Testosterone effects on spermatogenesis in the gonadotropin-releasing hormone-immunized rat

Active immunization of adult rats with a GnRH fusion protein was used to inhibit gonadotropin secretion and to establish an in vivo model for studying the hormonal control of spermatogenesis. The model was characterized in terms of the efficacy of the immunogen as well as the time course and nature of the immunological and biological responses. To study the reinitiation of spermatogenesis, testosterone (T) was chosen for this initial study as it is known to restore spermatogenesis in gonadotropin-deficient rats. Adult Sprague-Dawley rats were actively immunized with a proprietory GnRH immunogen (BA-11, 100 μg s.c.) every 4 wk. After 12 wk, 58% of animals showed markedly suppressed testicular size, as assessed by scrotal palpation, and were classed as responders. Serum LH, FSH, and T as well as the testicular elongated spermatid count (ESC) and epididymal sperm were undetectable in responding animals. Marked reductions in testicular (29% of control), prostatic (8% of control), and epididymal (32% of control) weights were seen. Spermatogenesis was severely disrupted with no evidence of progression beyond round spermatids. To study the action of T in GnRH-immunized animals, T (defined by lengths of s.c. silastic implant, T3-T24 cm) was given to responding animals. Animals were killed 2, 8, and 12 wk after T24 administration. In response to T24, serum T levels increased to 4 times control levels, serum FSH levels were restored to 65% of control levels by 2 wk, and serum LH remained undetectable. Testicular weight increased to 80% of control levels at 12 wk (p <0.05 vs. control). Epididymal and prostatic weights were normalized by T. ESC increased to 82% of control values at 12 wk (110 ± 10 vs. control 134 ± 8 million/testis, p = 0.001). Spermatogenesis was histologically normal after 8 wk of T24 treatment. To study the time course and dose response of T action, animals were immunized with another GnRH immunogen (BA-17), which yielded an 87% response rate at 12 wk. Testicular weight increased by Day 5 of T24 treatment, and a clear dose- response effect was apparent. The restoration of ESC was delayed compared to that of testicular weight (no restoration at 2 wk) and required ≥ T6 treatment. Rats immunized for 20 wk and then given T24 treatment showed a similar pattern of restoration in testicular weight and ESC. Serum FSH was normalized by Day 2 of T treatment by doses ≥ 3 cm. We conclude, first, that the GnRH immunization procedure using the BA-17 immunogen, by withdrawing gonadotropins, results in evidence of severe regression of spermatogenesis in ~90% of rats within 12 wk and provides a convenient and highly efficient in vivo model for the study of spermatogenesis. The ability of the regressed testis to respond to T even after many weeks of profound regression indicates that a stable, albeit severely reduced, germ cell population is maintained and will respond to hormonal stimuli. Such a model of gonadotropin deficiency is ideal for the study of the action of hormones potentially regulating spermatogenesis. Our second conclusion is that T substantially, but not quantitatively, restores spermatogenesis either by the partial restoration of testicular T levels and/or by the concomitant restoration of serum FSH levels via stimulation of FSH secretion by GnRH-independent mechanisms.