In Wistar rats, daily injections of melatonin 1-3 h before lights-out on a 12L:12D cycle induce greater testicular inhibition than SD, with melatonin-injected rats having testes ca. 50% smaller than those of controls. Young Fischer 344 (F344) rats are more strongly inhibited than other strains by blinding or SD. For example, males in SD for 2-6 wk have testes averaging up to 50% smaller than males in long days (LD). Collectively, these data show that strains of laboratory rats have the neuroanatomical connections that mediate reproductive sensitivity to photoperiod but that these connections normally range from nonfunctional in adults to functional in young rats of some strains.
In theory, peripubertal rodents should be more strongly affected by inhibitory photoperiods than are adults, because young rodents are less experienced and have lower reserves of energy, and thus are more likely to fail to raise offspring or even to die in a reproductive attempt. Higher sensitivity of younger animals to inhibitory photoperiods has been reported in rats and some other rodents, including the strongly photoperiodic Siberian hamster. Because a typical rodent can expect only one or two reproductive opportunities in its life, the regulation of puberty by photoperiod is probably more important than adult photoresponsiveness for many species. For most rodents, the correct timing of puberty may be their single most important reproductive decision. buy levaquin online
While pubertal inhibition in response to SD or similar treatments is robust in F344 rats, reproduction is not inhibited as strongly as in the classically photoresponsive rodents—speeiesofhamsters£mdvoles,orspeeiesof Pero-myscus (deer mice and white-footed mice). We hypothesize that F344 rats have two components to photoresponsive-ness—oneanobligote inhibiSionbySD,cmdthefther a facultative inhibition in response to biologically relevant challenges.