Auditory Efferent System Modulates Mosquito Hearing

The performance of vertebrate ears is controlled by auditory efferents that originate in the brain and innervate the ear, synapsing onto hair cell somata and auditory afferent fibers [1, 2, 3]. Efferent activity can provide protection from noise and facilitate the detection and discrimination of sound by modulating mechanical amplification by hair cells and transmitter release as well as auditory afferent action potential firing [1, 2, 3]. Insect auditory organs are thought to lack efferent control [4, 5, 6, 7], but when we inspected mosquito ears, we obtained evidence for its existence. Antibodies against synaptic proteins recognized rows of bouton-like puncta running along the dendrites and axons of mosquito auditory sensory neurons. Electron microscopy identified synaptic and non-synaptic sites of vesicle release, and some of the innervating fibers co-labeled with somata in the CNS. Octopamine, GABA, and serotonin were identified as efferent neurotransmitters or neuromodulators that affect auditory frequency tuning, mechanical amplification, and sound-evoked potentials. Mosquito brains thus modulate mosquito ears, extending the use of auditory efferent systems from vertebrates to invertebrates and adding new levels of complexity to mosquito sound detection and communication.