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: It is concluded therefore that , unlike the ventrolateral B3 cells which mediate a methyldopa - induced hypotension via descending projections , the midline serotonin B3 cells in the medulla contribute to the hypotensive action of methyldopa , either by way of an ascending projection which does not pass through the median forebrain bundle , or through a projection restricted to the caudal brainstem .
### Output Text:
It is concluded therefore that , unlike the ventrolateral B3 cells which mediate a <span class="chemical">methyldopa</span> - induced hypotension via descending projections , the midline <span class="chemical">serotonin</span> B3 cells in the medulla contribute to the hypotensive action of <span class="chemical">methyldopa</span> , either by way of an ascending projection which does not pass through the median forebrain bundle , or through a projection restricted to the caudal brainstem . 