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: beta4 - / - mice were less sensitive to the effects of nicotine both at low doses , measured as decreased exploration in an open field , and at high doses , measured as sensitivity to nicotine - induced seizures .
### Output Text:
beta4 - / - mice were less sensitive to the effects of <span class="chemical">nicotine</span> both at low doses , measured as decreased exploration in an open field , and at high doses , measured as sensitivity to <span class="chemical">nicotine</span> - induced seizures . 