Reproductive and no reproductive responsiveness to photoperiod in laboratory rats

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Abstract
Nelson RJ, Moffatt CA, Goldman BD. Reproductive and no reproductive responsiveness to photoperiod in laboratory rats. J. Pineal. Res. 1994; 17: 123–131 Abstract Laboratory rats (Rattus norvegicus) have been traditionally considered nonphotoperiodic because reproductive function is unaffected by day length. However, at least three experimental manipulations of rats—perinatal androgen injection, per pubertal androgen implants, and peripubertal olfactory bulbectomy—have been reported to unmask reproductive responsiveness to photoperiod. The physiological means by which early testosterone treatment or olfactory bluestem affect the expression of photoperiodic were hypothesized to operate through similar underlying mechanism(s) that involved gonadotropin and prolactin blood levels. Short day lengths reduce blood levels of gonadotropins in so-called photoperiodic rodent species. Reduced prolactin levels result in virtually all reproductively photoperiodic species housed in short day lengths. In Experiment 1, male weanling rats either were olfactory-bulbectomized or received a sham-procedure and housed for 10 weeks in long (LD 16: 8) or short (LD 8: 16) days. Short-day rats reduced body mass, testicular sperm counts, and the size of their reproductive systems; olfactory bluestem amplified this inhibitory effect for some parameters including testicular and epididymal sperm counts. However, neither short days nor olfactory bluestem affected blood titers of follicle stimulating hormone (FSH) or prorating. Pelage density was also unaffected by photoperiod, but rats retained their juvenile fur color; i. e., short-day rats remained white, but long-day rats became yellowish. In Experiment 2, male rats were injected with testosterone at 3 days of age, then housed in long or short days until 10 weeks of age. Day length alone did not affect any experimental parameter measured in Experiment 2 except fur color; again, short-day rats retained their juvenile fur color. Perinatal testosterone treatment combined with short day lengths suppressed reproductive organ size and function. Blood plasma concentrations of FSH, but not prolactin were reduced at 6 and 10 weeks of age by early testosterone injection. In Experiment 3, male offspring of rats born in our lab were weaned at 21 days of age and implanted with a Silastic capsule that was either empty or filled with testosterone. Animals were housed for 10 weeks in long or short days. Photoperiod did not affect reproductive organ size or function in this experiment; however, pelage color was again affected by day length. There were no significant interactions between photoperiod and hormonal treatment. Rats bearing testosterone capsules displayed reduced reproductive organ size and function. Blood levels of FSH were reduced at 6, but not 10, weeks of age in these animals. Prolactin concentrations were unaffected by testosterone treatment. Taken together, these results suggest that laboratory rats retain some vestiges of photoperiodic responsiveness, indicating that the physiological capability to measure day length is extant, but that the effects of photoperiodic regulation on reproduction are minor and probably of limited functional significance in the laboratory setting.