Maintaining a high physiological rate of testosterone secretion in the adult ram by estradiol immunoneutralization partially prevents testicular regression

Abstract

The seasonal testicular cycle of 10 adult Dorset × Leicester × Suffolk rams was entrained to alternating 4-mo periods of long-inhibitory (16 h light) and short-stimulatory (8 h light) day lengths. Beginning in the third month of a short-day period, when the testes were fully redeveloped, five rams were passively immunized with a low titre (1:200) of estradiol antiserum for 13 wk. During this time the testes of the remaining (control) rams regressed almost completely. Estradiol immunoneutralization reliably maintained most characteristics of testosterone secretion (mean and base-line concentrations, and pulse amplitude) at "breeding season" values and had a minor but persistent effect on the hypothalamic–pituitary axis. When both the immunization period and the following 6 mo were considered, LH pulses were 50% larger in the immunized rams than in the control rams. Testicular regression was partially prevented in the immunized rams. However, their 1.5- to 1.7-cm larger scrotal circumference during most of the regression stage was not associated with a greater daily sperm output (based on sperm voided in urine) early in the redevelopment stage. Mean LH and FSH concentrations were increased by 200% in all rams during testicular recrudescence, and maximum testis size was achieved 1 mo earlier in the immunized rams than in the control rams. Immunization of rams against estradiol effectively maintained a high physiological rate of testosterone secretion, but was not able to prevent the normal decline in quantitative spermatogenesis. Key words: Estradiol immunoneutralization, testicular regression, testosterone, adult ram