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Thus , the purpose of the present study was to investigate the role of DH serotonergic and gamma - aminobutyric acid ( GABA ) ergic mechanisms in the expression of contextual fear 24 h after conditioning , reflected by fear - potentiated startle ( FPS ) and freezing behavior .
 Chemical: gamma - aminobutyric acid; Chemical: GABA
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: gamma - aminobutyric acid; Chemical: GABA