tokens: The effects of two unselective potassium ( K ( + ) - ) channel blockers , quinine ( 12 . 5 , 25 and 50 mg / kg ) and 4 - aminopyridine ( 1 and 2 mg / kg ) , on conditioned place preference and biphasic changes in motor activity induced by morphine ( 10 mg / kg ) were tested in Wistar rats .