4397
Methisergide ( 0 . 5 mg / kg , i . p . ) , a blocker of serotonin receptors , prevented the development of the antinociceptive effect of serotonin .
 serotonin receptors
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: GENE 
Text: {0} 
Answer:
 serotonin receptors