<i>EX VIVO</i> MAPPING OF NIGRO-THALAMIC DOPAMINERGIC TERMINALS REVEALS REGIONAL SPECIALIZATION AND SYNAPTIC MARKERS IN THE HUMAN THALAMUS

The thalamus plays a crucial role in integrating subcortical inputs and relaying them to cortical circuits, yet the extent and specificity of dopaminergic innervation to human thalamic nuclei remain incompletely understood. Recent tractographybased studies suggest the existence of direct nigro-thalamic pathways, but direct histological evidence is still limited. In this study, we provide an ex vivo mapping of dopaminergic projections to the human thalamus using high-resolution immunohistochemistry and confocal microscopy. Human brain specimens were processed to detect multiple dopaminergic markers, including tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), and aromatic L-amino acid decarboxylase (AADC), across anatomically defined thalamic nuclei. Multilabel immunofluorescence and 3D reconstruction allowed precise identification of axonal arborization and synaptic varicosities within the mediodorsal (MD), and ventral anterior (VA)/ventral lateral (VL) nuclei. Quantitative analysis revealed a significant expression of TH+/VMAT2+ fibers and D2R staining in the MD/VA/VL nuclei, supporting the notion of region-specific dopaminergic input. These results provide the first immunohistological confirmation of selective dopaminergic innervation of the human thalamus. This structural evidence complements prior neuroimaging findings and suggests a potential role for the nigro- thalamic pathway in modulating thalamo-cortical circuits involved in executive and cognitive functions.