Question at position 18 Which structure releases melatonin? Suprachiasmatic nucleusPineal glandAdrenal glandPituitary glandClear my selection

Question

BlackTom AI · Accepted Answer

Melatonin is synthesized and secreted by the pineal gland.

1) Relevant knowledge points and concepts
- Melatonin is an indoleamine hormone derived from tryptophan via serotonin (enzymes: AANAT and HIOMT/ASMT). 
- It is produced primarily in the pineal gland, a small endocrine organ in the brain. 
- Circadian timing of melatonin secretion is controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus, which conveys light/dark information to the pineal via sympathetic pathways. 
- Other listed structures (adrenal gland, pituitary) secrete different hormones (cortisol/epinephrine and pituitary tropic hormones, respectively).

2) Step-by-step reasoning
Step 1: Identify which structure is an endocrine gland known for melatonin synthesis — the pineal gland.  
Step 2: Recall that the SCN is the master circadian clock but does not itself secrete melatonin; instead it regulates pineal activity.  
Step 3: Exclude adrenal and pituitary because their primary secretions are not melatonin.  
Step 4: Conclude that the pineal gland releases melatonin, particularly at night when SCN-driven inhibition is relieved.

3) Intermediate biochemical steps (explicit)
- Tryptophan → (hydroxylation/decaboxylation) → Serotonin  
- Serotonin → (N-acetyltransferase, AANAT) → N-acetylserotonin  
- N-acetylserotonin → (hydroxyindole O-methyltransferase, HIOMT/ASMT) → Melatonin  
These enzymatic steps occur in pinealocytes of the pineal gland.

4) Why the correct answer is correct
The pineal gland contains the enzymes and cellular machinery for converting serotonin into melatonin and is the established primary source of circulating melatonin in humans. Its secretion increases at night and decreases in light, matching melatonin’s role in signaling night to the body.

5) Why the other options are incorrect
- Suprachiasmatic nucleus: controls timing of melatonin release (circadian pacemaker), but does not produce melatonin.  
- Adrenal gland: secretes cortisol, aldosterone, epinephrine/norepinephrine — not melatonin.  
- Pituitary gland: secretes growth hormone, ACTH, TSH, LH, FSH, prolactin, etc., not melatonin.

6) Conclusion
Therefore, the structure that releases melatonin is the pineal gland, which matches the provided correct answer: Pineal gland.

Question at position 18 Which structure releases melatonin? Suprachiasmatic nucleusPineal glandAdrenal glandPituitary glandClear my selection单项选择题

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