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 type - 2 serotonin antagonists may be clinically useful in attenuating opiate - induced rigidity , although further studies will be necessary to assess the interaction of possibly enhanced CNS , cardiovascular , and respiratory depression .
### Output Text:
Pretreatment with type - 2 <span class="chemical">serotonin</span> antagonists may be clinically useful in attenuating opiate - induced rigidity , although further studies will be necessary to assess the interaction of possibly enhanced CNS , cardiovascular , and respiratory depression . 