“This podcast is the first of what I hope to be a series of “journal club” discussions with scientists about an important scientific paper. In this episode Jason Wallach and I discuss the strange origin of the head twitch response and how it became the most important behavioral correlate for 5HT2A agonism in non-human animals.”

Overview

Hamilton and Wallach go through the old LSD mouse papers — the origin of the Head-Twitch Response (HTR) — and how it became the main behavioral marker for 5-HT2A activation.
The discussion quickly goes beyond just the data, asking how we can even measure a psychedelic effect, what animal models actually tell us, and whether the subjective experience itself is necessary for therapy.

https://commons.wikimedia.org/wiki/File:Head-twitch_response_induced_by_a_psychedelic_drug_in_mice.webm

Notes

The 1956 paper is bizarre for modern science. it reads like something from the 18th century — no methods, no strain information, no statistics. they just write things like 10–60% of mice twitched.
still, it’s the first report of LSD producing a distinct behavioral response in animals.
LSD (5–100 µg IV) caused a fast head shake within 5–10 minutes, lasting up to 2 hours.
mescaline, yohimbine, serotonin — no effect.
they tried random follow-up experiments:
- indole before LSD made the twitch permanent in 5–30% of mice
- isolation for 2–3 weeks then regrouping made ~80% show the effect for weeks
- reserpine suppressed it while given, then it came back
- both researchers were not from that field and never continued, probably unaware how important their finding was
LSD was still used mainly as a psychotomimetic, a model for psychosis.

The HTR today

the head-twitch response is still common in research, used as an indicator for 5-HT2A receptor activity
a 2020 paper by Halberstadt et al. quantified it across many substances and found very strong correlations:
- mouse HTR potency vs human hallucinogenic dose: r = 0.9448 (n = 36)
- HTR vs rat drug-discrimination potency: r ≈ 0.95 for LSD-trained rats, r ≈ 0.96 for DOM-trained rats
in other words, the mouse HTR ED50 predicts human hallucinogenic potency with high accuracy across classic psychedelics.
examples of HTR ED50 values (mice):
LSD ≈ 52.9 µg/kg
DOM ≈ 1.75 µmol/kg
DMT ≈ 10 mg/kg
these values match well with the corresponding human dose ranges (micrograms to hundreds of milligrams).
the HTR dose–response is often non-monotonic, rising and then decreasing at higher doses, likely due to 5-HT2C modulation of 5-HT2A output.
false positives / negatives:
- Lisuride is a 5-HT2A agonist but not hallucinogenic and fails to produce HTR — shows functional selectivity.
- metabolism and mixed receptor activity can also interfere with interpretation.

What are we even modelling?

we have no idea what it’s like to be a mouse.
even if you could see a “movie” of its experience, you’d still interpret it through human thinking.
to truly understand it, you’d have to become the mouse.
so what are we actually modelling?
the HTR only shows that a drug activates the receptor — not what the experience is like.
the same problem exists for human disorders like major depression.
diagnoses are based on symptoms, not unified biology.
two people can both “have depression” but be completely different physiologically.
rating scales are rough tools — they rely on interpretation and self-report.
it’s strange to expect a mouse swimming test or head-twitch model to map neatly onto human emotional experience.

Psychological vs biological

reduction of experimental conditions to purely physiological causes feels too simplistic and denies any psychological dimension.
psychedelics might be the weird exception here — the “delusional” quality itself can change perspective, which might be why they help.
studies where drug-naive people rate the importance of their psychedelic experience show it often ranks among the top five experiences of their lives.
how could that not be relevant to mental health outcomes?
these are not magic bullets, but they can reset perception and outlook.
with proper guidance, that shift might sustain itself.
you cannot separate the psychological from the biological.
one influences the other constantly.

Back to the mouse model

even if you could somehow see what the mouse experiences, you’d still analyse it through your own cognition.
there’s no equivalent to a “head-twitch” in humans.
false positives or negatives can occur due to metabolic differences between species, or due to how the behavior is defined.
for instance, Lisuride is active in receptor binding assays but produces no HTR, while some compounds may twitch but lack hallucinogenic activity
the dose–response curve of HTR is bell-shaped, which matches the discussion in the podcast — lower doses trigger HTR strongly, higher doses reduce it again.

Therapy and subjectivity

can the subjective experience be removed and still keep the therapeutic effect? maybe partially.
ketamine shows it might work to some degree.
but completely ignoring the experience seems wrong — it’s part of what creates meaning and reorganization.
people often describe psychedelic experiences as moments of seeing life “through another lens.”
it’s not everything, but probably a necessary part of the process.
psychedelics are not one-size-fits-all drugs. each compound has its own therapeutic profile, and will need its own approach — something Alexander Shulgin already said decades ago.

Other points

Forced Swim Test: absurd on the surface, but still predicts antidepressant activity, even for ketamine. it’s reliable because of statistics, not realism.
neuroplasticity is probably the shared mechanism.
depression involves prefrontal atrophy from chronic stress.
psychedelics reopen those circuits for repair — the intense, confusing subjective state might be the conscious side of that process.
MDMA for PTSD: shows how subjective experience and biology intertwine.
MDMA allows trauma recall without fear, leading to memory reconsolidation and new emotional associations.
it wouldn’t work the same way without that felt safety.

Main takeaway

the head-twitch response is a translational model. it measures receptor-level potency, not experience. it’s useful but limited — and reminds us that psychedelic therapy can’t be reduced to pharmacology alone. to measure psychedelics, you need to include the subject too.

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