By focusing on cortical regions known to be affected in AD dementia, subtle but reliable atrophy is identifiable in asymptomatic individuals nearly a decade before dementia, making this measure a potentially important imaging biomarker of early neurodegeneration.

Output properties of neurons are greatly shaped by voltage-gated ion channels, whose biophysical properties and localization within axodendritic compartments serve to significantly transform the original input. The hyperpolarization-activated current, Ih, is mediated by HCN channels and plays a fundamental role in influencing neuronal excitability by regulating both membrane potential and input resistance. In neurons such as cortical and hippocampal pyramidal neurons, the subcellular localization of HCN channels plays critical functional role, yet mechanisms controlling HCN channel trafficking are not fully understood. Because ion channel function and localization are often influenced by interacting proteins, we generated a knockout mouse lacking the HCN channel auxiliary subunit, TRIP8b. Eliminating expression of TRIP8b dramatically reduced Ih expression in hippocampal pyramidal neurons. Loss of Ih-dependent membrane voltage properties was attributable to reduction of HCN channels on the neuronal surface, and there was a striking disruption of the normal expression pattern of HCN channels in pyramidal neuron dendrites. In heterologous cells and neurons, absence of TRIP8b increased HCN subunit targeting to and degradation by lysosomes. Mice lacking TRIP8b demonstrated motor learning deficits and enhanced resistance to multiple tasks of behavioral despair with high predictive validity for antidepressant efficacy. We observed similar resistance to behavioral despair in distinct mutant mice lacking HCN1 or HCN2. These data demonstrate that interaction with the auxiliary subunit TRIP8b is a major mechanism underlying proper expression of HCN channels and Ih in vivo, and suggest that targeting Ih may provide a novel approach to treatment of depression.

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In nondemented individuals, entorhinal cortex atrophy is associated with risk of Alzheimer disease.

The concept of amnestic mild cognitive impairment (MCI) describes older people who show a decline predominantly in memory function, but who do not meet criteria for dementia. Because such individuals are at high risk for developing Alzheimer's disease, they are of great interest for understanding the prodromal stages of the disease process. The mechanism underlying memory dysfunction in people with MCI is not fully understood. The present study uses quantitative, high-resolution structural MRI techniques to investigate, in vivo, the anatomical substrate of memory dysfunction associated with MCI. Changes in brain structures were assessed with two imaging techniques: (i) whole-brain, voxelbased morphometry to determine regions of reduced white matter volume and (ii) sensitive volumetric segmentation of the entorhinal cortex and hippocampus, gray matter regions that are critically important for memory function. In participants with amnestic MCI, compared with age-matched controls, results showed a significant decrease in white matter volume in the region of the parahippocampal gyrus that includes the perforant path. There was also significant atrophy in both the entorhinal cortex and the hippocampus. Regression models demonstrated that both hippocampal volume and parahippocampal white matter volume were significant predictors of declarative memory performance. These results suggest that, in addition to hippocampal atrophy, disruption of parahippocampal white matter fibers contributes to memory decline in elderly individuals with MCI by partially disconnecting the hippocampus from incoming sensory information.entorhinal cortex ͉ imaging ͉ parahippocampal gyrus ͉ perforant path ͉ voxel-based morphometry O ne of the first clinically recognized symptoms of Alzheimer's disease (AD) is diminished performance on tests assessing declarative memory function. The brain regions critically important for this type of memory are the hippocampus and related mesial temporal lobe structures (1-3). Previous histopathological findings (4-10) indicate that the entorhinal cortex and hippocampus are pathologically involved very early in patients with AD and in individuals with mild cognitive impairment (MCI) who are at high risk for developing AD (11,12). Specifically, two of these studies (7, 9) found a loss of entorhinal cortex layer II neurons in patients with AD and in those with MCI, compared with controls. These neurons receive multimodal sensory input from primary sensory and association cortices and project this information to the hippocampus via the perforant path, a white matter tract located in the anterior medial portion of the parahippocampal gyrus (13-15). Therefore, the loss of layer II neurons in the entorhinal cortex could essentially cause a disconnection of information flow to the hippocampus (10). In addition, damage to the white matter of the parahippocampal gyrus could disrupt afferent connections to the entorhinal cortex and ultimately disconnect multimodal sensory input to the hippocampus, information vital to th...