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: To investigate the relationship of prostaglandins ( PGs ) to seizure induction , the effects of six PG synthetase inhibitors on convulsions induced by flurothyl , picrotoxin , pentetrazol ( PTZ ) , electroshock or bicuculline were evaluated .
### Output Text:
To investigate the relationship of <span class="chemical">prostaglandins</span> ( <span class="chemical">PGs</span> ) to seizure induction , the effects of six PG synthetase inhibitors on convulsions induced by <span class="chemical">flurothyl</span> , <span class="chemical">picrotoxin</span> , <span class="chemical">pentetrazol</span> ( <span class="chemical">PTZ</span> ) , electroshock or <span class="chemical">bicuculline</span> were evaluated . 