Abstract
To study the function of clock‐gene–expressing neurons, the tetanus‐toxin light chain (TeTxLC), which blocks chemical synaptic transmission, was expressed under the control of promoters of the clock genes period (per) and timeless (tim), each fused to GAL4‐encoding sequences. Although TeTxLC did not affect cycling of a clock‐gene product at the gross level, it disrupted the rhythmic behavior of adult Drosophila. In constant darkness, the proportion of rhythmic flies was reduced in flies expressing active TeTxLC compared to controls, including those expressing inactive toxin. The behavior of TeTxLC‐expressing flies was less synchronized to light:dark cycles than that of controls. To determine which neurons are responsible for these effects on behavior, the toxin was also expressed in restricted subsets of per/tim‐expressing, laterally located pacemaker neurons by expressing TeTxLC under the control of a driver in which GAL4‐encoding sequences are fused to the promoter of the pigment dispersing factor (pdf) gene. pdf‐gal4–driven TeTxLC expression had relatively little effect on behavioral rhythms, implying that per/tim neurons other than pdf‐expressing lateral neurons participate in the generation of rhythmic behavior. In another set of experiments, period gene products were expressed under the control of per‐gal4 or tim‐gal4. This resulted in an increased level of PER protein in many brain cells and reduction of bioluminescence cycling reported by a per‐luciferase transgene, especially in the case of per expression affected by tim‐gal4. This indicates a disruption of the transcriptional feedback loop that is a part of the oscillatory mechanism underlying Drosophila's circadian rhythms. Consistent with this molecular defect, the proportion of rhythmic individuals in constant darkness was subnormal in flies expressing PER under the control of tim‐gal4, and their behavior in light:dark cycles was abnormal. © 2000 John Wiley & Sons, Inc. J Neurobiol 43: 207–233, 2000