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: In addition , the role of dopamine receptors in mediating nicotine - induced locomotor stimulation was investigated by examining the effects of selective D1 and D2 dopamine receptor antagonists on activity induced by nicotine .
### Output Text:
In addition , the role of <span class="chemical">dopamine</span> receptors in mediating <span class="chemical">nicotine</span> - induced locomotor stimulation was investigated by examining the effects of selective D1 and D2 <span class="chemical">dopamine</span> receptor antagonists on activity induced by <span class="chemical">nicotine</span> . 