630 



PHYSIOLOGY OF GONADS 



prolactin. They had previously shown that 

 there was an immediate fall in the pituitary 

 content of prolactin after nursing (Gros- 

 vcnor and Turner, 1957b) and therefore 

 concluded that their findings were contrary 

 to the hypothesis that oxytocin is a hor- 

 monal link in the discharge of prolactin. 

 This, however, cannot be regarded as con- 

 clusive because of the difficulties of relating 

 pituitary content of a hormone to blood 

 levels of the hormone and also the difficulty 

 of determining the physiologic dose of oxy- 

 tocin, for if the oxytocin is carried directly 

 from the neurohypophysis into the anterior 

 lobe, then the concentration in the blood 

 reaching the anterior lobe may be relatively 

 great (see also Cowie and Folley, 1957). 



Other theories of the reflex maintenance 

 of milk secretion have been put forward. In 

 1953 Tverskoi, observing that repeated in- 

 jections of oxytocin were galactopoietic in 

 the goat, suggested that alveolar contraction 

 stimulated sensory nerve endings in the 

 alveolar walls which reflcxly caused the re- 

 lease of prolactin. It is obvious that his 

 observations could be explained on the basis 

 of the Benson-Folley theory of direct pitui- 

 tary stimulation by oxytocin. This possi- 

 bility was indeed considered by Tverskoi. 

 but rejected on the grounds that oxytocin 

 did not affect the prolactin content of the 

 pituitary (Meites and Turner, 1948). In 

 1957 Tverskoi found it necessary to revise 

 his theory, having found that full lactation 

 could be maintained in the goat after com- 

 plete and repeated denervation of the udder 

 provided oxytocin was regularly given to 

 evoke milk ejection. He then suggested that 

 alveolar contraction stimulates the syn- 

 thetic activities of the mammary epithelium 

 causing an uptake of prolactin from the 

 blood, the fall in the blood prolactin level 

 then stimulating the further production of 

 prolactin by the anterior lobe. Although 

 these latter observations of Tverskoi might 

 again be explained on the basis of direct 

 pituitary stimulation by exogenous oxy- 

 tocin, more recent studies on goats have 

 cast doubts on the validity of such an ex- 

 planation. Tverskoi (1958) and Denannir 

 and Martinet (1959a, b, 1960) have shown 

 that lactating goats will continue to lactate, 

 giving nonnal or onlv niodcratelv reduced 



milk yields after section of all nervous con- 

 nections between the udder and brain (cord 

 section, radicotomy, bilateral sympathec- 

 tomy) and without their receiving oxytocin 

 and in the absence of conditioned milk- 

 ejection reflexes. It has already been noted 

 that milk ejection in such animals may re- 

 sult from mechanical stimulation of the 

 myoepithelial cells by udder massage (see 

 page 624) , but the release of the galacto- 

 poietic complex from the anterior pituitary 

 would seem in these goats to have been in- 

 dependent of neurohormonal reflex activi- 

 ties. AVhether in such animals the release is 

 spontaneous or dependent on the level of 

 hormones in the blood as suggested by 

 Tverskoi (1957) is a matter for further re- 

 search. 



VI. Pharmacologic Blockade of the Re- 

 flexes Concerned in the Maintenance 

 of Milk Secretion and Milk Ejection 



Various attempts have been made to 

 investigate the mechanism controlling re- 

 lease of anterior pituitary hormones by the 

 use of dibenamine, atropine, and other 

 drugs. In reviewing such experiments, Harris 

 (1955) concluded that there was no con- 

 vincing evidence of the participation of 

 adrenergic, cholinergic, or histaminergic 

 agents in the control of gonadotrophic and 

 adrenocorticotrophic hormone release. Re- 

 cently Grosvenor and Turner (1957a) re- 

 ported that various ergot alkaloids, diben- 

 amine, and atropine blocked milk ejection 

 in the rat; the ergot alkaloids doing so 

 within 10 minutes of administration, the 

 atropine and dibenamine within 2 to 4 hours. 

 Inasmuch as milk ejection occurred in re- 

 sponse to exogenous oxytocin, it was con- 

 cluded that these drugs acted centrally, and 

 the presence of adrenergic and cholinergic 

 links in the neurohormone arc was postu- 

 lated to be responsible for the discharge of 

 oxytocin. Later, on tlie basis of assays of 

 jntuitary prolactin after nursing in drug- 

 injected lactating rats, it was suggested 

 that cholinergic and adrenergic links are 

 iinohcd in the reflex resi)onsible for pro- 

 lactin release (Grosvenor and Turner, 

 1958a). Ergot alkaloids, however, adminis- 

 tered in our laboratory to lactating rats had 

 no significant effect on the lactational per- 



