2893
As naloxone and clonidine do not appear to interact with the same receptor site , the observed functional antagonism suggests the release of an endogenous opiate by clonidine or alpha - methyldopa and the possible role of the opiate in the central control of sympathetic tone .
 naloxone, clonidine, clonidine, alpha - methyldopa
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 naloxone, clonidine, clonidine, alpha - methyldopa