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Estrous cycle, pregnancy, and parity enhance performance of rats in the object recognition or object placement tasks

J Paris, Psychology, The University at Albany - SUNY, Albany, United States
C Frye, Psychology, The University at Albany - SUNY, Albany, United States

Correspondence: Cheryl Frye, Email: cafrye{at}albany.edu

Abstract

Ovarian hormone elevations are associated with enhanced learning/memory. During behavioral estrus or pregnancy, progestins, such as progesterone and its metabolite, 5-pregnan-3-ol-20-one (3,5-THP), are elevated due, in part, to corpora luteal and placental secretion. During "pseudopregnancy," the induction of corpora luteal functioning results in a hormonal milieu analogous to pregnancy, which ceases after about 12 days, due to lack of placental formation. Multiparity is also associated with enhanced learning/memory, perhaps due to prior steroid exposure during pregnancy. Given evidence that progestins and/or parity may influence cognition, we investigated how natural alterations in progestin milieu influence cognitive performance. In Experiment 1, virgin rats (nulliparous) or rats with two prior pregnancies (multiparous) were assessed on the object placement and recognition tasks, when in high estrogen/progesterone (behavioral estrus) or low estrogen/progesterone (diestrus) phases of the estrous cycle. In Experiment 2, primiparous or multiparous rats were tested in the object placement and recognition tasks when not pregnant, pseudopregnant, or pregnant (between gestational days 6-12). In Experiment 3, pregnant primiparous or multiparous rats were assessed daily in the object placement or recognition tasks. Females in natural states associated with higher endogenous progestins (behavioral estrus, pregnancy, multiparity) outperformed rats in low progestin states (diestrus, non-pregnancy, nulliparity) on the object placement and recognition tasks. In earlier pregnancy, multiparous, compared to primiparous, rats had lower corticosterone, but higher estrogen levels, concomitant with better object placement performance. From gestational day 13 until post-partum, primiparous rats had higher 3,5-THP levels and improved object placement performance compared to multiparous rats.