UC San Diego

Animals integrate external cues, such as light, with intrinsic molecular rhythms of the circadian clock network to regulate physiology and behavior according to the perceived time of day. While much is known about synaptic connectivity within the clock network, surprisingly little is known about how this connectivity is functionally determined at the level of neurotransmitter specification and how this attribute can be modified by environmental cues. In the present study, we defined the classical neurotransmitter systems used by the neurons of the Drosophila circadian clock network, and tested how a selected subgroup can be modified by exposure to light. We observed the abundance of VGluT (a marker of glutamatergic signaling) and VAChT (a marker of cholinergic signaling) in various cell-types within the clock network, while also observed diversity of neurotransmitter identity within the clock network. Additionally, we discovered that chronic increase in photoperiod selectively increases expression of VAChT in large ventral lateral neurons (lLNvs), reflecting long-term form of neurotransmitter plasticity. Our findings reinforce other research from mammals, suggesting that environmental cues can modulate connectivity of the circadian clock network through changes in neurotransmitter identity. To our knowledge, this is the first time neurotransmitter plasticity has been reported in adult invertebrates.