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: Mice sensitive to the convulsant effects of nicotine had greater alpha - bungarotoxin binding in the hippocampus than seizure insensitive mice .
### Output Text:
Mice sensitive to the convulsant effects of <span class="chemical">nicotine</span> had greater alpha - bungarotoxin binding in the hippocampus than seizure insensitive mice . 