Postpartum depression in rats: differences in swim test immobility, sucrose preference and nurturing behaviors

Postpartum depression (PPD) is a common disorder affecting both mothers and their offspring. Studies of PPD in laboratory animals have typically assessed either immobility on forced swim testing or sucrose preference in ovariectomized rats following hormone supplementation and withdrawal or in stress models. To date, few studies have related these measures to maternal behaviors, a potential indicator of depressive-like activity postpartum. Because a single measure may be insufficient to characterize depression, the present study determined the distribution of depressive-like behaviors in Sprague-Dawley rats postpartum. Nurturing and non-nurturing behaviors exhibited by undisturbed dams were recorded during the first 12 days postpartum, and immobility in the forced swim test and sucrose preference were determined thereafter. A median-split analysis indicated that 19% of dams exhibited high sucrose preference and low immobility, 30% exhibited either only high immobility or only low sucrose preference, and 21% exhibited both high immobility and low preference. Dams exhibiting depressive-like activity on either or both tests displayed increased self-directed behaviors and decreased active nurturing during the dark phase of the diurnal cycle. This is the first study to characterize undisturbed nurturing and non-nurturing behaviors, and use both sucrose preference and immobility in the forced swim test, to classify PPD endophenotypes exhibited by rat dams following parturition. The present study underscores the idea that multiple tests should be used to characterize depressive-like behavior, which is highly heterogeneous in both the human and laboratory animal populations.