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 alpha3 and beta4 nicotinic acetylcholine receptor subunits are necessary for nicotine - induced seizures and hypolocomotion in mice .
### Output Text:
The alpha3 and beta4 nicotinic <span class="chemical">acetylcholine</span> receptor subunits are necessary for <span class="chemical">nicotine</span> - induced seizures and hypolocomotion in mice . 