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: Sub - chronic low dose gamma - vinyl GABA ( vigabatrin ) inhibits cocaine - induced increases in nucleus accumbens dopamine .
### Output Text:
Sub - chronic low dose <span class="chemical">gamma - vinyl GABA</span> ( <span class="chemical">vigabatrin</span> ) inhibits <span class="chemical">cocaine</span> - induced increases in nucleus accumbens <span class="chemical">dopamine</span> . 