Blocking synaptic transmission with tetanus toxin light chain reveals modes of neurotransmission in the PDF-positive circadian clock neurons of Drosophila melanogaster

► The PDF-positive neurons are the major component of the Drosophila clock network. ► They also use another neurotransmitter(s) (UNT) for controlling behavioral rhythms. ► Flies with blocked UNT pathway show increased PDF levels and complex rhythms in LL. ► Some clock neurons show TIM expression rhythms associated with the complex rhythms. ► The UNT pathway may be involved in the clock neuronal network together with PDF.