Adult hippocampal cell proliferation is suppressed with estrogen withdrawal after a hormone-simulated pregnancy

Estradiol withdrawal after pregnancy is hypothesized to precipitate depressive symptoms in vulnerable women. A hormone-simulated pregnancy was induced in female rats and the effects of a ‘postpartum’ drop in estradiol on hippocampal cell proliferation were examined. All groups were ovariectomized or given sham surgery prior to treatment. Rats were randomly assigned to ‘postpartum’, ‘postpartum’ + EB (estradiol benzoate), ‘postpartum’ + DPN (diarylpropionitrile; an ERβ agonist), ‘postpartum’ + IMI (imipramine; a tricyclic antidepressant), sham, ovariectomized (OVX), sham + IMI or OVX + IMI groups. All ‘postpartum’ groups received hormone injections (estradiol and progesterone) over 23 days to simulate pregnancy, while IMI groups also received daily imipramine injections. After day 23, ‘postpartum’ rats were withdrawn from the hormone-simulated pregnancy (mimicking the postpartum drop in gonadal hormones), while other ‘postpartum’ treatment groups received daily injections of DPN, EB or IMI. On day 3 ‘postpartum’ all rats were injected with bromodeoxyuridine (BrdU; a DNA synthesis marker) and perfused 24 h later to assess cell proliferation and cell death in the dentate gyrus. ‘Postpartum’ hormone withdrawal decreased hippocampal cell proliferation in the ‘postpartum’ and ‘postpartum’ + EB groups only. Chronic imipramine significantly increased hippocampal cell proliferation in sham + IMI, but not OVX + IMI rats suggesting that imipramine's effects to increase hippocampal cell proliferation in female rats is related to reproductive status. Cell death (pyknotic cells) was decreased only in the ‘postpartum’ group. Together, these results suggest an important, though complex, role for gonadal hormones in the cellular changes accompanying this model of postpartum depression.