1. From the Departments of Psychology (B.X., Z.Z., T.G.B., R.F.J., F.G., A.K.J.), Pharmacology (A.K.J.), and The Cardiovascular Center (A.K.J.), University of Iowa, Iowa City, IA; and Department of Physiology (M.H.) and Evelyn F. McKnight Brain Institute (M.H.), University of Arizona, Tucson, AZ.