Circadian control of prothoracicotropic hormone release in an adult insect and the induction of its rhythmicity by light cues

Jul 3, 2016Comparative biochemistry and physiology. Part A, Molecular & integrative physiology

Daily rhythm control of a hormone that triggers insect development and how light sets this rhythm

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Abstract

Prothoracicotropic hormone (PTTH) is present in the adult hemolymph of Rhodnius prolixus and is released in a daily rhythm.

The daily rhythm of PTTH release is controlled by the circadian clock, with a free running period of approximately 24 hours that adjusts with temperature.
Continuous exposure to constant light for three weeks completely stops PTTH release.
Transitioning insects from constant light to constant darkness quickly restores rhythmic PTTH release, indicating sensitivity to light cues.
PTTH levels in the hemolymph display a daily rhythm that aligns with the rhythm of PTTH release by the brain.
These findings suggest that PTTH functions as a neurohormone in adult insects and may play a role in regulating adult physiology.

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The insect neuropeptide prothoracicotropic hormone (PTTH) is a critical regulator of larval development. We recently demonstrated that PTTH is also present in adult Rhodnius prolixus and is released by adult brains in vitro with a clear daily rhythm during egg development. Here, we employ a well-established in vitro bioassay, to show that the daily rhythm of PTTH release by brains in vitro is under circadian control since it persists in aperiodic conditions with a free running period of around 24h that is temperature compensated. Prolonged exposure (3weeks) of insects to continuous constant light (LL) completely eliminated PTTH release. Subsequent transfer of such insects from LL to constant darkness (DD) rapidly induced rhythmic PTTH release, indicating that the circadian rhythm of PTTH release is induced by photic cues. Western analysis identified PTTH in the adult hemolymph, suggesting that PTTH acts as a functional neurohormone in the adult insect. Dot blot analysis revealed that PTTH levels in the hemolymph also cycled with a daily rhythm that persisted in DD and was synchronous with the rhythm of PTTH release by brains in vitro. We conclude that the previously documented photosensitive clock in the brain regulates rhythmic PTTH release and thus generates the rhythm seen in the hemolymph. These results emphasize the importance of rhythmic PTTH release in the adult insect and support a role for PTTH in adult physiology and possibly within the adult circadian system.

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Circadian control of prothoracicotropic hormone release in an adult insect and the induction of its rhythmicity by light cues

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