More recent studies have added a host of additional physiological outcomes related to stress and depressive behavior, including changes in dopamine signaling in different brain regions

(Heidbreder et al., 2000), altered heart rate and cardiac function (Späni et al., 2003 and Carnevali et al., 2012), and neurogenesis (Stranahan et al., 2006 and Lieberwirth and Wang, 2012). Which outcomes are affected by isolation depend in part on the age at which isolation occurs (reviewed in Hall, 1998), and there are sex differences in the effects of social isolation. These suggest that isolation may be stressful for females but not necessarily to the same extent for males (Hatch et al., 1965, Palanza, 2001 and Palanza et al., VE-822 2001). Assessing the impacts of both isolation and crowding share the problem of what to consider as the control comparison, as anxiety and other behavioral outcomes vary along a continuum of group sizes FRAX597 in vitro (Botelho et al., 2007). In recent decades, prairie voles have become a popular model for studying social behaviors because of their unusual capacity to form socially monogamous pair-bonds with opposite sex mates (Getz et al., 1981). An additional

advantage of this species is that the effects of social manipulations can be contextualized in terms of findings from field populations and semi-natural settings (e.g. Ophir et al., 2008 and Mabry et al., 2011). In wild prairie voles, cohabitation with a mate or a mate and undispersed offspring is common (Getz and Hofmann,

1986), and reproductively naïve prairie voles are affiliative towards their same-sex cage mates. In the lab, separation of adult prairie voles from a sibling cage-mate for 1–2 months reduced sucrose consumption (a measure of anhedonia), and was associated with increased plasma levels of oxytocin, CORT, and ACTH, as well as increased activity of oxytocin neurons in the hypothalamus following a resident intruder test. These effects were more profound in females (Grippo et al., 2007). Further work has shown that social isolation from a sibling also leads to changes in cardiac function associated with cardiovascular disease crotamiton (Grippo et al., 2011 and Peuler et al., 2012), and immobility in the forced swim test (Grippo et al., 2008) – considered a measure of depressive behavior. Some physiological and behavioral sequelae were prevented or ameliorated by exposure to environmental enrichment, or by peripheral administration of oxytocin (Grippo et al., 2009 and Grippo et al., 2014), as has been demonstrated in rats (Hellemans et al., 2004). Social isolation of prairie voles from weaning has been associated with higher circulating CORT, and greater CRF immunoreactivity in the paraventricular nucleus (PVN) of the hypothalamus (Ruscio et al., 2007).