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: OBJECTIVES : Given its preclinical success for treating substance abuse and the increased risk of visual field defects ( VFD ) associated with cumulative lifetime exposure , we explored the effects of sub - chronic low dose GVG on cocaine - induced increases in nucleus accumbens ( NAcc ) dopamine ( DA ) .
### Output Text:
OBJECTIVES : Given its preclinical success for treating substance abuse and the increased risk of visual field defects ( VFD ) associated with cumulative lifetime exposure , we explored the effects of sub - chronic low dose <span class="chemical">GVG</span> on <span class="chemical">cocaine</span> - induced increases in nucleus accumbens ( NAcc ) <span class="chemical">dopamine</span> ( <span class="chemical">DA</span> ) . 