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: Nociceptive effects induced by intrathecal administration of prostaglandin D2 , E2 , or F2 alpha to conscious mice .
### Output Text:
Nociceptive effects induced by intrathecal administration of <span class="chemical">prostaglandin D2 , E2 , or F2 alpha</span> to conscious mice . 