Exposure and/or sensitivity to stress have been implicated as conferring risk for development of Alzheimer's disease (AD). Although the basis for such a link remains unclear, we previously reported differential involvement of corticotropin-releasing factor receptor (CRFR) 1 and 2 in acute stress-induced tau phosphorylation (tau-P) and solubility in the hippocampus. Here we examined the role of CRFRs in tau-P induced by repeated stress and the structural manifestations of altered tau solubility. Robust tau-P responses were seen in WT and CRFR2 null mice exposed to repeated stress, which were sustained at even 24 h after the final stress exposure. A portion of phosphorylated tau in these mice was sequestered in detergent-soluble cellular fractions. In contrast, CRFR1 and CRFR double-KO mice did not exhibit repeated stress-induced alterations in tau-P or solubility. Similarly, treatment with CRFR1 antagonist attenuated repeated stress-induced tau-P. Using histochemical approaches in a transgenic CRFR1 reporter mouse line, we found substantial overlap between hippocampal CRFR1 expression and cells positive for phosphorylated tau after exposure to repeated stress. Ultrastructural analysis of negatively stained extracts from WT and CRFR2 null mice identified globular aggregates that displayed positive immunogold labeling for tau-P, as well as conformational changes in tau (MC1) seen in early AD. Given that repeated stress exposure results in chronic increases in hippocampal tau-P and its sequestration in an insoluble (and potentially prepathogenic) form, our data may define a link between stress and an AD-related pathogenic mechanism.neurofibrillary tangles | western blot | pathology | electron microscopy A lzheimer's disease (AD) is definitively characterized by the presence of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs). NFTs are insoluble intracellular inclusions composed of aggregated hyperphosphorylated forms of the cytoskeletal protein tau that accumulate initially within the hippocampal formation (1). A critical step in NFT formation is thought to be the transition of these phosphorylated tau species into aggregated insoluble fibrils. Hyperphosphorylated tau has a reduced ability to bind microtubules and has been reported to self-aggregate into paired helical filaments (PHFs), which compose NFTs (2, 3). The development of NFTs is positively correlated with cognitive decline and neuronal loss in AD (4, 5).The majority of AD cases are of sporadic origin, with age the primary risk factor. In addition, recent work has implicated environmental influences, such as stress, as also conferring risk for the development of AD (6, 7). In line with this, stress exposure can increase Aβ production and induce deficits in hippocampal cell proliferation and contextual memory in AD transgenic mice (8, 9). Moreover, exposure to a variety of physiological stressors can activate tau kinases and induce tau phosphorylation (tau-P) in rodents (reviewed in refs. 10-16). We previously reported that acute exposure to an emo...