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: Pretreatment with the D2 agonist PHNO enhanced nicotine - induced hyperactivity , whereas the D1 agonist SKF 38393 had no effect .
### Output Text:
Pretreatment with the D2 agonist <span class="chemical">PHNO</span> enhanced <span class="chemical">nicotine</span> - induced hyperactivity , whereas the D1 agonist <span class="chemical">SKF 38393</span> had no effect . 