Cb. Lazier et al., ANDROGEN INHIBITION OF VITELLOGENIN GENE-EXPRESSION IN TILAPIA (OREOCHROMIS-NILOTICUS), General and comparative endocrinology, 104(3), 1996, pp. 321-329
Treatment of mature female tilapia (Oreochromis niloticus) with high l
evels of androgen (17 alpha-methyltestosterone, 17 alpha MT) results i
n a pronounced decline in plasma vitellogenin levels as determined by
gel electrophoresis. Total RNA extracted from livers of treated fish a
nd vehicle-injected controls was analyzed by Northern and slot blot hy
bridization using an oligonucleotide complementary to a sequence in th
e 3' end of tilapia vitellogenin mRNA. The probe revealed an mRNA of 6
.5 kb in liver from the control mature female fish which was decreased
by 85% by androgen treatment. As expected, estradiol (E(2)) treatment
induced the 6.5-kb mRNA in mature male tilapia. The antiestrogen, tam
oxifen, strongly decreased vitellogenin mRNA levels in mature females.
Radioimmunoassay of serum from control and 17 alpha MT-treated female
tilapia showed a marked reduction in serum E(2) levels, from 11.4 +/-
2.6 ng/ml in controls to 2.2 +/- 0.13 ng/ml in treated fish. Tamoxife
n, however, resulted in increased serum E(2) levels, probably by block
ing E(2) negative feedback. The serum E(2)-lowering effect of 17 alpha
MT suggests an inhibitory site of action on gonadotropin production a
t the hypothalamic-pituitary axis, possibly through an androgen recept
or or through an estrogen receptor after local aromatization of 17 alp
ha MT. (C) 1996 Academic Press, Inc.