Normal differentiation of prolactin cells in neonatal rats requires a maternal signal specific to early lactation.

This laboratory has previously demonstrated that PRL-secreting cells are virtually nonexistent on day 3 but appear in appreciable numbers on day 4 of neonatal life. The purpose of the present study was to determine whether this explosive appearance of PRL cells is due to maternal influences specific to the first 4 days of lactation. Litters of 1-day-old rat pups were placed with foster mothers that had been lactating for either 1 or 4 days. Four days later (at 5 days of age), the anterior pituitaries from these pups were removed, dispersed into individual cells with trypsin, and subjected to reverse hemolytic plaque assays for PRL and GH release. We found that the proportion of PRL-releasing cells in pituitaries from pups placed with day 1 mothers (4.2 +/- 0.6%; mean +/- SE; n = 4 experiments) was similar to that in 5-day-old pups that had not been fostered (3.4 +/- 0.6%; n = 3 experiments). In contrast, placing 1-day-old pups with day 4 mothers significantly reduced (P less than 0.01) the fraction that released PRL to 0.6 +/- 0.2% of all pituitary cells (n = 4 experiments). This effect appeared to be specific to PRL cells, since the percentage of all pituitary cells that secreted GH was not different between litters fostered onto day 1 or day 4 dams (40.0 +/- 3.8% and 41.1 +/- 3.9%, respectively; n = 3 experiments). Furthermore, the suppression of PRL cell expression did not result from a gross nutritional deficit, since the rates of body wt gain during the foster period were not different between the two groups (12.4 +/- 0.5 and 12.9 +/- 0.8 g/litter.day, respectively; n = 4 experiments). In a second design, newborn pups were placed with females that had been lactating for 0, 2, 4, or 7 days and analyzed for PRL- and GH-releasing cells at 5 days of age. In two trials of this experiment, the fractions of pituitary cells that secreted PRL were 5.2, 6.3, 5.9, and 1.2%, respectively for trial 1 and 3.3, 4.1, 0.7, and 0.6%, respectively for trial 2. As in the first experiment, the proportion of GH-releasing cells and the rate of growth were not different among the four groups. Taken together, these results indicate that normal differentiation of PRL cells in neonatal rats is triggered by a maternal signal present during the first few days of lactation, and that the magnitude of this signal declines with the progression of lactation.