Ecdysone synthesis and release by the brain-ring gland complex of blowfly larvae

Abstract

Brain-ring gland complexes dissected from larvae of the blowfly Calliphora vicina secrete ecdysteroids when incubated in vitro. The secretion is linear over several hours. By HPLC it was shown that the major secretory product is ecdysone. Secretion is due to biosynthesis and not to release of stored ecdysone. The rate of ecdysone secretion appears to depend on the physiological age of the donor larvae: while all brain-ring gland complexes isolated from larvae of different ages in the second half of 3rd instar secreted ecdysone, the highest rates were observed around pupariation. By contrast, isolated ring glands without the brain secreted ecdysone only when explanted from larvae ready for pupariation. Thus the overall effect of the brain appears to be stimulating. Synthesis of ecdysone leading to the so-called commitment peak(s) in mid-instar larvae is likely to be under different control than the synthesis of ecdysone initiating pupariation. By addition of pharmacological agents the secretion of ecdysone from brain-ring gland complexes can be modulated in vitro. The observed effects may be classified into three groups: (1) increase of ecdysone release: caused by atropine, picrotoxin, β-endorphin, and activators of protein kinase C; (2) no influence: observed with tetrodotoxin, aldrin, dieldrin, DDT, insulin, and crustacean moult-inhibiting hormone; (3) decrease of ecdysone release: caused by GABA, eserine, cobalt and magnesium ions, TMB-8, nifedipin, and azadirachtin. So far we detected only one substance which decouples ecdysone biosynthesis from its release: the limonoid azadirachtin inhibits secretion of ecdysone but apparently is without effect on its biosynthesis. The neurotransmitter GABA caused a decrease and its antagonist picrotoxin an increase of ecsysone secretion. Thus GABA is likely to be involved in the control of ecdysone biosynthesis and release. Other experiments indicate that the neurotransmitter acetylcholine is also inhibitory. The observed effects of neuroactive compounds demonstrate the existence and importance of the neuronal component in the complex control of ecdysone secretion.