Median Raphe projection into the dorsal dentate gyrus modulates anxiety behavior and coping with learning under stress

The Median Raphe (MR) provides a major, mainly serotonergic, input to the hippocampus, particularly influencing the dentate gyrus (DG), but its specific role remains unclear. Here, focusing on the dorsal DG, we investigated its contribution to anxiety-like behavior and learning under stress, using a pathway-specific chemogenetic approach in male Sprague-Dawley rats. Retrograde Cre-expressing AAV was injected into the dorsal DG, and Cre-dependent excitatory (hM3Dq) or inhibitory (hM4Di) DREADDs were expressed in MR neurons. Behavioral assessments included the Open Field (OF), Elevated Plus Maze (EPM), and an extended Two-Way Shuttle Avoidance task (eTWSA). MR-dorsal DG pathway activation had no significant impact, but its inhibition decreased anxiety-like behavior in the EPM and improved eTWSA performance, particularly in behaviorally “diverged” animals. Knocked down in the dorsal DG EphA7, a molecule involved in basket cell synapse stabilization on DG granule cells, prevented the anxiolytic effects and behavioral divergence induced by MR-dorsal DG inhibition, but did not prevent improved eTWSA performance. These findings suggest a dissociation between mechanisms of emotional regulation and stress-related learning mechanisms within the dorsal DG, with the MR input to DG basket interneurons modulating anxiety but not learning flexibility. The results highlight a subregion- and pathway-specific role of the MR-hippocampal input in modulating behavior.