The Habenula's role in major depressive disorder: recent insights from preclinical and human studies

The habenula is a small epithalamic structure composed of two distinct subregions, the medial (MHb) and lateral (LHb) habenula. It serves as a critical hub for integrating fronto-limbic and brainstem signals to regulate motivation, mood, and reward processing. Therefore, it is unsurprising that dysfunction of the habenula has been implicated in several mood disorders including major depressive disorder (MDD), a debilitating mood disorder marked by low mood and feelings of hopelessness. This review synthesizes recent advances in understanding the habenula's neurocircuitry, molecular landscape, and role in MDD pathophysiology, while evaluating its potential as a therapeutic target. Specifically, emerging evidence highlights subregion-specific pathology. Indeed, in MDD and in animal models of depression, the MHb has been shown to exhibit marked downregulation of calcium-dependent activator protein for secretion 2 (CAPS2) and deficits in nicotinic acetylcholine receptor-mediated signaling. While in the LHb, dysregulated expression profiles of inward-rectifying potassium channel Kir4.1, the β isoform of calcium/calmodulin-dependent protein kinase II (CaMKIIβ), protein phosphatase 2 A (PP2A), and small nucleolar RNA SNORA69 have been found in animal models of depression and MDD postmortem studies. Structural imaging and postmortem neurohistological studies in MDD patients have further revealed habenular volume changes, reduced neuronal cell counts, diminished cell area, and abnormal functional connectivity. As research unravels the habenula's complexities, its potential in treating mood disorders grows increasingly salient, offering new avenues for intervention in mental health.