Light and serotonin phase-shift the circadian clock in the cricket optic lobe in vitro

Light and serotonin were found to cause phase shifts of the circadian neural activity rhythm in the optic lobe of the cricket Gryllus bimaculatus cultured in vitro. The two phase-shifting agents yielded phase-response curves different in shape. Light induced phase delay and advance in the early and late subjective night, respectively, and almost no shifts in the subjective day, whereas serotonin phase-advances the clock during the subjective day and induced delay shifts during the subjective night. The largest phase advance and delay occurred at circadian time 21 and 12, respectively, for light, and circadian time 3 and 18, respectively, for serotonin. Quipazine, a nonspecific serotonin agonist, induced phase advance and phase delay at circadian time 3 and 18, respectively, like serotonin. (±)8-OH-DPAT, a specific 5-HT(1A) agonist, phase delayed by 2 h at the subjective night, but produced no significant phase shifts at the subjective day. When NAN-190, a specific 5-HT(1A) antagonist, was applied together with quipazine, it completely blocked the phase delay at circadian time 18, whereas it had no effect on the advance shifts induced by quipazine. The results suggest that the phase dependency of serotonin-induced phase shifts of the clock may be partly attributable to the daily change in receptor type.