Early gonadotropin secretion and sexual maturation in bull calves and ram lambs

Abstract

Regulation of the early rise in gonadotropin secretion in bull calves and its involvement in the reproductive development of bull calves and ram lambs were studied. Development of the testes, prostate and seminal vesicles was assessed from birth to puberty by ultrasonagraphy and image intensity analysis in bull calves and ram lambs. A GnRH superagonist ( Leuprolide acetate) was used to suppress early gonadotropin secretion in bull calves and ram lambs, to study the importance of early gonadotropin secretion for testicular development. GnRH treatment was used to induce LH secretion in the early postnatal period in bull calves and its effects on testicular and endocrine development were studied. Histology and flowcytometry were used to evaluate testicular development in these studies. The role of estradiol in the negative feedback regulation of early gonadotropin secretion was studied in bull calves by using an aromatase inhibitor (ICI 16949A) and a nonsteroidal antiestrogen (ICI 182,780). The involvement of opioidergic and dopaminergic neuronal systems in the regulation of the early rise in gonadotropin secretion was studied in bull calves by using their receptor antagonists (naloxone and pimozide, respectively). Suppressing the early increase in gonadotropin secretion stressed its importance for testicular development in bull calves. The timing of the early increase in secretion of gonadotropins is also important; by inducing higher gonadotropin secretion early in the life of bull calves, reproductive development was enhanced. In ram lambs, high gonadotropin secretion up to 11 weeks of age did not appear critical for reproductive development. We were unable demonstrate a role for estradiol in the negative feedback regulation of early gonadotropin secretion in bull calves. Opioidergic and dopaminergic neuronal systems appeared to be important in the regulation of gonadotropin secretion in bull calves towards the end of the early increase in LH secretion; opioids may be critical for termination of the early rise in LH secretion. Ultrasonography and computer assisted image intensity analysis appeared to be very useful techniques in assessment of reproductive development in bull calves and ram lambs. Image intensity analysis showed promise for identifying critical stages in development of the reproductive system.