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: They suggest that , in normal conscious rats , the central tachycardia of bromocriptine appears to predominate and to mask the bradycardia of this agonist at peripheral dopamine D2 receptors .
### Output Text:
They suggest that , in normal conscious rats , the central tachycardia of <span class="chemical">bromocriptine</span> appears to predominate and to mask the bradycardia of this agonist at peripheral <span class="chemical">dopamine</span> D2 receptors . 