450 



WILLIAM G. VAN DER ICLOOT 



fascinating t5 knjw the source and nature of the influence which can so 

 effc ti-.ely and selectively inhibit the neurons of the brain. 



It will be remembered that bw temperatures act to promote the return of 

 chclinesterase to the brain. During diapause there is an accumulation of a 

 cholinergic substance (probably acetylch ')line) in the brain. The accumulation 

 is gradual at room temperature, rapid at bw temperatures. The chemical, 

 electrical and endocrine changes in the brain during diapause are summarized 

 in Fig. 2. It was proposed that the cholinergic substance accumulates until 

 a critical level is reached and the reappearance of cholinesterase is evoked. This 

 idea is being tested, and the data so far suggest that it is right, but not enough 

 experiments ha\e been completed t3 be sure. The results already available 

 show that ch':^linesterase returns t3 chilled brains implanted into isolated 

 pupal abdomens, which contain no known endocrine organ. This fact argues 

 that the changes leading to t'le reappearance of chclinesterase take place 

 within the cells of the brain and are not triggered from without. 



Fig. 2. A diagrammatic summary of the physiology of the Cecmpia brain during 

 metamorphosis. The time scale during diapause is, of course, greatly condensed. 



For explanation see text. , release of brain hormone; , electrical 



activity of the brain; , cholinesterase in the brain; — • — , titer of cholin- 

 ergic substance in the brain. (Reprinted from the Biol. Bull.) 



To summarize, the neurosecretory cells of the Cecropia silkworm are inhi- 

 bited; this is the cause of the pupal diapause. The inhibition is achieved by an 

 enzymatic change in the brain neurons which may persist for months. Inhibi- 

 tion of this type must be exceptional, because few animals can afford to shut 

 down a considerable portion of the central nervous system for weeks on end. 

 For the silkworm, however, the method guarantees the safe passage of winter 

 in a developmental stage where there seems to be little need for a brain. 



INHIBITORY NEURONS IN NEUROENDOCRINE SYSTEMS 

 Viviparous Insects 

 In species in which the females hold fertilized eggs in a uterus or in an 



