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 results indicate that acute nicotine injection induces a pronounced hyperactivity in rats habituated to the test environment .
### Output Text:
The results indicate that acute <span class="chemical">nicotine</span> injection induces a pronounced hyperactivity in rats habituated to the test environment . 