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: GABA involvement in naloxone induced reversal of respiratory paralysis produced by thiopental .
### Output Text:
<span class="chemical">GABA</span> involvement in <span class="chemical">naloxone</span> induced reversal of respiratory paralysis produced by <span class="chemical">thiopental</span> . 