Mechanical feedback amplification in Drosophila hearing is independent of synaptic transmission

Vertebrate inner-ear hair cells use mechanical feedback to amplify sound-induced vibrations. The gain of this 'cochlear amplifier' is centrally controlled via efferent fibres that, making synaptic contacts with the hair cells, modulate the feedback gain. The sensory neurons of the Drosophila ear likewise employ mechanical feedback to assist sound-evoked vibrations, yet whether this neuron-based feedback is also subject to efferent control has remained uncertain. We show here that the function of Drosophila auditory neurons is independent of efferent modulation, and that no synaptic transmission is needed to control the gain of mechanical feedback amplification. Immunohistochemical, mechanical and electrophysiological analyses revealed that the Drosophila auditory organ lacks peripheral synapses and efferent innervations, and that blocking synaptic transmission in a pan-neural manner does not affect the afferent electrical activity of the sensory neurons or the mechanical feedback gain. Hence, unlike the cochlear amplifier of vertebrates, mechanical feedback amplification in Drosophila is not associated with an efferent control system but seems to be a purely local process that is solely controlled peripherally within the ear itself.