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: Together , these results suggest that the beta4 and the alpha3 subunits are mediators of nicotine - induced seizures and hypolocomotion .
### Output Text:
Together , these results suggest that the beta4 and the alpha3 subunits are mediators of <span class="chemical">nicotine</span> - induced seizures and hypolocomotion . 