γ-amino butyric acid and glutamic acid in Manduca sexta: Proposed roles in insect development

Abstract

1. In the tobacco hornworm, Manduca sexta, the haemolymph titres of γ-amino butyruc acid (GABA) and glutamic acid range from 0.03–0.40 × 10 −3 M and 0.1–7.8 × 10 −3 M, respectively, during the last larval instar. 2. Glutamic acid in hemolymph does not appear to be derived from haemocytes. 3. Increases in GABA and glutamate haemolymph titres occur sequentially and independently of changes in the titres of ecdysone and juvenile hormone. 4. A single injection of GABA sufficient to initially raise the haemolymph titre to 4 mM on each day of the larval-pupal transformation causes a 12–18 hr delay in the time of the larval-pupal ecdysis. 5. A single injection of glutamate sufficient to initially raise the haemolymph titre by 2 mM on each day of the larval-pupal transformation was without apparent effect on co-ordinated body movements or on the time of the larval-pupil ecdysis. 6. When GABAase is injected into hornworms whose GABA titres are rising (pink stripe + 1 day, through pink stripe + 3 days), haemolymph GABA titres decline, rates of haemolymph GABA turnover increase and the time of the larval-pupil ecdysis is advanced significantly. 7. When hornworms are administered GABAase at the pink stripe stage in development, a time when the titre of haemolymph GABA is minimal, the time of ecdysis is normal. 8. A possible role for GABA during the larval-pupal transformation may be in preventing premature initiation of pre-ecdysial behavior, while a proposed role for glutamic acid is to potentiate those coordinated neuro-muscular activities necessary for successful ecdysis which had been inhibited by GABA.