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: For example , the symptoms of RLS can be dramatically improved by levodopa and dopamine agonists , whereas central dopamine D2 receptor antagonists can induce or aggravate RLS symptoms .
### Output Text:
For example , the symptoms of RLS can be dramatically improved by <span class="chemical">levodopa</span> and <span class="chemical">dopamine</span> agonists , whereas central <span class="chemical">dopamine</span> D2 receptor antagonists can induce or aggravate RLS symptoms . 