Striped bass are seasonal breeding fish, spawning once a year during the spring. All 3-yr-old males are sexually mature; however, 60–64% of the fish mature earlier as 1- or 2-yr-old animals. The endocrine basis underlying early maturity in 2-yr-old males was studied at the molecular level by monitoring changes in pituitary βFSH and βLH mRNA levels by ribonuclease protection assay, and correlating these changes to stages of testicular development. In maturing males, the mRNA levels of βFSH were elevated during early spermatogenesis, whereas βLH mRNA levels peaked during spermiation. The appearance of spermatozoa in the testis was associated with a decrease in βFSH mRNA and a rise in βLH mRNA abundance. Immature males had lower levels of βLH mRNA than maturing males, but there were no differences in βFSH mRNA levels between immature and maturing males. The regulation of gonadotropin gene expression in 2-yr-old males was studied by the chronic administration of GnRH analogue (GnRHa) and testosterone (T), with or without pimozide (P) supplementation. In immature males, the combination of T and GnRHa stimulated a three- to fivefold increase in βFSH and βLH mRNA levels, but the same treatment had no effect on gonadotropin gene expression in maturing males. In addition, the coadministration of P to immature males suppressed the stimulatory effect of GnRHa and T on βFSH and βLH mRNA levels, suggesting that dopamine may have a novel role in regulating gonadotropin gene expression.
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