NEUROSECRETION 



1053 



neurosecretory material and in the posterior lobe 

 hormones suggests a far closer relationship than 

 simply that of a carrier substance, b) it is not clear 

 why the carrier substance is released into the blood 

 stream also, and c) it is not clear why the carrier 

 substance should be subsequently demonstrable as 

 posterior lobe hormone (340). These questions and 

 the histochemical results of Sloper become clearer on 

 the supposition of an identity between the neurosecre- 

 tory material and the posterior lobe hormones. See 

 also the more recent discussion by Acher (i). 



There can no longer be any doubt that neurosecre- 

 tory material and the posterior lobe hormones are 

 produced by nerve cells in the hypothalamic nuclei 

 and that these substances are transported along the 

 axons in fiber tracts passing to the posterior lobe of the 

 pituitary. It is not clear why certain species do not 

 store neurosecretory material in the hypothalamic 

 nuclei and why specific segments of the neurosecretory 

 pathways are completely devoid of this material. This 

 latter ob.servation has led several investigators to 

 deny the axonal transport of neurosecretory material 

 (84, 304). This concept has also been challenged on 

 the basis that for simple physical reasons the axon 

 would be unable to move or press such material along 

 its length. The dynamic fluidity of cytoplasm as 

 demonstrated by photography of tissue culture prep- 

 arations contradicts such purely physical objections. 



Estimation of the status of neurosecretory activity 

 cannot be made solely from a knowledge of the 

 amount of neurosecretory material present. Subnor- 

 mal amounts of neurosecretory material may represent 

 either a condition of lowered functional activity or 

 accelerated release of neurosecretory material asso- 

 ciated with hyperactivity. Hyperactivity is indicated 

 clearly when a decreased content of neurosecretory 

 material is associated with cytological evidence of 

 increased cellular activity. 



Only meager information is currently available 

 with respect to the electrical activity in neurosecretory 

 fibers. Several considerations indicate that such fibers 

 should be capable of impulse conduction and possess 

 excitatory capacity. Injection of iiypertonic saline 

 into the carotid artery is followed, after a very short 

 latent period, by release of antidiuretic substance into 

 the blood stream (184, 332) with simultaneous in- 

 creased electrical activity in the supraoptic nucleus 

 (228). Furthermore, the release of iiormones from the 

 posterior lobe is dependent upon the continuity of 

 fibers in the stalk. The conductional capacits' of 

 neurosecretorv fibers has been demonstrated in verte- 



TABLE 2. Content of Posterior Lobe Hormones in the Hypo- 

 thalamus Expressed as Per Cent of the Total Amount Pres- 

 ent in the Corresponding Pituitary Gland 



Posterior Lobe Hormones 



* From Hild & Zetler (170). 

 t From Adamsons et at. (6). 



TABLE 3. Studies Correlating the Alteration in Neurose- 

 cretory Material and Posterior Lobe Hormones Under \'ar- 

 ious Experimental Conditions 



* Numbers indicate text references. 



brates by Potter & Lowenstein (258) and in inverte- 

 brates by Mil burn [quoted by Bliss (49)]. 



The exact mechanism of release of posterior lobe 

 hormones, as well as the role of the pituicytes, awaits 

 further investigation. Research in neurosecretion has 

 led to a fundamentally new interpretation of the 

 function of the posterior lobe, namely that it serves 

 only as a depot or reservoir for the posterior lobe 

 hormones (165, 237). Hormones are released from 

 this depot according to the needs of the organism. 

 The release or depletion of hormones from this depot, 

 requiring intact nerve connections (165), occurs 

 considerably more rapidly than its reaccumulation 

 (17, 237, 317). Although it is fairly certain liiat the 

 pituicytes do not synthesize the posterior lobe hor- 

 mones, it seems clearthat they are concerned somehow 

 in the mechanism of release (46, 124, 165, 179, 189, 



