601
Antinociceptive effect of morphine was reduced in chronically treated rats ( 39 + / - 10 vs . 18 + / - 5 au ) while the combination - induced antinociception was remained similar as an acute treatment ( 298 + / - 7 vs . 280 + / - 17 au ) .
 morphine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 morphine