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: Evidence for an involvement of D1 and D2 dopamine receptors in mediating nicotine - induced hyperactivity in rats .
### Output Text:
Evidence for an involvement of D1 and D2 <span class="chemical">dopamine</span> receptors in mediating <span class="chemical">nicotine</span> - induced hyperactivity in rats . 