Carrier-mediated transport of labeled oxytocin from brain to blood.

The transport of 125I-oxytocin from brain to blood was investigated in mice after intraventricular injection of radioactively labeled oxytocin with or without unlabeled candidate inhibitors. Residual radioactivity in the brain detected after decapitation was the principal determinant of transport activity. The half-time disappearance from the central nervous system of labeled oxytocin was 19.1 min. Inhibition by 10 nmol/mouse of oxytocin showed a saturable component to transport. A 10-nmol dose of tyrosine-melanocyte-stimulating hormone release inhibiting factor (Tyr-MIF-1) and pressinamide also significantly inhibited transport of labeled oxytocin (p less than 0.05). There was no inhibition of the system by a 10-nmol dose of tyrosine, iodotyrosine, MIF-1, or arginine vasopressin. Studies performed with 125I-oxytocin injected simultaneously with 131I-Tyr-MIF-1 with or without unlabeled oxytocin or Tyr-MIF-1 were consistent with both peptides being transported by the previously described peptide transport system-1 (PTS-1). Pretreatment with aluminum (100 mg/kg of elemental aluminum given 60-90 min before intraventricular injection), previously shown to inhibit PTS-1 and some other transport systems, inhibited the transport of labeled oxytocin. Radioactivity collected from the blood after intraventricular injection of 125I-oxytocin eluted on HPLC at the same position as the labeled oxytocin standard and differently from tyrosine, Tyr-MIF-1, MIF-1 and tocinamide. It is concluded that a saturable system exists for the transport of intact oxytocin from brain to blood which appears to be the previously described PTS-1.