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Altered plasma corticosterone responses during lactation in mice of two inbred strains
Plasma corticosterone levels during lactation were examined in inbred A/J (A) and C57BL/6J (C) mice. Following shock, lactating A females showed a diminished corticosterone response relative to nonlactating controls on day 5 of lactation, but not on day 12. In contrast, plasma corticosterone levels of lactating C females 5 and 15 min following shock were lower than those of controls on day 12, but not on day 5. Consistent with these findings, lactating A females had significantly lower plasma levels of corticosterone 15 min following nonnociceptive treatments (involving distrubance, exposure to novelty, and pup manipulation) when tested on day 5 then they did when tested on day 12, whereas lactating C females had lower levels on day 12 than on day 5. Lactating females of each strain showed the same response to shock on day 12 if they reared A pups as they did if they reared C pups. This indicates that the difference found between the strains on day 12 was due to a difference in some property of A and C mothers, rather than to a difference in a characteristic, such as suckling patterns, of the pups of the two strains. It was also found that at long intervals following shock (45 and 90 min) 5-day lactating C females had higher levels of plasma corticosterone than did controls. There was no effect of lactation on resting adrenocorticoid levels in A or C females at either 5 or 12 days. The results indicate that laboratory mice show changes in hypothalamo-pituitary-adrenal activity during lactation which both vary with strain of the mother, and differ in several respects from those previously reported for rats.
Plasma corticosterone levels during lactation were examined in inbred A/J (A) and C57BL/6J (C) mice. Following shock, lactating A females showed a diminished corticosterone response relative to nonlactating controls on day 5 of lactation, but not on day 12. In contrast, plasma corticosterone levels of lactating C females 5 and 15 min following shock were lower than those of controls on day 12, but not on day 5. Consistent with these findings, lactating A females had significantly lower plasma levels of corticosterone 15 min following nonnociceptive treatments (involving distrubance, exposure to novelty, and pup manipulation) when tested on day 5 then they did when tested on day 12, whereas lactating C females had lower levels on day 12 than on day 5. Lactating females of each strain showed the same response to shock on day 12 if they reared A pups as they did if they reared C pups. This indicates that the difference found between the strains on day 12 was due to a difference in some property of A and C mothers, rather than to a difference in a characteristic, such as suckling patterns, of the pups of the two strains. It was also found that at long intervals following shock (45 and 90 min) 5-day lactating C females had higher levels of plasma corticosterone than did controls. There was no effect of lactation on resting adrenocorticoid levels in A or C females at either 5 or 12 days. The results indicate that laboratory mice show changes in hypothalamo-pituitary-adrenal activity during lactation which both vary with strain of the mother, and differ in several respects from those previously reported for rats.