Oxytocin and neurophysin in plasma and CSF during suckling in the guinea-pig

Cerebrospinal fluid (CSF) obtained from the cisterna magna of conscious unrestrained guinea-pigs was assayed for oxytocin (OT), vasopressin (AVP) and neurophysin (NP). Basal levels of OT and AVP were approximately 5 and 15 fmol/ml, respectively, whereas NP levels were considerably higher (around 1 pmol/ml); there was no significant differences between males, females and lactating females. In lactating females, simultaneous samples of blood and CSF were obtained during suckling episodes. Despite large increases in plasma OT and NP at milk ejection, CSF levels remained unchanged throughout. Intracerebroventricular injection of 100 fmol OT produced an 8-fold rise in the concentration of this peptide in cisternal fluid within 10 min. Since CSF levels of OT and NP do not change when these peptides are released from the pituitary, we conclude that the presence of these peptides in CSF reflects activity of extrapituitary terminals and that these terminals are not activated during suckling.