In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: 50 mg / kg , i . v . thiopental produced respiratory arrest with further increase in GABA and decrease in glutamate again in cortex and brain stem without affecting any of the amino acids studied in four regions of rat brain .
### Output Text:
50 mg / kg , i . v . <span class="chemical">thiopental</span> produced respiratory arrest with further increase in <span class="chemical">GABA</span> and decrease in <span class="chemical">glutamate</span> again in cortex and brain stem without affecting any of the <span class="chemical">amino acids</span> studied in four regions of rat brain . 