SEX DIFFERENCES IN THE OUTCOME OF JUVENILE SOCIAL ISOLATION ON HPA AXIS FUNCTION IN RATS M. G. PISU, a A. GARAU, b G. BOERO, b F. BIGGIO, b V. PIBIRI, b R. DORE, by V. LOCCI, c E. PACI, b P. PORCU a AND M. SERRA a,b,d * a Neuroscience Institute, National Research Council of Italy (CNR), Cagliari, Italy b Department of Life and Environment Sciences, University of Cagliari, Cagliari, Italy c Department of Biomedical Sciences, University of Sassari, 07100 Sassari, Italy d Center of Excellence for the Neurobiology of Dependence, University of Cagliari, Cagliari, Italy Abstract—Women are more likely than men to suffer from anxiety disorders and major depression. These disorders share hyperresponsiveness to stress as an etiological fac- tor. Thus, sex differences in brain arousal systems and their regulation by chronic stress may account for the increased vulnerability to these disorders in women. Social isolation is a model of early life stress that results in neurobiological alterations leading to increased anxiety-like and depressive-like behaviors. Here we investigated the sex dif- ference in the effects of post-weaning social isolation on acute stress sensitivity and behavior in rats. In both sexes, social isolation at weaning reduced basal levels of the neu- roactive steroid allopregnanolone in the brain and of corti- costerone in plasma. Moreover, acute stress increased plasma corticosterone levels in both group-housed and socially isolated male and female rats; however this effect was greater in male than female rats subjected to social iso- lation. Intriguingly, group-housed female rats showed no change in plasma and brain levels of allopregnanolone after acute foot-shock stress. The absence of stress-induced effects on allopregnanolone synthesis might be due to the physiologically higher levels of this hormone in females vs. males. Accordingly, increasing allopregnanolone levels in male rats blunted the response to foot-shock stress in these animals. Socially isolated male, but not female, rats also display depressive-like behavior and increased hip- pocampal brain-derived neurotrophic factor (BDNF). The ovarian steroids could ‘‘buffer” the effect of this adverse experience in females on these parameters. Finally, the dexamethasone (DEX) suppression test indicated that the chronic stress associated with social isolation impairs feed- back inhibition in both sexes in which an increase in the abundance of glucocorticoid receptors (GRs) in the hip- pocampus was found. Altogether, these results demon- strate that social isolation affects neuroendocrine reactivity to stress, plasticity and emotionality in a sexually dimorphic manner. Ó 2016 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: social isolation, allopregnanolone, corticos- terone, HPA axis, depressive-like behavior sex difference. INTRODUCTION Sex differences are particularly noticeable in neuropsychiatric diseases (Vesga-Lopez et al., 2008; Conley and Rudolph, 2009), and anxiety disorders and depression are almost twice as recurrent in females com- pared with males (Kessler, 2003). A mutual underlying feature of these psychiatric disorders, which are more prevalent in females, is the association with stress (Kendler et al., 2000). These differences may be under- lined by sexual dimorphisms/diergisms due to the pres- ence of sex hormones that influence brain function with regard to the hypothalamic-pituitary–adrenal (HPA) axis activity and its responses (Patchev and Almeida, 1998; Young, 1998; Rhodes and Rubin, 1999). Indeed, a dys- regulation of the HPA axis is one of the most commonly described alterations that correlates with symptoms of mood disorders (Pariante and Lightman, 2008; Ferna´ndez-Guasti et al., 2012; Spijker and van Rossum, 2012). Social isolation is a model of prolonged mild stress and is associated with marked behavioral alterations such as increased locomotor activity, anxiety, and aggression in laboratory animals (Fone and Porkes, 2008). The 3a-hydroxy ring A-reduced steroid metabolite of progesterone, allopregnanolone, by binding with high affinity to membrane-bound GABA A receptors acts as potent positive allosteric modulator, significantly aug- menting GABA-activated chloride ion currents, and thus enhancing GABA A -mediated inhibitory neurotransmission (Morrow et al., 1987; Lambert et al., 1995). We have shown that social isolation in male rats results in a marked decrease in the brain and plasma concentrations of allo- pregnanolone and is accompanied to an enhanced neu- rosteroidogenic response to acute stressful stimuli and http://dx.doi.org/10.1016/j.neuroscience.2016.02.009 0306-4522/Ó 2016 IBRO. Published by Elsevier Ltd. All rights reserved. * Correspondence to: M. Serra, Department of Life and Environmental Sciences, Section of Neuroscience and Anthropology, University of Cagliari, Cagliari 09100, Italy. Tel: +39-070-675-4136; fax: +39- 070-675-4166. E-mail address: mserra@unica.it (M. Serra). y Present address: Department of Internal Medicine I, University of Lu¨beck, Lu¨beck, Germany. Abbreviations: 2-AG, 2-arachidonylglycerol; BDNF, brain-derived neurotrophic factor; CRH, corticotrophin releasing hormone; DEX, dexamethasone; eCBs, endocannabinoids; GRs, glucocorticoid receptor; HPA, hypothalamic-pituitary–adrenal; PTSD, posttraumatic stress disorder. Neuroscience 320 (2016) 172–182 172