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Auditory Processing in Insect Brains

Final Report Summary - AUDITORYFLY (Auditory Processing in Insect Brains)

Summary

Fruit flies Drosophila melanogaster use sound to communicate during courtship. Male flies produce sounds with one wing and females perceive this sound with auditory receptors located in the antenna. These receptors send auditory information to a brain region called antennal mechano-sensory and motor center (AMMC). AMMC neurons project to another auditory region (ventro-lateral protocerebrum, VLP). The goal of the project was to physiologically characterise auditory neurons and to discover new auditory types of neurons in the brain centres of female flies. Characterising known neurons and discovering new types of auditory neurons will shed light on how the brain processes acoustic information.

We characterized physiologically and anatomically neurons in the auditory system of the Drosophila melanogaster. We used the patch clamp technique (an electrophysiological method) to measure cell responses to sound. A previous study (Tootoonian et al., J Neurosci, 32:787-798, 2012) showed that at least some auditory cell types in Drosophila auditory system do not produce action potentials. For this reason we chose the patch clamp method since it can detect all changes of the membrane potential, contrarily to extracellular methods which only measure action potentials. In addition, as the patch clamp technique is an intracellular method we could label the recorded neurons by injecting a dye for further anatomical analysis.
Neurons were stimulated with sounds during recordings. We used pure tones (frequency range: 100-700 Hz, white noise: 80-1000 Hz, synthetic pulses, and natural fly songs).

We recorded and analysed 10 cell types in the AMMC and VLP and 1 cell type in a higher brain area. All of these neurons responded to auditory stimulation. Nine out of eleven types were never characterized before and we named them according to the anatomical properties: “V” for neurons with processes in VLP only, and “AV” when processes were in both AMMC and VLP. Two neurons types only projected to VLP (V1 and V2) and six projected to AMMC and VLP (types AV1 to AV6). The neuron in the high brain area was named HB1. Neurons A1 and B2 were described before in Tootoonian et al., J Neurosci, 32:787-798, 2012.

All neurons responded to low frequency sounds (100-1000 Hz) with a peak response between 200 and 300 Hz which is in the range of frequencies found in natural fly song. Other response properties varied greatly across cell types. Some neurons mostly responded to stimulus onset while others produced a sustained response during the entire stimulus duration. Some neurons were most sensitive to stimuli with inter-pulse intervals corresponding to that of the male song (about 35 ms). In addition, we found that some auditory neurons produced action potentials which was not known before. Using separate genetic markers for axons and dendrites we could identify the polarity of each neuron type and thus infer the direction of information transfer within the neuronal network. Examination of axonal and dendritic trees revealed that each cell type is unique and innervates different parts of the auditory regions. For example AV1 projects from AMMC to VLP and is likely a projection neuron. In addition we built a computational model that can predict the neuron response to synthetic sounds and natural songs.

All together our data set composed of physiological, anatomical, and behavioural data will represent a nearly complete description of the auditory processing – from perception to behavioural response – of one animal model. This will help to understand brain function in general and acoustic communication in particular.