| Research Abstract |
The mechanism of coupling between the two optic lobe circadian clocks was investigated with special reference to the role of serotonin and pigment-dispersing factor (PDF). Light-induced electrical activities of medulla bilateral neuron (MBNs) that convey coupling information showed daily variations, being more intense during the night than the day. Microinjection of serotonin into the optic lobe caused a dose- and time-dependent inhibition of spontaneous and light-induced responses, mimicking the day state. Application of metergoline, a non-selective serotonin antagonist, increased spontaneous activity and light-evoked responses, with higher effect during the day. Further pharmacological investigation revealed that this serotonergic inhibition was mediated by 5-HT_7 like receptors. We then purified the PDF from the cricket brain and made an antibody by immunizing the rabbit. The antibody labeled two groups of neurons in the optic lobe, one located near the lamina and the other near proximal medulla. PDF injected into the optic lobe enhanced the MBN's photo-responsiveness during the day, while little change was induced during the night. The enhancing effect of PDF during the day was suppressed by serotonin. PDF also induced the phase shifts of the locomotor rhythm in a phase dependent manner : it induces phase advance during the late subjective day to early subjective night and delays during the late subjective night to early subjective day. These facts suggest that in the cricket's optic lobe serotonin is released during the daytime and sets the day state in the MBNs, PDF is released during the late day to night and enhances MBNs' photo-responsiveness to set their night state. The dual regulation of MBNs activity and the clock phase by serotonin and PDF may play central role in the stable coupling between the two optic lobe pacemakers.
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