4280
When the l - methadone - sensitive , opioid receptor is blocked by naloxone or tolerance has developed , than l - methadone can produce behavioral effects by a nonopioid mechanism .
 opioid receptor
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: GENE 
Text: {0} 
Answer:
 opioid receptor