Modulation of neural circuits: how stimulus context shapes innate behavior in Drosophila

• Innate behavior is flexible and subject to modulation by stimulus context.
• Fruit odors modulate Drosophila's innate aversion to CO2 by multiple mechanisms, operating at every known station of the CO2 olfactory circuit.
• Satiety state regulates feeding behavior by altering olfactory and gustatory sensitivity.
• Satiety state is communicated by a variety of metabolic cues, which regulate neuromodulator signaling to influence neural circuit function.

Remarkable advances have been made in recent years in our understanding of innate behavior and the underlying neural circuits. In particular, a wealth of neuromodulatory mechanisms have been uncovered that can alter the input–output relationship of a hereditary neural circuit. It is now clear that this inbuilt flexibility allows animals to modify their behavioral responses according to environmental cues, metabolic demands and physiological states. Here, we discuss recent insights into how modulation of neural circuits impacts innate behavior, with a special focus on how environmental cues and internal physiological states shape different aspects of feeding behavior in Drosophila.