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 stimulant action of nicotine was blocked by the central nicotinic antagonist mecamylamine but not by the peripheral nicotinic blocker hexamethonium , indicating that the response is probably mediated by central nicotinic receptors .
### Output Text:
The stimulant action of <span class="chemical">nicotine</span> was blocked by the central nicotinic antagonist <span class="chemical">mecamylamine</span> but not by the peripheral nicotinic blocker <span class="chemical">hexamethonium</span> , indicating that the response is probably mediated by central nicotinic receptors . 