TRH transport to rat milk.

Neonatal rat possess several unique features in relation to TRH. Thus, the hypothalamus does not control pituitary TSH secretion, neonatal plasma does not degrade TRH and a significant quantity of this neurohormone is present in upper part of gastrointestinal tract, contractile response of duodenum to TRH being dependent on suckling. Therefore, a possible role of exogenous TRH from maternal milk was studied. Repeated or single injection of 50 micrograms TRH to lactating rats resulted in an increase of serum TSH and decrease of pituitary TSH content in sucklings. Repeated TRH administration to thyroidectomized lactating rats prevented a decrease of plasma T4 in their pups. After intravenous administration of 3H-TRH the total radioactivity of plasma and milk equalized at 25 min and increased continuously in milk during 2 h study. The presence of unaltered 3H-TRH in rat milk and gastric content of pups was confirmed by paper chromatography, electrophoresis and specific antibody precipitation. TRH degrading activity of rat milk, though qualitatively similar, represents one fifth of plasma activity in vitro. It is concluded that there is one way transport of TRH between plasma and milk and that this neurohormone is transferred to rat milk in a biologically active form.