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: Dopamine D2 receptors were elevated in the striatum , whereas serotonin 2C , but not serotonin 1A , receptors were elevated in the orbital frontal cortex .
### Output Text:
<span class="chemical">Dopamine</span> D2 receptors were elevated in the striatum , whereas <span class="chemical">serotonin</span> 2C , but not <span class="chemical">serotonin</span> 1A , receptors were elevated in the orbital frontal cortex . 