Relationship between prolactin secretion and hypothalamic prolactin releasing factor in pregnant and puerperium rats

The present study attempted to elucidate stimulatory factor(s) in the rat hypothalamus which controls prolactin secretion from the anterior pituitary. Rat serum prolactin was elevated so much in late pregnancy that we prepared the hypothalamic extract of late pregnant rats. Prolactin levels in serum and pituitary by radioimmunoassay. After injection of this extract into a lactating rat 48-60 hr after delivery, the serum prolactin level was elevated significantly one to four hr later and the pituitary prolactin level declined two hr later. On the other hand, the hypothalamic extract of normal female rats prepared in a similar manner inhibited prolactin secretion from the anterior pituitary in the lactating rat as described by other authors. These data indicate that the prolactin releasing factor may consist in the hypothalamus of late pregnant rat, and be predominant over the prolactin inhibiting factor during late pregnancy. Prolactin secretion was also investigated in lactating and non-lactating puerperium rats. Prolactin in serum and pituitary declined with days after delivery in non-lactating rats, but not in lactating rats. The presumed factor for such prolactin release in lactating rats is considered to be the prolactin releasing factor.