Mice: Progesterone and the regulation of strain differences in pregnancy-induced nest building

Four experiments with 113 C57BL/6J and 80 DBA/2J female mice showed that pregnant DBA/2J Ss built significantly larger and more completely enclosed nests than did pregnant C57BL/6J Ss. This strain difference was restricted to the last half of gestation and was not observed during either the virgin state or lactation. Genotype-based differences in pregnancy-induced nest building were not related to circulating levels of progesterone (P), core temperature, or body weight. Exposure to supplemental P (100, 200, or 400 |mg, sc) during pregnancy elevated nest building exhibited by pregnant C57BL Ss but did not induce DBA-like levels of the behavior. Also, virgin DBA Ss built larger nests in response to P than did C57BL Ss. Findings suggest that differences in the sensitivity of central neural tissue to steroid hormones may account for genotypically determined variation in patterns of pregnancy-induced nest building. (27 ref)