[1] Department of Physiology, University of Toronto, Toronto, Ont. M5S 1A8, Canada
[2] Department of Pharmacology, University of Toronto, Toronto, Ont. M5S 1A8, Canada
[3] Dept. of Physiology, Anaesthesia, University of Toronto, Toronto, Ont. M5S 1A8, Canada
[4] Department of Neurosciences, University of New Mexico, Health Sciences Center, Albuquerque, NM 87131-5223, United States