The etiology of depression is still poorly understood, but two major causative hypotheses have been put forth: the monoamine deficiency and the stress hypotheses of depression. We evaluate these hypotheses using animal models of endogenous depression and chronic stress. The endogenously depressed rat and its control strain were developed by bidirectional selective breeding from the Wistar–Kyoto (WKY) rat, an accepted model of major depressive disorder (MDD). The WKY More Immobile (WMI) substrain shows high immobility/despair-like behavior in the forced swim test (FST), while the control substrain, WKY Less Immobile (WLI), shows no depressive behavior in the FST. Chronic stress responses were investigated by using Brown Norway, Fischer 344, Lewis and WKY, genetically and behaviorally distinct strains of rats. Animals were either not stressed (NS) or exposed to chronic restraint stress (CRS). Genome-wide microarray analyses identified differentially expressed genes in hippocampi and amygdalae of the endogenous depression and the chronic stress models. No significant difference was observed in the expression of monoaminergic transmission-related genes in either model. Furthermore, very few genes showed overlapping changes in the WMI vs WLI and CRS vs NS comparisons, strongly suggesting divergence between endogenous depressive behavior- and chronic stress-related molecular mechanisms. Taken together, these results posit that although chronic stress may induce depressive behavior, its molecular underpinnings differ from those of endogenous depression in animals and possibly in humans, suggesting the need for different treatments. The identification of novel endogenous depression-related and chronic stress response genes suggests that unexplored molecular mechanisms could be targeted for the development of novel therapeutic agents.

The Wistar-Kyoto (WKY) rat exhibits physiological and behavioral similarities to endophenotypes of human depression. In the forced swim test (FST), a well-characterized antidepressant-reversible test for behavioral despair in rodents, WKYs express characteristics of behavioral despair; increased immobility, and decreased climbing. To map genetic loci linked to behavior in the FST, we conducted a quantitative trait loci (QTL) analysis of the segregating F2 generation of a WKY x Fisher 344 (F344) reciprocal intercross. Using linear-model-based genome scans to include covariate (sex or lineage)-by-QTL interaction effects, four significant QTL influencing climbing behavior were identified. In addition, we identified three, seven, and two suggestive QTL for climbing, immobility, and swimming, respectively. One of these loci was pleiotropic, affecting both immobility and climbing. As found in human linkage studies, several of these QTL showed sex- and/or lineage-dependent effects. A simultaneous search strategy identified three epistatic locus pairs for climbing. Multiple regression analysis was employed to characterize the joint contributions of these QTL and to clarify the sex- and lineage-dependent effects. As expected for complex traits, FST behavior is influenced by multiple QTL of small effect, each contributing 5%-10%, accounting for a total 10%-30% of the phenotypic variance. A number of loci mapped in this study share overlapping candidate regions with previously identified emotionality QTL in mice as well as with susceptibility loci recognized by linkage or genome scan analyses for major depression or bipolar disorder in humans. The presence of these loci across species suggests that these QTL may represent universal genetic factors contributing to mood disorders.

