Experience-facilitated improvements in pup retrieval; evidence for an epigenetic effect

Abstract

The quality and quantity of maternal care received during infancy are highly predictive of successful infant development. It has been well established, primarily in rats, that the combination of hormonal and infant stimuli at birth modifies neural circuits that regulate maternal responsiveness. During subsequent interactions, infant stimuli are more likely to elicit rapid maternal responsiveness. Some species, such as humans, can display maternal care in the absence of the endocrine events of pregnancy and birth. Similarly, virgin C57BL/6J female mice, display maternal care toward infants, and experience with infants elicits long-lasting increases in maternal care. We hypothesized that these experience-induced changes in behavior may be mediated by chromatin modifications, which in turn change expression of genes that promote maternal care. One site of action is the medial preoptic area (MPOA). To test our hypothesis we treated virgin female mice with sodium butyrate, a histone deacetylase inhibitor. This treatment potentiated maternal responsiveness as well as the expression of several genes: estrogen receptor β (Esr2), oxytocin (Oxt), and cyclicAMP response element binding protein (CREB) binding protein (Crebbp; a histone acetyltransferase) in the MPOA. These data suggest that experience induces high levels of maternal care via epigenetic modifications.

Introduction

In humans and other mammals, experience with infants has substantial effects on the quality of subsequent maternal care, which in turn affects infant development. In human mothers, more contact with infants during the first hours–days postpartum is associated with increased maternal responding, reduced infant crying, and an increase in secure infant attachment (Bystrova et al., 2009, Erlandsson et al., 2007, Kennell and Klaus, 1998, Klaus et al., 1972). For at risk mothers, greater infant contact is significantly correlated with a decreased incidence of child abuse/neglect (Buranasin, 1991, O'Connor et al., 1980). Therefore, understanding how experience with infants can produce changes in maternal care is essential for understanding how these mechanisms might fail in mothers that fail to bond with their infants.

The mechanisms through which mother–infant interactions act on the brain to alter subsequent maternal responsiveness have been best characterized in rats, in which the combination of hormonal and infant stimuli at birth permanently enhances maternal responsiveness (Fleming and Korsmit, 1996, Fleming et al., 1999, Numan, 2006). For example, postpartum rats are highly responsive to pups and will learn to press a lever or traverse a novel environment to retrieve pups back to the nest (Fleming et al., 1994, Lee et al., 2000, Stern and Mackinnon, 1976). Even in the absence of continued hormone or infant exposure, female rats show long-lasting changes in maternal responsiveness (Bridges, 1975, Bridges, 1977, Bridges, 1978, Orpen and Fleming, 1987, Orpen et al., 1987, Scanlan et al., 2006).

The medial preoptic area (MPOA), is the critical neural site that responds to both hormonal and sensory inputs from pups and regulates behavior (Arrati et al., 2006, Fleming et al., 1983, Gray and Brooks, 1984, Jacobson et al., 1980, Kalinichev et al., 2000, Lee and Brown, 2007, Lee et al., 2000, Numan, 1974, Numan and Callahan, 1980, Numan et al., 1977, Numan et al., 1988). The MPOA undergoes a variety of changes pre- versus post-partum that promote maternal responsiveness (Afonso et al., 2009, Febo et al., 2005, Fleming and Korsmit, 1996, Kim et al., 2010, Kuroda et al., 2007, Meddle et al., 2007, Numan and Numan, 1994, Numan and Numan, 1995, Numan and Numan, 1997, Numan et al., 1998, Seifritz et al., 2003, Stack et al., 2002). Thus, interaction with infants in the context of these changes, likely modifies the MPOA such that during subsequent interactions, infant stimuli come to elicit maternal responsiveness more effectively.

However, not all species rely on the hormonal stimulation of birth to respond to infants. For example, when foster pups are scattered in the home cage of virgin laboratory mice, they respond to pups (retrieve, lick, crouch) within 15 min (Calamandrei and Keverne, 1994, Gandelman, 1973a, Gandelman, 1973b, Gandelman and Vom Saal, 1975, Kuroda et al., 2008, Larsen et al., 2008, Leussis et al., 2008, Lucas et al., 1998, Mann et al., 1983, Noirot, 1972, Okabe et al., 2011, Stolzenberg and Rissman, 2011, Thomas and Palmiter, 1997). We have recently shown that experience with pups can initiate and sustain maternal responsiveness in a novel T-maze (high levels of maternal responsiveness) in virgin C57BL6/J mice. Moreover, behaviors displayed by virgins are not significantly different from postpartum females (Stolzenberg and Rissman, 2011). In addition, only 4 days of experience (for just 2 h/day) with pups is required for females to show high levels of maternal responsiveness, whereas mice with 50% less pup experience (2 h/day for 2 days) do not show this experience-induced increase in high levels of maternal responsiveness.

A critical question then is how this subtle difference in maternal experience can induce high levels of maternal responsiveness. We hypothesize that, in the absence of pregnancy and parturition, multiple experiences with infants may allow the transcription of genes that are typically regulated by the combination of hormonal stimulation and mother–infant interaction at birth to be activated in virgin mice. We speculated that epigenetic control of gene expression might contribute to the acute regulation of gene expression in response to experiences (Sweatt, 2009). One mechanism through which experience-dependent behavioral modifications are consolidated is epigenetic histone acetylation. Addition of acetyl groups, by histone acetyltransferases (HATs), to the histone proteins around which DNA is wrapped increases the sensitivity of DNA to transcriptional regulation.

In order to explore these hypotheses, we used a histone deacetylase inhibitor (HDAC inhibitor), sodium butyrate (SB), which inhibits HDAC activity and increases histone acetylation (Roozendaal et al., 2010). In the first experiment we examined whether HDAC inhibition would potentiate the effects of maternal experience on subsequent maternal behavior. In experiments 2 and 3 we examined the specificity of SB effects on maternal responsiveness. Finally, as a first step toward addressing how HDAC inhibition might mediate effects, we asked whether SB upregulated the expression of genes that are known to be associated with maternal experience.