IEGs (immediate early genes) are definitely involved in the therapeutic biology of ketamine, psychedelics, and SSRIs. But they’re usually more like the brain’s “first-wave transcriptional relay + plasticity work order” than a single magic switch that is the therapy. Humans love magic switches. Biology does not.
IEGs like Fos/c-Fos, Jun, Egr1, Egr2, Arc, Npas4 turn on within minutes of strong neural activation and kick off downstream gene programs that reshape synapses and circuits. They’re used constantly as activity-to-plasticity markers because they light up the specific cells/circuits a drug engages.
Ketamine’s rapid antidepressant action is strongly tied to synaptic plasticity signaling (classically: AMPA throughput → BDNF/TrkB → mTOR/translation and related pathways). Those pathways drive expression of plasticity-related genes including Arc in some contexts, and Arc is often discussed as part of the synaptogenesis machinery downstream of mTOR.
That said, ketamine’s IEG responses can be region- and gene-specific (some studies even report antidepressant-relevant changes via downregulating Egr1 in hippocampus under certain conditions). So “IEGs” aren’t a single direction or a single mechanism here; they’re a family of fast responders sitting inside a broader signaling-and-translation program.
How “responsible” are they? Likely important for the durable rewiring piece, but ketamine’s core causal spine is usually framed upstream (BDNF/TrkB, mTOR/ERK, synapse restoration), with IEGs as early transcriptional/effector steps that help consolidate changes.
Psychedelics reliably induce IEGs (c-Fos, Arc, Egr1/2, etc.), and those effects are greatly reduced/absent when 5-HT2A signaling is blocked or genetically removed in animal models.
There’s also evidence for longer-lasting epigenomic/chromatin changes after psychedelic exposure (e.g., DOI) at regulatory regions tied to synaptic assembly genes, which is exactly the kind of longer-tail biology you’d expect to be set up by early transcription-factor waves (IEGs included).
How “responsible” are they? Very likely necessary for converting the acute receptor hit into longer-term plasticity programs in preclinical models, but still not “the therapy” by themselves. In humans, therapeutic outcomes also depend heavily on network state, context, and learning. IEGs are a biological gateway for plasticity; what gets written during that plastic window is… inconveniently influenced by experience.
With SSRIs, the clinical effect is delayed, and that delay lines up with transcriptional remodeling more than acute monoamine changes. A particularly on-the-nose example: chronic fluoxetine induces an AP-1 transcriptional program (AP-1 is formed by IEG-family transcription factors like c-Fos and c-Jun) that regulates remodeling genes including p11 (S100a10), and AP-1 function was reported as required for the antidepressant effect in vivo in that work.
Separately, chronic antidepressant treatment has been shown to induce Arc expression in specific forebrain regions, consistent with long-term synaptic adaptation.
How “responsible” are they? For SSRIs, an IEG-linked transcription factor program (AP-1) looks closer to a causal gate than in the other two classes, because it’s tied to the onset phase and experimentally implicated as required in at least one strong line of evidence.