1. Department of Pharmacology, MRC Centre for Synaptic Plasticity, School of Medical Sciences, University Walk, University of Bristol, Bristol, BS8 1TD, UK, Department of Anatomy, MRC Centre for Synaptic Plasticity, School of Medical Sciences, University of Bristol, BS8 1TD, UK, and Neuroscience Research, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, Indiana 46285, USA