The Wistar-Kyoto (WKY) rat strain demonstrates endogenous hormonal and behavioral abnormalities that emulate many of those found in symptom-presenting depressive patients. Evidence suggests that the WKY strain may harbor heterogeneity not found in other inbred strains, including greater behavioral and genetic variability. We took advantage of this variability and selectively bred WKY for 'depressive' behavior using immobility in the forced swim test (FST) as a functional selector. Successive generations of selective breeding resulted in rats that exhibited the extremes of immobility in the FST: 'WKY most immobile' (WMI) and 'WKY least immobile' (WLI). Male WMI rats also showed significantly decreased activity in the open field test (OFT). Plasma corticosterone (CORT) response to restraint stress was significantly lower and less variable in WMI compared to WLI males. Subacute treatment of males with several classes of antidepressant had different effects on FST behavior in the two substrains. Both desipramine (10 mg/kg body weight), a tricyclic antidepressant, and phenelzine (7.5 mg/kg), a monoamine oxidase inhibitor, significantly and drastically decreased FST immobility in WMI. In contrast, WLI showed a limited response to these antidepressants. Neither substrain responded to fluoxetine (10 mg/kg), a selective serotonin reuptake inhibitor. These data show that selective breeding of WKY rats has resulted in two substrains with reduced variability and differing responsiveness to antidepressants, which represent a novel means to dissect the molecular mechanisms underlying depressive behavior. Molecular Psychiatry (2003) WKY rats are hyper-reactive to stress and show dysregulation of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-thyroid (HPT) axes. We previously showed that compared to Wi rats, basal plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels of WKY rats remain significantly higher for several hours after the diurnal peak. 2 They also display increased steadystate levels of anterior pituitary pro-opiomelanocortin (POMC) mRNA and exaggerated plasma ACTH responses to acute and chronic stress compared to other rat strains. 3,4 A number of laboratories have found that WKY rats exhibit depressive-like behavior in a wide range of accepted behavioral paradigms. Increased immobility in the forced swim test (FST), considered as a measure of depressive-like behavior, 5-7 has been found in many studies. [8][9][10][11][12] Among other assessments of depressive or anxious behavior, WKY rats also show decreased activity in the open field test (OFT), a measure of anxiety/fear, 9,13 and in the defensive burying test (DB), a potential measure of passive coping. 9 Several studies have found that WKY rats show reduced responsiveness to tricyclic antidepressants (TCA) compared to Brown Norway, Lewis, Fisher 344 (F344), 11 and Sprague Dawley (SD) 14 rats, while others have reported increased responsiveness to TCA antidepressants. 15 Fluoxetine (selective serotonin reuptake inhibito...

Early-onset major depressive disorder (MDD) is a serious and prevalent psychiatric illness in adolescents and young adults. Current treatments are not optimally effective. Biological markers of early-onset MDD could increase diagnostic specificity, but no such biomarker exists. Our innovative approach to biomarker discovery for early-onset MDD combined results from genome-wide transcriptomic profiles in the blood of two animal models of depression, representing the genetic and the environmental, stress-related, etiology of MDD. We carried out unbiased analyses of this combined set of 26 candidate blood transcriptomic markers in a sample of 15–19-year-old subjects with MDD (N=14) and subjects with no disorder (ND, N=14). A panel of 11 blood markers differentiated participants with early-onset MDD from the ND group. Additionally, a separate but partially overlapping panel of 18 transcripts distinguished subjects with MDD with or without comorbid anxiety. Four transcripts, discovered from the chronic stress animal model, correlated with maltreatment scores in youths. These pilot data suggest that our approach can lead to clinically valid diagnostic panels of blood transcripts for early-onset MDD, which could reduce diagnostic heterogeneity in this population and has the potential to advance individualized treatment strategies.

An objective, laboratory-based diagnostic tool could increase the diagnostic accuracy of major depressive disorders (MDDs), identify factors that characterize patients and promote individualized therapy. The goal of this study was to assess a blood-based biomarker panel, which showed promise in adolescents with MDD, in adult primary care patients with MDD and age-, gender- and race-matched nondepressed (ND) controls. Patients with MDD received cognitive behavioral therapy (CBT) and clinical assessment using self-reported depression with the Patient Health Questionnaire–9 (PHQ-9). The measures, including blood RNA collection, were obtained before and after 18 weeks of CBT. Blood transcript levels of nine markers of ADCY3, DGKA, FAM46A, IGSF4A/CADM1, KIAA1539, MARCKS, PSME1, RAPH1 and TLR7, differed significantly between participants with MDD (N=32) and ND controls (N=32) at baseline (q< 0.05). Abundance of the DGKA, KIAA1539 and RAPH1 transcripts remained significantly different between subjects with MDD and ND controls even after post-CBT remission (defined as PHQ-9 <5). The ROC area under the curve for these transcripts demonstrated high discriminative ability between MDD and ND participants, regardless of their current clinical status. Before CBT, significant co-expression network of specific transcripts existed in MDD subjects who subsequently remitted in response to CBT, but not in those who remained depressed. Thus, blood levels of different transcript panels may identify the depressed from the nondepressed among primary care patients, during a depressive episode or in remission, or follow and predict response to CBT in depressed individuals.