This work was supported by the Adler Foundation, the McGowan Charitable Fund, gifts from the Powell and Wright Foundations, a Howard Hughes Medial Institute predoctoral fellowship (C.L.H.), and National Institutes of Health Grants GM38093 (F.M.B.) and NS24054 (W.C.M.). We thank Drs. Mark von Zastrow and Nigel Bunnett for the use of epifluorescence and confocal microscopes and for helpful discussions. We thank Janice Valletta for technical assistance. Mark Bunin was most helpful in preparing the hippocampal cultures.