LAYER-SPECIFIC CORTICAL AFFERENTATION OF THE PARAVENTRICULAR THALAMIC NUCLEUS

The paraventricular thalamic nucleus (PVT) integrates various hypothalamic and brainstem signals related to arousal, motivation and homeostasis with cortical inputs. Increase or decrease in PVT-activity exerts profound, long-lasting effects on behaviour. The regional distribution of the various afferents within the PVT remains poorly characterized.
Among cortical regions the medial prefrontal cortex (mPFC) has been shown to significantly influence PVT activity and PVT-dependent behaviours. Earlier studies described predominantly, modulatory, layer 6 (L6) mPFC-PVT projections and report few or no robost layer 5 (L5) mPFC-PVT connections that could account for the pronounced top-down mPFC control on PVT.
To resolve this contradiction, we co-injected the classical retrograde tracer cholera toxin B (CTB) and a retrograde viral construct into the PVT. We show that these tracers preferentially label distinct corticothalamic pathways: CTB primarily labelled neurons in mPFC L6, whereas viral labelling was enriched in L5. Analysis of subcortical excitatory, inhibitory and cortical L5, L6 axonal distribution in the PVT using anterograde viral tracing in vGLUT2-Cre, vGAT-Cre and cortical layer-specific Cre-driver mouse lines revealed significant segregation of these inputs. Most subcortical and L5 afferents preferentially innervated the PVT core, which contains densely packed calretinin‑expressing (CR+) neurons. In contrast, L6 inputs were abundant in the transition zone between PVT and the mediodorsal nucleus.
These findings demonstrate that subcortical excitatory/inhibitory afferents converge with L5 cortical axons in the CR+ PVT-core, placing it as a critical bottleneck in the subcortex-cortex communication related to affective behaviour.