Elucidation of Serotonin-related Profiles in Aedes Aegypti Digestive Tract Predicts Novel Putative Roles of Serotonin

Despite its biological importance, understanding of serotonin roles in insect physiology is limited. To address this, the current study aimed to answer basic questions associated with serotonin-related distribution patterns in the Aedes aegypti (yellow fever mosquito) digestive tract. The rationale behind such analyses is based on the fact that serotonin effects are determined by its distribution and availability. Therefore, investigating such patterns is a pivotal step towards elucidating the serotonergic functions and significance. Two stages of the mosquito development cycle, those of larva and adult (female), were the focus of investigations. Their differential modes of feeding (continuous vs. discontinuous), diets, gut morphology and physiology make them a useful system to study and understand the fundamentals of the serotonergic mechanisms that regulate food processing in insects. Immunolocalization studies were the main approach carried out, and have not only identified differential aspects of the serotonergic network between the two stages but, importantly, have provided evidence assigning novel putative roles for serotonin in Aedes aegypti physiology. Overall, the midgut of both mosquito stages is innervated with serotonergic axons. However, while in the larval midgut, axon patterns vary significantly between the anterior and posterior section, in the female adult midgut, such differences were not observed. The posterior midgut (pMG) in both mosquito stages is richly innervated with serotonergic axons containing multiple but small release sites that, apart from regulating the gut epithelial cells, may mediate the Malpighian tubules (MT) functions. This statement is corroborated by AaSer-1 (a putative serotonin receptor) expression in the MT of both larvae and adults. Since MT are resting on the pMG surface, it is likely that the serotonergic source affecting them originates from the pMG. Thus, a new model has been proposed which attests (previously questioned) serotonin roles in diuresis. To further elaborate on the serotonergic-related profiles and significance during blood digestion, the expression of AaSer-1 in the adult female gut was investigated. While AaSer-1-specific immunofluorescence in sucrose-fed female adults is rather ‘unremarkable’ and mainly limited to the MT, in the blood-fed mosquitoes, expression patterns suggest receptor involvement in the nuclear, tracheal, muscular, and MT functions during blood digestion and absorption. Vol-4, Issue-3 PP. 136-151 ISSN: 2394-5788 137 | P a g e 3 0 M a r c h 2 0 1 7 w w w . g j a r . o r g Associations of AaSer-1 with tracheoblast formations and mediation of nuclear functions during blood digestion indicate novel putative roles of serotonin in mosquito (insect) physiology. Taken together, results provide strong evidence for the serotonin significance and complexity in the regulation of mosquito feeding-related processes.