Sex Hormones’ Regulation of Rodent Physical Activity: A Review

J. Timothy Lightfoot

In past studies physical activity was mostly thought of as a voluntary activity, but newer literature suggests that is regulated by biological factors. These factors include but are not limited to genetics and sex hormones.

In humans, females are generally less active than males. However, many rodent studies have shown that the female rodents are 20-50% more active each day compared to male rodents.

Some of the first studies showed that ovariectomies significantly decreased female rat activity, and this was similar to decreased running wheel activity in males following castration. The activity was restored when either ovarian tissue or testes were implanted in males or females, although the increase in activity was greater when the ovarian tissue was implanted. Therefore, future studies wanted to look at three specific sex hormones: estrogen, progesterone, and testosterone.

ESTROGEN: A study with female voles (who undergo induced estrus when exposed to males and who do not require progesterone for sexual receptivity), showed that the effect of estradiol on physical activity is linked to an increased number of estradiol receptors in the brain. These receptors can be in the alpha or beta isoform, and are located in the medial preoptic area and the anterior hypothalamus. Although it was determined that estrogenic activation of the ERalpha-pathway is the primary mediator in increased running wheel activity, the mechanism for it is unclear.

PROGESTERONE: When injected with estradiol, progesterone did not influence the activity of rats. However, when animals first received an injection of estrogen their activity increased. After progesterone was then injected, the activity sharply decreased. Once the progesterone injections were stopped, the activity increased once again. This suggested that the decrease in activity that progesterone causes was mediated through direct interference with estrogen. This would help explain why the variable activity pattern occurs in female rats.

TESTOSTERONE: When capsules of testosterone were implanted in castrated male rats, their locomoter activity increased. Other studies found that testosterone injections significantly increased running wheel activity in animals, however not as much as estradiol injections. In addition, testosterone implants in castrated males restored physical activity, but no increase in physical activity levels were seen when testosterone supplementation was given to intact animals.

In conclusion, female rodents are more active than male rodents due to sex hormones. This mechanism is mediated through an estrogen-alpha receptor pathway. This pathway requires the aromatization of testosterone into estrogen within the male animals.

 

~LNM