Thirst is a strongly motivated internal state that is represented in central brain circuits that are only partially understood. Water seeking is a discrete step of the thirst behavioral sequence that is amenable to uncovering the mechanisms for motivational properties such as goal-oriented behavior, value encoding, and behavioral competition. In Drosophila water seeking is regulated by the NPY-like neuropeptide NPF, however the circuitry for NPF-dependent water seeking is unknown. To uncover the downstream circuitry, we identified the NPF receptor NPFR and the neurons it is expressed in as being acutely critical for thirsty water seeking in males. Refinement of the NPFR pattern uncovered a role for a single neuron, the L1-l, in promoting thirsty water seeking. The L1-l neuron increases its activity in thirsty flies and is involved in the regulation of dopaminergic neurons in long-term memory formation. Thus, NPFR and its ligand NPF, already known for its role in feeding behavior, are also important for a second ingestive behavior. Significance Statement Understanding how a single motivated behavior is represented in the neural circuitry of the brain will help uncover drive-specific and universal encoding mechanisms for motivation. Thirst is useful because it is relatively simple compared to feeding behavior and it is strongly motivated. The fly Drosophila, with its straightforward genetics and complete connectome, is helpful in determining a complete circuit for thirsty water seeking. Here we discover a single neuron in the fly brain, the L1-l, that actively receives input from the NPY-like neuropeptide NPF to promote water seeking. Prior findings suggest that the L1-l modulates valence inputs into sensory processing centers, suggesting a similar function in thirsty seeking.
Loss of function Frazzled/DCC mutants disrupt synaptogenesis in the Giant Fiber (GF) System of Drosophila . We observe weaker physiology in loss-of-function (LOF) male and female specimens, characterized by longer latencies and reduced response frequencies between the GFs and the motor neurons. These physiological phenotypes are linked to a loss of gap junctions in the GFs, specifically the loss of the shaking-B(neural+16) isoform of innexin in the presynaptic terminal. We present evidence of Frazzled's role in gap junction regulation by utilizing the UAS-GAL4 system in Drosophila to rescue mutant phenotypes. Expression of various UAS-Frazzled constructs in a Frazzled LOF background was used to dissect the role of different parts of the Frazzled receptor in the assembly of electrical synapses. Expressing Frazzledβs intracellular domain in Frazzled LOF mutants rescued axon pathfinding and synaptogenesis. This is supported by the complementary result that Frazzled fails to rescue synaptic function when the transcriptional activation domain is disrupted, as shown by the deletion of the highly conserved intracellular P3 domain or by a construct with a point-mutation in the highly conserved P3 domain known to be required for transcriptional activation. A computational model clarifies the role of gap junctions and the function of the Giant Fiber System. The present work shows how various domains of a guidance molecule regulate synaptogenesis through the regulation of synaptic components. Significance Statement Loss of function Frazzled/DCC mutants demonstrate that the gene regulates synaptogenesis in the Giant Fiber (GF) System of Drosophila. In frazzled loss of function mutants, we observe weaker physiology, characterized by longer latencies and reduced response frequencies between the GFs and the motor neurons. These physiological phenotypes are linked to a loss of gap junctions in the GFs. A GF computational model is provided to test the role of gap junctions and the function of the Giant Fiber System. We present evidence of Frazzled's role in gap junction regulation by utilizing the UAS-GAL4 system in Drosophila to rescue mutant phenotypes. We also show that this effect can be computationally modeled, supporting our findings and presenting a novel role of Frazzled.
flypapers