Stress resilience and homeostatic response
The close relationship between circadian rhythms and mood disorders warranted a systemic investigation of transcriptomic changes in the principal clock (SCN) and brain regions that modulate mood. We focused on the Hb because it is a semi-autonomous oscillator with a strong role in stress regulation and mood disorders 14. We found that chronic stress elicited a robust change in the gene expression in the SCN and to a lesser degree in the Hb (Fig. 2–3). This transcriptomic signature was specific to resilient mice and was strongest at ZT16. In contrast, comparing susceptible and stress-naïve mice did not reveal strong differential expression patterns. Large differential upregulation in resilient mice was associated with enrichment and predicted activation of signalling and plasticity-related pathways in both nuclei (Fig. 4–5).
Resilience is an active homeostatic process and involves transcriptional changes in a diffuse network of interconnected brain regions 56. The current understanding is that chronic stress imposes an adaptive challenge that requires behavioural flexibility and physiological homeostatic buffering 57. The inability to elicit such a response leads to failure in stress-coping and strongly contributes to associated stress-induced pathologies 57,58. Resilient mice do not show behavioural deficits likely because of large transcriptomic changes such as those observed in our study. Early studies found that resilient mice exhibited large and unique upregulation of neurotrophic factors and stress-responsive genes in the prefrontal cortex 31. Others found strong gene upregulation in resilient mice relative to control and susceptible mice in midbrain regions such as the nucleus accumbens and VTA 59,60. The physiological consequences of large transcriptomic changes in resilient mice are also likely responsible for observed differences in circuit-specific neural activity between resilient and susceptible mice. For example, VTA dopaminergic (DA) cells in susceptible, but not resilient, mice exhibit elevated firing 61. The pathophysiologically elevated firing is counteracted in resilient mice by compensatory mechanisms that are absent in susceptible mice 62. Furthermore, our findings that large transcriptomic changes occur in resilient mice at a specific night time point (ZT16) add a significant temporal dimension to the current understanding of the molecular mechanisms that drive stress-resilience. Our data, along with previously published physiological and circuit studies, suggest that temporal patterns of regional activity may play a crucial role in driving a particular behavioural phenotype 15. Our data provides further evidence of a tight association between large transcriptional changes and successful stress-coping mechanisms. Moreover, future studies with sampling performed over several day/night cycles following chronic stress would allow for stability estimation of this transcriptomic signature.
SCN transcriptomic profile
We observed significant upregulation of Adora2a in the SCN of resilient mice at ZT16. Adenosine is known to regulate Per1 and Per2 expression via A1/A2A receptor activation which is hypothesised to integrate light and sleep signalling to track homeostatic sleep pressure and regulate sleep architecture - both of which were previously found to be differentially impacted in resilient and susceptible mice 34,36,63. Upregulation of Adora2a in the SCN of resilient mice at ZT16 may be a subcomponent of adaptive homeostatic response that maintains consolidated sleep 34,36. Interestingly, the upregulation of A2A receptors in the forebrain is associated with depressive-like behaviour and decreased synaptic plasticity following unpredictable chronic mild stress (CMS) 64. Resilient mice displayed strong upregulation of Ntrk1 and Ngfr in the SCN at ZT16. NGF binds to Ntrk1 and Ngfr 65,66. NGF levels are significantly lower in depressed patients 67 while increasing its levels has antidepressant properties 68,69. Furthermore, BDNF, which has functional roles in synaptic plasticity, stress response, and antidepressant action in several regions, also binds to Ngfr 70,71. Ngfr is specifically synthesised in cholinergic neurons which strongly modulate SCN neuronal excitability and circadian rhythmicity at night 72–74.
We also observed high upregulation of cholinergic muscarinic receptors Chrm 1–5. These receptors have a functional role in fear and emotional responsiveness 75,76. Cholinergic signalling in the SCN can induce phase shifts in locomotor rhythms 77. However, at present it is unclear how increased muscarinic receptor expression at ZT16 in the SCN drives resilience. A recent study demonstrated that the stress-associated neuropeptide CRH increases cholinergic interneuron firing that causes potentiation of dopamine transmission 78,79. Besides cholinergic receptors, Drd2 and Crh were upregulated in the SCN of resilient mice at ZT16. Dopaminergic input from VTA to SCN modulates circadian entrainment through activation of excitatory Drd1 receptors 80. The putative homeostatic role of inhibitory Drd2 in circadian regulation in resilient mice is currently unknown. Our observation that Crh is upregulated in resilient mice adds to the recent findings that CRH-expressing neurons are also present in the SCN 81. CRH increases gating of BDNF signalling 82. Thus, increased CRH expression at ZT16 may be responsible for putative BDNF-induced antidepressant-like effects in stress-resilient mice. The resilient phenotype has also been associated with increased cAMP and GPCR-mediated signalling 59,83. These are a subset of enriched plasticity-related pathways in our dataset that are predicted to be activated in the SCN of resilient mice. However, the functional consequences of these changes are currently unclear since plasticity-related transcription factor Creb3l1 was downregulated, while many others, such as Gbx1, were strongly upregulated in the resilient mice. Previously, Creb downregulation has been associated with stress resilience 84,85. Overall, few studies have examined stress effects on SCN physiology and those predominantly focused on the expression of clock genes 7,86–88. Our observations highlight potentially novel molecular mechanisms in the SCN that promote resilience to stress that warrant further investigation.
