Effects of genotype on differential behavioral responsiveness to progesterone and 5alpha-dihdroprogesterone in mice.

Thirty female CD-1 mice, 30 female Swiss-Webster mice, 45 hybrid female mice of the strain SWCD1F1, AND 45 HYBRId female mice of the strain CD1SWF1 were ovariectomized and administered estradiol benzoate once weekly for 6 weeks. Estrogen injections were followed 2 days later by injections of progesterone, dihydroprogesterone (DHP), or oil and the animals were tested for receptivity 7 hr later. Over the six tests, there was a progressive increase in the frequency of lordosis responses in all strains following progesterone treatment. However, lordosis scores varied widely across animals within strains. Following DHP treatment, lordosis frequency was not increased in the Swiss-Webster strain. Females in the other strains did show a progressive increase in lordosis frequency over weeks. The data indicate that the hybrid strains develop the potential to respond to DHP and thus behave like the CD-1 strain, suggesting that sensitivity to DHP is a dominant trait.