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: Nicotine ( 0 . 25 mg / kg ) pretreatment blocked the caffeine - but not pentylenetetrazole - induced anxiety .
### Output Text:
<span class="chemical">Nicotine</span> ( 0 . 25 mg / kg ) pretreatment blocked the <span class="chemical">caffeine</span> - but not <span class="chemical">pentylenetetrazole</span> - induced anxiety . 