On different occasions separated by at least 1 week , dogs received intravenous ( IV ) administrations of tramadol ( 6.81 , 10.0 mg kg ( -1 ) ) , tapentadol ( 2.15 , 4.64 , 6.81 mg kg ( -1 ) ) or morphine ( 0.464 , 0.681 , 1.0 mg kg ( -1 ) ) with subsequent measurement of tail withdrawal latencies from a thermal stimulus ( for each treatment n = 5 ) .