c-Fos expression in the paraventricular nucleus during the hibernation bout. A: Coronal section of the brain of a 13-lined ground squirrel 1.5 mm posterior to the level of the bregma. Insert area is shown enlarged in B–D. The PVN showed no c-fos activity in ART (B), in ET (C), and in LT (D); however, in IBA strong c-fos expression was detected mainly in the parvocellular neurons of the nucleus (E). 3V, third ventricle; cc, corpus callosum; LH, lateral hypothalamus; LV, lateral ventricle; oc, optic chiasm; PVN, paraventricular nucleus. Scale bar = 50 μm.