LACTOTROPE DIFFERENTIATION IN RATS IS MODULATED BY A MILK-BORNE SIGNAL TRANSFERRED TO THE NEONATAL CIRCULATION

We have previously reported that normal differentiation of PRL-secreti ng cells in neonatal rats requires a maternal signal specific to the f irst few days of lactation. Subsequently, we found that milk proteins from this period had a greater capacity than those from later in lacta tion to stimulate lactotrope differentiation in vitro, suggesting that the maternal signal was a milk-borne peptide. We reasoned in the pres ent study that if this were the case, the signal should be present in the serum of suckling pups, and its level should reflect that in mothe rs' milk. To test this hypothesis, we first compared serum from 3-day- old pups with that from newborn pups that had not yet suckled. Anterio r pituitary cells from 1-day-old pups were cultured for 6 days in the presence of various serum samples (0.01-1.0% by volume). Lactotrope di fferentiation was assessed using a reverse hemolytic plaque assay for PRL. Serum from 3-day-old pups was more effective (P < 0.01; n = 4) th an that from unsuckled newborns at stimulating lactotrope differentiat ion in vitro (to 3.5 +/- 0.5% and 1.5 +/- 0.5% of all anterior pituita ry cells, respectively). Next, we tested whether this increase in seru m bioactivity in 3-day-old pups required maternal influences restricte d to the first few days of lactation. Newborn pups were placed with fo ster mothers that had been lactating for 0 or 7 days. When the pups we re 3 days old, their serum was tested for lactotrope-differentiating a ctivity. Serum from pups placed with day 0 foster mothers (control) in creased the abundance of PRL cells to 2.4 +/- 0.3% of all cells (P < 0 .01; n = 3), whereas serum from pups placed with day 7 foster mothers failed to stimulate lactotrope differentiation. In similar experiments , litters placed the day after birth with day 7 foster mothers were se parated on day 3 postpartum for 3 h, after which littermates were allo wed to suckle for 90 min on different foster mothers on day 3 or 9 of lactation. This brief suckling bout increased (P < 0.01; n = 3) the ca pacity of serum from pups on day 3 mothers to stimulate PRL cell diffe rentiation (to 3.9 +/- 0.4%) above that of serum from littermates plac ed with day 9 foster mothers (to 1.8 +/- 0.2%). Finally, the diffefent iating activities in pup serum and mothers' milk eluted from gel filtr ation chromatography in identical fractions (4-8 kilodaltons). The pre sent results along with our previous findings indicate that lactotrope differentiation in rats is modulated by a maternal signal that is tra nsferred to the circulation of neonatal pups via ingestion of mothers' milk early in lactation.