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: The concentration of GABA was only slightly but significantly decreased in the colliculi without modifications in the other areas .
### Output Text:
The concentration of <span class="chemical">GABA</span> was only slightly but significantly decreased in the colliculi without modifications in the other areas . 