1328 



HORMONAL REGULATION OF BEHAVIOR 



milk formation can be induced in single 

 ducts of the proprely prepared rabbit mam- 

 mary gland by small local injections of pro- 

 lactin (Bradley and Clarke, 1956). How- 

 ever, it is now clear that other hormones 

 normally play a role in the maintenance of 

 lactation. Prolactin alone cannot maintain 

 complete milk production in rats hypophy- 

 sectomized on the fourth day of lactation, 

 whereas adrenocorticotrophic hormone 

 (ACTH) and probably growth hormone 

 have synergistic effects with prolactin on 

 milk production (Cowie, 1957; Williams, 

 1945). 



The recent reviews and experiments by 

 Folley (1956), and by Lyons, Li and John- 

 son (1958) make it plain that a consider- 

 able number of hormones secreted by the 

 pituitary gland and by the ovary partici- 

 pate in the onset and maintenance of milk 

 secretion. There is some evidence that at 

 least one of these hormones, in addition to 

 prolactin, is released in response to suckling 

 stimulation. Gregoire (1947a, b) found that 

 the involution of the thymus which occurs 

 during pregnancy can be maintained after 

 parturition by suckling, wdiereas the thy- 

 mus regenerates if suckling is prevented. 

 Injected prolactin does not maintain the 

 involution of the thymus in the absence of 

 suckling. This suggests that the effect of 

 suckling on the condition of the thymus is 

 through the stimulation of the secretion of 

 some other hormone, probably ACTH (Fol- 

 ley, 1956). 



Further details of the effects of various 

 hormones on the mammary gland are con- 

 tained in the chapter by Cowie and Folley. 

 It is sufficient for our purposes to point 

 out that the effects of suckling stimulation 

 are probably somewhat more complex than 

 the stimulation of the secretion of a single 

 hormone by the pituitary. 



Loeb and his co-workers found that, when 

 the mammary gland of a laetating guinea 

 l^ig was ligated on one side, so that suckling 

 took place only on the other side, the gland 

 on the ligated side regressed, whereas only 

 the one on the suckled side was maintained 

 in a secretory condition (Kuramitsu and 

 Loeb, 1921; Hesselberg and Loeb, 1937a, 

 b). They concluded that the stimulation 

 of secretory activity in the mammary gland 

 by suckling is a local effect, probably de- 



pendent on milk removal. Somewhat sim- 

 ilar observations were made on the rat by 

 Weichert (1942) who noted that, when a 

 laetating rat has a small litter, the pups 

 tend to use the anterior nipples only. In 

 the third week of lactation, the mammary 

 tissue in the neighborhood of the suckled 

 nipples is engorged whereas that in the 

 neighborhood of the nonsuckled nipples is 

 regressed. This regression is not prevented 

 by prolactin injection. 



Such data seem to contradict numerous 

 observations (already referred to above) 

 indicating that maintenance of the mam- 

 mary gland through suckling stimulation is 

 a systemic effect, and that nonsuckled as 

 well as suckled glands in the same animal 

 are maintained by the suckling stimulus 

 (Selye, 1934; Selye, Collip and Thomson, 

 1934; Turner and Reineke, 1936). This con- 

 tradiction can readily be resolved by a 

 consideration of the effect of mammary 

 engorgement on the access of circulating 

 hormones to the mammary tissue. Williams 

 (1941) ligated at their midpoints the main 

 ducts of some of the mammary glands in 

 laetating mice, thus preventing milk drain- 

 age beyond the ligation, while permitting it 

 up to the ligation. He found that the ob- 

 structed portion of the gland became in- 

 flamed and necrotic, and that the mainte- 

 nance of the milk-secreting parenchyma in 

 response to suckling stimulation w^as con- 

 siderably reduced. Selye, Collip and Thom- 

 son (1934) tied off the milk duct of the 

 mammary gland on one side of laetating 

 rats and removed the nipples on the other 

 side. They found that secretion was con- 

 tinued in both glands, indicating that stim- 

 ulation on one side maintained secretion on 

 the other side, and also that secretion could 

 be maintained in the engorged gland. How- 

 ever, after about 3 weeks, the milk pressure 

 in the gland seemed to damage the secretory 

 epithelium, resulting in its involution. Cross 

 and Silver (1956) and Cross (1957) ligated 

 the teat duct in rats, resulting in a maximal 

 engorgement of the duct after 24 hours. At 

 this stage, if the ligatures were loosened, 

 milk ejection could be elicited by intra- 

 venously injected oxytocin. After 40 hours 

 of such ligation, no response to intravenous 

 oxytocin was observed, and the mammary 

 gland became whitish (instead of pinkish). 



