Seasonal variations in serotonin immunoreactivity and ultrastructure in the pineal organ of the Japanese grass lizard, with special reference to environmental temperature

The seasonal variations in serotonin immunoreactivity and ultrastructure of the secretory rudimentary photoreceptor cells (SRPC) were studied in the p ineal organ of the Japanese grass lizard, Takydromus tachydromoides in rela tion to the environmental temperature. Our results clearly demonstrated tha t serotonin immunoreactivity in the lizard pineal organ displayed seasonal variations under an artificial photoperiod of LD 12:12 and natural temperat ure in the laboratory. Immunoreactivity became intense with increase in tem perature from spring to summer, showing the strongest reaction in the summe r, and subsequently became weak with the drop in temperature to winter. Als o, the SRPC of the lizard showed distinct seasonal variations in number and size of the dense-cored vesicles correlated to the serotonin immunoreactiv ity. In contrast, the changes in size of the lysosomes and nucleoli of the SRPC were inversely proportional to that of the dense-cored vesicles. Furth ermore, the lysosomes ingested some dense-cored vesicles after the autumn, and they coalesced to form huge autophagosomes or residual bodies during th e winter. The present study provided serotonin-immunohistochemical and ultr astructural evidence for seasonal variations in the secretory activity of t he lizard pineal organ in accordance with changes in the environmental temp erature, However, there may be few functional relationships between the pin eal gland and the reproductive organs in the male Japanese lizard in relati on to environmental temperature. (C) 1999 Harcourt Publishers Ltd.