Evening Activity Termination Timing in Drosophila melanogaster : Effects of Pigment-Dispersing Factor-Containing Large and Small Ventrolateral Neurons

In animals, the brain contains circadian clock neurons that regulate activity rhythms. The fruit fly Drosophila melanogaster exhibits a bimodal activity pattern characterized by two peaks, in the morning (M) and evening (E), known as the M and E peaks. These activity peaks are orchestrated by a network of approximately 240 clock neurons. The neuropeptide pigment-dispersing factor (PDF) is expressed in two sets of clock neurons, the large ventrolateral neurons (l-LN v ) and the small ventrolateral neurons (s-LN v ). Mutants of Pdf , as well as flies lacking PDF neurons, exhibit a characteristic E activity that is commonly simplified to a phase-advanced pattern under 12 h:12 h light-dark cycles. Previous studies have demonstrated that l-LN v neurons regulate the phase of the E peak; however, this effect is evident only under long photoperiod conditions. Therefore, the E peak phenotype observed in Pdf mutants remains incompletely explained. In this study, we employed genetic cell ablation and Pdf RNA interference using Gal4 lines specific to l-LN v neurons in a well-controlled genetic background. Under long photoperiod conditions, flies lacking l-LN v , s-LN v , or both neuronal groups exhibited an early termination of E activity prior to lights-off, resulting in a phase-advanced E peak. Similar results were obtained in Pdf knockdown flies. Notably, l-LN v neurons had a stronger effect on the timing of E activity termination than s-LN v neurons. These findings demonstrate that LN v neurons control the phase of E activity by modulating the timing of its offset, providing new insights into the neuronal mechanisms that shape daily activity patterns.