= Pineal gland
The \b[pineal gland] is a small neuroendocrine organ of the <epithalamus>, attached to the roof of the <third ventricle>. It secretes <melatonin>, linking a neural timing system to a chemical signal distributed beyond the <brain>. Its anatomical position is distinct from that of the nearby <habenula>.<Pineal Gland in NLM MeSH>[D010870 scope note and anatomical hierarchy.]{ref}
A classic experiment located a strong daily rhythm at an enzymatic step in pineal metabolism. In 1970, <D. C. Klein> and <J. L. Weller> reported that rat pineal <serotonin N-acetyltransferase> activity was more than 15 times higher at night than during the day. The rhythm persisted in complete darkness and in blinded animals, whereas constant illumination suppressed it. Their experiments in isolated tissue also implicated <norepinephrine> in regulation of the enzyme activity.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Science169,1093–1095(1970), abstract: lighting conditions, enzyme rhythm and in-vitro regulation.]{ref}
The darkness control is particularly illuminating. A mechanism that merely switched on whenever light was absent would predict a continuously high output in constant darkness. Continued rhythmic variation instead supports an internally generated timing signal. It does not show that the <pineal gland> alone generates the organism's entire <circadian rhythm>: the measurement concerns one gland and one metabolic activity in an intact timing system. Likewise, the reported activity ratio is an enzyme-assay result, not a universal ratio of human blood <melatonin> concentrations.
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