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: Selective stimulation of the DA D1 receptor may provide better integration of neural inputs transmitted to the internal segment of the globus pallidus ( referred to as the basal ganglia output ) compared with levodopa and selective DA D2 receptor agonist .
### Output Text:
Selective stimulation of the <span class="chemical">DA</span> D1 receptor may provide better integration of neural inputs transmitted to the internal segment of the globus pallidus ( referred to as the basal ganglia output ) compared with <span class="chemical">levodopa</span> and selective <span class="chemical">DA</span> D2 receptor agonist . 