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: Previous experiments in this laboratory have shown that microinjection of methyldopa onto the ventrolateral cells of the B3 serotonin neurons in the medulla elicits a hypotensive response mediated by a projection descending into the spinal cord .
### Output Text:
Previous experiments in this laboratory have shown that microinjection of <span class="chemical">methyldopa</span> onto the ventrolateral cells of the B3 <span class="chemical">serotonin</span> neurons in the medulla elicits a hypotensive response mediated by a projection descending into the spinal cord . 