Dissection of the Drosophila neuropeptide F circuit using a high-throughput two-choice assay

Significance

The perception and processing of rewarding events are essential for organismal survival. In Drosophila, several groups of neurons have been shown to mediate reward perception or processing. However, a complete description of the reward circuit is missing. Here, we describe a simple two-choice, high-throughput assay suitable for performing large neuronal activation screens for neural circuits involved in reward perception/processing. We characterized this assay using activation of neuropeptide F (NPF) neurons, a known rewarding experience for flies. Furthermore, using genetic intersectional strategies, we subdivided the NPF neurons into different classes and showed that the activation of a subset of small NPF neurons located in the dorsomedial brain is sufficient to trigger preference.

Abstract

In their classic experiments, Olds and Milner showed that rats learn to lever press to receive an electric stimulus in specific brain regions. This led to the identification of mammalian reward centers. Our interest in defining the neuronal substrates of reward perception in the fruit fly Drosophila melanogaster prompted us to develop a simpler experimental approach wherein flies could implement behavior that induces self-stimulation of specific neurons in their brains. The high-throughput assay employs optogenetic activation of neurons when the fly occupies a specific area of a behavioral chamber, and the flies’ preferential occupation of this area reflects their choosing to experience optogenetic stimulation. Flies in which neuropeptide F (NPF) neurons are activated display preference for the illuminated side of the chamber. We show that optogenetic activation of NPF neuron is rewarding in olfactory conditioning experiments and that the preference for NPF neuron activation is dependent on NPF signaling. Finally, we identify a small subset of NPF-expressing neurons located in the dorsomedial posterior brain that are sufficient to elicit preference in our assay. This assay provides the means for carrying out unbiased screens to map reward neurons in flies.