Serotonin modulation in the male Aedes aegypti ear influences hearing

Abstract

Male Aedes aegypti (Ae. aegypti) mosquitoes rely on hearing to identify conspecific females for mating, with the male attraction to the sound of flying females (“phonotaxis”) an important behavior in the initial courtship stage. Hearing thus represents a promising target for novel methods of mosquito control, and hearing behaviors (such as male phonotaxis) can be targeted via the use of sound traps. These traps unfortunately have proven to be relatively ineffective during field deployment. Shifting the target from hearing behavior to hearing function could therefore offer a novel method of interfering with Ae. aegypti mating. Numerous neurotransmitters, including serotonin (5-hydroxytryptamine, or 5-HT) and octopamine, are expressed in the male ear, with modulation of the latter proven to influence the mechanical responses of the ear to sound. The effect of serotonin modulation however remains underexplored despite its significant role in determining many key behaviors and biological processes of animals. Here we investigated the influence of serotonin on the Ae. aegypti hearing function and behaviors. Using immunohistochemistry, we found significant expression of serotonin in the male and female Ae. aegypti ears. In the male ear, presynaptic sites identified via antibody labelling showed only partial overlap with serotonin. Next, we used RT-qPCR to identify and quantify the expression levels of three different serotonin receptor families (5-HT1, 5-HT2, and 5-HT7) in the mosquito heads and ears. Although all receptors were identified in the ears of both sexes, those from the 5-HT7 family were significantly more expressed in the ears relative to the heads. We then thoracically injected serotonin-related compounds into the mosquitoes and found a significant, reversible effect of serotonin exposure on the male ear mechanical tuning frequency. Finally, oral administration of a serotonin-synthesis inhibitor altered male phonotaxis. The mosquito serotonergic system and its receptors thus represent interesting targets for novel methods of mosquito, and thus disease, control.

Monoaminergic Systems in Flight-Induced Potentiation of Phonotactic Behavior in Female Crickets Gryllus bimaculatus

“…AMPT completely prevented the phonotactic response, which was clearly below the control level observed in non-flyers. Mosquitoes Aedes aegypti treated with AMTP similarly demonstrated a significant reduction of phonotactic response [ 58 ]. The combined evidence suggests that 5-HT positively regulates phonotactic behavior and may be involved in the mechanisms of flight-induced potentiation of phonotactic behavior in crickets.…”

“…However, mosquitoes possess serotonergic innervation of the primary auditory neurons of the Johnston organ (JO). This innervation is believed to be involved in the efferent control of hearing [ 75 , 76 ], and several types of 5-HT receptors (5-HT 1 , 5-HT 2 , and 5-HT 7 ) are expressed there [ 58 ]. Thoracically injected 5-HT caused an increase in the mechanical tuning frequency of the JO, reversed by the injection of methiothepin [ 58 ].…”

“…This innervation is believed to be involved in the efferent control of hearing [ 75 , 76 ], and several types of 5-HT receptors (5-HT 1 , 5-HT 2 , and 5-HT 7 ) are expressed there [ 58 ]. Thoracically injected 5-HT caused an increase in the mechanical tuning frequency of the JO, reversed by the injection of methiothepin [ 58 ]. The opposite effect, a clear decrease in the mechanical tuning frequency of a male mosquito JO, was observed after depleting the 5-HT content via AMTP.…”

“…The opposite effect, a clear decrease in the mechanical tuning frequency of a male mosquito JO, was observed after depleting the 5-HT content via AMTP. The authors of [ 58 ] concluded that changes in the 5-HT level alter the phonotactic profiles of male mosquitoes via modulation of their hearing function.…”

Characterization of a novel RNAi yeast insecticide that silences mosquito 5-HT1 receptor genes

“…CRISPR-Cas9-mediated silencing of 5-HTR2B in adult A. aegypti was recently shown to inhibit growth through reduced lipid accumulation 38 , whereas the deletion of 5-HTR7A resulted in significant larval and pupal mortality 39 . More recently, inhibition of the serotonergic system in adult A. aegypti altered male phonotaxis, a behavior that’s associated with hearing impairment and has implications on mating behavior 40 . The results from these few functional studies of mosquito 5-HTRs are beginning to unravel their potential as novel putative insecticide targets, highlighting the need for further functional analyses of 5-HTR genes in mosquitoes.…”