A photosensitive circadian oscillator in an insect endocrine gland: photic induction of rhythmic steroidogenesis in vitro

The pail-ed prothoracic glands of the insect Rhodnius prolixus each comprise a group of about 200 structurally identical cells. The synthesis (and release) of steroid moulting hormones (ecdysteroids) by these glands is under circadian control in vivo. We monitored ecdysteroid synthesis by single glands during long-term incubations in vitro. Synthesis is rhythmic in vitro and persists in continuous darkness. Glands which are arrhythmic (from prolonged continuous light) respond to transfer to darkness in vitro with the initiation of a free-running circadian rhythm of ecdysteroid synthesis. Therefore, the glands possess a light-sensitive circadian oscillator. These properties are conventionally associated with nervous tissue of animals. It is suggested that rhythmicity is synchronized within the gland by the known structural and electrical coupling between its component cells. The glands share properties with known pacemakers such as the avian pineal. However, the glands in vivo receive input from both light cues and the cerebral neuropeptide, prothoracicotropic hormone. Rhythmic release of this neuropeptide is controlled by a second oscillator located in the brain. We conclude that the pacemaker in the endocrine system of R. prolixus comprises at least three oscillators, one in each prothoracic gland and one in the brain, which are coupled hormonally. We conclude that the prothoracic gland is an important component of the circadian system controlling development in R. prolixus and that peripheral endocrine glands may play a more active role in the generation of animal circadian organization than has been thought.