Medial Habenula-interpeduncular Nucleus Circuit Contributes To Anhedonia-like Behavior In A Rat Model Of Depression

The habenula is a nuclear complex composed of the lateral habenula(LHb)and medial habenula(MHb),two distinct structures.Much progress has been made to emphasize the important role of LHb in the pathogenesis of depression.Previous studies in our laboratory expressed that the LHb is a key regulator of the activity of dorsal raphe nuclei(DRN)serotonergic neurons in affective disorders such as depression.However in recent years,the role of MHb has begun to gain increasing attention.The MHb connects to the interpeduncular nucleus(IPN)both morphologically and functionally.The IPN is an important midbrain structure that receives projections from the MHb via the internal portion of the FR,and in turn,regulates the activity of monoaminergic neurons such as dopamine and serotonin neurons in the ventral tegmental area(VTA)/substantia nigra pars compacta(SNc)and DRN,respectively.The MHb-IPN pathway plays an important role in regulating higher brain functions,including cognition,reward and decision making.Its dysfunction may be a pathological mechanism in neuropsychiatric disorders such as ADHD,fear,anxiety,and nicotineaddiction and withdrawal.Thus,the study of the MHb-IPN pathway is helpful for further understanding the role of the Hb and molecular mechanisms in depression.Method:1.To observe the neuronal activity of the MHb-IPN pathway in depression model,the chronic unpredictable mild stress(CUMS)-exposed rats were used for studies.The cytochrome c oxidase(CCO)activity was detected by histochemical staining and redox reaction.2.To observe the changes of substance P of the IPN and the dopamine function of the nucleus accumbens nucleus(NAc),we used ELISA analysis to detect the SP content in the IPN in CUMS-exposed rats.Western Blot method was used to detect the expression of NAc dopamine D2 receptor in the NAc.3.To observe the effect of MHb-lesion on depression-like behaviors in CUMS-exposed rats.The percentage of sucrose consumption in sucrose preference test(SPT)and the change of immobility time in Forced Swimming Test(FST)were recorded.4.To observe the effect of MHb-lesion on the IPN activity and on the D2 R in the NAc,the cytochrome C oxidase activity in the IPN was measured by redox reaction.The SP content in the IPN was detected by ELISA analysis.The expression of D2 R protein in the NAc was detected by Western Blot.5.To evaluate the effect of the SP in the IPN on anhedonia-like behavior,the cannula was implanted in the IPN in rats.SPT was conducted after the SP microinjection.Western Blot was used to detect the expression of D2 R protein in the NAc.6.To test the hypothesis that SP release increases the neuronal activity of the IPN,we recorded firing rate extracellularly from IPN neurons using an in vitro slice preparation and validated the pharmacological signature of IPN neurons responding to SP perfusion.Results:1.Performing the CUMS procedure for 28 days significantly reduced sucrose consumption in the SPT and a markedly longer immobility time in the FST.The CCO activity of the MHb-IPN in the CUMS-induced depression model group was significantly increased compared with the control group.2.In the enzyme-linked immunosorbent assay(ELISA)analysis,we found that SP concentration in the IPN was significantly elevated in CUMS-exposed rats compared with that of the control group and the expression of D2 R in the NAc was significantly decreased.3.Regarding the SPT,the MHb lesions group consumed more sucrose than the MHb-sham group suggesting a difference in their hedonic state.Regarding the FST,no significant difference in immobility time was detected between the two groups.4.The MHb lesions group had less CCO activity in the IPN than the MHb sham group Compared with sham-operated group.MHb lesions in CUMS-exposed rats markedly decreased SP release to the IPN,as the concentration of SP in the IPN was significantly decreased compared with that of the MHb sham group.Meanwhile,the expression of D2 R in the NAc was significantly increased compared with that of the MHb sham group.5.The SP microinjection into the IPN caused a significantly decrease of sucrose consumption in SPT in rats compared with the control group.The expression of D2 R in the NAc was significantly decreased compared with that of the control group.6.On the whole,mean firing rate of 48 IPN neurons were increased after SP perfusion.These results reveal an excitatory effect of SP on the firing rate of IPN neurons.Conclusion:In our study,CUMS exposure in rats generated a realistic model of depression which elevated the neuronal activity of the MHb-IPN pathway and was coupled with the elevation of SP content in the IPN and the decreased expression of D2 R protein.The CUMS rats with MHb lesions exhibited a reversal of anhedonia-like behavior.In addition,we found that CCO activity and SP content were down-regulated in the IPN and the D2 R expression was incresasd in the NAc,which wasaccompanied by the recovery of anhedonia-like behavior.Finally,firing recordings of IPN neurons in brain slices showed that the spontaneous activity of IPN neurons was significantly increased in response to SP perfusion.Collectively,our results suggested that in CUMS-exposed rats,an increase in the neuronal activity of the MHb-IPN circuit was involved in the performance of anhedonia-like behavior and that SP might mediate this effect of the MHb on the IPN neurons.