It is of interest that an initial maternal

It is of interest that an initial maternal experience in virgin mice produces both changes in subsequent maternal behavior as well as possible changes in neural gene expression. Stolzenberg et al. (2012) proposed that experience-based changes in maternal responsiveness may be mediated by chromatin modifications which may then promote changes in gene expression. They found that treatment of virgin female mice with the histone deacetylase inhibitor, sodium butyrate, both stimulated maternal care and the expression of several genes, including estrogen receptor-β and oxytocin, but not estrogen receptor-α. It would be of interest to determine whether prior maternal experience in parous rats would result in similar increases in gene expression, and most notably, whether Esr1 expression is modified by prior parity in specific flunixin meglumine regions. Based upon the results of the present study, however, one would predict that long-term changes in Esr1 expression would not be found in previously parous rats. In a related study in mice that examined the involvement of estrogen in experience-induced maternal behavior, it was shown that in the absence of estrogen exposure, maternal experience produced long-lasting modifications in maternal responsiveness (Stolzenberg and Rissman, 2011). Hence, it appears that in mice and rats the effects of maternal experience on subsequent maternal care may not be dependent upon an involvement of estrogens and estrogen receptor alpha. A recent review of the effects of epigenetic mechanisms and experience in the hormonal and non-hormonal regulation of maternal behavior provides a substantive analysis of these processes and relationships (Stolzenberg and Champagne, 2016). Dramatic differences in body weight were found across development in the Esr1 KOs and WT rats. The increased body weight in the KO females both supports a role for Esr1 in weight regulation and validates the accuracy of genetic screening during the postnatal period in our study. Rumi et al. (2014), earlier reported that Esr1 KO female rats were statistically heavier at about 10 weeks of age, whereas knockout males were lighter. We found that by 60 days of age the Esr1 KO females weighed significantly more than WT subjects and that at 120 days of age that Esr1 KO rats weighed >500 g, approximately 50% more than WT subjects. It is well established that estrogens help to maintain lower weights by increasing metabolic rates and decreasing food consumption (Drewett, 1973; Asarian and Geary, 2002). While we did not measure food intake or metabolic rate, it seems likely that these factors were altered in the Esr1 KO subjects. Another study found that white adipose tissue in Esr1 KO male and female mice increased with age more than that found in WT male and female mice (Heine et al., 2000). Heine et al. (2000) reported that Esr1 KO male mice did not display increased energy intake, but had a decrease in energy expenditure compared to that of WT males, indicating that lower activity in Esr1 KO mice might contribute to their obesity. In another study in mice, the decrease in food intake normally observed in cycling females was prevented with an Esr1 antagonist in ovariectomized female rats given estradiol or in cycling female rats, but no effect was found on food intake following treatment with an estrogen receptor beta antagonist (Santollo et al., 2010). It is possible, although not established, that Esr1 KO female rats may have lower activity levels that could inhibit their motivation to engage in pup-directed behaviors. Overall, the results demonstrate that the estrogen receptor alpha gene is not critical for the expression of maternal behavior in nulliparous female rats, but may be involved in other behavioral processes, such as motivation and food regulation. Since the subjects in this study were born with their KO genotype, it would be interesting to examine a conditional KO that could be induced at different developmental time points and then compare maternal memory in both WT and conditional KO rats. Furthermore, a conditional region-specific Esr1 KO, such as one directed at the MPOA in mice using siRNA to silence ER-α (Ribeiro et al., 2012), an area vital for the expression of maternal behavior (Numan et al., 1977; Numan and Stolzenberg, 2009) would help clarify a possible involvement of Esr1 in this brain region in rats and its effects on maternal responsiveness and maternal memory. It is also important to employ these and other models to address these issues. One other approach, for example, would be to interfere with maternal memory by administering previous parous rats with a selective estrogen receptor-alpha antagonist. Preliminary results from our laboratory using this approach failed to block maternal memory with the ER-α antagonist methyl-piperidino-pyrazole hydrate (MPP; Zhou et al., 2009), thus providing support that the ER-α receptor may not be critical for the expression of maternal memory (Gallagher & Bridges, unpublished findings). Finally, it is noted that the present study is unique in that it utilized the Esr1 KO to investigate the expression of maternal behavior in rats. Use of this KO model provides the opportunity to explore the role of Esr1 in this and related physiological and behavioral processes.