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: Pre - treatment with a dose of 30 mg / kg 2h before picrotoxin microperfusion prevented seizures in the 75 % of the rats .
### Output Text:
Pre - treatment with a dose of 30 mg / kg 2h before <span class="chemical">picrotoxin</span> microperfusion prevented seizures in the 75 % of the rats . 