Photoperiod is generally considered the major cue in the regulation of reproduction in temperate zone rodents. However, both field and laboratory studies show that typical temperate zone rodents often use a combination of environmental cues, including photoperiod, food abundance, and temperature, to regulate reproduction seasonally. Thus, species of rodents following seasonal reproductive strategies may rely differentially on photoperiod and other environmental cues.
Laboratory rats are derived from temperate zone ancestors, but adult laboratory rats are not traditionally considered reproductively photoperiodic. However, a reproductive response to short days (SD) and/or melatonin treatment can be induced in adult male Sprague-Dawley, Wistar, and some other strains of rats by several procedures: food deprivation, neonatal androgen treatment, chronic exposure to exogenous testosterone (T), or olfactory bulbectomy, but see). buy ortho tri-cyclen online
In contrast, puberty can be delayed in some rats by treatment with photoperiod alone. Constant dark, blinding, or very short days (light < 2 h) can delay puberty in some strains of rats, but the effects reported have been slight, on the order of a 1- to 2-wk delay in vaginal opening or a 0-20% smaller testis size relative to controls. In Wistar rats, testicular development is inhibited by constant dark (testes 30% smaller than those of controls after 8 wk). In the more biologically reasonable condition of SD (6-8 h light), testicular development of Wistar or Sprague-Dawley rats is either uninhibited (Wistar: no difference after 7 wk; Sprague-Dawley: no difference from 2 to 10 wk ) or moderately inhibited (Wistar: 30% smaller after 4 wk ).