Abstract
This study showed that in adult Drosophila melanogaster, the type of sugarâeither present within the crop lumen or in the bathing solution of the cropâhad no effect on crop muscle contraction. What is important, however, is the volume within the crop lumen. Electrophysiological recordings demonstrated that exogenous applications of serotonin on crop muscles increases both the amplitude and the frequency of crop contraction rate, while adipokinetic hormone mainly enhances the crop contraction frequency. Conversely, octopamine virtually silenced the overall crop activity. The present study reports for the first time an analysis of serotonin effects along the gut-brain axis in adult D. melanogaster. Injection of serotonin into the brain between the interocellar area shows that brain applications of serotonin decrease the frequency of crop activity. Based on our results, we propose that there are two different, opposite pathways for crop motility control governed by serotonin: excitatory when added in the abdomen (i.e., directly bathing the crop) and inhibitory when supplied within the brain (i.e., by injection). Finally, our results point to a double brain-gut serotonergic circuitry suggesting that not only the brain can affect gut functions, but the gut can also affect the central nervous system. On the basis of our results, and data in the literature, a possible mechanism for these two discrete serotonergic functions is suggested.
Highlights
Neural control systems governing the regulation of meal size are subjects of great interest in both mammalian [1,2,3,4] and insect systems [5,6,7,8]
Feeding the flies different sugars showed that the mean rate of contraction was 16.95 for sucrose, 16.40 for glucose and 18.25 cont/min for fructose and that the type of sugar ingested had no effect on crop contractions (Fig 2)
We showed that in adult Drosophila melanogaster, the type of sugarâeither present within the crop lumen or in the bathing solution of the cropâhad no effect on crop muscle contraction, which differed from P. regina, other mechanisms are mainly involved in the crop contraction such as the volume, as above discussed, and as the major aim of our study, the possible involvement of neurotransmitters/neuromodulators
Summary
Neural control systems governing the regulation of meal size are subjects of great interest in both mammalian [1,2,3,4] and insect systems [5,6,7,8]. Research in two adult dipteran species, Phormia regina [9,10,11] and Drosophila melanogaster [12,13,14,15,16,17], has been instructive in identifying the sensory, motor, and integrative components of the neural control systems for feeding. Crop muscle control in the adult fruit fly, Drosophila melanogaster had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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