tokens: However , artificially ventilated rats , pretreated with MK - 801 , were more sensitive ( lethal cocaine dose , 76 . 6 + / - 8 . 0 mg / kg , n = 5 ) than vehicle pretreated rats ( 129 . 4 + / - 15 . 8 mg / kg , n = 6 ) , indicating that MK - 801 may increase both the respiratory and the cardiac toxicity of cocaine in urethane anesthetized rats .