Early maturity in the male striped bass, Morone saxatilis: Follicle-stimulating hormone and luteinizing hormone gene expression and their regulation by gonadotropin-releasing hormone analogue and testosterone

Striped bass are seasonal breeding fish, spawning once a year during the sp ring. All 3-yr-old males are sexually mature; however, 60-64% of the fish m ature earlier as 1- or 2-yr-old animals. The endocrine basis underlying ear ly maturity in 2-yr-old males was studied at the molecular level by monitor ing changes in pituitary beta FSH and beta LH mRNA levels by ribonuclease p rotection assay, and correlating these changes to stages of testicular deve lopment. In maturing males, the mRNA levels of beta FSH were elevated durin g early spermatogenesis, whereas beta LH mRNA levels peaked during spermiat ion. The appearance of spermatozoa in the testis was associated with a decr ease in beta FSH mRNA and a rise in beta LH mRNA abundance. Immature males had lower levels of beta LH mRNA than maturing males, but there were no dif ferences in beta FSH mRNA levels between immature and maturing males. The r egulation of gonadotropin gene expression in 2-yr-old males was studied by the chronic administration of GnRH analogue (GnRHa) and testosterone CT), w ith or without pimozide (P) supplementation. In immature males, the combina tion of T and GnRHa stimulated a three- to fivefold increase in beta FSH an d beta LH 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 beta FSH and beta LH mRNA levels, suggesting that dopamine may have a novel role in regulating gonadotropin gene expression.