Agonist
What is an agonist?
An agonist is a molecule that binds to a receptor on a cell and activates it, triggering a biological response. Think of it as a key that fits into a lock and turns it — the lock being the receptor, and the turning being the cellular response.
In your body, natural agonists include neurotransmitters like serotonin, dopamine, and norepinephrine. These molecules are produced by your own neurons to send signals across synapses. A drug that acts as an agonist mimics this natural signaling — it's an outside key that fits the same lock.
Types of agonists
Full agonist
Produces the maximum possible response when binding to a receptor. Activates the receptor as fully as the natural neurotransmitter would (or more).
Partial agonist
Binds to the receptor but produces a submaximal response — it turns the lock, but not all the way. Partial agonists can actually act as functional antagonists in the presence of full agonists by competing for receptor binding.
Inverse agonist
Binds to the receptor but produces the opposite effect of a normal agonist. Instead of activating the receptor, it reduces the receptor's baseline activity below its resting level.
Why it matters for microdosing
Classical psychedelics are 5-HT2A agonists
Psilocybin (via its active metabolite psilocin), LSD, mescaline, and DMT all work primarily by binding to and activating the 5-HT2A serotonin receptor. They are agonists at this receptor — they mimic serotonin's action, but with different binding characteristics that produce distinct effects.
The agonist profile matters
Not all 5-HT2A agonists are the same:
- LSD — very potent agonist with high binding affinity; also acts on dopamine and other receptors
- Psilocin — selective serotonin receptor agonist with lower binding affinity
- Mescaline — phenethylamine agonist with broader receptor profile
- DMT — tryptamine agonist also active at sigma-1 receptors
These differences in agonist profile contribute to the different subjective qualities of each substance.
Functional selectivity
Modern pharmacology recognizes that agonists can activate the same receptor in different ways, triggering different intracellular signaling pathways. This is called "biased agonism" or "functional selectivity." It may explain why different psychedelics that all target 5-HT2A produce somewhat different effects.
Agonists vs. other drug types
| Type | Action | Example |
|---|---|---|
| Agonist | Binds and activates receptor | Psilocin at 5-HT2A |
| Antagonist | Binds but blocks activation | Ketanserin at 5-HT2A |
| Partial agonist | Binds, partially activates | Some atypical antipsychotics |
| Inverse agonist | Binds, reduces baseline activity | Some research compounds |
| Allosteric modulator | Binds elsewhere, changes receptor behavior | Benzodiazepines at GABA-A |
Practical implications
- Understanding that psychedelics are agonists explains why tolerance develops — repeated receptor activation triggers downregulation
- It explains why SSRIs can blunt effects — chronic SSRI use changes serotonin receptor sensitivity
- It explains why cross-tolerance exists between different psychedelics — they're all agonists at the same receptor