In rats , morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone ( 3mg/kg , which produced 90 % mu opioid receptor occupancy ) , while 100 and 300 mg/kg LY2456302 ( which produced 56 % and 87 % mu opioid receptor occupancy , respectively ) only partially blocked morphine-induced mydriasis .