Table 1.

Key Characteristics of Included Studies.

Study	Type	Neurobiological Measure(s)	SI Stress	PTSD Measure(s)	Main Findings
Pibiri F. et al. (2008) [42]	Isolated and group housed (groups of 5) adult male Swiss-Webster mice	-5aReductase Type I (5aRI) -Allopregnanolone (Allo) -S-norfluoxetine, selective brain steroidogenic stimulant (SBSS)	3–4 weeks of isolation of the experimental group	Inescapable electric foot shock (unconditioned stimulus) Acoustic tone (conditioned stimulus)	SI enhances contextual fear responses and impairs fear extinction and this finding relates to Allo and 5aRI downregulation in mPFC, hippocampus and amygdala. Allo downregulation may be reversed by S-norfluoxetine
Conrad K.L. et al. (2011) [43]	Isolated and group housed (n = 9–11) male C57Bl/6J mice (7–8 weeks old)	-dlBNST, subregion of extended amygdala serving as relay of corticolimbic information to paraventricular nucleus of the hypothalamus	-Acute SI (1 day) -Chronic SI housing (6–8 weeks)	Electrophysiological tetanus protocol	Acutely and chronically isolated animals presented blunting of dlBNST longterm potentiation compared to group housed animals. Moreover, they exhibited anxiety-like behavior in the novel open field
Daskalakis N.P. et al. (2014) [46]	Wistar rat pups (male/female till pnd9, after that only male rat pups) divided into four groups (n = 8–10 per group) (i) repeated maternal separation (MS) in home-environment (HOME-SEP), with the pups remaining together (ii) repeated-MS in a novel-environment (NOVEL-SEP), the pups were individually housed in a novel-environment; (iii) repeated handling of daily brief (15 min instead of 8 h) MS in the home-all together or in a novel-environment individually (HOME-HAN and NOVEL-HAN); (iv) no-separation/no-handling (NON-SEP/NON-HAN) control condition.	-Amygdala c-Fos expression -Pituitary ACTH-release -Adrenal CORT secretion	Prolonged isolation from siblings in a novel-environment during repeated-maternal separation between post-natal-day (pnd) 3–5	Contextual fear-conditioning on pnd 240 in a fear-conditioning box. Floor of the box consisted of stainless steel rods, connected to a shock generator. Rats were individually placed in the shock box. After 2 min, one electric foot shock was given and 2 min later, the rat returned to its homecage. Re-exposure: 24 h later, the same procedure was repeated however without delivery of the foot shock	The stress of prolonged isolation from siblings in a novel-environment during repeated-MS (NOVEL-SEP) activates the amygdala fear network with enduring consequences for behavioral and endocrine fear-reactions. NOVEL-SEP pups exhibited increased amygdala activation and pituitary ACTH-release that persisted from early-life into adulthood, while adrenal CORT-secretion was reduced.
Locci A. et al. (2017) [44]	Swiss-Webster male mice Isolated at pnd 25 Isolated at pnd 45	-S-fluoxetine-Selective Serotonin Reuptake Inhibitor (SSRI) -Allo analogs: ganaxolone, BR351, BR297 -PEA-endocannabinoid	Isolation at pnd21 (early adolescence) and at pnd45	Male intruder mouse was placed in a resident home cage and resident–intruder interactions were videotaped. aggressive behavior of SI mice was characterized by exploratory activity around the intruder, rearing and tail rattle, wrestling and/or a violent biting attacks.	Isolation of early adolescent mice induced more severe aggression than isolation later in life. all drugs had better effect on late onset aggression while S-fluoxetine had the lowest response rate in treating aggression, compared to endocannabinoid PEA with the highest response rate.
Stein J.Y. et al. (2018a) [51]	Longitudinal study 83 Israeli ex-POWs	Telomere Length (TL) measured at T2 (2015) utilizing the Southern Blot measured in total white blood cells obtained from 10 mL of blood	Loneliness and perceived Social Support measured at T1 (1991) with -UCLA-loneliness scale-Müller’s social network approach/depression measured with Symptom Checklist-90/PTSD measured with PTSD-Inventory (PTSD-I)	Captivity on the Egyptian and Syrian Fronts during the Yom-Kippur War in 1973 (T0)	Lack of perceived social support and loneliness at T1 were both negatively correlated with TL at T2. Depression was positively correlated with PTSD, lack of perceived social support, and loneliness. Lack of perceived social support and loneliness were positively correlated.
Stein J.Y. et al. (2018b) [52]	Longitudinal Study 99 Israeli ex-POWs	TL measured at T2 (2015) utilizing the Southern Blot measured in total white blood cells obtained from 10 mL of blood	At T1 (1991) self-reports of captivity suffering in terms of: (a) weight loss, (b) physical abuse, (c) solitary confinement Homecoming experience in terms of: (a) received social support (b) loss of place in the family (c) sense of accusation (d) loneliness	Captivity during the Yom-Kippur War in 1973 (T0)	When all the study variables were accounted for, solitary confinement in captivity and interpersonal factors at homecoming (i.e., loss of place in the family, sense of loneliness, sense of being accused by society at homecoming) significantly contributed to shorter TL
Morena M. et al. (2018) [47]	Sprague-Dawley male rats All rats individually housed Rats were divided to a group exposed to extinction procedure and rats never exposed to extinction procedure. One week after trauma, exposed to extinction rats were subjected to three spaced extinction sessions, mimicking human exposure therapy	-e-CB -AEA FAAH inhibitor URB597 increases AEA concentration	3 days individually housed before the experiment	5 inescapable footshocks	-Rats never exposed to the repeated extinction procedure presented decreased hippocampal AEA levels when measured immediately after the SI session URB597, through indirect activation of CB1 receptors, enhanced the consolidation of extinction, as well as ameliorated the trauma-induced alterations in social behavior by increasing the levels of social interaction.
Boero G. et al. (2018) [48]	Isolated and group housed (groups of 5) male Sprague-Dawley rats at pnd25	COR; CBG; CRH; ACTH; mMR; mGR; CB1R; AEA; 2-AG.	30 days of isolation (between pnd25–pnd55)	5 min of acute foot-shock stress at pnd55	SI decreased plasma total CORT and CBG concentration levels, however, after acute stress exposure, SI rats showed long-lasting CORT, ACTH and CRH responses indicating dysregulation of the HPA axis; SI also induced downregulation of hippocampal mMR and upregulation of hippocampal and hypothalamic mGR; Observed overexpression in GR mRNA was linked to stress-induced hippocampal neuronal loss; SI also affected the hypothalamic eCB system: compared to group-housed rats, basal levels of AEA and CBR1 were increased, while basal levels of 2-AG were decreased in socially isolated rats suggesting that social isolation alters eCB-mediated signaling in the hypothalamus, likely inducing an impairment in glutamatergic and GABAergic inputs that control CRH release, contributing to the impairment of the feedback inhibition of the HPA axis.
Algamal M. et al. (2018) [45]	C57BL/6 male mice (aged 8–10 weeks) stressed group (n = 17) and group housed (n = 15) (controls)	-Hippocampal volume; hippocampal BDNF and CRH; Plasma CORT levels- amygdala Pro-BDNF, its signaling receptor P75NTR and NMDA receptor 1; proinflammatory cytokines; hypothalamic FKBP51	Single housing for the experimental stress group of rats for the 21 days of the stress experiment and for the following 6 months till the end of the study.	Repeated unpredictable stress (RUS) paradigm involved 21 days of (i) daily unstable social housing with an alternate congener, (ii) unpredictable repetitive exposures to danger-related predator odor (fox urine, TMT), while restraint for 30 min, (iii) physical trauma in the form of five repeated inescapable footshocks, (iv) lack of social support, single housed post stress	6 months after RUS: stressed animals showed lower CORT and ACTH levels and reduced levels of FKBP51 in the hypothalamus, suggesting blunted HPA-axis reactivity; twofold increase of proinflammatory cytokines IL-1β and IFN-γ was observed; volume of the CA1 region of the dorsal hippocampus was significantly reduced and hippocampal BDNF levels were significantly downregulated; in the amygdala reduction in the levels of ProBDNF and P75NTR were observed. Stressed mice demonstrated recall of traumatic memories, passive stress coping behavior, acute anxiety, and weight gain deficits when compared to control mice.
Cheng W. et al. (2019) [49]	Wistar neonates male rats divided into four groups (n = 25 for each group): (a) control group, (b) neonatal isolation (NI) (c) single-prolonged stress (SPS) (d) NI+SPS group	-Plasma CORT levels; hippocampal and amygdala GR expression; Synapsin1 protein; PSD95; NLG proteins-1 and -2	NI between pnd2–pnd9 for 1 h per day	SPS on pnd56: 2 h of restraint, followed by 20 min of forced swim, followed by exposure to ether vapor leading to loss of consciousness	Plasma CORT response to SPS was significantly greater in the NI+SPS group compared with NI rats; GR immunoreactivity was higher in the dentate gyrus of the hippocampus and lower in basolateral amygdala (BLA) for the NI+SPS group compared to the SPS group; Synapsin1 reduced in the BLA and the hippocampal dentate gyrus of NI+SPS-group when compared with SPS-group rats; PSD-95 hippocampal expression and the ratio of NLG-1/-2 in rats of the NI+SPS group was significantly higher than that of rats in the SPS group; Finally, NI +SPS exacerbated the increased anxiety levels and impaired spatial memory than NI or SPS alone.

