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: However , when given intravesically , PGE2 may stimulate micturition by releasing tachykinins from nerves in and / or immediately below the urothelium .
### Output Text:
However , when given intravesically , <span class="chemical">PGE2</span> may stimulate micturition by releasing <span class="chemical">tachykinins</span> from nerves in and / or immediately below the urothelium . 