Daily variations in crustacean hyperglycaemic hormone and serotonin immunoreactivity during the development of crayfish.

The present study investigated changes in crustacean hyperglycaemic hormone (CHH) and serotonin (5-hydroxytryptamine, 5-HT) immunoreactivity in the retina and the X-organ/sinus gland complex (XO-SG) of the crayfish Procambarus clarkii at two developmental stages, post-embryonic stage two (PO2) and the juvenile stage, at three different times of day, under a photoperiod cycle of 12 h:12 h L:D, using qualitative and quantitative immunohistochemical methods. In the retina, CHH immunoreactivity is located in the tapetal cells, while 5-HT immunoreactivity is found in the retinular cells. In the XO-SG, CHH-immunoreactivity is localized to the CHH-producing cell perikarya and in their axons and endings in the sinus gland, while 5-HT immunoreactivity is restricted to axon endings branching into the perikarya of the CHH-producing cells. A stereological analysis demonstrates that the PO2 and juvenile stages show significant differences in the amount of the immunoreactive CHH and 5-HT material at the three selected time points, indicating daily and related changes in the levels of CHH and 5-HT in the XO-SG and the retina. Our findings therefore support the idea that daily rhythms in the secretory activity of the XO-SG complex affect the circadian sensitivity of the eye. Furthermore, the differences found between the PO2 and juvenile stages suggest that both CHH and 5-HT are key factors in the development of the circadian rhythm of retinal sensitivity.