CENTRAL CONTROL OF PITUITARY SECRETION 



1 02 I 



control procedures for assessing hypophysial portal 

 vessel regeneration left the finding open to doubt, and 

 especially since several workers (17, 352, 355) found 

 that stalk section results in histological signs of thyroid 

 atrophy. More recently it has been observed (81) 

 that effective stalk section (with little or no portal 

 vessel regeneration) results in a 4-hr. thyroidal uptake 

 of I''" in the ferret of about one-third normal, althous;h 

 stalk section which was followed by obvious portal 

 vessel regeneration did not decrease the uptake below 

 normal. Less valid criteria for assessing the absolute 

 level of thyroid activity (48-hr. uptake of P'" and rate 

 of release of hormonal radioacti\ity from the thyroid) 

 gave much the same result in the rabbit (47). These 

 results indicate that effective stalk section results in a 

 reduction of the basal level of thyroid activity although 

 not to that seen in the hypophysectomized animal, 

 and that it is interruption of the portal vessels of the 

 stalk that is responsible for this effect. Now, since in 

 recent years it has become apparent that various 

 noxious stimuli inhibit the activity of the thyroid 

 gland (46), it became of interest to see the effect of 

 pituitary stalk section on this stress response. In a 

 study performed on rabbits Brown-Grant et al. (47) 

 found that the operation of stalk section with plate 

 insertion largely abolished the ih\roid inhibition to 

 restraint but had little effect on the inhibition follow- 

 ing laparotomy. In simply stalk-sectioned rabbits the 

 thyroid response to restraint was still present in 1 1 out 

 of 1 2 animals and these rabbits were found, post- 

 mortem, to possess regeneration of the portal vessels. 

 These findings are consistent with the hypothesis of 

 Fortier (105) regarding the dual control of ACTH 

 release. In the same work it was found that injection 

 of thyroxine still inhibits the secretion of TSH in the 

 absence of anatoinical connections between the 

 hypothalamus and pituitary, thus indicating that a 

 direct action of thyroid hormone on anterior pituitary 

 cells is involved in the feed-back mechanism (see 

 below). 



Transplants of pituitary tissue under the median 

 eminence of the tuber cinereum were found to main- 

 tain the thyroid in a normal state, as judged histo- 

 logically (161). In these cases the pituitary grafts were 

 vascularized by the hypophysial portal system (see 

 fig. 5), but in control animals in which the grafts 

 were placed in the subarachnoid space, outside the 

 range of the portal vessels, thyroid atrophy occurred. 

 This latter finding has been the usual experience of 

 workers who have studied the thyrotrophic activity of 

 pituitary tissue transplanted to a site remote from the 

 sella turcica. Schweizer & Long (307) found intra- 



ocular transplants of the anterior pituitary maintained 

 some thyrotrophic activity in a few of their animals 

 i)ut at a greatly reduced level. Greer el al. (134) ob- 

 .served that pituitary transplants in hypophysec- 

 tomized mice did not maintain the weight of the 

 thyroid above that of Inpophysectomized controls but 

 that the uptake of I''" per unit thyroid weight and the 

 thyroid-serum iodide ratio was two thirds the level of 

 the intact controls, von Euler & Holmgren (345), 

 working on hypophysectomized rabbits bearing 

 pituitary transplants, found thyroid activity was re- 

 duced, although it was higher than in hvpophysec- 

 tomized control animals. They found further that the 

 thyrotrophic activity of the pituitary transplants was 

 no longer modified by exposure to cold, or by anes- 

 thesia, but was still inhibited l)y injection of thyroxine. 

 These workers confirmed the finding that thvroxine 

 may exert an effect directly on the pituitary s;land by 

 injecting minute amounts of thyroxine into the gland 

 in normal rabbits (344). 



In summary, it may be said that the anterior 

 pituitary gland, disconnected from the central nervous 

 system, still maintains a residual secretion of TSH 

 and that this secretion may be inhibited by a raised 

 blood level of thyroxine or by stress stimuli which in- 

 volve tissue damage or cause a raised blood level of 

 adrenal steroids. The effects exerted by the nervous 

 system over TSH secretion would appear to be to 

 maintain the normal rate of .secretion and to modify 

 this rate in response to stimuli actinsj through the 

 central nervous system. 



Lactogenic Hormone Secretion After Pituitary Stalk Sec- 

 tion or Transplantation. The nervous system may be 

 related to lactation in two ways. First, it is well 

 established now that the stimulus of suckling, and the 

 conditioned reflexes that may be associated with this 

 process, evokes nervous reflex release of oxytocic 

 hormone and that this hormone causes contraction of 

 the myoepithelial cells of mammary tissue and so a 

 positive milk ejection from the breast to the young. 

 Second, as first suggested by Selye (309), the nervous 

 stimulation of suckling may be of importance in main- 

 taining secretion of the lactogenic hormone and 

 thereby milk secretion. Both the neurohypophysis and 

 adenohypophysis may therefore exert an individual 

 and specific action on mammary tissue. It is possible 

 that the two lobes of the pituitary are interrelated in 

 lactation in yet a further way since Benson & Folley 

 (25) have found that the mammary glands of lac- 

 tating female rats, from which the litters had been 

 removed, were maintained in a more active state if 

 repeated injections of oxytocin were given. Oxytocin 



