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: Further , 5 , 7 - DHT lesion of serotonin nerves travelling in the median forebrain bundle , one of the main ascending pathways from the B3 serotonin cells , did not affect the fall in blood pressure associated with a midline B3 serotonin methyldopa injection .
### Output Text:
Further , <span class="chemical">5 , 7 - DHT</span> lesion of <span class="chemical">serotonin</span> nerves travelling in the median forebrain bundle , one of the main ascending pathways from the B3 <span class="chemical">serotonin</span> cells , did not affect the fall in blood pressure associated with a midline B3 <span class="chemical">serotonin</span> <span class="chemical">methyldopa</span> injection . 