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: These findings suggest that tonic occupation of dopamine receptors impairs learning by competition with phasic dopamine signals .
### Output Text:
These findings suggest that tonic occupation of <span class="chemical">dopamine</span> receptors impairs learning by competition with phasic <span class="chemical">dopamine</span> signals . 