Regulation of Olfactory Associative Memory by the Circadian Clock Output Signal Pigment-Dispersing Factor (PDF)

Abstract

Dissociation between the output of the circadian clock and external environmental cues is a major cause of human cognitive dysfunction. While the effects of ablation of the molecular clock on memory have been studied in many systems, little has been done to test the role of specific clock circuit output signals. To address this gap, we examined the effects of mutation of Pigment-dispersing factor (Pdf) and its receptor, Pdfr on associative memory in male and female Drosophila. Loss of PDF signaling significantly decreases the ability to form associative memory. Appetitive short-term memory (STM), which in wildtype is time-of-day (TOD)independent, is decreased across the day by mutation of Pdf or Pdfr, but more substantially in the morning than in the evening. This defect is due to PDFR expression in adult neurons outside the core clock circuit and the mushroom body Kenyon cells. The acquisition of a TOD difference in mutants implies the existence of multiple oscillators that act to normalize memory formation across the day for appetitive processes. Interestingly, aversive STM requires PDF but not PDFR, suggesting that there are valence-specific pathways downstream of PDF that regulate memory formation. These data argue that the circadian clock uses circuit-specific and molecularly diverse output pathways to enhance the ability of animals to optimize responses to changing conditions. SIGNIFICANCE STATEMENT From humans to invertebrates, cognitive processes are influenced by organisms' internal circadian clocks, the pace of which is linked to the solar cycle. Disruption of this link is increasingly common (e.g. jetlag, social jetlag disorders) and causes cognitive impairments that 5 are costly and long-lasting. A detailed understanding of how the internal clock regulates cognition is critical for the development of therapeutic methods. Here, we show for the first time that olfactory associative memory in Drosophila requires signaling by Pigment-dispersing factor (PDF), a neuromodulatory signaling peptide produced only by circadian clock circuit neurons. We also find a novel role for the clock circuit in stabilizing appetitive sucrose/odor memory across the day.

Light/Clock Influences Membrane Potential Dynamics to Regulate Sleep States

“…In Drosophila , synaptic plasticity that regulates sleep was induced by specific sequences of spikes that occur during spontaneous activity in clock neurons ( 12 ). Moreover, based on recent studies, the interaction of light and clock information influences memory learning, possibly mediated by sleep ( 13 , 14 ). These studies made an interesting link between light and clock information in forming neural coding, which is based on “neural activity” of circadian clock networks.…”

Neonicotinoids disrupt memory, circadian behaviour and sleep

“…Furthermore, given that neonicotinoids appear to disrupt circadian rhythmicity, and given that there is circadian regulation of memory formation with time of day influencing performance, neonicotinoid-induced disruption to the clock may contribute to the memory deficits 61 . The time-of-day differences in mushroom body memory are thought to result from day/night differences in PDF and PDFR signalling 30 , therefore the loss sLNv dorsal terminal PDF rhythmic signalling of PDF caused by the neonicotinoids may provide another route to the insecticide induced reductions in memory we report.…”

“…Memory formation is also reliant on circadian rhythms 29 , 30 and sleep 31 , 32 . Work in honeybees and bumblebees has shown that neonicotinioids can reduce behavioural rhythmicity and disrupt sleep behaviour in these important pollinators 21 , 33 .…”

The Power of Drosophila melanogaster for Modeling Neonicotinoid Effects on Pollinators and Identifying Novel Mechanisms

“…This is thought to help them find new foraging patches, as most flowers are nectar-rich in the morning (Bloch et al, 2017). Likewise, Drosophila has been shown to demonstrate time-ofday memories and circadian change in olfaction and olfactory memory (Lyons and Roman, 2009;Chouhan et al, 2015;Flyer-Adams et al, 2020), showing that these behaviors, underlying mechanisms and potential impacts of neonicotinoids can be modeled using this system.…”

“…Therefore this study demonstrates that neonicotinoid exposure disrupts the day/night differences in clock neuron synaptic PDF neuropeptide availability and structural plasticity and decreases circadian rhythms. Interestingly, recent work suggests circadian changes in PDF signaling from the sLNv to the mushroom body may mediate time-of-day effects on Drosophila olfactory shock memory (Flyer-Adams et al, 2020). Therefore the disruption of sLNv synaptic PDF neuropeptide levels and structural plasticity may contribute to or be responsible for the deficits in mushroom body mediated olfactory shock memory seen.…”