Endocrine basis for two types of individual differences in lordosis reflex intensity.

We have identified individual differences in two independent types of lordosis reflex intensity. Overall lordosis reflex intensity, averaged across 9.4 +/- 0.99 days (range = 1-18 days), differs among cycling individuals and is a biomarker for the rate and pattern of reproductive aging. Although estrogen is necessary to permit lordosis reflex responding and manifests individual differences throughout the estrous cycle, it was individual differences in progesterone that correlated negatively with the individual variation in overall lordosis reflex intensity; females with intense overall lordosis reflexes had low progesterone throughout the estrous cycle. Independent of their overall reflex intensity, females also displayed individual variation in lordosis reflex intensity on proestrus. Among females that still maintained a cyclic lordosis reflex, those with intense lordosis reflexes on proestrus had high levels of luteinizing hormone (LH) at lights out. Among females that displayed intense lordosis reflexes throughout the estrous cycle (i.e., had entered constant lordosis), those with intense proestrous lordosis reflexes had low levels of LH at lights out on proestrus. Females who had entered constant lordosis tended to have lower overall progesterone than females who still maintained cyclic patterns of lordosis. Thus, constant lordosis was identified as an important component of the changes of reproductive aging.