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: CONCLUSIONS : Our results indicate that trazodone at 2 . 5 - 20 mg / kg does not block pre - and postsynaptic striatal D2 DA receptors , while at 30 , 40 and 50 mg / kg it blocks postsynaptic striatal D2 DA receptors .
### Output Text:
CONCLUSIONS : Our results indicate that <span class="chemical">trazodone</span> at 2 . 5 - 20 mg / kg does not block pre - and postsynaptic striatal D2 DA receptors , while at 30 , 40 and 50 mg / kg it blocks postsynaptic striatal D2 DA receptors . 