The neuropeptide, arginine vasopressin, exhibits a prominent daily rhythm in the cerebrospinal fluid (CSF) of several mammals. The CSF rhythm is an endogenously generated, circadian rhythm and is entrained to the 24 h period by the daily light-dark cycle. The circadian regulation of vasopressin in CSF is effectively insulated from the osmotic regulation of the peptide in blood. The CSF vasopressin rhythm is produced by a neural system anatomically separate from the classical hypothalamo-neurohypophyseal system. The suprachiasmatic nuclei, the site of a hypothalamic circadian pacemaker, are an important component of this newly defined neural system and are necessary for rhythm generation. These findings support the concept that brain vasopressin is both functionally and anatomically compartmentalized for specific actions within the CNS.
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