Lipotropin, melanotropin and endorphin: in vivo catabolism and entry into cerebrospinal fluid.

Anesthetized rabbits were given intravenous injections of either beta-lipotropin (beta-LPH), beta-melanotropin (beta-MSH) or beta-endorphin. The postinjection concentrations of these peptides in plasma and cerebrospinal fluid (CSF) were measured by radioimmunoassay (RIA). The plasma disappearance half-times were 13.7 min for beta-LPH, 5.1 min for beta-MSH, and 4.8 min for beta-endorphin. Circulating beta-LPH is cleaved to peptides tentatively identified as gamma-LPH and beta-endorphin. Each of these peptides appeared in the CSF within 2 min postinjection. The maximum CSF to plasma ratios were 0.08 for beta-LPH, 1.48 for beta-MSH,and 0.23 for beta-endorphin.