Rosbash and his colleagues theorized that if dual circadian clocks existed, they would likely occupy different anatomical positions in the fly brain. They knew that the flies' circadian-clock neurons included one distinctive cluster called the ventral lateral neurons and another called the dorsal lateral neurons. The ventral lateral neurons were known to express an important circadian signaling molecule called pigment dispersing factor (PDF). The dorsal lateral neurons did not express a known signaling molecule, but were part of a larger group of circadian neurons that express a gene for a circadian photoreceptor protein known as cryptochrome.
Starting with those parameters, Rosbash and his colleagues used genetic techniques to selectively deliver cell death genes to ablate specific groups of neurons. They then observed how these genetic deletions affected the flies' activity. The experiments revealed that the PDF-expressing ventral lateral neurons govern the morning activity peak; while another group of neurons, including the dorsal lateral neurons, governs the evening activity peak.
In additional experiments, the researchers selectively disrupted the internal clock machinery of the circadian neurons. The studies showed that one set of circadian neurons drives the other. "It's as if there is a wiring circuit from one set to the next and, under natural light conditions, one can regulate the physiology of another," said Rosbash. The scientists theorize that this type of coupling between two circadian oscillators coordinates the two activity peaks and aids responses to environmental factors such as seasonal changes in lightness and darkness.
Rosbash said that there is evidence that mammals, including humans, also possess su
'"/>
Contact: Jennifer Michalowski
michalow@hhmi.org
301-215-8576
Howard Hughes Medical Institute
13-Oct-2004