ABSTRACT
Circadian rhythms are an adaptive response of organisms to the environment that enables them to measure time. Circadian rhythms are some of the most studied biological rhythms. Serotonin (5HT) has been proposed as their modulator of circadian rhythms, playing a pivotal role in their establishment. However, 5HT concentrations are altered in insect organisms when they feed on some plant extracts. Insects show a variety of rhythms. The larval stage of the lepidopteran Spodoptera frugiperda is a pest of economically important crops. As a response, plants have developed secondary metabolites, such as azadirachtin, obtained from Azadirachta indica. We assessed the circadian rhythm of 5HT in the brain and digestive tube of larvae of S. frugiperda; furthermore, the effect of A. indica extract on the oscillations was evaluated. 5HT modulates the rhythms of locomotor activity, and if extracts of A. indica alter the concentration of 5HT, it can indirectly alter the rhythms of locomotor activity, as well as peristaltic movements of the intestine. Larvae were exposed to a 12 h:12 h light-dark (LD) photoperiod, and half of them remained for 72 h under constant darkness (DD). Tissue samples were obtained at six different times during a single 24 h period, and the amount of 5HT was quantified by high-performance liquid chromatography (HPLC). Data were statistically compared by a one-way ANOVA followed by a Tukey post hoc test and subjected to Cosinor analysis for assessment of 24 h rhythmicity. The results showed that the A. indica methanolic extract had an effect on the 5HT concentration of the brain and digestive tube of the larvae. In the brain, the 5HT increase in larvae fed with the extract could alter memory, learning, sleep, and locomotor activity processes. Whereas in the intestine, the 5HT decrease in the larvae fed with the extract could decrease peristalsis movements and, therefore, indirectly influence the antifeedant effect.