ACTH = pituitary adrenocorticotropic hormone; AEA = endocannabinoid anandamide; 2-AG = 2-arachidonoyglycerol; Allo = allopregnanolone; BDNF = Brain Derived Neurotrophic Factor; BLA = basolateral amygdala; CB = endocannabinoid system; CB1R = cannabinoid receptors type 1; CBG = corticosterone binding globulin; CORT: corticosterone; CRH = corticotrophine releasing hormone; dlBNST = dorsolateral bed nucleus of stria terminalis; e-CB = endocannabinoid system; e-FAAH = Fatty-acid amide hydrolase of anandamide; FKBP51 = FK506 binding protein 51; GABAergic = gamma-aminobutyric acidergic; HOME-HAN = repeated handling of brief maternal separation in home environment with al pups together; HOME-SEP: repeated maternal separation in home-environment, with the pups remaining together; IL-1β = proinflammatory interleukin 1β; IFNγ = proinflammatory interferon-γ; MS = maternal separation; mPFC = medial prefrontal cortex; mGR = membrane glucocorticoid receptors; mMR = membrane mineralcorticoid receptors; NI = neonatal isolation; NLG1,NLG-2 = neuroligin proteins 1–2; NOVEL-HAN = repeated handling of daily brief (15 min instead of 8 h) MS in the in a novel-environment individually; NOVEL-SEP = repeated MS, pups were individually housed in a novel-environment; PEA = N-palmitoylethanolamine; pnd = postnatal day; POW = Prisoner of War; 5aRI = 5aReductase Type I; PSD95 = postsynaptic density protein 95; P75NTR = signaling receptor of ProBDNF; RUS = repeated unpredictable stress; SBSS = selective brain steroidogenic stimulant; SSRI = Selective Serotonine Reuptake Inhibitor; SPS = single-prolonged-stress; TL = telomere length; TMT = Trimethylthiazoline, component of fox urine, URB597 = FAAH inhibitor.