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al., 1991). Taken together, it is highly possible that cortisol at least plays
some role in fl ounder metamorphosis. Adrenal corticosteroids seem to
enhance the effects of thyroid hormone by augmenting nuclear binding
capacity of tadpole tail nuclei to thyroid hormone (Niki et al., 1981; Suzuki
and Kikuyama, 1983). Such works are needed to know the mechanism of
permissive action of cortisol in fl ounder metamorphosis.
4.2.7.2 Sex steroids
de Jesus et al. (1992) showed modulation of thyroid hormone action by sex
steroids in a study using in vitro culture of fi n rays of the Japanese fl ounder.
Neither estradiol nor testosterone, had any direct effect on the shortening
of fi n rays. However, when either of the hormones was added together
with thyroid hormone, fi n-ray shortening was retarded compared with
that treated with thyroid hormone alone. Teststerone was more potent in
the inhibition than estradiol. In addition, when fl ounders were reared in
seawater containing either estradiol or testosterone, both steroids delayed
metamorphosis, again with testosterone being more potent in the inhibition
of metamorphosis. On the other hand, tissue levels of these hormones
are kept rather low and both steroids do not show any signifi cant change
throughout the metamorphosis (de Jesus et al., 1992). In addition, the
hormone doses used in the in vitro study were apparently higher than the
tissue hormone levels in the fl ounder during spontaneous metamorphosis.
Thus, the physiological signifi cance of sex steroids on modulation of thyroid
hormone action in metamorphosis is not clear.
4.2.7.3 Prolactin and growth hormone
Prolactin is known to antagonize thyroid hormone action in amphibian
metamorphosis (Yoshizato and Yasumasu, 1972a, b; Jaffe and Geschwind,
1974; Derby, 1975; Dent, 1988). When the hormone was added to in vitro
culture of the elongated fi n rays of the fl ounder, prolactin also exhibited
an antagonistic effect on the fi n-ray shortening stimulated by thyroid
hormone, though prolactin alone did not exert any effect (de Jesus et al.,
1994). Injection of prolactin into prometamorphic larvae also delayed the
resorption of the dorsal fi n rays, though other metamorphic parameters
such as eye migration and settling behavior were not affected (de Jesus et
al., 1994). Furthermore, the same authors found that expression level of
prolactin gene in the pituitary estimated by in situ hybridization steadily
increased during metamorphosis and became highest at post climax. Steady
increase in the volume of prolactin cells in the pituitary until climax of
metamorphosis of the fl ounder was also reported by Hiroi et al. (1997).
Above mentioned antimetamorphic and/or antagonistic action of prolactin
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