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: Binding of nicotine to nicotinic acetylcholine receptors ( nAChRs ) elicits a series of dose - dependent behaviors that go from altered exploration , sedation , and tremors , to seizures and death .
### Output Text:
Binding of <span class="chemical">nicotine</span> to nicotinic <span class="chemical">acetylcholine</span> receptors ( nAChRs ) elicits a series of dose - dependent behaviors that go from altered exploration , sedation , and tremors , to seizures and death . 