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: We suggest that trazodone ( 5 , 10 and 20 mg / kg ) , by blocking the 5 - HT 2C receptors , releases the nigrostriatal DAergic neurons from tonic inhibition caused by 5 - HT , and thereby potentiates dexamphetamine stereotypy and antagonizes haloperidol catalepsy .
### Output Text:
We suggest that <span class="chemical">trazodone</span> ( 5 , 10 and 20 mg / kg ) , by blocking the <span class="chemical">5 - HT</span> 2C receptors , releases the nigrostriatal DAergic neurons from tonic inhibition caused by <span class="chemical">5 - HT</span> , and thereby potentiates <span class="chemical">dexamphetamine</span> stereotypy and antagonizes <span class="chemical">haloperidol</span> catalepsy . 