The effects of long-term testosterone, gonadotropin-releasing hormone agonist and pimozide treatments on testicular development and luteinizing hormone levels in juvenile and early maturing striped bass, Morone saxatilis

Abstract

The present study was conducted to test the responsiveness of the juvenile male reproductive axis to hormonal stimulation and to compare it to that of early maturing males. Long-term treatments with various combinations of T, GnRHa and pimozide did not result in an increased incidence of early maturing males, but did stimulate spermatogenesis slightly in juvenile fish. In early maturing males, the treatments appeared to be inhibitory since they resulted in a reduction of the GSI and a lower incidence of spermiating males. In early maturing males, pituitary LH content was elevated by GnRHa treatments alone while in juvenile males a combination of T and GnRHa was needed to increase the levels of LH in the pituitary. Thus, T may play an important role during puberty by potentiating the effects of GnRH on LH synthesis. In both juvenile and early maturing males, plasma LH levels could be increased only by high doses of GnRHa (in combination with T). Therefore, LH synthesis and release probably require different levels of GnRH stimulation. A GnRH challenge (single injection of 50 μg GnRHa/kg) at the end of the experiment resulted in a dramatic elevation of plasma LH levels in almost all animals. This finding demonstrates that pituitaries from juvenile and early maturing males were responsive to GnRHa stimulation, even after long-term hormonal treatments. The addition of pimozide did not affect the T- and GnRHa-induced increase in pituitary LH content but inhibited the release of LH in response to a GnRHa challenge. In conclusion, high doses of GnRHa in combination with T can increase plasma LH levels in juvenile males but do not induce complete testicular maturation. Factors other than T, GnRHa or LH are probably involved in the induction and completion of spermatogenesis.