Short photoperiods evoke testicular apoptosis in white-footed mice (Peromyscus leucopus)

Many small, nontropical mammals stop breeding during winter. Chronic exposu re of males to short days (<12.5 h light/day) causes the testes to atrophy and both steroidogenesis and gametogenesis to decrease. Male white-footed m ice (Peromyscus leucopus) exposed to inhibitory short day lengths provide a natural animal model to study the cellular mechanisms regulating testicula r regression. In the present study, the possible role of apoptosis was asse ssed during naturally occurring, short day-induced gonadal regression in wh ite-footed mice by in situ terminal transferase-mediated end labeling (TUNE L), quantitative DNA 3'-end-labeling autoradiography (laddering) of DNA fra gments, and quantification of Fas protein expression, an early initiator of apoptosis. Sexually mature male mice were exposed to short (8 h of light, 16 h of darkness) or long (16 h of light, 8 h of darkness) day lengths for 2, 4, 6, 8, or 10 weeks; gonads were then removed and processed for detecti on of apoptotic activity. In common with previous studies, the first signif icant reduction in relative testis mass was observed at week 10 of short da y exposure. A 2- to 3-fold increase in apoptotic (TUNEL-positive) germ cell s per seminiferous tubule was observed in the testes of mice exposed to sho rt days for 4, 6, 8, or 10 weeks compared with the testes of long day anima ls. The extent of 3'-end labeling of low mol wt DNA increased with 4- 8 wee ks of short day exposure. Western blot analysis revealed an up-regulation o f the Fas protein in the testes of short day males at 4, 8, and 10 weeks. F as staining was primarily localized to spermatocytes and spermatids. Plasma testosterone concentrations decreased in short compared with long day anim als after 6, 8, or 10 weeks. The increase in TUNEL positive-labeled germ ce lls, testicular DNA fragmentation, and up-regulation of the Fas protein bef ore short day reductions of testis mass and function suggest that apoptosis is important for the mediation of photoperiod-induced testicular regressio n in whitefooted mice.