Neuroendocrine control of diapause hormone secretion in the silkworm, Bombyx mori

Abstract

To clarify the control mechanism of diapause hormone (DH) secretion in the silkworm Bombyx mori a series of anatomical and pharmacological experiments were carried out. The arrangement of `diapause' and `non-diapause' eggs in the ovarioles of the moths was determined by the coloration method to estimate the accumulation of 3-hydroxykynurenine in the eggs. The females destined to lay non-diapause eggs (non-diapause producers) had diapause eggs in their ovaries if their subesophageal ganglions (Sg) had been surgically removed at 2 days after larval–pupal ecdysis or later. In contrast when the surgical extirpation extended to the brain and the corpora cardiaca (CC)–corpora allata (CA) complex in addition to the Sg, the non-diapause producers had no diapause eggs. When the Sg was removed from the females destined to lay diapause eggs (diapause-producers), diapause eggs appeared in response to the treatment at 2 days after larval–pupal ecdysis, but the appearance of diapause eggs was delayed by 2 days when the brain–CC–CA complex was included among the organs removed. These observations suggested that DH is produced in Sg and transferred to the CC–CA complex, and that the secretion of DH from the complex is suppressed in non-diapause producers. The pattern of diapause and non-diapause eggs induced by the transection of the subesophageal connective in diapause and non-diapause producers suggested a regenerative and secretory capacity of the neurosecretory cells after the operation. The appearance of diapause eggs in non-diapause producers with transected protocerebrum of the brain confirmed that there was an inhibitory center in the protocerebrum. Changes in parts of the ovarioles containing diapause and non-diapause eggs with time of injection of γ-aminobutyric acid (GABA) and picrotoxin suggested that a GABAergic inhibitory mechanism in DH secretion may be active in non-diapause producers but inactive in diapause producers throughout the pupal stage.