Ecdysone receptor B1 in Bombyx mori L. (Lepidoptera:Bombycidae) prothoracic gland under various organ culture conditions

During the early last larval instar, hormone secretory activity of prothoracic glands in Bombyx mori L. is undetectable despite the high prothoracicotropic hormone (PTTH) level in hemolymph. Early reports indicated that the inhibition of gland activity is caused by 20-hydroxyecdysteroid (20E) peak on day 2 of the fourth larval instar and juvenile hormone (JH) only resumes this inactive period during the early days of 5th larval instar. Following these reports, prothoracic glands also become a target of its hormone, 20E. Actions of 20E are mediated via heterodimeric ecdysone receptor complex which consists of ecdysone receptor (EcR) and its partner molecule, ultraspiracle (USP). This study was designed to determine the expression pattern of ecdysone receptor B1 (EcR-B1) in the prothoracic glands of B. mori during last larval instar and early pupal stage. Organ culture experiments were carried out to clarify the effects of juvenile hormone and 20-hydroxyecdysone on the EcR-B1 expression. Immunocytochemical studies revealed the presence of EcR-B1 in prothoracic gland cells. A rise of EcR-B1 immoreactivity was detected on day 3 of 5th instar in which glands recover the secretion activity. Juvenile hormone administration to culture medium maintained EcR-B1 immunoreactivity especially on day 6 and day 9 when compared with control but in contrast to this result, an ecdysone application to culture medium weakened the immunoreactivity. These results suggested that negative feedback effect of ecdysone on secretory activity of the glands may be exerted by means of ecdysone receptor and it may also involve consistent and regular 20E secretion during the fifth instar.