Temporal Changes in Testicular Histology and Steroidogenesis in Juvenile Bank Voles (Clethrionomys glareolus, Schreber) Subjected to Different Photoperiods

The temporal relationships of testicular steroidogenesis, gametogenesis, and interstitial cell histo0logy during testicular maturation and regression induced by photoperiod were investigated. Juvenile bank voles (18–22 days old) were killed immediately or after exposure to either a long [18-h light-6-h dark (18L:6D)] or a short (6L:18D) photoperiod for 1, 2, 4, and 6 weeks. Testicular pregnenolone (Δ⁵-P), progesterone (Δ⁴-P), 17αa-hydroxyprogesterone (17-Δ⁴-P) and testosterone (T) concentrations were determined, and the histological state of the Leydig cells and seminiferous epithelium were evaluated. After 2 weeks of exposure to the short photoperiod, marked histological changes were evident in both testicular compartments, and a decline in the testicular T level was observed. The intratesticular concentrations of Δ⁵-P remained low and that of 17-Δ⁴-P declined during the first 2 weeks of exposure to the short photoperiod. After 4 and 6 weeks in the short photoperiod, spermatogenesis was virtually absent and the mean concentrations of all four steroids were very low, indicating a marked suppression of T synthesis. In voles subjected to the long photoperiod, there was an increase in Δ⁵-P, Δ⁴-P, and T levels as the testes developed. Despite considerable individual variation in testicular steroid concentrations, the relative amounts of the various steroids were similar after 4–6 weeks of exposure to long days, Δ⁵-P and 17-Δ⁴-P concentrations being lower than those of Δ^4-P and T. These findings indicate that the enzymes catalyzing cholesterol side chain cleavage, C₁₇-C₂₀-lyase and/or 17αhydroxylase, may be rate limiting in the biosynthesis of T in this species. The results suggest that short photoperiods inhibit androgen synthesis in the bank vole by modifying cholesterol side chain cleavage, and possibly C₁₇-C₂₀-lyase and/or 17αhydroxylase activities in the testis.