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: Previous studies have suggested that repeated exposure of rats to the drug or to the experimental environment is necessary to observe nicotine - induced locomotor stimulation .
### Output Text:
Previous studies have suggested that repeated exposure of rats to the drug or to the experimental environment is necessary to observe <span class="chemical">nicotine</span> - induced locomotor stimulation . 