Protein hormones in cerebrospinal fluid: evidence for retrograde transport of prolactin from the pituitary to the brain in man.

Plasma and cerebrospinal fluid (CSF) levels of two hormones of similar molecular size, pituitary prolactin (PRL) and human chorionic somatomammotropin (hCS), and of the bigger hormone human chorionic gonadotropin (hCG) were measured in six pregnant women without pituitary disease. For all three hormones, the plasma and CSF levels were closely correlated. The plasma/CSF concentration ratio for hCG (571 +/- 378, mean +/- SD) was significantly different (P less than 0.01) from the hCS ratio (24.6 +/- 6.1); the hCS ratio was significantly different (P less than 0.005) from the PRL ratio (7.2 +/- 1.5). We conclude that (1) the CSF concentration of a protein hormone depends on the plasma concentration and on its molecular size, and (2) pituitary hormones reach the CSF not only via filtration of peripheral blood at the choroid plexuses, but also more directly via retrograde transport from the pituitary to the brain.