1. Department of Discovery Medicinal Chemistry, G. D. Searle Research and Development, 4901 Searle Parkway, Skokie, Illinois 60077, G. D. Searle Research and Development, 700 Chesterfield Parkway North, Chesterfield, Missouri 63198, and Department of Discovery Pharmacology, Searle Research and Development, 800 North Lindbergh Boulevard, St. Louis, Missouri 63167