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: We examined the role of the beta4 subunits in nicotine - induced seizures and hypolocomotion in beta4 homozygous null ( beta4 - / - ) and alpha3 heterozygous ( + / - ) mice .
### Output Text:
We examined the role of the beta4 subunits in <span class="chemical">nicotine</span> - induced seizures and hypolocomotion in beta4 homozygous null ( beta4 - / - ) and alpha3 heterozygous ( + / - ) mice . 