Regulation of Depression-like Behaviors in Mice by Connexin Hemichannels and the Lateral Septum


Student thesis: Doctoral Thesis

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Award date3 Aug 2022


Major depressive disorder is a leading mood disorder affecting over 300 million people worldwide. The prevalence of depression is still increasing due to multiple factors, such as the ongoing COVID-19 pandemic. Most people afflicted with depression could not receive effective treatments. Current interventions of depression are somewhat effective but have significant drawbacks. For example, over two-thirds of patients with depression did not respond to classical antidepressant drugs. Understanding the neurobiology of depression may promote the development of therapeutic interventions for depression.

Emerging evidence suggests that neuroinflammation has a crucial role in depression. As the main mediators of inflammatory processes in the brain, glial cells can also modulate neurotransmission and neural activity through neuron-glia crosstalk. For example, connexin hemichannel in astrocytes can control the transport of small molecules such as glutamate. Glutamatergic dysfunction and excitatory-inhibitory (E/I) imbalance are core neural endophenotypes of depression. Recent studies have demonstrated that glial hemichannels are dysregulated in depression. Based on these findings, I hypothesized that blockade of hemichannel could produce antidepressant-like effects. Our collaborator screened and identified a novel connexin hemichannel inhibitor, small molecule D4. In the present study, I investigated the therapeutic potential of D4 in depression. Using ex vivo dye uptake, I first showed that D4 reduced the hemichannel opening induced by systemic inflammation 24 hours after a single-dose injection of lipopolysaccharide (LPS). Next, I found that D4 treatment prevented the LPS-induced depression-like behaviors as evidenced by the reduction of immobility in the tail suspension test (TST) and forced swim test (FST). D4 decreased astrocyte proliferation in depressed-like mice induced by repeated LPS challenges. Furthermore, D4 treatment rescued depressive-like symptoms in mice subjected to chronic restraint stress (CRS). To test whether D4 affects neural activity, I measured c-FOS expression (a marker for neuronal activation) in depression-related brain regions one hour after TST. I found that D4 restored CRS-induced hypoactivation in several limbic regions, including the hippocampus and lateral septum. Together, these results suggest that D4 produces antidepressant-like effects by suppressing neuroinflammation and regulating neural activity.

The septum is an evolutionarily conserved brain region in the limbic system. However, its role in emotional regulation and depression remains unclear. To further understand the circuit mechanisms of depression, I studied the functions of the lateral septum (LS) in depression. First, I confirmed that LS was recruited by multiple stressful tests, including TST, FST, and SPT. Somatostatin-expressing (SST) neurons and calcium/calmodulin-dependent protein kinase type II subunit alpha-expressing (CaMKIIα) neurons are the main subtypes of neurons in the LS. Interestingly, in mice subjected to TST, both SST neurons and CaMKIIα neurons in the LS were activated. Using microendoscopic calcium imaging in freely behaving mice, I found that LSSST neurons showed significant increases in their activity during mobile states in the TST but not in the open field test, a behavioral test used for assessing anxiety and locomotion in mice. Optogenetic activation of LSSST neurons induced a significant reduction in TST immobility in normal mice. In addition, optogenetic LSSST activation also decreased TST immobility in mice injected with LPS. Chemogenetic manipulation showed that activation of LSSST neurons improved CRS-induced depressive behavior in the FST. A similar effect was observed in the chemogenetic activation of LSCaMKIIα neurons. Using viral tracing, I investigated the projection pattern of LS with other depression-associated regions. I showed that LS receives inputs from the hippocampus and projects to downstream regions such as the hypothalamus and ventral tegmental area. Modulation of LS activity also had distinctive effects on these targets. Collectively, my work reveals important neural mechanisms by which the LS contributes to depression.

Overall, findings from my thesis uncover the potential antidepressant-like effects of a novel hemichannel inhibitor D4 and further our current understanding of the lateral septum as a neural substrate in the limbic system that contributes to the regulation of depression.

    Research areas

  • Depression, Antidepressant, Hemichannel inhibitor, D4, Lateral septum, Somatostatin-expressing neuron, CaMKIIα-expressing neuron