The CRH-ACTH-cortisol axis demonstrates a circadian rhythm with peak cortisol levels at approximately 8 AM. The neuroendocrine driver generating this rhythm is:

• A Light-driven serotonin release from the dorsal raphe nucleus stimulating hypothalamic CRH neurons
• B Morning rise in blood glucose stimulating cortisol secretion through a glycostatic feedback
• C Entrainment of the suprachiasmatic nucleus (SCN) output to paraventricular nucleus (PVN) CRH neurons via neural projections ✓
• D Nocturnal melatonin suppression allowing diurnal CRH pulsatility

Explanation

The circadian cortisol rhythm is driven by the master circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus, which receives photic input from the retinohypothalamic tract. SCN neurons project (directly and via intermediate neurons) to the paraventricular nucleus (PVN), where CRH neurons receive time-of-day signals. CRH pulses increase in the early morning, driving ACTH and cortisol surges. Melatonin does have reciprocal relationships with the HPA axis but is not the primary driver; blood glucose feedback is a minor regulatory input.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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