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Circadian regulation of a daily rhythm of release of prothoracicotropic hormone from the brain retrocerebral complex of Rhodnius prolixus (hemiptera) during larval-adult development.

A daily rhythm of release of prothoracicotropic hormone (PTTH) has been reported throughout most of larval-adult development in Rhodnius prolixus. PTTH released by explanted brain-retrocerebral complexes was quantified using an in vitro bioassay in which the PTTH released into the incubation medium was assayed by its ability to stimulate ecdysteroid synthesis in arrhythmic prothoracic glands (PGs). The present article employs this assay to reveal that the daily rhythm of PTTH release is under circadian control. The rhythm free-runs in both continuous darkness (DD) and continuous light (LL) with a period length close to 24 hr, which is temperature compensated. The rhythm appears to damp out more rapidly in LL than in DD. It is argued that the circadian clock regulating PTTH release is in the brain and is entrained by extra-retinal photoreception. It is suggested that this "PTTH clock" is coupled in vivo to the clock previously described in the PGs that regulates rhythmic ecdysteroid synthesis by the circadian rhythm of release of PTTH. This coupling appears to be tight, since the rhythm of PTTH release retains close synchrony with the rhythm of ecdysteroid synthesis under both DD and LL. It is concluded that these two coupled clocks comprise a multioscillator system that drives the rhythms in ecdysteroid synthesis and the hemolymph ecdysteroid titer and consequently imposes temporal order on ecdysteroid-dependent developmental events.