Rhythmic Steroidogenesis by the Prothoracic Glands of the Insect Rhodnius Prolixus in the Absence of Rhythmic Neuropeptide Input: Implications for the Role of Prothoracicotropic Hormone

Circadian rhythms have been reported both in the synthesis of the insect steroid moulting hormones (ecdysteroids) by the prothoracic glands (PGs) and in the release of the cerebral neuropeptide, prothoracicotropic hormone (PTTH). PTTH is known to activate steroidogenesis early in development, but the function of continued rhythmic release is unknown. The functional relationship between these two hormonal rhythms was examined. We report the properties of the rhythm of steroidogenesis by PGs of animals in which PTTH release was prevented by decapitation or by injection of a sublethal dose of tetrodotoxin. Rhythmic steroidogenesis by PGs was maintained in both cases; the rhythm retained entrainment to a light-dark cycle and free-ran in continuous light or darkness. It is inferred that rhythmic neuropeptide input is not required to drive rhythmic steroidogenesis and that in its absence, steroidogenesis becomes entrained by light cues. In both decapitated and paralyzed animals, the rhythm of steroidogenesis showed a reversal of phase from that of intact animals under all conditions of illumination tested. We infer that the rhythm of PTTH appears to entrain rhythmic steroidogenesis and entrainment by PTTH dominates entrainment by light in vivo. Similarities to other circadian systems are discussed, in which neurochemical agents entrain overt rhythms to a phase displaced by 12 hr from that for light. It is concluded that the function of PTTH is not confined to initial activation of steroidogenesis early in development, as previously thought, but continues throughout development as a central element in the circadian organization of the endocrine system that regulates development.