Study On The Stress Theory Of Mood Disorders:from Postmortem Study To Animal Models

Aim:Mood disorders are common diseases.Major depressive disorder(MDD)and bipolar disorder(BD)are two important types of mood disorders,which are major public health burdens.It is urgent to reveal the pathogenesis of mood disorders in order to find novel therapeutic target.The stress theory is an important theory of the pathogenesis of mood disorders,and an important basis to model various kinds of animal models for depression.Mood disorders are characterized by sex-related hyperactivity of stress-related brain circuits.In this network,the hyothalamo-pitutary-adrenal(HPA)axis and the locus coeruleus(LC)are two important hubs.There is a wealth of evidence showing that these two systems are activated in mood disorders.The HPA activity has been linked to clinical symptoms in mood disorders and the LC is activating the HPA-axis.Therefore,it is very important to analysis possible differences in HPA-axis activation between BD and MDD patients.Recently,it was suggested that the stress-related molecule oxytocin(OXT)and the tyrosine kinase receptor ErbB4 may be involved in the pathogenesis of BD.In the present work,we focused on the difference of these molecules respectively the hypothalamus and LC in MDD and BD.We studied(1)the differential expression of corticotropin-releasing hormone(CRH)and OXT in the postmortem hypothalamus tissue of MDD and BD patients;(2)the differential expression of Tyrosine Hydroxylase(TH)and ErbB4 in postmortem LC tissue of MDD and BD patients;(3)the effect of sex steroids on vulnerability of HPA axis in stress response in SD rats,and discussed how they modeled different kinds of depression.Material and methods:Experiment-1:Differential expression of CRH and OXT was determined in the hypothalamus of MDD and BD.Human postmortem hypothalamus material was obtained from the Netherlands Brain Bank(MDD,n=9;BD,n=6;MDD control group,n=9;BD control group,n=6).Patients in the control group and their respective mood disorder subgroup were strictly matched for age,sex,the time of death,death season,postmortem delay(PMD),pH of cerebrospinal fluid(CSF)and fixation time.Quantitative immunocytochemistry(ICH)was used to analyze the total peptide content of CRH and OXT in the paraventricular nucleus(PVN),while quantitative in situ hybridization was used to measure transcript levels of CRH and OXT at their respective protein-peak levels.Double labeling of CRH immunoreactivity(-ir)and OXT-ir was used to determine whether there was a co-localization of CRH and OXT in PVN neurons.Experiment 2:Differentiatial expression of TH-ir and ErbB4-ir was determined in the LC of MDD and BD.Human postmortem hypothalamus material was obtained from the Netherlands Brain Bank(MDD,n=10;BD,n=10;MDD control group,n=10;BD control group,n=10,eight patients in control groups were overlapping,12 control patients in total).Patients in the control group and their respective mood disorder subgroup were strictly matched for age,sex,the time of death,death season,PMD,CSF-pH,and fixation time.Postmortem mid-level LC sections were studied by ICH for TH-ir and ErbB4-ir.Double labeling of TH-ir and ErbB4-ir was used to study the co-localization of TH and ErbB4 in LC.Experiment 3:Sex hormones affect acute and chronic stress responses in sexually dimorphic patterns.This has consequences for depression models.Sprague-Dawley(SD)rats were used for this animal model study.Gonadectomized or sham-operated male and female rats were exposed to foot shock stress or chronic unpredictable mild stress(CUMS)or no stressor.For all rats,plasma sex hormones,i.e.testosterone(T)and estradiol(E2),hypothalamic T or E2,and the mRNA expression of stress-related molecules were determined,including CRH,OXT,arginine vasopressin(AVP),glucocorticoid receptor(GR),mineralocorticoid receptor(MR),estrogen receptor-a(ERa),estrogen ER?,androgen receptor(AR)and aromatase(ARO).Results;Experiment 1:CRH-ir showed a trend of increase in MDD(P=0.058)and a trend pf decrease in BD patients(p=0.093)in the hypothalamic PVN,while CRH-mRNA showed a significant increase in BD patients(P=0.037).OXT-ir and OXT-mRNA were both elevated in BD(P=0.024,and P=0.055,respectively),but not in MDD patients(p=0.258 and p=0.847,respectively).The ratio of OXT-ir/CRH-ir was significantly increased in the BD group compared with its controls(P=0.006),while this ratio did not show a significant difference between the MDD group and its controls(p=0.847).There was no co-localization of CRH and OXT in the hypothalamic PVN.Experiment 2:Enhanced levels of TH-ir were found in the LC of MDD patients as compared to matched controls(p=0.004),but not in BD patients compared with controls(p=0.900).ErbB4-ir showed no differences between mood disorder patients and controls in the LC(p = 0.800 and p = 0.630,respectively).The co-location staining of ErbB4 and TH showed that both of them were expressed in LC neurons.Experiment 3:No significant correlation was observed in terms of plasma sex hormones,brain sex steroids,and hypothalamic stress-related molecule mRNAs(p>0.113)in intact or gonadectomized,male or female,rats.Male and female rats,either intact or gonadectomized and exposed to acute or chronic stress,showed different patterns of stress-related molecule changes(including CRH,OXT,AVP,AR,ERa,ER?,ARO,MR).Conclusion:A clearly activated OXT system was found in BD patients.The unbalanced expression of hypothalamic OXT and CRH might be the pathological basis for depressive and maniac symptoms in BD patients,and a key factor for the difference in symptoms between MDD and BD.Possible mechanisms should be studies in the future.LC was only activated in MDD but not in BD,similar to the difference in HPA activity between MDD and BD.ErbB4-irdid not change in the mood disorders,which is different from findings in an animal model.In SD rats,the brain sex steroid levels were independent of peripheral sex hormones,which was surprising.The acute stress model of ovariectomised female rats mat be used as a model for depression in women in perimenopausal and postpartum stage,in which the change in peripheral sex hormone levels increases their vulnerability to stress;the intact CUMS female rats may be used as a model for depression in women in the reproductive age,who are more vulnerable to stress and depression than man;the ovariectomized CUMS rats may be used as a model to study mood disorders of women before puberty or post-menopausal stage.An animal model for BD is currently not available.