CHOLINESTERASE IN THE BRAIN OF THE CECROPIA SILKMOTH DURING METAMORPHOSIS AND PUPAL DIAPAUSE

¹ Department of Zoology, The University of Michigan, Ann Arbor, Michigan 48104

1. The localization and properties of cholinesterase in the brain of the Cecropia silkmoth were investigated by histochemical and quantitative spectrophotometric methods utilizing acetylthiocholine as substrate.

2. During pupal diapause, substantial activity was visualized in neuropile. At the outset of adult development, activity was also detected in adjacent regions occupied by neuronal or glial cell bodies.

3. Only one form of cholinesterase was detected with certainty. On the basis of substrate-activity relationships for acetylthiocholine, propionylthiocholine and butyrylthiocholine, and on the basis of its sensitivity to certain selective esterase inhibitors, the enzyme has properties of an acetylcholinesterase.

4. Substantial and essentially unchanging enzymatic activity was detected during pupation and most of pupal diapause, when the brain becomes endocrinologically inactive. However, an approximate doubling in activity was detected during storage of diapausing pupae at 6° C., apparently signalling the recovery of neuroendocrine competency by the brain. Subsequent growth and morphogenesis of adult brain were found to be accompanied by a six-fold further increase in activity.

5. Cholinesterase activity also persists during diapause in the Cynthia and Polyphemus silkmoths.

6. Consideration of the properties and optimal assay conditions for this enzyme in pupal brain assists in explaining previous reports that it was undetectable.

7. The presence of substantial cholinesterase activity throughout metamorphosis shows that a generalized disappearance and reappearance of the enzyme cannot be responsible for inactivation and reactivation of the neurosecretory mechanism that controls the onset and termination of diapause.

8. In the light of evidence for multiple forms of esterase in silkmoth brain, the present findings do not preclude a possible role for one or more esterases as part of the physiological mechanism controlling neurosecretion and diapause.