Dysregulation of dopaminergic transmission has been widely involved in the pathophysiology of depression45. Depressive disorders symptoms such as social avoidance, anhedonia, lost of appetite, helplessness and amotivation have been consistently associated with dysfunctions of dopamine signalling and functions35,46−48. Now, it is increasingly accepted that males and females respond differently to chronic stress49,50 suggesting that the DA system could exhibit sex-specific morphological and molecular adaptations to chronic stress in a sex-specific fashion. Here, we report findings showing morphological and molecular alterations in mesocortical and mesolimbic dopaminergic pathways of both stressed male and female mice and provide new insights into the implication of dopaminergic circuits in mediating stress susceptibility in a sex-specific fashion.
Neuroanatomical and morphological changes in several brain regions have been consistently associated with MDD. For instance, post-mortem studies on MDD patients showed a decrease of neuronal soma size in layer V and VI compared to healthy subjects51,52. Smaller volume of dorsal striatal gray matter was also shown to correlate with suicidal ideation in adolescent53. Similarly, susceptible mice to CSDS exhibit lower spine density in the mPFC and increased spine density in the NAc and the VTA54 associated with higher and lower volume in the VTA and NAc, respectively55. Here, we provide results showing that CSDS does not change the number of DA cells in the VTA but rather induces an important remodelling of the DA mesolimbic and mesocortical circuits in both sexes. In the NAc, our findings suggest that chronic social stress increases DA axons density in the shell, but not in the core. DA projections form the VTA target mainly the shell part of the NAc composed of MSN expressing either DRD1, DRD2 or both receptors56,57. Importantly, lower frequency of excitatory input onto DRD1 and higher frequency onto DRD2 expressing MSNs has been previously reported in the NAc of susceptible mice to CSDS. Accordingly, optogenetic stimulation of DRD1 MSNs was associated with the expression of resilience while the optogenetic inhibition of DRD2 MSNs was shown to induce depressive-like behaviors following CSDS58. Accordingly, an increase in projections density in the NAc shell as supported by our results could cause an imbalance in dopamine regulation in this region and drive the behavioral effects of CSDS. More work will be required to determine whether these changes impact DA transmission at DRD1 or DRD2 MSNs in males and females.
In contrast, we found a significant decrease of axonal dopaminergic density in the layer V and VI of mPFC in susceptible mice of both males and females. DA neurons from the VTA project mainly on layer V and VI of the mPFC59. Interestingly, optogenetic inhibition of the mesocortical DA pathway was previously shown to promote susceptibility to CSDS in males35. Functionally, speaking, the lower density of DA axons observed in our study is likely to decrease DA signalling in this region and promote social withdrawal in males and females after CSDS. Importantly, while providing interesting insights into the morphological impact of CSDS on DA circuits, approach using immunohistochemistry for TH carries limitations that could interfere with the interpretation of our results. For instance, it has been estimated that 10% of TH + axons in the mPFC originates from axons from the locus coeruleus (LC)60. However, mPFC inputs from the LC are mainly located in layers II/III38 and as such, may not interfere with our main findings in layers V/VI. Nevertheless, the results of our study suggest that CSDS induces morphological alterations in DA axons and could lead to distinct functional impairments in dopaminergic signalling in the NAc and mPFC.
Importantly, our findings show that morphological changes in DA circuits induced by chronic social stress associate with molecular changes affecting intracellular signalling in DA neurons and their targeted brain regions. Transcriptional profiling studies showed before that chronic stress including CSDS induces a global reorganization of transcriptional structures of DA neurons and its targeted brain regions61,62. Importantly, these alterations have also been associated with changes within several intracellular signalling pathways including MAPK43,63,64, AKT65,66, mTOR67 and GTPase pathways68,69. Of particular interest, molecular markers such as pERK and c-Fos have been used as proxies of neuronal activity in several mouse models of chronic and acute stress49,70,71. Here, we provide evidence for a downregulation of pERK signalling in TH + neurons of susceptible male mice specifically. This is in contrast with results from previous studies showing elevated ERK activity in neurons of the VTA after CSDS64,71. Noticeably, these effects were observed in a small proportion of DA neurons in the VTA (approx. 6%) while most of the pERK + neurons in the VTA did not express TH which could explain the discrepancies between our study and others. Thus, it is likely that the impact of CSDS on ERK signalling in the VTA may be relevant to the activity of different neuronal populations with distinct neuronal connectivity patterns. In contrast, we found a trend toward an increase in c-Fos expression in the VTA of susceptible female mice which is consistent with previous observations in females after CSDS49 and supports the sex-specific involvement of these two intracellular signalling pathways in the processing emotional responses to social cues differently72.
Functionally, the pathway-specific morphological changes observed mPFC and NAc projections along with the molecular changes in pERK and c-Fos expression in the VTA following CSDS are likely to impact DA signalling in a pathway specific fashion. Our analysis quantifying pERK and c-Fos expression in the mPFC and NAc support this conclusion although indirectly. In the NAc, our results suggest that CSDS increases pERK levels in the core of male susceptible mice and the shell of susceptible female mice. Previous studies showed that stress activates mainly the shell structure of the NAc73,74 while drugs seeking activates the core structure75. The pERK expression patterns observed in the core and shell suggest that both structures integrate emotional stressors differently in males and females and suggest that distinct neuronal circuits relevant to these sub-regions in the NAc may contribute differently to the expression of stress in males and females. In the mPFC, our findings support a downregulation of ERK signaling in susceptible mice as shown before after CSDS76. It has been shown that ERK activation contribute to upregulation of dendritic spine density in the mPFC and improved learning and memory76,77. However, our findings are also in contrast with other studies showing a significant increase in pERK expression in females following chronic variable stress78 and suggest that different stress types may impose different impact on the activity of the mPFC in males and females. These discrepancies could be explained in part by the nature of both stress paradigms, CSDS reproducing psychosocial stress while CVS being composed of physical stressors79. While these results suggest that the pathways-specific morphological and molecular changes affecting the mesolimbic and mesocortical pathways in males and females may lead to changes in DA signalling in the mPFC and NAc, our results remain indirect and more work will be required to determine the functional impact of these changes in males and females.
To conclude, results from our study provide new insights on the impact of CSDS on dopaminergic sub-circuits and the contribution of mesolimbic and mesocortical pathways on the expression of depressive-like behaviors associating these effects with morphological and molecular alterations in males and females. In doing so, we increase our understanding of the functional and molecular mechanisms from which sexual dimorphism emerges, defining the clinical characteristics of mood disorders. Ultimately, we will be in a better position to develop therapeutic approaches more suited to treat depression in men and women.