Activation of a Habenulo–Raphe Circuit Is Critical for the Behavioral and Neurochemical Consequences of Uncontrollable Stress in the Male Rat

Stress, per se, increases Fos-IR in the LHb–DRN pathway. A, Schematic showing FG injected directly into the DRN prior HC, IS, or ES (left). Photomicrographs show the site of injection at 4× (top) and 10× (bottom) magnification. Scale bars, 200 μm. Fluoro-Gold traveled retrogradely to the LHb prior to rats receiving HC, IS, or ES (right). B, Two weeks after Fluoro-Gold injection, rats received HC, IS, or ES and were killed 2 h later. Total FG (left), Fos-IR (middle), and the percentage of FG cells also expressing Fos (FG+Fos; right) were quantified in the MHb, LHbM, and LHbL (top). The total number of FG-labeled cells in each habenula subregion (FG+Fos) did not differ across treatment groups (left). Total Fos-IR was increased in the LHbM and LHbL of rats that received either IS or ES (middle). Similarly, activation of the LHb–DRN pathway (% FG+Fos) was increased in rats that received either IS or ES (right). Photomicrographs (bottom) of representative LHbM tissue samples are shown for rats that received HC (top), IS (middle), or ES (bottom). Images in the first three panels are taken at 20× (scale bar, 50 um) and magnified to 40× at the far right (scale bar, 50 μm). FG reliably labeled cell bodies in the LHbM 2 weeks after injection (left). Increased Fos-IR is evident in rats that received IS or ES, relative to HC (middle). IS and ES also increased LHb–DRN pathway activation relative to HC, as measured by the colocalization of FG and Fos-IR (right). Arrows indicate colocalized FG and Fos-IR (magenta spot). N = 12/group, two habenula sections/rat.