# Video research-update insert: Serotonin deficiency hypothesis of MDD

Editorial script and storyboard insert, 5 October 2026. No rendered video. Read alongside the original review; no new grade is announced.

## Narration

Serotonergic perturbations can reveal relapse vulnerability and altered release capacity, but a simple baseline-deficiency account of all major depression requires evidence beyond antidepressant efficacy or candidate-gene association. State, subtype and measurement specificity remain central.

In the 1990 study, tryptophan depletion transiently reversed remission in a subset. Drug-maintained vulnerability does not establish low serotonin as the initial disease cause. [P2184795]

In the 2004 study, depletion induced symptoms and altered regional metabolism in remitted patients versus controls. Susceptibility after past disease is not prospective proof of an initiating deficiency. [P15289275]

In the 2022 study, release capacity was lower in the studied depressive-episode group. Small cross-sectional challenge study cannot establish temporal precedence or universal deficiency. [P36635177]

The next discriminating question is: Does a measurable deficit precede a first untreated episode? Release differences may be state effects or downstream consequences of illness. Longitudinal never-medicated at-risk and incident-case cohorts with repeated validated release measurements, matched controls and treatment histories.

The timeline shows selected changes in evidence, not inevitable progress or proof of a single cause. The review’s archived grade remains unchanged; the depth of source reading is documented in the update.

## Storyboard and source locks

| Insert | Visual | Narration anchor | Sources |
|---|---|---|---|
| Opening | Claim and bounded update statement | First paragraph | Review claim and source ledger |
| Timeline 1990 | Dated study card with design, result and limit | Tryptophan depletion transiently reversed remission in a subset. | [P2184795](https://pubmed.ncbi.nlm.nih.gov/2184795/) |
| Timeline 2004 | Dated study card with design, result and limit | Depletion induced symptoms and altered regional metabolism in remitted patients versus controls. | [P15289275](https://pubmed.ncbi.nlm.nih.gov/15289275/) |
| Timeline 2022 | Dated study card with design, result and limit | Release capacity was lower in the studied depressive-episode group. | [P36635177](https://pubmed.ncbi.nlm.nih.gov/36635177/) |
| G1 | Question → competing explanation → discriminating test | Does a measurable deficit precede a first untreated episode? | [P36635177](https://pubmed.ncbi.nlm.nih.gov/36635177/), [P30845820](https://pubmed.ncbi.nlm.nih.gov/30845820/), [P7944875](https://pubmed.ncbi.nlm.nih.gov/7944875/) |
| G2 | Question → competing explanation → discriminating test | Which biologic subtype is vulnerable to depletion? | [P2184795](https://pubmed.ncbi.nlm.nih.gov/2184795/), [P15289275](https://pubmed.ncbi.nlm.nih.gov/15289275/), [P7944875](https://pubmed.ncbi.nlm.nih.gov/7944875/) |
| G3 | Question → competing explanation → discriminating test | How specific is the release measurement? | [P36635177](https://pubmed.ncbi.nlm.nih.gov/36635177/) |
| G4 | Question → competing explanation → discriminating test | Does treatment work through serotonin normalization or another route? | [P2184795](https://pubmed.ncbi.nlm.nih.gov/2184795/), [P19755572](https://pubmed.ncbi.nlm.nih.gov/19755572/), [P36635177](https://pubmed.ncbi.nlm.nih.gov/36635177/) |

## Editorial constraints

- Use the primary-source links shown above and keep the inference limit on screen with each milestone.
- Do not narrate an abstract check as complete-paper examination, a model result as human proof, or an unresolved inference as absence of all research.
- Preserve trial endpoint hierarchy, cohort reuse, correction notices and online/issue dates.
- Use original graphics made from this ledger. Publisher figure imagery has not been cleared or supplied for video reuse.
- Confirm any change in verdict through a separate documented HSEF review; this insert makes none.