The animals were randomly divided into four treatments : D ( @ g kg ( -@ ) of dexmedetomidine ) , DM ( dexmedetomidine @ g kg ( -@ ) and methadone @ mg kg ( -@ ) ) ; DMO ( dexmedetomidine @ g kg ( -@ ) and morphine @ mg kg ( -@ ) ) , and DT ( dexmedetomidine @ g kg ( -@ ) and tramadol @ mg kg ( -@ ) ) .