PARENTAL BEHAVIOR 



1341 



B. HORMONE SECRETION AS A BEHAVIORAL 

 RESPONSE 



At various points in our discussion, it has 

 become apparent that external stimuli of 

 various kinds, including some produced by 

 other members of the species, may be in- 

 fluential in eliciting the secretion of various 

 hormones. It may be helpful if we briefly 

 discuss the physiologic basis of such re- 



1. Xeural Control of Hormone Secretion 



Both the anterior lobe and the posterior 

 lobe of the pituitary gland are connected 

 to the hypothalamus, althougli in quite 

 different ways. 



It will be recalled that electrical stimula- 

 tion of the anterior hypothalamus (An- 

 dersson, 1951b), or of the pituitary stalk 

 (Cross and Harris, 1950), induces milk 

 ejection due to the secretion of oxytocin by 

 the posterior pituitary. The principal func- 

 tional connection between the brain and the 

 posterior lobe is by way of the hypothala- 

 mohypophyseal nerve tracts in the pituitary 

 stalk (Green, 1951a). (The secretory cells 

 of the neurohypophysis are themselves neu- 

 ral in embryonic origin.) Harris (1947b) 

 implanted electrodes with their tips in 

 various locations in the hypothalamus, pi- 

 tuitary stalk, and neurohypophysis; the 

 electrodes were connected to a coil im- 

 bedded in the skull, so that they could be 

 activated by holding a second coil near the 

 animal's head without actually touching 

 the animal. Harris was able to produce an 

 antidiuretic effect in water-loaded animals 

 by stimulating the neurohypophysis, the 

 supra-opticohypophyseal tract of the hypo- 

 thalamus, or any part of the intervening 

 nerve pathway. The effects of such electri- 

 cal stimulation could be duplicated by the 

 injection of posterior pituitary extracts. 

 This and other evidence (Harris, 1955) 

 indicates that the secretory activity of the 

 posterior lobe of the pituitary gland is 

 under neural control, and that the nerve 

 connection between the hypothalamus and 

 the neurohypophysis plays an essential role 

 in the regulation of neurohypophyseal ac- 

 tivity. 



In contrast to the situation in the pos- 



terior hyi)ophysis, the anterior lobe is 

 very sparsely innervated, both in mammals 

 (Rasmussen, 1938; Green, 1951b) and in 

 birds (Wingstrand, 1951). On the basis of 

 a thorough survey of the evidence, Harris 

 (1955) concludes that there is no evidence 

 of a secretomotor innervation of the secre- 

 tory cells of the anterior hypophysis. 



The principal connection between the hy- 

 pothalamus and the anterior hypophysis 

 appears to be by a portal blood system. In 

 mammals, small branches of the internal 

 carotid arteries form a plexus at the base 

 of the pituitary stalk, from which capillary 

 loops arise and penetrate into the tissue of 

 the median eminence, where they come into 

 intimate relationship with the nerve fibers 

 of various hypothalamic tracts (Harris, 

 1955). The vessels of this plexus merge to 

 form i^ortal vessels which lie on the surface 

 of the pituitary stalk. Lower down on the 

 stalk, these trunks divide again to dis- 

 tribute their blood to the cells of the an- 

 terior hypophysis. The arrangement in birds 

 is similar, except that the capillary network 

 lies on the surface of the median eminence, 

 and its relationship with the nerve fibers 

 of the hypothalamus is accomplished by 

 the looping of nerve fibers from deeper- 

 lying cells to the surface and back (Benoit 

 and Assenmacher, 1955). 



Electrical stimulation applied directly to 

 the anterior hypophysis of the female rabbit 

 is ineffective in inducing ovulation (equiva- 

 lent to stimulation of LH secretion) , whereas 

 stimulation of tlie hypothalamus does in- 

 duce this response (Markee, Sawyer, and 

 Hollinshead, 1946; Harris, 1948). Since di- 

 rect stimulation of the pituitary stalk is 

 also ineffective, it seems that the effect of 

 electrically stimulating the hypothalamus 

 is to cause the transmission to the anterior 

 pituitary gland of an excitatory effect by 

 way of structures which are not themselves 

 sensitive to electrical stimulation. (This, 

 it will be seen, is in contrast to the neural 

 excitation of the posterior pituitary.) Harris 

 (1947a) and Green and Harris (1947) sug- 

 gested that hypothalamic control of an- 

 terior pituitary secretion is accomplished 

 through neurosecretory products of hyjio- 

 tlialamic cells, which are carried by tlie 

 portal system fi'om the liyjiothalanuis down 



