Furthermore, in both rodents and humans, differences among individuals in anxiety or the experimental instillation of anxious states has been observed to affect somatosensory functioning ( Jorum, 1988 van Meeteren et al., 1997 Kain et al., 2000 Rhudy and Meagher, 2000 Geerse et al., 2006 Devall et al., 2009 Aron et al., 2012 Corral-Frias et al., 2013). Such innate or “trait” anxiety could also contribute to heterogeneity in the anxiety-related behavior of postpartum laboratory rats, and there is a burgeoning body of research demonstrating the stability of emotional traits (including anxiety) in individual non-human animals across the lifespan ( Burtt, 1967 Lister, 1987 Clarke and Boinski, 1995 Leibsch et al., 1998 Henniger et al., 2000 Gosling, 2001 Landgraf and Wigger, 2002 Cavigelli et al., 2007 Uher et al., 2008 Quinn et al., 2011 Curley et al., 2012 Cavigelli et al., 2013). This is suggested by the fact noted above that women are differentially susceptible to anxiety dysregulation during the postpartum period, with one of the best predictors of their postpartum anxiety being their history of anxiety before giving birth ( Engle et al., 1990 Hundley et al., 1998 O’Connor et al., 2002 Heron et al., 2004 Britton, 2008 Grant et al., 2008). However, postpartum female rats may be heterogeneous in how their anxiety is affected by physical contact with neonates. Studies on this topic in laboratory rats have provided valuable information about what can be expected for the anxiety-related behaviors of most postpartum females in response to infant contact. A similar anxiolytic effect of recent suckling or non-suckling contact with infants has been found in human mothers ( Heinrichs et al., 2001). In rats, this reduction depends on recent suckling or non-suckling physical contact with offspring, and dams’ anxiety-related behavior rises to levels found in nulliparous females if the litter is removed even for a few hours before testing ( Lonstein, 2005 Figueira et al., 2008 Smith and Lonstein, 2008 Miller and Lonstein, 2011). Indeed, most studies find that anxiety-related behavior in early postpartum laboratory rodents is lower than what is found in females that have not given birth (see Lonstein, 2007 for review). While the early postpartum period has been characterized for some women as a time of particular susceptibility to anxiety and other types of emotional dysregulation, the majority of women and other female animals studied show stable or even improved emotional regulation during the postpartum period ( Neumann, 2003 Heron et al., 2004 Ross and McLean, 2006 Lonstein, 2007). This flux involves salient changes in how females process social stimuli, most obviously resulting in heightened positive responses to neonates, as well as changes in the new mother’s emotional state that help or hinder these positive responses. The onset and maintenance of motherhood is a time of tremendous neurobehavioral flux for female mammals (Numan et al., 2006 Lonstein et al., 2013 Sisk et al., 2013). This effect is particularly true for females with the lowest anxiety, may be mediated by central noradrenergic systems, and has implications for their ability to attend to their offspring. Thus, a primary effect of recent contact with offspring on anxiety-related behavior in postpartum rats is to shift females away from their innate anxiety to a more moderate level of responding. There was no relationship between females’ anxiety and dorsal raphe TPH2. This reduction in anxiety in the least-anxious females after litter removal was associated with lower brainstem DBH. Specifically, dams reverted back to their pre-mating levels of anxiety such that offspring removal slightly increased anxiety in the most-anxious females but greatly lowered anxiety in the least-anxious females. Removal of the offspring before testing, however, differentially affected anxiety based on dams’ innate anxiety. It was found that anxiety-related behavior in the two groups did not differ when dams were permitted contact with offspring before testing. Levels of dopamine β-hydroxylase (DBH, norepinephrine synthesizing enzyme) and tryptophan hydroxylase- 2 (TPH2, serotonin synthesizing enzyme) were measured in the brainstem and dorsal raphe, respectively. Anxiety was assessed again postpartum after females were permitted or prevented from contacting their offspring 4 h before testing. Anxiety behavior was assessed in a large group of nulliparous female rats and the least-anxious and most-anxious tertiles were mated. Factors contributing to differences among females in their susceptibility to the anxiety-modulating effect of offspring contact are unknown, but could include their innate anxiety and brain monoaminergic activity. This includes a suppression of anxiety-related behaviors that requires recent physical contact with offspring. In female mammals, the postpartum period involves dramatic shifts in many socioemotional behaviors.
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