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: Specific dopamine - 1 , ( DA1 ) and dopamine - 2 ( DA2 ) receptor agonists are now under clinical investigation .
### Output Text:
Specific <span class="chemical">dopamine</span> - 1 , ( <span class="chemical">DA1</span> ) and <span class="chemical">dopamine</span> - 2 ( <span class="chemical">DA2</span> ) receptor agonists are now under clinical investigation . 