1. Laboratory of Neurosciences (W.D., Z.G., H.J., M.P.M.), National Institute on Aging Gerontology Research Center, Baltimore, Maryland 21224;
2. Comparative Medicine Section (M.W.), National Institute on Aging Gerontology Research Center, Baltimore, Maryland 21224;
3. Department of Neuroscience (M.P.M.), Johns Hopkins University School of Medicine, Baltimore, Maryland 21205