Analysis of the mechanism underlying the rhythm reversal from diurnal to nocturnal in the cricket Gryllus bimaculatus, with special reference to the role of serotonin

The cricket, Gryllus bimaculatus, shows a rhythm reversal from diurnal to nocturnal in about a week after the imaginai molt. In the present study, we investigated the role of serotonin (5-HT) in the rhythm reversal. The 5-HT content in the brain measured by HPLC equipped with an electrochemical detector gradually increased after the imaginai molt, and in fully nocturnal adults it was about 2 times of nymphal level. We then examined the effects of 5,7-dihydroxytryptamine (5,7-DHT), a selective neurotoxine to serotonergic neurons, on the locomotor rhythm. In most animals with 5,7-DHT (25 μM or 250 μM, 32.2 nl) injected into the brain, daytime activity significantly increased even after the rhythm reversal, while nighttime activity was not significantly affected, forming rather diurnal pattern. The serotonin content in the brain of animals injected with 250 μM 5,7-DHT was reduced by about 30%. On the basis of these results, possible involvement of 5-HT in the neural mechanism controlling the locomotor rhythm is discussed.