Effect of octopamine on the frequency tuning of the auditory system in <i>Culex pipiens pipiens</i> mosquito (Diptera, Culicidae)

Abstract

We aimed at demonstrating the direct physiological effects of octopamine on the auditory responses of the Johnston’s organ sensory neurons in Culex pipiens pipiens mosquitoes. Responses to acoustic stimulation were measured as the frequency-threshold curves and as an instantly recorded autoexcitation frequency before and after the injection of octopamine (or chlordimeform, an agonist of octopamine receptors). The autoexcitation frequency was assessed by including the auditory neurons into a positive feedback loop, when the amplified neuronal response was used to drive the stimulating loudspeaker. Our results indicate that: octopamine affects the properties of the auditory system both in male and female mosquitoes; in female mosquitoes, octopamine significantly reduces the sensitivity of auditory neurons below 90 Hz and produces little effect in the high-frequency range (100 Hz and above); in male mosquitoes, octopamine significantly raises the tuning frequency of the auditory system (ratio of tuning frequencies before and after the injection 1.32–1.55). There is a strong sexual dimorphism in the octopaminergic modulation of the mosquito auditory system. The observed physiological effects of octopamine, both in male and female mosquitoes, cannot be fully explained by the modulation of mechanical stiffness of the antenna and must include the changes in frequency tuning of the auditory neurons.