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: As a result , dopamine release and content , as well as its metabolites contents were decreased in caudate putamen .
### Output Text:
As a result , <span class="chemical">dopamine</span> release and content , as well as its metabolites contents were decreased in caudate putamen . 