Identification and characterization of interneurons in drosophila gustatory circuitry

Precise control of feeding behaviour is essential for the survival of animals and is largely determined
by internal nutritional state and food palatability. While progress has been made towards the
identification of gustatory circuitry, it remains largely elusive how information regarding internal
nutritional state is integrated into the feeding circuitry to modulate feeding behavior. Here we identify
a new class of interneurons (VM neurons) within the gustatory circuitry of drosophila that
modulate sweet sensitivity of the animal in a starvation-dependent manner. VM neurons likely receive
direct input from Gr5a sweet receptor neurons and VM neurites are present in closed proximity
to the branches of insulin producing cells. Conditional silencing of VM neurons impairs sweet
sensitivity of starved animals while artificial activation is sufficient to elicit proboscis extension.
Interestingly silencing of insulin producing cells (IPCs) increases sweet sensitivity, suggesting that
IPCs exert a modulatory effect on the sensitivity of the gustatory circuitry. in a manner opposite to
VM neurons. Consistent with our hypothesis that VM neurons representing a direct target of IPCs in
the gustatory circuit, blocking insulin signaling in VM neurons increases sweet sensitivity in fed
animals. Taken together, we identified a neuromodulatory pathway that insulin signaling translates
information of internal nutritional state into changes in gustatory circuit sensitivity. This mechanism
enables animals rapidly adjust their feeding behaviours to maintain a homeostatic nutritional
state.