1324 



HORMONAL REGULATION OF BEHAVIOR 



epithelial cells plays some role here. Linzell 

 (1955j found that oxytocin applied locally 

 in very small amounts induced local con- 

 traction of the alveoli. Acetylcholine also 

 produced local contraction. Although the 

 mammary glands of sheep and goats are 

 extensively innervated by sympathetic 

 nerve fibers (but seem to receive no para- 

 sympathetic innervation) (Linzell, 1959), 

 these fibers are vasomotor in their effect ; no 

 secretomotor fibers to the mammary gland 

 are known (Harris, 1958) and contraction 

 of the myo-epithelial cells is not induced 

 by stimulating the existing nerves. It there- 

 fore seems very likely that the effector end 

 of the milk-ejection reflex consists of the 

 direct action of oxytocin in the alveoli, 

 where it causes contraction by way of its 

 effect on the myo-epithelial cells of the al- 

 veolar walls. Further light may be cast on 

 this mechanism by a consideration of the 

 effects of emotional disturbance on the milk 

 ejection reflex. 



Emotional inhibition of milk ejection. 

 Ely and Petersen (1941) reported that 

 stimuli which seemed to frighten lactating 

 cows inhibited the milk-ejection reflex. 

 Whittlestone (1951, quoted by Whittle- 

 stone, 1954) found that electric shocks ad- 

 ministered to a nursing sow interfered with 

 milk ejection, and that the degree of in- 

 terference was roughly proportional to the 

 intensity of the shock. According to Cross 

 (1952, 1953), emotional disturbance in- 

 duced in lactating rabbits by electric shock, 

 unfamiliar handling procedures and sur- 

 roundings, forcible restraint during nursing, 

 etc., greatly reduced the amount of milk 

 got by the suckling pups. 



Braude and Mitchell (1952) found that 

 adrenaline, injected before oxytocin, inhibits 

 the occurrence of the milk-ejection response 

 which normally follows oxytocin injection. 

 Adrenaline also inhibits milk ejection in the 

 cow and in the rabbit (Ely and Petersen, 

 1941; Cross, 1953). This influence of 

 adrenaline is, of course, consistent with the 

 effects of emotional disturbance. 



How does emotional disturbance inhibit 

 milk ejection? There are three general pos- 

 sibilities, (a) Emotional excitation might 

 block or inhibit the secretion of oxytocin by 

 influencing the central neural mechanisms 

 for its release, (b) Adrenaline might, by 



causing constriction of the blood vessels in 

 the mammary gland, prevent oxytocin cir- 

 culating in the blood from reaching the 

 myoepithelial cells of the milk-secreting 

 tubules, (c) Adrenaline might inhibit milk 

 ejection by a direct effect on the myo-epi- 

 thelial cells. 



The last possibility (direct inhibition of 

 the myoepithelial cells by adrenaline) seems 

 unlikely. Linzell (1955) found that oxy- 

 tocin caused local contraction when applied 

 in extremely small amounts to a mouse 

 mammary gland exposed by dissection. 

 Acetylcholine and other parasympathomi- 

 metic drugs were also effective in producing 

 contraction on local application. On the 

 other hand, adrenaline, when applied locally, 

 caused local vasoconstriction in the capil- 

 laries and arterioles, but no constriction of 

 alveoli or myo-epithelial cells. Adrenaline 

 did not prevent the contraction of the al- 

 veoli when oxytocin was applied locally at 

 the same locus to which adrenaline had just 

 been applied, and at which vasoconstriction 

 was at a maximum. Further, Cross (1955a t 

 and Yokoyama (1956) found that mechani- 

 cal taps on a lobule of the mammary gland 

 of a rabbit produced a rise in intralobular 

 milk pressure, even though adrenaline had 

 just been injected, and when as a conse- 

 quence of the adrenaline injection, milk 

 ejection could not be elicited by a suckling 

 stimulus. It therefore seems that the reac- 

 tivity of the contractile elements them- 

 selves is not suppressed by adrenaline. 



It is likely, however, that adrenaline can 

 interfere with the effect of injected oxytocin 

 on the milk-ejection reflex. Pi'ior systemic 

 injection of adrenaline inhibits the milk- 

 ejection response to subsequent injections 

 of oxytocin in the rabbit (Cross, 1953) and 

 sow (Whittlestone, 1954). In this connec- 

 tion it should be noted again that no se- 

 cretomotor fibers are known in the inner- 

 vation of the mammary gland (Harris, 

 1958), that there is no parasympathetic in- 

 nervation to the gland tissue, and that the 

 sympathetic innervation seems to consist 

 largely of vasomotor fibers (Linzell, 1959). 

 These observations, together with the fail- 

 ure of local application of adrenaline to in- 

 hil)it contraction of the myo-epithelial cells, 

 suggest that the peripheral effect of adrena- 

 line on milk ejection is largely through its 



