A new method of recording from the giant fiber of Drosophila melanogaster shows that the strength of its auditory inputs remains constant with age

Abstract

There have been relatively few studies of how central synapses age in adult Drosophila melanogaster. In this study we investigate the aging of the synaptic inputs to the Giant Fiber (GF) from the Johnstons Organ neurons (JONs). In previously published experiments an indirect assay of this synaptic connection was used; here we describe a new, more direct assay, which allows reliable detection of the GF action potential in the neck connective, and long term recording of its responses to sound. Genetic ablation using diphtheria toxin expressed in the GF with R68A06-GAL4 was used to show that these signals indeed arose from the GF and not from other descending neurons. As before, the sound-evoked action potentials (SEPs) in the antennal nerve were recorded via an electrode inserted at the base of the antenna. We then used this technique to quantify the response of the JONs to a high frequency sound pulse, and also the strength of the JON-GF synapse, in males and female of different ages. At no age was there any significant difference between males and females, for any of the parameters measured. Sensitivity of the JONs increased between 1 d and 10 d, with the sound intensity that elicited a half-maximal SEP decreasing by 40%. This measure almost doubled by 20 d and had increased 3-fold by 50 d compared to 10 d. Thus, JONs are most responsive around the period when most matings are taking place. The strength of the JON-GF synaptic connection itself was quite variable and did not change significantly with age. As a result, the GFs sensitivity to sound approximately followed the JONs sensitivity, but with greater variability.