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 , pretreatment with 5 , 10 and 20 mg / kg i . p . trazodone enhanced dexamphetamine stereotypy , and antagonized haloperidol catalepsy , ergometrine - induced WDS behavior and fluoxetine - induced penile erections .
### Output Text:
However , pretreatment with 5 , 10 and 20 mg / kg i . p . <span class="chemical">trazodone</span> enhanced <span class="chemical">dexamphetamine</span> stereotypy , and antagonized <span class="chemical">haloperidol</span> catalepsy , <span class="chemical">ergometrine</span> - induced WDS behavior and <span class="chemical">fluoxetine</span> - induced penile erections . 