1330 



HORMONAL REGULATION OF BEHAVIOR 



proximately the same level of secretory tis- 

 sue (48 per cent of section area) as did 

 oxytocin injection. Oxytocin injected into 

 hypophysectomized females had no effect 

 on the mammary secretory tissue. It should 

 be pointed out that prolactin has a local 

 lactogenic effect when introduced directly 

 into the milk-secreting ducts, whereas oxy- 

 tocin, as well as other anterior and posterior 

 principles, has no such effect. McCann, 

 Mack and Gale (1959) found that hypo- 

 thalamic lesions in the supra-opticohypo- 

 physeal tracts of lactating rats prevented 

 milk ejection and milk secretion. The loss 

 of milk secretion was noted even when the 

 anterior hypophysis and its portal vessels, 

 as well as the median eminence, all ap- 

 peared histologically normal. Oxytocin in- 

 jected into such animals delayed the invo- 

 lution of the mammary glands. Similarly, 

 Donovan and van der Werff ten Bosch 

 (1957) found that destruction of the pitu- 

 itary portal blood vessels did not result in 

 loss of lactation when oxytocin was ad- 

 ministered regularly after the operation, 

 and Desclin (1956a, b) found that oxytocin 

 injected into virgin rats made possible the 

 development of deciduomas in response to 

 irritation of the uterine wall. 



It must be concluded that oxytocin is 

 capable of acting as a neurohumor stimu- 

 lating the secretion of a lactogenic hormone 

 by the pituitary gland. 



Tverskoy (1953) milked a goat by inject- 

 ing oxytocin, then draining the cisterns di- 

 rectly with catheters. The milk yield con- 

 tinued normal, indicating that external 

 stimuli to the nipples were not necessary. 

 When he arranged the catheters for con- 

 tinuous drainage (without oxytocin injec- 

 tion) , the milk secretion declined. When the 

 catheters were arranged for continuous 

 drainage, but oxytocin was nevertheless ad- 

 ministered twice a day, the milk secretion 

 increased. Tverskoy interpreted this as in- 

 dicating that the rise in alveolar pressure, 

 caused by the oxytocin injection, was a 

 source of afferent sensory inflow which was 

 the stimulus for the reflex secretion of pro- 

 lactin. This is an ingenious interpretation, 

 but the simpler conclusion by Benson and 

 Folley from their experiment, namely that 



oxytocin acts directly as a neurohumor to 

 induce the secretion of prolactin, could ap- 

 ply to this experiment as well. 



Cross (1960) points out that other effects 

 of suckling stimulation than the specifically 

 endocrine ones may be related to the main- 

 tenance of milk secretion. Suckling induces 

 increased eating and weight gains in mother 

 rats, even when milk withdrawal is pre- 

 vented by cutting the milk ducts (Cotes 

 and Cross, 1954), and this effect cannot be 

 duplicated by prolactin injection. Nursing 

 mother rats consume more food than non- 

 nursing rats (Slonaker, 1925) (although 

 Bateman (1957) reports that lactating mice 

 spend no more time feeding than do non- 

 lactating ones) . 



Such effects might contribute to the 

 amount of milk secretion without reference 

 to the stimulation of pituitary hormone se- 

 cretion. Nevertheless, it is clear that the 

 stimulation provided by suckling causes the 

 release of lactogenic hormones from the pi- 

 tuitary gland, and it now seems likely that 

 this effect is not an effect separate from the 

 stimulation of oxytocin secretion, but rather 

 that the oxytocin secreted by the posterior 

 pituitary as a result of suckling is, at least 

 to some degree, involved in the stimulation 

 of prolactin secretion by the anterior pitu- 

 itary.^ 



4. Xursing Behavior and the Condition of 

 the Mammary Gland. 



Cross (1952) found that rabbits, which 

 normally nurse the young once a day, could 

 not be induced to nurse more often unless 

 loss of milk was prevented by sealing the 

 teats with collodion. He concluded that 

 mammary distension was a factor in the 

 motivation of nursing. Such a conclusion 

 seems reasonable, and there is considerable 

 evidence in its favor. Many observers have 

 noted that the frequency with which young 

 are nursed is in part a function of the stage 

 of lactation, and thus of the amount of milk 

 being produced, although it is not clear 



'Rothchild and Quilligan (1960) have failed to 

 verify either the induction of pseudopregnancy or 

 the formation of deciduomas by oxytocin injection 

 in the rat. They therefore independently suggested 

 the same explanation as had Tverskoy for the 

 type of results found bj- Benson and Folley. 



