The Effect Of Hot Temperature On Lactation Capacity And Mechanism In Swiss Mice

Reproduction is an important stage of an animal's life history.Animals take different energy strategies to cope with environmental temperature changes.Lactation capacity is a major factor that determines the reproductive performance and reproductive value of animals,which is significantly affected by temperature.However,the effects of environmental temperature changes on lactation capacity and milk energy output of the mammary glands and their regulatory mechanisms are still unclear.The female Swiss mice were used as an animal model in the present study.Experiment 1: metabolic rate and the thermal neutral zone(TNZ)were determined in the female Swiss mice at different temperatures.Experiment 2: the lactating Swiss mice were randomly assigned into four groups,21?(control group),27?(lower than TNZ critical point temperature),30?(critical point temperature in TNZ)and33?(higher than TNZ upper critical point temperature).The females were exposed to the above temperature throughout lactation,during which energy intake and milk energy output were determined.Experiment 3: female mice were randomly divided into four groups,21?group(the females were lactating at 21? throughout lactation),21-30? group(the females were lactating at 21? from day 0 to 10,but were transferred to 30? on day 11 and thereafter),30?(the females were lactating at 30? throughout lactation)and 30-21?(the females were lactating at 30? from day 0 to 10,but were transferred 21? on day 11 and thereafter).The changes of body mass,food intake,litter size,litter mass and milk energy output were examined during lactation.The expressions of gene and / or protein of a number of genes in the mammary glands associated with milk production were also examined.The results showed that: 1)the TNZ of Swiss mice ranged from 30? to32.5?;2)body mass,food intake,litter mass and milk energy output were significantly reduced in the females exposed to the upper critical point temperature of TNZ;3)food intake,litter mass and milk energy output were increased significantly in the females after being transferred from the 30? to 21? relative to those lactating at 30?,while they failed to recover to the levels that observed in the females lactating at 21?,indicating that both energy intake and milk energy output were constrained under the hot temperatures;4)in the females in 21-30?,30? and 30-21? groups,the down-regulation of gene and protein expression of several genes in the mammary glands were observed,such as Stat3,Stat5 a,and Stat5 b.Several genes related to milk protein synthesis were also down-regulated significantly in response to hottemperatures.The findings of the present study suggest that hot temperature may be the factors inducing the limitation on the mammary glands in female mice,which may fail to recover.The adaptive regulation of gene expression of Stat3,Stat5 a and Stat5 b in the mammary glands may play important roles in the response to hot temperature,which may be the molecular mechanisms of the limitation on the capacity to produce milk in the mammary glands in the small mammals exposed to hot temperature.