Inhibition of Manduca sexta corpora allata in vitro by a cerebral allatostatic neuropeptide.

When an in vitro assay system and radioimmunoassays specific for juvenile hormones (JH) I and III were used to probe the effect of day 4 last instar larval brains on JH synthesis by day 0 last instar larval corpora allata (CA) of the tobacco hornworm, Manduca sexta, a selective inhibition of JH I synthesis by the CA was observed. The nature of this inhibition suggested the presence of an allatostatin specific for the synthesis of JH I. Its occurrence in the day 4 brain was demonstrated by the ability of a crude brain extract to inhibit the CA in a dose-dependent manner. The allatostatic factor (ASF) appears to be a protein, based on its heat lability and pronase sensitivity, and it has apparent molecular weights of 6.8 and 13 kDa. Inhibition of JH I synthesis occurs within 1 min of exposure of the CA to the factor and is reversible by 6 h after this exposure. Thus it appears that a cerebral neuropeptide specifically inhibiting JH I synthesis by the CA is present in Manduca on day 4 of the last larval instar, a time when the hemolymph titer of JH must drop to ensure the occurrence of pupal commitment.