Reproduction and longevity in the hamster and rat

The role of estrogen deficiency in skin ageing and wound healing

The links between hormonal signalling and lifespan have been well documented in a range of model organisms. For example, in C. elegans or D. melanogaster, lifespan can be modulated by ablating germline cells, or manipulating reproductive history or pregnenolone signalling. In mammalian systems, however, hormonal contribution to longevity is less well understood. With increasing age human steroid hormone profiles change substantially, particularly following menopause in women. This article reviews recent links between steroid sex hormones and ageing, with special emphasis on the skin and wound repair. Estrogen, which substantially decreases with advancing age in both males and females, protects against multiple aspects of cellular ageing in rodent models, including oxidative damage, telomere shortening and cellular senescence. Estrogen's effects are particularly pronounced in the skin where cutaneous changes post-menopause are well documented, and can be partially reversed by classical Hormone Replacement Therapy (HRT). Our research shows that while chronological ageing has clear effects on skin wound healing, falling estrogen levels are the principle mediator of these effects. Thus, both HRT and topical estrogen replacement substantially accelerate healing in elderly humans, but are associated with unwanted deleterious effects, particularly cancer promotion. In fact, much current research effort is being invested in exploring the therapeutic potential of estrogen signalling manipulation to reverse age-associated pathology in peripheral tissues. In the case of the skin the differential targeting of estrogen receptors to promote healing in aged subjects is a real therapeutic possibility.

Evaluation of sex differences on mitochondrial bioenergetics and apoptosis in mice

It has been postulated that the differences in longevity observed between organisms of different sexes within a species can be attributed to differences in oxidative stress. It is generally accepted that differences are due to the higher female estrogen levels. However, in some species males live the same or longer despite their lower estrogen values. Therefore, in the present study, we analyze key parameters of mitochondrial bioenergetics, oxidative stress and apoptosis in the B6 (C57Bl/6J) mouse strain. There are no differences in longevity between males and females in this mouse strain, although estrogen levels are higher in females. We did not find any differences in heart, skeletal muscle and liver mitochondrial oxygen consumption (State 3 and State 4) and ATP content between male and female mice. Moreover, mitochondrial H(2)O(2) generation and oxidative stress levels determined by cytosolic protein carbonyls and concentration of 8-hydroxy-2'-deoxyguanosine in mitochondrial DNA were similar in both sexes. In addition, markers of apoptosis (caspase-3, caspase-9 and mono- and oligonucleosomes: the apoptosis index) were not different between male and female mice. These data show that there are no differences in mitochondrial bioenergetics, oxidative stress and apoptosis due to gender in this mouse strain according with the lack of differences in longevity. These results support the Mitochondrial Free Radical Theory of Aging, and indicate that oxidative stress generation independent of estrogen levels determines aging rate.

Factors limiting reproductive efficiency in selected laboratory animals

The mouse, rat, hamster, and rabbit have had remarkable success in surviving in the wild, which attests to their high reproductive capability. In fact, Parkes refers to mice conceiving at each postpartum estrus having the potential for 13 litters per year. Paufler et al. used GnRH to repeatedly cause ovulation in 27 rabbits a few days after parturition, resulting in an average pregnancy rate of 71.5%, 7.0 young born, 5.8 young weaned, and 50 young per doe per year. All four species produce an excess of spermatozoa relative to the requirements for fertilization. The rabbit is suitable for semen collection, artificial insemination, and most of the techniques one might wish to model for reproductive studies in both males and females. A major limitation to reproduction in these species is that reproductive capacity fails long before the finite oocyte population formed prenatally is depleted. The uterus of the aged female appears to be the major cause of reproductive failure, as fertilized eggs replaced in such a uterus usually soon deteriorate. With in vitro techniques, much is still to be learned about harvesting, maturing and fertilizing oocytes and identifying those most likely to result in formation of a healthy neonate.

The effects of age on substrate depletion and hormonal responses during submaximal exercise in hamsters

Senescent hamsters display a marked reduction in volume of voluntary running. The purpose of this study was to determine whether age differences exist in the pattern of fuel utilization during submaximal exercise, which may account for the reduction in voluntary running. Further, we determined the effects of age on muscle oxidative capacity to assess its relationship to endurance performance in senescent hamsters. Depletion of carbohydrate and lipid content of skeletal muscle and liver, and changes in blood concentration of various hormones and substrates during one hour of exercise at 60 percent of VO2 max served to assess age effects on utilization of metabolic substrates. Exercise produced equivalent depletion of muscle glycogen and similar rise in plasma free fatty acids in young and old hamsters. No exercise effects on skeletal muscle triglyceride concentration or on plasma glycerol, glucagon or catecholamine concentrations were noted. With palmitoyl carnitine as substrate (but not with pyruvate) State 3 respiration of cardiac and skeletal muscle homogenates was lower in old compared to young hamsters. Although old hamsters have a reduced capacity to oxidize lipids in vitro, few age differences in fuel use are evident in vivo during submaximal exercise. Thus, these minor age differences in substrate utilization do not likely account for the substantial reduction in the levels of spontaneous running in senescent hamsters.

Age-related decline in receptivity in normal, neonatally androgenized female and male hamsters

The pattern of female sexual behavior shown by aging male and female hamsters was related to exposure to perinatal androgen. Twenty-five female and 37 male hamsters in Experiment 1 was gonadectomized on the day of birth or at 43 days of age. The received 11 tests for receptivity between the ages of 28 and 66 wk. In Experiment 2, 83 females were implanted with a Silastic pellet containing 3, 1, 3 or 9% testosterone (T) or testosterone propionate (TP) by weight from the second to the tenth day of life, and tested approximately every 2 wk from 22 to 58 wk of age. Receptivity was induced by estradiol benzoate and progesterone injected before each 600 sec mating test. Normal females exhibited the highest lordosis scores, followed by minimally androgenized females and males castrated at birth. High doses of exogenous or endogenous perinatal T suppressed receptivity throughout the animal's lifespan, but did not accelerate the rate of decline in female behavior over time. Non-androgenized animals showed the greatest decrease in receptivity with advanced age.