Daily rhythm of responsiveness to prothoracicotropic hormone in prothoracic glands of rhodnius prolixus

The synthesis of ecdysteroids by the prothoracic glands (PGs) of Rhodnius prolixus occurs with a circadian rhythm throughout most of larval-adult development. This rhythm is generated by a circadian oscillator within the PGs. The principle regulator of the PGs is the cerebral neuropeptide prothoracicotropic hormone (PTTH), which is also known to be released with a circadian rhythm, but it has not been shown that these daily releases of PTTH are detected by the PGs. The present report examines the ability of PTTH to augment ecdysteroid synthesis in rhythmic PGs at various times of day. There is a daily rhythm in the responsiveness of PGs to PTTH; high responsiveness occurs around dusk and PGs are largely unresponsive to PTTH during the daytime. This rhythm phase-leads the rhythms of PTTH release and of ecdysteroid synthesis by several hours. The data are suggestive of a daily rhythm of up- and down-regulation of PTTH receptor availability in the PGs. It is concluded that the daily releases of PTTH are detected by the PGs and participate in the regulation of daily ecdysteroid synthesis. Therefore, PTTH plays a continuing role in the regulation of PGs throughout development. Arch. Copyright 1999 Wiley-Liss, Inc.