Brain-Wide Connectivity of GLP1R Neurons in the Thalamic Reticular Nucleus.

BACKGROUND: Glucagon-like peptide-1 receptor (GLP1R) is a G-protein-coupled receptor recognized for its essential role in metabolic homeostasis and insulin secretion. Emerging evidence suggests that central GLP-1 signaling also modulates sensory information processing and cognitive functions. The thalamic reticular nucleus (TRN) serves as a critical inhibitory hub that filters and prioritizes sensory transmission between the thalamus and the cerebral cortex. We have identified a distinct population of GLP1R-expressing neurons distributed within the TRN; however, their long-range structural connectivity remains largely uncharacterized.

METHODS: In this study we used Glp1r-Cre mice combined with viral-genetic tracing strategies to map the whole-brain inputs and outputs of GLP1R-linked neurons. To identify direct monosynaptic inputs, a Cre-dependent retrograde rabies virus system was employed. To delineate the efferent axonal projections, a Cre-dependent synaptophysin-based tracing strategy was employed.

RESULTS: GLP1R neurons exhibited a distinct rostrocaudal distribution, with most located in the middle region of the TRN. The starter neurons, identified by the colocalization of adeno-associated virus (AAV)-double-floxed inverted orientation (DIO)-enhanced green fluorescent protein (EGFP)-tumor virus A receptor (TVA), AAV-DIO-rabies virus glycoprotein from the CVS-N2c strain (N2cG), and rabies virus (RV)-envelope protein A (EnvA)-glycoprotein-deleted (ΔG)-mCherry, were primarily located in the TRN. Retrograde-labeled neurons were identified across numerous brain regions. Dense clusters of input neurons were observed in the primary and secondary motor cortices (M1 and M2, respectively) and the primary somatosensory cortex (S1). Substantial inputs were observed, including from the ventrolateral (VL), central lateral (CL), and posterior (Po) thalamic nuclei. Additionally, notable presynaptic labeling was detected in subcortical regions such as the zona incerta (ZI) and lateral hypothalamic area (LH), as well as midbrain structures including the substantia nigra pars reticulata (SNR) and the deep mesencephalic nucleus (DpMe). Anterograde synaptophysin-based mapping revealed that TRNGLP1R neurons selectively project to the ventral medial nucleus (VM), paracentral thalamic nucleus (PC), mediodorsal thalamus, lateral part (MDL), and lateral habenula (LHb) nuclei.

CONCLUSIONS: The results confirm the existence of complex long-range afferent and efferent circuits associated with TRNGLP1R neurons, providing a morphological basis for studying their role in integrating metabolic states with sensory gating.