Increased granulovacuolar degeneration in the thalamus and higher neurofibrillary tangle Braak stages in bipolar disorder

Aim Recent neuropathological studies suggest that the accumulation of neurodegenerative disease–associated proteins in subcortical structures may contribute to mood symptoms. Animal models have highlighted the role of the paraventricular thalamic nucleus (PVT) in bipolar disorder (BD) pathophysiology. However, neuropathological investigations in the thalamus in BD remain limited. This study aimed to examine neurodegenerative pathology in the thalamus and medial temporal region including the hippocampus in patients with BD. Methods Postmortem brain tissues of the thalamus and medial temporal region of nine patients with BD and nine age‐matched controls were obtained from Matsuzawa Hospital, with additional medial temporal samples of 14 BD cases acquired from the Stanley Foundation Brain Bank. Immunohistochemical analyses were performed using antibodies against phosphorylated tau, amyloid‐ β , α ‐synuclein, TDP‐43, and granulovacuolar degeneration (GVD) markers including CHMP2B and CK‐1 δ . Results The 23 BD cases exhibited a significantly greater burden of tau pathologies, including higher neurofibrillary tangle Braak stages ( P = 0.015) and more severe argyrophilic grain Saito stage ( P = 0.029), compared with the nine controls. Notably, CHMP2B‐positive GVD was significantly more frequently observed in the PVT of BD cases than in the controls (five of nine vs. zero of nine, P = 0.029). Conclusions These findings suggest that neurodegenerative processes, particularly tau pathology and CHMP2B‐positive GVD in the PVT may play a role in BD pathophysiology.