TESTOSTERONE EFFECTS ON SPERMATOGENESIS IN THE GONADOTROPIN-RELEASINGHORMONE-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 fo r studying the hormonal control of spermatogenesis. The model was char acterized in terms of the efficacy of the immunogen as well as the tim e 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 g onadotropin-deficient rats. Adult Sprague-Dawley rats were actively im munized with a proprietory GnRH immunogen (BA-11, 100 mu g s.c.) every 4 wk. After 12 wk, 58% of animals showed markedly suppressed testicul ar size, as assessed by scrotal palpation, and were classed as respond ers. Serum LH, FSH, and T as well as the testicular elongated spermati d count (ESC) and epididymal sperm were undetectable in responding ani mals. Marked reductions in testicular (29% of control), prostatic (8% of control), and epididymal (32% of control) weights were seen. Sperma togenesis was severely disrupted with no evidence of progression beyon d 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 time s control levels, serum FSH levels were restored to 65% of control lev els by 2 wk and serum LH remained undetectable. Testicular weight incr eased to 80% of control levels at 12 wk (p < 0.05 vs. control). Epidid ymal and prostatic weights were normalized by T. ESC increased to 82% of control values at 12 wk (110 +/- 10 vs. control 134 +/- 8 million/t estis, p 0.001). Spermatogenesis was histologically normal after 8 wk of T24 treatment. To study the time course and dose response of T acti on, animals were immunized with another GnRH immunogen (BA-17), which yielded an 87% response rate at 12 wk Testicular weight increased by D ay 5 of T24 treatment, and a clear dose-response effect was apparent. The restoration of ESC was delayed compared to that of testicular weig ht (no restoration at 2 wk) and required greater than or equal to Tb t reatment. 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 greater than or eq ual to 3 cm. We conclude, first, that the GnRH immunization procedure using the BA-17 immunogen, by withdrawing gonadotropins, results in ev idence of severe regression of spermatogenesis in similar to 90% of ra ts 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 in dicates that a stable, albeit severely reduced, germ cell population i s maintained and will respond to hormonal stimuli. Such a model of gon adotropin 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 eith er by the partial restoration of testicular T levels and/or by the con comitant restoration of serum FSH levels via stimulation of FSH secret ion by GnRH-independent mechanisms.