Circadian Organization of the Neuroendocrine System of an Adult Insect, Rhodnius Prolixus (STÄL) (HEMIPTERA)

Circadian clocks synchronize with external environmental cycles and regulate rhythms throughout the organism, creating an internal temporal organization of cellular and physiological processes. In the model insect Rhodnius prolixus the prothoracicotropic hormone (PTTH)-ecdysteroid axis is a central component of the larval circadian system. However, PTTH is considered a larval hormone and its only known target, the ecdysteroid-producing prothoracic glands, are absent in adults. Here, PTTH is demonstrated to be present in adult female Rhodnius and its synthesis and release during the period of egg development and oviposition were shown to fluctuate with a daily rhythm that is controlled by the circadian clock in the brain. Ecdysteroids are also present during this time and their levels in hemolymph and ovaries undergo synchronous daily variations that are likewise under clock control. Ovaries are the only adult tissue examined that both contained and released ecdysteroids. It is inferred that the ovaries generate the rhythm of ecdysteroids in the hemolymph. The parallel patterns of PTTH and ecdysteroid release suggest these processes are related and it is tempting to speculate that the PTTH-ecdysteroid axis persists in the adult leading to the orchestration of complex adult-specific processes, such as egg development and oviposition.