The reaccumulation after suckling of prolactin by the pituitary of spayed lactating rats was significantly increased after a single s.c. injection of acidic extract of rat hypothalamus (stalk-median eminence region) given at the start of the reaccumulation period. Injection of ammonium acetate, an extract of ovine hypothalamus rich in luteinizing hormone-releasing factor (LH-RF), extract of rat occipital cortex or oxytocin was ineffective. An extract of ovine hypothalamus rich in prolactin-inhibiting factor (PIF) (0·15 ml./rat) blocked the fall in pituitary prolactin concentration induced by suckling, but in the same or higher (0·3 ml.) doses neither increased nor depressed the reaccumulation of prolactin after suckling. A single s.c. injection after suckling of 2–8 mg. ovine prolactin in each of four instances significantly increased the amount of prolactin reaccumulated by the pituitary. The level attained was equal to or greater than that resulting from injecting rat hypothalamic extract. These results suggest that the increased blood level of prolactin after suckling stimulates the reaccumulation of prolactin in the pituitary possibly indirectly via a factor or factors in the hypothalamus.
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