Photic and hormonal Zeitgebers to a photosensitive prothoracic gland clock that regulates steroid synthesis in the insect Rhodnius prolixus (Hemiptera)

The prothoracic glands (PGs) in the insect Rhodnius prolixus contain a photosensitive circadian clock controlling synthesis of the steroid molting hormones. We examined the response of the PG clock to internal hormonal Zeitgebers. Immunohistochemistry revealed the depletion of nuclear PER from PG cells in both continuous light (LL) and continuous dark (DD). LL and DD PGs were incubated in vitro and exposed to a 1h pulse of brain neuropeptides. In LL PGs, but not in DD PGs, expression and circadian cycling of PER were reinitiated by both prothoracicotropic hormone (PTTH) and insulin-like peptide(s) (ILPs), and both peptides were capable of reinitiating rhythmic ecdysteroid synthesis. ILPs and PTTH both have known stimulatory effects on steroidogenesis by PGs. We infer at least two brain neuropeptide Zeitgebers are capable of contributing to entrainment of the PG clocks by acting as signals of darkness to the PGs and stimulating a rhythmic clock output (ecdysteroids).