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Early Life Influences on Emotional Reactivity Neuroscience and Biobehavioral Reviews 34 (2010) 808–820 Contents lists available at ScienceDirect Neuroscience and Biobehavioral Reviews journal homepage: www.elsevier.com/locate/neubiorev Review Early life influences on emotional reactivity: Evidence that social enrichment has greater effects than handling on anxiety-like behaviors, neuroendocrine responses to stress and central BDNF levels Francesca Cirulli a,∗, Alessandra Berry a, Luca Tommaso Bonsignore a, Francesca Capone a, Ivana D’Andrea a, Luigi Aloe b, Igor Branchi a, Enrico Alleva a a Section of Behavioural Neuroscience, Department of Cell Biology and Neuroscience, Istituto Superiore di Sanità, Viale Regina Elena 299, I-00161, Rome, Italy b Institute of Neurobiology and Molecular Medicine, CNR, Rome, Italy article info abstract Keywords: During the early post-natal phases the brain is experience-seeking and provided by a considerable plas- Handling ticity which allows a fine tuning between the external environment and the developing organism. Since Communal rearing the early work of Seymour Levine, an impressive amount of research has clearly shown that stressful Corticosterone experiences exert powerful effects on the brain and body development. These effects can last throughout Behavior the entire life span influencing brain function and increasing the risk for depression and anxiety disor- Ultrasonic vocalizations Depression ders. The mechanisms underlying the effects of early stress on the developing organism have been widely Anxiety studied in rodents through experimental manipulations of the post-natal environment, such as handling, Stress which have been shown to exert important effects on the emotional phenotype and the response to Brain-derived neurotrophic factor (BDNF) stress. In the present paper we review the relevant literature and present some original data indicating Mice that, compared to handling, which imposes an external manipulation on the mother–infant relation- ship, social enrichment, in the form of communal rearing, in mice has very profound effects on animal’s emotionality and the response to stress. These effects are also accompanied by important changes in central levels of brain-derived neurotrophic factor. The present data indicate that communal rearing has more pervasive effects than handling, strengthening previous data suggesting that it is a good animal model of reduced susceptibility to depression-like behavior. Overall, the availability of ever more sophis- ticated animal models represents a fundamental tool to translate basic research data into appropriate interventions for humans raised under traumatic or impoverished situations. © 2010 Elsevier Ltd. All rights reserved. Contents 1. Introduction ......................................................................................................................................... 809 2. Manipulating the mother–infant relationship ...................................................................................................... 809 3. Communal nesting as a form of early social enrichment ........................................................................................... 810 4. Neurotrophins as transducers of early experiences ................................................................................................ 811 5. Comparing handling vs communal rearing paradigms: evidence that social enrichment has more profound and differential effects than handling ................................................................................................................... 811 5.1. Materials and methods ...................................................................................................................... 812 5.1.1. Animals and breeding procedures ................................................................................................. 812 5.1.2. Experimental groups and manipulation procedures following birth ............................................................. 812 5.1.3. Ultrasonic vocalizations and drug treatments .................................................................................... 812 5.1.4. Plus-maze test ..................................................................................................................... 812 5.1.5. Exposure to repeated swimming stress ........................................................................................... 813 5.1.6. BDNF determination ............................................................................................................... 813 5.1.7. Statistical analysis ................................................................................................................. 813 ∗ Corresponding author. Tel.: +39 06 49902480; fax: +39 06 4957821. E-mail address: [email protected] (F. Cirulli). 0149-7634/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.neubiorev.2010.02.008 F. Cirulli et al. / Neuroscience and Biobehavioral Reviews 34 (2010) 808–820 809 5.2. Results ....................................................................................................................................... 813 5.2.1. Weight gain ........................................................................................................................ 813 5.2.2. Ultrasounds ........................................................................................................................ 813 5.2.3. Plus-maze test ..................................................................................................................... 813 5.2.4. Chronic swimming stress .......................................................................................................... 814 5.2.5. BDNF levels ........................................................................................................................ 815 5.3. Discussion.................................................................................................................................... 816 5.3.1. Short-term effects of early manipulations on mouse emotionality............................................................... 817 5.3.2. Long-term effects on emotionality and the neuroendocrine responses to stress ................................................ 817 5.3.3. Long-term effects of early manipulations on BDNF levels ........................................................................ 818 6. Conclusions .......................................................................................................................................... 818 Acknowledgements ................................................................................................................................ 818 References........................................................................................................................................... 818 1. Introduction Studies performed in altricial rodents (e.g. mice and rats) have clearly shown that maternal care is crucial for an adequate devel- In humans, quality of family life influences the development of opment of the pups, representing the most relevant source of individual differences in vulnerability to psychopathology through- early stimulation. Thus modifications of the maternal environment out life (Heim and Nemeroff, 2001). Not only severe conditions such may result in long-term changes in the pattern of neuroendocrine as physical or sexual abuse, but also persistent emotional neglect or and emotional/behavioral responses later in life (Cirulli et al., family conflict, can compromise growth, intellectual development 2009a). and lead to increased risk for adult obesity, depression and anxi- Wild rodents having pups are often forced to leave the nest ety disorders (Cicchetti and Toth, 1995; Heim and Nemeroff, 2001; for variable periods (hours) of time to provide their them- Rutter et al., 2001). selves food. This pattern of maternal attendance to the nest Animal and human studies have provided a wealth of data has been modeled in the laboratory settings by early handling showing the negative effects of chronic exposure to stress and/or (H), which consists of removing the pups from the mother and adversity on the developing brain (Plotsky and Meaney, 1993; Liu their cage and placing them in individual compartments for up et al., 1997; Heim and Nemeroff, 2001; Pryce et al., 2001; Roceri et to 15 min until weaning (Levine, 1957). Animals handled dur- al., 2004; Levine, 2005; Cirulli et al., 2009a,b). However, while there ing infancy show important changes in the functionality of the is no doubt that the developing brain is “experience-sensitive” and hypothalamic–pituitary–adrenal (HPA) axis (Levine, 1957)ina “experience-dependent”, the mechanisms that render a particular way such that the ability of the adult organism to respond, cope, experience “good” or “bad” are still in the process of being defined. and adapt to novel and/or stressful stimuli is increased (Meaney We are still away from fully understanding why the same expe- et al., 1991). For example, immediately after the exposure to an rience can be detrimental for some individuals, while having no electric shock, H rats (tested at adulthood) show a faster peak effect in others. A stressful experience cannot be considered as in the release of the stress hormones glucocorticoids (GC), and being necessarily negative, as it represents an adaptive physio- a rapid return to basal levels when compared to non-handled logical response to the
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