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: Receptor mechanisms of nicotine - induced locomotor hyperactivity in chronic nicotine - treated rats .
### Output Text:
Receptor mechanisms of <span class="chemical">nicotine</span> - induced locomotor hyperactivity in chronic <span class="chemical">nicotine</span> - treated rats . 