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: The concentrations of dopamine ( DA ) , serotonin and their metabolites decreased significantly after MA administration , which was inhibited by the alpha - TC and DFO pretreatment .
### Output Text:
The concentrations of <span class="chemical">dopamine</span> ( <span class="chemical">DA</span> ) , <span class="chemical">serotonin</span> and their metabolites decreased significantly after <span class="chemical">MA</span> administration , which was inhibited by the <span class="chemical">alpha - TC</span> and <span class="chemical">DFO</span> pretreatment . 