In vivo evidence for a controlled offset of melatonin synthesis at dawn by the suprachiasmatic nucleus in the rat

Prolonged norepinephrine stimulation of the pineal gland is sufficient to prevent the early morning decline of melatonin release.

• The central biological clock in the SCN regulates melatonin synthesis through a multi-synaptic pathway.
• GABA provides daytime inhibitory signals, while glutamate delivers nighttime stimulation to the PVN-pineal pathway.
• Increased GABA release at dawn onto PVN neurones may reduce norepinephrine stimulation of the pineal gland.
• Blockade of GABA signaling at dawn did not completely prevent the decline of melatonin release.
• GABAergic inhibition alone is not sufficient to fully explain the early morning decline in melatonin levels.

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