1. L. Kubin and R. O. Davies are in the Dept. of Animal Biology, School of Veterinary Medicine; A. I. Pack is in the Pulmonary and Critical Care Division, Dept. of Medicine; and are all members of the Center for Sleep and Respiratory Neurobiology, University of Pennsylvania, 991 Maloney Building/HUP, 3600 Spruce St., Philadelphia, PA 19104–4283, USA.