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 bee n reported throughout most of larval-adult development in Rhodnius pro lixus. PTTH released by explanted brain-retrocerebral complexes was qu antified using an in vitro bioassay in which the PTTH released into th e incubation medium was assayed by its ability to stimulate ecdysteroi d synthesis in arrhythmic prothoracic glands (PGs). The present articl e employs this assay to reveal that the daily rhythm of PTTH release i s under circadian control. The rhythm free-runs in both continuous dar kness (DD) and continuous light (LL) with a period length close to 24 hr, which is temperature compensated. The rhythm appears to damp out m ore rapidly in LL than in DD. It is argued that the circadian clock re gulating PTTH release is in the brain and is entrained by extraretinal photoreception. It is suggested that this ''PTTH clock'' is coupled i n vivo to the clock previously described in the PGs that regulates rhy thmic 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 compris e a multioscillator system that drives the rhythms in ecdysteroid synt hesis and the hemolymph ecdysteroid titer and consequently imposes tem poral order on ecdysteroid-dependent developmental events. (C) 1996 Ac ademic Press, Inc.