ORGANIZATION OF GLUTAMATERGIC AND GABAERGIC INPUTS TO THE THALAMIC NUCLEUS REUNIENS

The thalamic nucleus reuniens (RE) coordinates emotional memory related behaviors by orchestrating medial prefrontal–hippocampal communication and subcortical influences. However, the origin of subcortical glutamatergic and GABAergic inputs in RE is unknown. It is unclear how these inputs are organized within the RE and relative to cortical inputs of various origin.
Here we delineated RE using calretinin (CR) / calbindin (CB) immunostaining and examined the density of presynaptic terminals by immunohistochemistry for vesicular glutamate transporter 1 and 2 (vGluT1, vGluT2), and the vesicular GABA transporter (vGAT). The origins of inputs were resolved with Cre-dependent viral neuroanatomical tracing: retrograde labeling to map the afferent source regions and anterograde AAV tracing in vGluT1-Cre, vGluT2-Cre, and vGAT-Cre mice, complemented by layer-specific cortical Cre lines to assess laminar/regional specificity. vGluT1-immunostaining was broadly uniform within the RE and was not different compared to that of the adjacent nuclei. In contrast, RE as outlined by CB-labelling, displayed stronger vGLUT2- and vGAT-positive terminal staining relative to its neighbors. Viral tracing revealed widespread and selective, subcortical vGAT- and vGLUT2-positive inputs mainly from the hypothalamic and brainstem regions, respectively. Most of these inputs were compartmentalized within the RE. Cortical inputs arising from different cortical territories also displayed highly heterogeneous innervation pattern with the RE.
Together, these findings indicate that overall, the RE receives strong, and selective subcortical GABAergic and glutamatergic inputs but the different sets of cortical and subcortical inputs display robust compartmentalization within the RE resulting in a complex territorial mosaic specialized to integrate different afferents.