Availability of the Rabbit for Assay of the Hypophyseal Lactogenic Hormone. ∗

Two years ago, while investigating various preparations of the anterior pituitary for their lactogenic effect we employed castrated virgin rabbits, as had Corner 1 before us. While satisfied with the results in the rabbit, we prefer at present to use the virgin guinea pig ovariectomized in oestrus and injected immediately thereafter (Nelson and Pfiffner 2 ) or the 4 to 5 weeks old squab (Riddle 3 ) as more convenient forms. It would seem, however, that the same degree of satisfaction with the castrate virgin rabbit has not been experienced by some other investigators. This report restates our position on the action of the lactogenic hormone in rabbits, and submits further information concerning our experiences with this animal.

We have reported⁴ that a mature mammary gland in a virgin rabbit normally matured, then castrated, will respond by growth and lactation to the lactogenic hormone, but will merely atrophy following administration of potent growth hormone, or of potent gonadotropic hormone from pregnant mares'serum, or pregnant women's urine. That no continuing ovarian factor plays a rôle in the resultant growth of the ducts and alveoli of such positive reactions seems plausible since we always remove the ovaries just before beginning injections. Should ovarian secretions remain in the body after castration, their influence on the mammary apparatus could only be a synergistic one with the lactogenic hormone, since without administration of the latter the castrate rabbit's mammary glands gradually atrophy.

The rabbits used by us have been 5 1/2 to 6 months old virgin New Zealand Whites and Reds weighing around 3 kg. If the nipples of these just mature females show any appreciable growth over and above the immature or male-sized nipples, then lactation may be expected following treatment with lactogenic hormone. Precaution must be exercised to ascertain that the animal is not already secreting milk. Stripping of the glands throughout their extent and observation of a control biopsy spread assures one on this point. We have often observed “spontaneous” lactation in virgin rabbits isolated from males and other females, but we have never found anything but atrophy of the mammary glands of untreated castrate rabbits not lactating at the time of operation. Spontaneous lactation has been observed in young virgin rabbits, the ovaries of which upon serial sectioning showed no evidence that a corpus luteum had ever been formed (although a considerable amount of interstitial tissue around atretic follicles was present), and such animals have been observed to lactate continuously for a month following removal of the ovaries, apparently in response to lactogenic hormone from the animals'own hypophyses.