Spatio-temporal expression of TSH beta and FSH beta genes in normally metamorphosing, metamorphosed, and metamorphosis-arrested Hynobius retardatus.

Expression patterns of TSH beta and FSH beta genes were investigated in normally metamorphosing and metamorphosed individuals and in goitrogen-treated (TU-SPC), metamorphosis-arrested larvae of a Japanese salamander, Hynobius retardatus, a particular population of which had been reported to show neoteny. Cloned sequences for Hynobius TSH beta and FSH beta covered 66 and 56% of full-length genes, respectively, and both showed relatively high similarity to those of other vertebrates. TSH beta-expressing cells were observed at the ventro-caudal region of the pars distalis of controls, whereas an extraordinarily large number of cells which occupied the whole area of the pituitary glands strongly expressed TSH beta in TU-SPCs. In contrast, no FSH beta-expressing cells were observed in the pituitary glands of controls, whereas several positive cells were observed in the pituitary glands of TU-SPCs. Northern blot analyses revealed a rapid increase of TSH beta expression in the pituitary gland of TU-SPCs that finally reached the level of 20- to 30-fold as much as that of controls. Surprisingly, the expression of FSH beta also increased in the pituitary gland of TU-SPCs compared with that of controls. On the other hand, triiodothyronine administration to the control juveniles reduced the expression of TSH beta compared with saline-administrated controls, suggesting that thyroid hormone (TH) exerts a repressing effect on TSH beta gene. These results suggest that TSH beta overexpression in TU-SPCs must be caused by a deprivation of TH which plays a role in the negative feedback system and that FSH beta expression also might be enhanced by mechanisms similar to those of TSH beta, probably due to certain overlapping of the endocrinological regulation of TSH and FSH in H. retardatus.