THE CONTROL OF NEUROSECRETION AND DIAPAUSE BY PHYSIOLOGICAL CHANGES IN THE BRAIN OF THE CECROPIA SILKWORM

Abstract

1. The 26 neurosecretory cells in the brain of the Cecropia silkworm have been shown by Williams to undergo systematic changes in endocrine activity during metamorphosis. At the pupal molt the neurosecretory cells lose endocrine activity. Consequently, growth is checked and the animal enters pupal diapause. Endocrine activity is restored following prolonged exposure to low temperatures. When the brain hormone is released, diapause ends and the development of the adult moth begins.2. The electrical events in the brain as a whole have been studied in relation to these changes in endocrine activity. The larval brain shows abundant "spontaneous" electrical activity. However, on the day before the pupal molt the brain loses spontaneous activity and is no longer excited by direct electrical stimulation. The electrical silence of the brain persists throughout the months of diapause. Electrical activity reappears in the pupal brain a few days before the end of diapause.3. Cholinesterase (ChE) is present in high titer in the larval brain. However, two days before pupation the brain ChE begins a rapid decline, so at pupation ChE is undetectable in the brain. ChE does not reappear in the pupal brain until shortly before the recovery of electrical and endocrine activities. There is no evidence that an inhibitor of ChE is present in the diapausing brain. Evidently, ChE is absent or in low titer and is synthesized just before the onset of adult development.4. During diapause the cholinergic action of brain extracts steadily increases. The increase is slow in the brains of pupae stored at 25° C. and rapid in the brains of pupae stored at 15° or 6° C. Therefore the rate of accumulation of the cholinergic substance parallels the effectiveness of the temperature treatment in restoring the brain to endocrine activity.5. When ChE reappears in the brain, just before the onset of adult development, the titer of cholinergic substance in the brain falls to a low level. The low level is maintained until shortly before adult emergence, when the titer of cholinergic substance increases five-fold.6. Choline acetylase activity remains uniform in brain homogenates from unchilled and chilled pupae.7. The changes in electrical activity, ChE, and cholinergic substance during metamorphosis are peculiar to the brain. The ganglia of the ventral nerve cord retain normal function throughout diapause.8. ChE is synthesized in chilled pupal brains which are cultured at 25° C. in the absence of prothoracic glands. Under the same circumstances unchilled brains are unable to synthesize ChE. Moreover, exposure to the brain and the prothoracic gland hormones fails to promote the synthesis of ChE in unchilled pupal brains.9. ChE and electrical activity remain constant during the metamorphosis of Galleria, a non-diapausing insect. Two additional diapausing species, Telea polyphemus and Samia walkeri showed variations in brain ChE and electrical activity identical to those described in Cecropia.10. The evidence suggests that the pupal diapause of Cecropia results from the failure of the brain as a whole. The diapausing brain is electrically silent, apparently because of the absence of ChE. During diapause there is an accumulation of a cholinergic substance in the brain—an observation which suggests that the reappearance of ChE is elicited by high substrate concentrations. When ChE reappears, the brain becomes electrically active, the neurosecretory cells release brain hormone, and diapause is ended.