Inhibition of Nursing-Induced and Stress-Induced Fall in Pituitary Prolactin Concentration in Lactating Rats by Injection of Acid Extracts of Bovine Hypothalamus

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Abstract
The concentration of prolactin in the anterior pituitary glands of lactating rats killed with either ether or Nembutal after 10 ½ hr of isolation from their litters averaged 0.073 IU/mg (prenursed level). Thirty min of nursing, laparotomy with bleeding from the inferior vena cava during 5 min of ether anesthesia, and the stress of cervical stunning followed by decapitation, each reduced the pituitary prolactin concentration significantly below the prenursed level. Nembutal prevented the fall of prolactin concentration which resulted from laparotomy and bleeding. Nursing, followed by ether anesthesia and the stress of laparotomy and bleeding, did not lower pituitary prolactin content more than nursing alone. Injections of acid extracts of bovine stalk-median-eminence (1 SME/rat) inhibited the fall in prolactin concentration which followed the combined stimuli of nursing and stress. Treatment with extracts of bovine cerebral cortex, or with ovine prolactin (12 mg), did not. These results provide in vivo evidence for the existence of a hypothalamic inhibitory factor in the normal regulation of prolactin release in response to nursing or stress. (Endocrinology76: 883, 1965)