight healthy volunteers, using the 5-HT2A agonist radioligand [11C]Cimbi-36, found that a single oral dose of psilocybin at 3 to 30 mg produced dose-related 5-HT2A receptor occupancy of up to 72%, with plasma psilocin levels and occupancy conforming to a single-site binding model [s2]. Subjective psychedelic intensity correlated with both occupancy and psilocin levels [s2].

That study establishes the pharmacology: stimulating 5-HT2A is what produces the psychedelic experience [s2]. It does not establish that a receptor difference in depression explains why the drugs might help, and the October review does not make that claim either.

The two findings sit in an unresolved relationship. If 5-HT2A binding is lower in unmedicated depression [s1], the implication for a drug that works by stimulating that receptor is not obvious in either direction, and no study cited here tests it.

What to watch

Whether studies using agonist radiotracers reproduce the antagonist-tracer finding; whether the correlation between binding and depression severity holds in larger single-cohort samples rather than across study-level regression; and whether receptor imaging before psychedelic treatment predicts who responds — a question this literature raises and has not answered.

This article describes neuroimaging research and is informational only. It is not medical advice and does not recommend any treatment.

Sources

• [s1] Cortical 5-HT2A receptors in depression and suicide: a systematic review and meta-analysis of in vivo and post-mortem imaging studies, Molecular Psychiatry, 2025;30(12):6045-6062, published online 2025-10-06.
• [s2] Psychedelic effects of psilocybin correlate with serotonin 2A receptor occupancy and plasma psilocin levels, Neuropsychopharmacology, 2019;44(7):1328-1334, published online 2019-01-26.