Vasopressin Exhibits a Rhythmic Daily Pattern in Cerebrospinal Fluid but not in Blood

Examination of vasopressin concentrations in cerebrospinal fluid continuously withdrawn from free-ranging cats revealed a large daily rhythm, with high concentrations occurring during daylight hours. A similar daily pattern of the peptide was not detected in the circulation. The rhythmic fluctuation of vasopressin in the cerebrospinal fluid further implicates this body fluid as a vehicle of neuroendocrine communication and provides additional evidence that vasopressin may act by means of the cerebrospinal fluid to modulate behavioral processes, such as memory.