House mice (Mus musculus) and laboratory strains of rats (Rattus norvegicus) have been traditionally considered nonphotoperiodic because their reproductive systems are unaffected by day length (photoperiod). In rats, however, at least three experimental manipulations, perinatal testosterone injection, chronic peripubertal testosterone exposure, or peripubertal olfactory bulbectomy, have revealed latent reproductive photoperiodism. The effectiveness of these experimental treatments may be unique to albino rats. Alternatively, these experimental manipulations may unmask the ability to discriminate short from long days in several "nonphotoperiodic" species and, thus, reveal clues to common physiological mechanisms underlying reproductive responsiveness to photoperiod. In the present study, male house mice were 1) subjected to olfactory bulbectomy or a sham operation at 23 days of age, 2) injected with testosterone or the oil vehicle at 3 days of age, or 3) implanted subcutaneously with an empty Silastic capsule or one filled with testosterone at 22 days of age. All mice were subsequently housed either in LD 16:8 or LD 4:20 photoperiods. The physiological mechanisms necessary to discriminate long from short day lengths are extant in house mice. Testicular mass was significantly reduced in short-day bulbectomized males when assessed 6 weeks postoperatively, but not when measured 10 weeks after surgery. Similarly, mice injected with testosterone when 3 days old and reared in short days had smaller testes as compared to testosterone-treated males housed in long days. Mice implanted with testosterone capsules regressed their reproductive systems regardless of photoperiod. Other reproductive organ weights followed the same general pattern of results as for testicular mass. Body mass was not affected by day length. Taken together, these results indicate that mice process photoperiodic information, but that it is normally uncoupled from reproduction. These data also suggest that it may be more appropriate to categorize traits, not individuals or species, as responsive to photoperiod. © 1990.
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