Hb transcriptomic profile
Adcy1 and Adcy9 were upregulated in the Hb of resilient mice relative to controls at ZT16. Adenylyl cyclases play a pivotal role in both cAMP-mediated and the ERK½-CREB signalling cascade which are known to modulate neuronal physiology 89–91. Overexpression of Adcy1 in the forebrain prevents BDNF downregulation in stressed mice, thereby promoting stress resiliency 92. Adcy1 forebrain overexpression induces greater long-term potentiation (LTP) 91. Moreover, we observed ZT16-specific upregulation of other plasticity-modulating genes, such as Grin2a, Grin2b, Cacna1h, Camk2b, and Ppp3ca which correlate with large predicted activation of synaptogenesis and LTP signalling pathways. Resilient mice exhibit greater hippocampal LTP relative to susceptible mice because of higher expression of Grin2b and Camk2 93. Upregulation of plasticity-related genes may be a part of a larger homeostatic response associated with stress resiliency 94,95. We also observed upregulation of Gad1, Gabra5, Kcnq3 and Npy1r genes in stress-resilient mice. Sub-chronic mild stress decreases long-term depression (LTD) of GABAergic neurons in the LHb 96. Furthermore, male mice with Kcnq3 mutation that inhibits GABA binding exhibit reduced self-grooming, behaviour indicative of apathy 32,97. Neuropeptide Y action through its Y1 receptor has been associated with stress resilience and anxiolysis in several brain regions, including the Hb 98,99. In the LHb, Npy1r-expressing neurons play a critical role in chronic stress adaptation by driving palatable food intake and reducing anxiety-like behaviour 100. Thus, it is possible that elevated Npy1r expression at ZT16 in the Hb drives increased food intake in resilient mice to compensate for lower levels of adipose tissue and serum leptin 101.
Clock genes
Since our analysis revealed a strong effect of stress on the SCN and Hb at a specific time point, we examined the rhythmic expression of clock genes in both of these nuclei. Chronic stress induces arrhythmicity of Arntl in the SCN of both resilient and susceptible mice which might imply that putative homeostatic adaptations in resilient mice do not protect against stress-induced disruption in rhythmic expression of this clock gene. Despite previous observation that knockdown of Arntl leads to behavioural despair, anxiety and helplessness following CMS, our results suggest that Arntl arrhythmicity in the SCN does not directly impact mood regulation 7. Rhythmic Cry expression in susceptible, but not resilient, mice is unexpected as it suggests a compensatory mechanism that maintains rhythmic expression of some genes. Another study found that CMS in rats induces perturbation of Per2 in the LHb, without affecting the SCN rhythms 102.
Previous work has shown that exposure to CSDS in the day or night induces differences in locomotor activity and tissue specific Per2 expression profile103,104. In contrast no difference in locomotor rhythms or Per2 expression were observed in mice kept in constant dim red light where clock rhythms are likely free running86,105. These observations imply that exposure during different light cycles modulate rhythmic behavioural and gene expression profiles. Future studies will investigate whether the transcriptomic profile of mice exposed to chronic stress at night induces similar transcriptomic profiles observed in resilient mice at ZT16.
In our study, Per1, Per3, and Cry1 were arrhythmic in the Hb of stress-naïve mice, but not resilient and susceptible mice. Although unexpected, these results can be attributed to the subdivisional differences at the level of MHb and LHb 10,24–26. Clock gene expression in the MHb has yet to be systematically investigated in mice, though it has been reported previously in hamsters and rats 106,107. Despite both the LHb and MHb showing diurnal variation in firing 24,25, the LHb displays more evident rhythms of higher amplitude than the MHb 10. In the present study we did not differentiate between the two subregions and the reduced spatial resolution might have contributed to the observed arrhythmicity of clock genes in stress-naïve mice. Moreover, different stressors have been previously shown to differentially affect neural dynamics 32. Thus, the differential effect of CSDS and CMS on the transcriptomic landscape may be driven by the stressor type. Higher sampling resolution would, however, be required to accurately describe effects on the phase of core clock gene expression, thus warranting future studies.