8840
The opioid antagonist naltrexone ( 0 . 01 - 1 . 0 mg / kg ) antagonized the discriminative stimulus effects of heroin , but naltrexone at doses up to 10 mg / kg had no effect on the discriminative stimulus effects of cocaine .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: GENE 
Text: {0} 
Answer:
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