PARENTAL BEHAVIOR 



1329 



Histologic examination showed that the 

 capillary bed was collapsed and almost 

 without blood cells. However, oxytocin di- 

 rectly applied to the alveoli still caused 

 local contraction. Therefore, the engorge- 

 ment had resulted in a failure of blood cir- 

 culation of the mammary glands. It seems 

 likely that obstruction of circulation in the 

 mammary gland caused by engorgement by 

 nonwithdrawn milk is adequate to explain 

 the failure of suckling stimulation in some 

 animals so treated to maintain milk secre- 

 tion. It should be noted that the guinea pig 

 produces relatively little milk, and does not 

 have large cisterns for the storage of milk. 

 Therefore, it might be expected that local 

 engorgement would have more serious ef- 

 fects in such a species than in some others. 



The mechanism of suckling-induced lac- 

 tation. The material just presented, plus 

 the fact that suckling stimulation induces 

 pseudopregnancy (Selye and McKeown, 

 1934b) and prolongs gestation (Weichert, 

 1939), clearly indicates that suckling stim- 

 ulation induces the secretion of prolactin. 

 We may now inquire into the mechanism of 

 this effect. 



Herold (1939) reported that section of 

 the pituitary stalk in lactating rats was 

 followed by the gradual cessation of milk 

 secretion and involution of the mammary 

 gland, although the young continued at- 

 temps at suckling. Jacobsohn and Westman 

 (1945) and Jacobsohn (1949) sectioned the 

 pituitary stalk in rats and in rabbits, and, 

 like Herold, found that all the young died. 

 However, the involution of the mammary 

 gland was only partial, some of the alveoli 

 in each animal remaining secretory. This is 

 in contrast to the result of forcible weaning 

 of the young or of hypophysectomy, both 

 of which treatments result in cessation of 

 milk secretion and complete involution of 

 the mammary gland. These experiments 

 seem to imply the continuation, after stalk 

 section, of some secretion of lactation-stim- 

 ulating hormone (s) by the pituitary gland. 

 Desclin (1940) found that the young of 13 

 out of 22 lactating rats whose pituitary 

 stalks were transected 5 to 6 days post- 

 partum survived, although theii- weight was 

 abnormally low. Everett (1954, 1956) found 

 tiiat pituitary glands transplanted to the 

 renal capsule continued to seercte prolactin, 



and he suggested that neural connections 

 are unnecessary for the maintenance of the 

 secretion of this hormone, and that possibly 

 the neural effect is inhibition under appro- 

 priate stimulation (Nikitovitch-Winer and 

 Everett, 1958) . This is consistent with the 

 results of Jacobsohn's observations, except 

 that the amount of prolactin apparently 

 secreted when the stalk is transected in lac- 

 tating rats is small compared to the amount 

 produced under suckling stimulation. Roth- 

 child (1960a) showed that rat pituitaries 

 autotransplanted to the renal capsule can 

 secrete enough prolactin to maintain lac- 

 tation, again at a lower level than in intact 

 lactating animals. He also showed that 

 suckling stimuli cause a reduction of gona- 

 dotrojihin secretion, and that this effect is 

 probably independent of the effect of such 

 stimuli on prolactin secretion.^ 



Although the experiments cited thus far 

 make it clear that both milk ejection and 

 lactation occur as a response to suckling 

 stimulation, it is not so clear whether both 

 phenomena are primary effects of suckling 

 stimulation (Folley, 1947). Petersen (1948) 

 suggested that suckling might stimulate 

 lactation by inducing the secretion of a 

 posterior pituitary hormone which in turn 

 might stimulate the secretion of the lacto- 

 genic hormone (s). Benson and Folley (1956, 

 1957) have provided evidence that oxyto- 

 cin, the secretion of which is undoubtedly 

 stimulated by suckling (see above), itself 

 stimulates the secretion of prolactin. They 

 removed rat litters at 4 days postpartum, 

 when the treatment of the mothers began. 

 (Oxytocin was injected, at various dosage 

 levels, for 9 days, at the end of which time 

 the oxytocin-treated animals had more se- 

 cretory tissue in their mammary glands (40 

 to 46 per cent of the area of histologic sec- 

 tions) than did the controls (about 16 j^er 

 cent). The diameter of the alveoli was also 

 greater in the oxytocin-treated animals 

 (about 50 ijl) than in the controls (about 

 15 fi) . Injected prolactin maintained ap- 



•'' These experiments were described in a series 

 of important papers on the corpus luteum-pituitary 

 relationship, which appeared too late to be fully 

 considered in the preparation of this chapter 

 Rothchild, 1960b: Rothchild and Dickey, 1960; 

 Rothcliild and QuiUigan, I960: Quilhgan and 

 Rothchild, 1960). 



