PENK/PDYN And Their Receptors In The Frontal Cortex In Depression And The Role Of MOR In The Early Stage Of Chronic Stress

Background:Depression is one of the most serious mental diseases in the world.According to the new report from WTO in 2017,there are 3.5 million people suffering from it all over the world.In clinical trials,depression has become one of the world ' s most disabling diseases.Endogenous opioids is the opiate-like substance,which is naturally produced in the brain of mammals.There are three classical precursors of opioids:proopiomelanocortin(POMC),proenkephalin(PENK),and prodynorphin(PDYN).Opioids execute their biological effects via three main opioid receptors,?(MOR),?(DOR)and?(KOR)after the precursors experience a series of cutting and exist as some small peptides.Many studies have reported that endogenous opioids are involved in the pathogenesis of depression.Currently,most of researches focused on the role of opioids and their receptors in the amygdala,accumbens and hippocampus.However,little attention is gained on the advanced regulating center—frontal cortex.In addition,some results of MOR are inconsistent with each other: both MOR agonist and MOR KO could improve the depressive-like behaviors of animals,which means that the role of MOR in depression needs to be clarified in the future.In order to clarify these two questions,a classic UCMS(unpredictable chronic mild stress)rat model of depression was applied and the expressions of PENK/PDYN and their receptors(MOR,DOR,KOR)in the frontal cortex were tested by q PCR in UCMS for 3 and 6weeks respectively.After that,two sub brain areas of frontal cortex in depressionpatients: ACC and DLPFC(from brain bank in Netherlands),which are closely associated with depression,were obtained and above genes were also tested in the same way.After that,to determine the role of MOR in the early stage of depression,the effects of MOR agonist(Methadone)/ antagonist(Naloxone)on the depressive-like behaviors and the expressions of CRH/AVP in the hypothalamus were measured in the rats of UCMS for 3 weeks.Methods:1.UCMS model: 40 Male Wistar rats,weight between 250 g to 320 g,were randomly divided into a control and a UCMS group.During the stress procedure(3/6week),all stressed rats were raised individually,while non-stressed rats were housed six per cage in a separate room.2.The expression of PENK/PDYN,MOR,DOR,KOR in the frontal cortex in 3 / 6 week UCMS rats: after the last behavior tests,all the rats were scarified.The expression of PENK/PDYN,MOR,DOR,KOR in the frontal cortex in the UCMS 3/6 week and control groups were detected with the method of q PCR.3.The expression of PENK/PDYN,MOR,DOR,KOR in DLPFC and ACC in depression patients: 24 ACC(anterior cingulate cortex)brain tissues from 12 depression patients(7 Bipolar and 5 Major depressive disorder)and 12 matched control patients and 28 DLPFC brain tissues from 14 depression patients(9 Bipolar and 5 Major depressive disorder)and 14 matched control patients were detected by q PCR.4.Effect of MOR agonist(Methadone)/ antagonist(Naloxone)on behaviors and HPA axis: 40 animals randomly were divided into 4 groups: control,UCMS,UCMS+Methadone(0.5mg/kg),UCMS+Naloxone(2.5mg/kg).The animals in Methadone/ Naloxone group were subcutaneously injected once a day for three weeks.The expression of MOR in the frontal cortex,hippocampus,striatum and hypothalamus and CRH/AVP in the hypothalamus among these four groups were detected by q PCR.Results:1.The depressive bhaviors and q PCR results in 3/6week UCMS: Compared to the control group,3/6 week UCMS group showed decreased weight gain,sucrose intakeand time in the open arm(P<0.01).In the q PCR tset,compared to the control group,MOR m RNA significantly increased(P<0.01),while PENK/PDYN/MOR/KOR showed no change(P>0.05)in the UCMS 3 week group.Different from UCMS 3week animals,an opposite change(MOR significantly decreased,P<0.01)was observed in the UCMS 6 week group In addition,PENK and PDYN significantly decreased and increased(P<0.001),respectively.while no changes in DOR and KOR were found in the UCMS 6 week group.2.q PCR detection in DLPFC and ACC in depression patients: In ACC,compared with control patients,depression patients showed decreased MOR expression(a decrease of74%,P<0.01)among these data,major depression patients showed markedly decreased MOR(decreased of 30%,P<0.01)and KOR(decreased 40%,P<0.01)compared with control patients;Bipolar patients showed no significant differences.In DLPFC,compared with control patients,all the gene showed no differences(P>0.05).There is no correlation between MOR,KOR expression and age,postmortem delay,fixed time and brain weight in both DLPFC and ACC.3.Effect of MOR agonist(Methadone)/ antagonist(Naloxone)on behaviors and CRH/AVP expression in hypothalamus in UCMS 3 week rats: q PCR result showed that MOR significantly increased in frontal cortex,hippocampus,striatum and hypothalamus(P<0.01).Behavior tests showed both Methadone and Naloxone improved depressive behavior: compared with the UCMS group,weight gain,sucrose intake,time in the open arm are higher at some time point(P<0.01),while the immobility time of forced swim test is lower(P<0.001).q PCR data showed that compared to control group,CRH and AVP show the tendence of increased,but no significance difference(P>0.05).Compared to UCMS group,UCMS+Methadone group showed increased CRH/AVP,CRH increased 62%(P<0.05),AVP increased 21%(P?0.05).while UCMS+Naloxone group showed decreased CRH/AVP,CRH decreased 46%(P ? 0.05),AVP decreased80%(P?0.05).Conclusions:1.The expression of MOR in the frontal cortex has shown firstly increased in the rats of UCMS 3 week but then decreased in the rats of UCMS 6 week.Meanwhile,decreased PENK and increased PDYN were also observed in the rats of UCMS 6 week.These changes indicate that,as an important brain area,the frontal cortex involves in the opioid-related pathogenesis of depression.2.In the study of depression patients,the significantly decrease of MOR in the ACC in major depression patients confirmed the changes of MOR in the frontal cortex in the rats of UCMS 6 week,which indicates that ACC would be a critical cutting-point for studying the frontal cortex involved and opioid-related pathological mechanism of depression in the future.3.The changes of MOR in frontal cortex that first increase in the rats of UCMS 3 week then decease in the rats of UCMS 6week indicate that MOR plays different roles in the early stage and the opposite change of MOR in early/late stage may be the potential mechanism of the presented inconsistent results of MOR in depression.4.MOR antagonist(Naloxone)not only improved depressive-like Behaviors but also decreased the AVP expression in hypothalamus in the rats of UCMS 3 week,which indicates that MOR antagonist may provide the protective effect for a promising therapeutic strategy in early stage of depression.However,MOR agonist(Methadone)also improved the depressive behaviors,however,significantly increased the of CRH expression in hypothalamus,which is different from what we expected.The potential mechanism of it needs further study to be clarified.