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: Midline B3 serotonin nerves in rat medulla are involved in hypotensive effect of methyldopa .
### Output Text:
Midline B3 <span class="chemical">serotonin</span> nerves in rat medulla are involved in hypotensive effect of <span class="chemical">methyldopa</span> . 