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: However , intraspinal injection of 5 , 7 - DHT to produce a more selective lesion of only descending serotonin projections in the spinal cord did not affect this hypotension .
### Output Text:
However , intraspinal injection of <span class="chemical">5 , 7 - DHT</span> to produce a more selective lesion of only descending <span class="chemical">serotonin</span> projections in the spinal cord did not affect this hypotension . 