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: Concentration of dopamine and its metabolites were decreased in the striatum of substantia nigra lesioned rats .
### Output Text:
Concentration of <span class="chemical">dopamine</span> and its metabolites were decreased in the striatum of substantia nigra lesioned rats . 