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: 25 mg / kg , i . v . thiopental produced anesthesia without altering respiratory rate , increased GABA , decreased glutamate , and had no effect on aspartate or glycine levels compared to controls in rat cortex and brain stem .
### Output Text:
25 mg / kg , i . v . <span class="chemical">thiopental</span> produced anesthesia without altering respiratory rate , increased <span class="chemical">GABA</span> , decreased <span class="chemical">glutamate</span> , and had no effect on <span class="chemical">aspartate</span> or <span class="chemical">glycine</span> levels compared to controls in rat cortex and brain stem . 