The effects of short day exposure on seasonal and circadian reproductive rhythms of female golden hamsters

Abstract

Four groups of female golden hamsters were exposed to short photoperiods (SP, LD 10:14) for 4, 14, 20, or 27 weeks and tested for physiological markers (uterine weight and estrous cycles) and behavioral (lordosis, approach and aggressive behaviors) measures while in contact with a stud male. After behavioral testing, females were ovariectomized and, during the next 2 weeks, were tested twice more (with a stud male) after replacement with 0.33 μg (low dose) and 1.0 μg (high dose) EB plus progesterone (500 μg). Results show that, after 14 weeks of SP conditions, uterine weights and percentage of females showing normal estrous cycles are at a minimum. This is mirrored by minimal levels of lordoses and maximal levels of aggressive and approach behavior at week 14. Physiological measures did not fully recover (to preregression levels) until week 27; however, behavioral measures show an earlier recovery by week 20. SP exposure also affects the circadian patterning of behaviors: Females that show lordosis at week 14 did so later in the day than did females tested at other weeks. Females in the regressed state also fail to show a significant decrease in approach behaviors (and a significant increase in receptive behaviors) over the couse of the circadian day, a pattern seen in nonregressed females. Following hormone replacement with the low EB (+P) dose, females do not become receptive; however, at the higher dose, all but the week 14 group show increased receptivity. In contrast, females show more approach behaviors in response to the low drug dose; all but the 14 week group show a marked decrease in approach behaviors in response to the higher drug dose. Females are highly aggressive after 14 weeks of SP exposure regardless of hormone level. The relationship between behavioral and physiological measures, between low versus high hormone replacement and between the different behavioral systems, and the effects on these of short-day exposure, are discussed.