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    • A recent framework for conceptualizing ELS posits that adver

    2018-11-07

    A recent framework for conceptualizing ELS posits that adverse early experiences fall along dimensions of threat and deprivation (McLaughlin et al., 2014; Sheridan and McLaughlin, 2014). Here, threat is conceptualized as an atypical experience posing a direct physical danger to a developing order AMN-107 (e.g., physical/sexual abuse), whereas deprivation is operationalized as the absence of expected social, cognitive and affective environmental inputs and enrichment during development (e.g., neglect). Consideration of the nature of adversities experienced is an important step forward in the field. We would add that caregiver deprivation (i.e., emotional neglect or institutional care) also poses a direct threat to a (semi-)altricial organism’s survival: caregiver deprivation involves a lack of protection from outside threats and a lack/absence of physiological and affective regulation from a caregiver. Caregiver deprivation can take many forms across species––e.g., removing a maternal figure from a nest, rearing in isolation from the rest of a group, or institutionalization in humans (i.e., being reared in orphanage care). We will focus largely on ELS in the form of caregiver deprivation in this review, because it allows for the ability to draw parallels across non-human and human literatures. ELS is often associated with the development of a host of cognitive, social and affective deficits, which precede the development of mental illness later in life (Gee and Casey, 2015; Green et al., 2010; Gunnar and Quevedo, 2007; Masten and Cicchetti, 2010). Most frequently, caregiver deprivation is linked with the development of social withdrawal, poor regulatory abilities, and higher risk for internalizing illness such as depression and anhedonia, anxiety disorders, as well as externalizing disorders and behavioral problems (Conti et al., 2012; Corcoran et al., 2012; Ellis et al., 2004; Gee and Casey, 2015; Gunnar and Quevedo, 2007; Lupien et al., 2009; Pechtel and Pizzagalli, 2011; Romeo et al., 2003; Sánchez et al., 2001; Tottenham and Sheridan, 2009; Zeanah et al., 2009). The link between caregiver deprivation and downstream mental health consequences may be mediated by ELS induced changes in the functional development of the amygdala and striatum and their interaction, both of which undergo massive change throughout childhood and adolescence, lending them to be plastic and subject to environmental influence (Gee and Casey, 2015; Masten and Cicchetti, 2010).
    Amygdala and striatum: anatomical considerations The amygdala and striatum are two subcortical structures critical for affective valuation and learning across species. The amygdala is comprised of approximately thirteen nuclei and subnuclei (rodents: LeDoux, 2000 non-human primates: Pitkänen and Amaral, 1998; Pitkänen et al., 1997). The basolateral complex of the amygdala (the lateral nucleus, basal nucleus, accessory basal nucleus; BLA) and the central nucleus (CeA) are most often implicated in affective valuation, and relay information regarding associations between environmental stimuli and potential outcomes to connected regions (LeDoux, 2000; Pitkänen et al., 1997). The BLA receives sensory input from thalamic nuclei, auditory and sensory cortices as well as the hippocampus, and provides both direct and indirect signals to the central nucleus of the amygdala (Pitkänen et al., 1997). The majority of BLA neurons are excitatory glutamatergic cells which project to other amygdala nuclei including the CeA, the ventral hippocampus (anterior in humans), prefrontal cortex and importantly, to the ventral striatum (nucleus accumbens (NAcc)). The CeA is the major output structure of the amygdala, containing primarily inhibitory GABAergic neurons and controls autonomic responses to incentive-laden stimuli (Davis and Whalen, 2001; Phelps and LeDoux, 2005). The striatum is the primary input region of the basal ganglia (Delgado, 2007) and receives input from a host of prefrontal cortical and subcortical structures including orbitofrontal cortex (OFC), ventromedial PFC (vmPFC), portions of anterior cingulate cortex (ACC), the hippocampus, and importantly, the amygdala (Alexander et al., 1986; Haber and Behrens, 2014; Haber and Knutson, 2010; Middleton and Strick, 2000; Pennartz et al., 2011; Sesack and Grace, 2010). The striatum can be segregated along a dorsal-ventral divide (though see Voorn et al., 2004) The dorsal striatum is comprised of the caudate nucleus and the putamen, whereas the ventral striatum is comprised of the nucleus accumbens (consisting of medial (core) and lateral (shell) divisions), and ventral portions of the caudate nucleus and putamen (Delgado, 2007; Haber and Knutson, 2010; Meredith and Pattiselanno, 1996; Zaborszky et al., 1985). The ventral striatum receives excitatory glutamatergic input from cortical regions, thalamus, ventral hippocampus (anterior in humans), and the BLA (Haber and Behrens, 2014; Tovote et al., 2015). Importantly, dopaminergic (DA) input from midbrain nuclei (e.g., ventral tegmental area) heavily innervate the ventral striatum, and these connections are thought to mediate ventral striatal function in appetitive behaviors (Haber et al., 2000; Haber and Knutson, 2010; Sesack and Grace, 2010)