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 effect appears to be mediated by central nicotine receptors , possibly located on dopaminergic neurons , and also requires the activation of both D1 and D2 dopamine receptors .
### Output Text:
The effect appears to be mediated by central <span class="chemical">nicotine</span> receptors , possibly located on dopaminergic neurons , and also requires the activation of both D1 and D2 <span class="chemical">dopamine</span> receptors . 