Reproductive and nonreproductive responsiveness to photoperiod in laboratory rats

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, peripubertal 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 bulbectomy affect the expression of photoperiodism 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 bulbectomy amplified this inhibitory effect for some parameters including testicular and epididymal sperm counts. However, neither short days nor olfactory bulbectomy affected blood titers of follicle stimulating hormone (FSH) or prolactin. 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.(ABSTRACT TRUNCATED AT 250 WORDS)