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: However , methylphenidate is a dopamine agonist via the inhibition of uptake of dopamine , and therefore dopaminergic systems in the brainstem ( mainly the midbrain ) and the spinal cord were unlikely to participate in the onset of this syndrome .
### Output Text:
However , <span class="chemical">methylphenidate</span> is a <span class="chemical">dopamine</span> agonist via the inhibition of uptake of <span class="chemical">dopamine</span> , and therefore dopaminergic systems in the brainstem ( mainly the midbrain ) and the spinal cord were unlikely to participate in the onset of this syndrome . 