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: Systemic pretreatment with ketanserin , a relatively specific type - 2 serotonin receptor antagonist , significantly attenuated the muscle rigidity produced in rats by the potent short - acting opiate agonist alfentanil .
### Output Text:
Systemic pretreatment with <span class="chemical">ketanserin</span> , a relatively specific type - 2 <span class="chemical">serotonin</span> receptor antagonist , significantly attenuated the muscle rigidity produced in rats by the potent short - acting opiate agonist <span class="chemical">alfentanil</span> . 