CSF vasopressin rhythm is effectively insulated from osmotic regulation of plasma vasopressin

By using our method for continuous removal of cisternal cerebrospinal fluid (CSF) and intermittent sampling of blood from unanesthetized freely moving cats, we investigated the effect of osmotic-induced changes in plasma vasopressin on the daily rhythm of CSF vasopressin. Examination of the daily profiles of vasopressin and osmolality in the CSF and plasma of six euhydrated animals showed that CSF vasopressin concentrations exhibit a clear daily rhythm, whereas CSF osmolality and plasma vasopressin and osmolality do not exhibit such daily variation. A 48-h period of water deprivation caused marked sustained elevations in plasma vasopressin concentrations, which returned to basal levels on rehydration. In contrast, water deprivation had only a small effect on the CSF vasopressin rhythm. Although there was a significant elevation of the normally low nighttime CSF vasopressin levels during water deprivation in three of the four animals studied, high daytime vasopressin levels were unaltered and the daily rhythm was clearly evident before, during, and after the period of water removal in all animals. Changes between plasma vasopressin and osmolality were significantly correlated in all animals. Changes between plasma and CSF osmolality were significantly correlated in three of the four animals. The data indicate that the circadian regulation of the CSF vasopressin rhythm is effectively insulated from the osmotic regulation of plasma vasopressin.