Treatment of the killifish, Fundulus heteroclitus, with ovine thyrotropin (oTSH) produced elevations in serum thyroxin (T4) that varied seasonally with a strong inverse correlation to environmental photoperiod (P less than 0.001). At 20 degrees C, oTSH (0.2 IU) increased serum T4 to levels ranging from 1.5 micrograms/100 ml in midsummer to 7.5 micrograms/100 ml in midwinter despite relatively stable resting levels throughout the two years of study. Similar rates of clearance of T4 in summer and winter suggest that an alteration in thyroid response to oTSH accounts for this change. The serum T4 response to oTSH is both photoperiod- and temperature-dependent. In laboratory studies, fish exposed to a short artificial photoperiod (LD 8:16) for one month responded to oTSH with T4 levels about twice as high as those in fish exposed to long days (LD 14:10). On the other hand the T4 response to oTSH was sharply reduced at 5 degrees C from that seen at 20 degrees C. This suggests that, in nature, seasonal changes in photoperiod and temperature have opposite effects on thyroidal responsiveness to TSH. Overall, the annual variation in the T4 response to oTSH appears to be driven by photoperiod, inasmuch as the alterations preceded major temperature changes in the wild and can be shown to occur at constant temperature in captivity. Photoperiodic induction of changes in thyroid sensitivity may aid in the maintenance of basal T4 levels under changing thermal conditions.

Prolactin release from prolactin-cell-containing tissue, the rostral pars distalis (RPD) of the tilapia pituitary, is sensitive to estrogens. This study was undertaken to evaluate the effect of two estrogenic pesticides, p,p'-DDT and o,p'-DDD, on the release of both tilapia prolactins (PRL177 and PRL188) from the RPD in static incubation, and to examine the effect of exposing tilapia fry to o,p' -DDD on growth and development. We found thatp,p' -DDT stimulated the release of both PRL177 and PRL188, and that o,p'-DDD inhibited the release of PRL188 at the concentrations employed. We also found that 8 months after exposing fry to o,p'-DDD for 28 days, the body weight and liver weight of males and females were significantly different from those of untreated control animals (P<0.01). Gonad weight and pituitary weight were not affected. We also found that 8 months after exposing fry to the estrogen ethynylestradiol (EE2), the body weight of males was significantly lower (P<0.01) and the body weight of females was significantly higher (P<0.01) than that of the respective untreated controls. Treatment with EE2 also significantly lowered (P<0.05) the liver weight of males and significantly increased (P<0.01) the gonad and pituitary weights of females compared with their respective controls. The lowest dose of o,p' -DDD used in the in vivo study induced a significant increase in the number of males, an effect opposite to that of EE2.