Influence of Testosterone and/or Luteinizing Hormone Releasing Hormone Analogue on Precocious Sexual Development in the Juvenile Rainbow Trout 1

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Abstract
The influence of testosterone, LHRH agonist and combinations of these hormones on gonadotropic hormone (GtH) levels in the sexually immature trout was investigated. Both the steroid and releasing hormone preparations, testosterone in Silastic capsules and cholesterol-pelleted LHRH-A, were formulated for sustained release and long-term biological action following a single hormone implantation. Marked increases in pituitary GtH followed testosterone and/or testosterone and LHRH analog treatment combined, but the low pituitary GtH level in controls remained unchanged after LHRH analog administration alone. Plasma GtH titers increased with time after testosterone treatment, indicating a positive steroid feedback effect by androgen on GtH in the juvenile rainbow trout. When combined with testosterone treatment, LHRH analog augmented plasma GtH levels compared to fish receiving testosterone treatment alone. In males the elevated plasma GtH levels were associated with testes stimulation and onset of spermatogenesis; in females, however, no significant stimulation of the ovaries was observed. The testosterone stimulus is sufficient to induce onset of sexual development in immature males but not females. Whereas LHRH analog releases GtH from the testosterone-primed trout pituitary, LHRH treatment alone under these conditions fails to stimulate the juvenile trout reproductive system.