A time-dependent peak of oxytocin exists in cerebrospinal fluid but not in plasma of humans.

Oxytocin (OT) and the oxytocin-neurophysin (OT-Np) were measured by RIA in samples of cerebrospinal fluid (CSF) obtained sequentially at 0600, 1200, 1800, and 2400 h from six patients in whom intrathecal catheters were temporarily placed for CSF rhinorrhea. The highest levels of OT in CSF were found at 1200 h. An analysis of variance of sequential measures of the concentration of OT in samples of CSF obtained every 6 h over a 30-h period showed the mean levels (+/- SEM) of OT at 1200 h, 6.41 +/- 1.13 microU/ml and 5.06 +/- 0.58 microU/ml, to be significantly higher (p less than 0.05) than mean levels of OT at 0600 h, 2.50 +/- 0.65 microU/ml; 1800 h, 2.63 +/- 0.61 microU/ml and 2.64 +/- 1.21 microU/ml; and 2400 h, 2.86 +/- 1.13 microU/ml. The levels of OT-Np in CSF did not show a similar peak. In three of the patients simultaneous samples of blood were obtained for measurement of the same peptides, but no corresponding peak of OT or its Np was found in plasma of these three patients. The level of OT in CSF at all times was also significantly higher (p less than 0.05) than the level of OT in plasma of these three patients. Levels of OT and OT-Np were measured by RIA of samples of plasma obtained hourly for a 24-h period from six healthy men and six healthy women. No diurnal variation of OT or its Np in the plasma of men or women was found. This pattern of OT in the CSF of humans is similar to the pattern of OT in the CSF of the Rhesus monkey, but in contrast to the lack of a clearly defined peak of OT in the CSF of the cat or the rat. These observations in humans reinforce the differences among species of the secretion of OT in the CSF.