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: OBJECTIVES : The present study investigates the effects of nicotine on anxiety induced by caffeine and another anxiogenic drug , pentylenetetrazole , in mice .
### Output Text:
OBJECTIVES : The present study investigates the effects of <span class="chemical">nicotine</span> on anxiety induced by <span class="chemical">caffeine</span> and another anxiogenic drug , <span class="chemical">pentylenetetrazole</span> , in mice . 